Intensive Course on BNBC 2020

BNBC 2020

Intensive Course on BNBC 2020

বিএসসি ইন সিভিল ইঞ্জিনিয়ারিং পাশ করার পর যদি কেউ চাকরী বা প্রফেশনাল প্র্যাক্টিস করা শুরু করতে চায় তাহলে তার উচিত অন্তত বাংলাদেশ ন্যাশনাল বিল্ডিং কোড টা ভাল ভাবে অধ্যয়ন করা । সব না বুঝলে ও পুরো কোড টা একবার দেখতে হবে । কোথায় কি আছে জানতে হবে । অন্তত এটুকু বুঝতে হবে – কোডের কোন অংশ কোথায় প্রয়োজন হবে । যা বুঝতে পারছ না নোট করে রাখ । সব একবারে বুঝতে পারার কোন প্রয়োজন নেই ।  এই কোর্সটা তোমাদের প্রফেশনাল জীবনে পথ চলাকে সহজ করার জন্য আমাদের প্রয়াস। আশা করি ভাল লাগবে, উপকারে আসবে।

Civil Engineers who want to start their professional careers in Bangladesh after completing B.Sc. in Civil Engineering should study the Bangladesh National Building Code (BNBC) thoroughly. Even if you don’t understand some parts of BNBC, try to complete reading all the related Geotechnical and Structural related portions. You have to know the contents – which part, where are they located. If you don’t understand something, please note it down. Now, the question is which version of BNBC should be studied. We suggest studying BNBC 2020.  This course is our endeavour to assist you in your professional development. We hope that you would like it and it would benefit you.

What you'll learn

  • Serviceability limits, drift and deflection limits
  • Earthquake resistant design concept of RCC structures
  • Earthquake load calculation as per BNBC 2020
  • Wind load calculation as per BNBC 2020
  • Seismic detailing of RCC structures as per BNBC 2020
  • Soils and foundations as per BNBC 2020

Special Gift with The Course

Wind load calculation excel program

Prerequisite / Eligibility

  • BSc, MSc or PhD in Civil Engineering
  • Level 4 or 4th year student of bachelor’s in civil engineering

Course Teacher

Professor Dr. Jahangir Alam, Department of Civil Engineering, BUET, Dhaka, Bangladesh

Brief Biography of Course Teacher

Education

  • PhD in Geotechnical Earthquake Engineering, the University of Tokyo, Japan, 2005
  • MSc in Civil and Geotechnical Engineering, BUET, Dhaka, Bangladesh, 2002
  • BSc in Civil Engineering, BUET, Dhaka, Bangladesh, 1998

Biography

Professor Dr. Engr. Md. Jahangir Alam is faculty member at the Department of Civil Engineering, BUET, Dhaka-1000, since 1999. He completed his PhD from University of Tokyo in Geotechnical Earthquake Engineering in 2005 as a Monbu-Kagaku-sho Scholar. He was research fellow in Ecole Centrale Paris in 2008. Professor Jahangir did his BSc in Civil Engg with major in Structure and MSc in Civil and Geotechnical Engineering from BUET.

Professor Jahangir has multidisciplinary research interests and has publications in international journals and conferences. His current research topics are “Risk Sensitive Land Use Planning of Mega City”, Physical and Numerical Modeling of Liquefaction Hazard, Mitigation against Seismic Liquefaction, Cyclic Behavior of Non-plastic silt, Reinforced Earth, Earthquake Resistant Foundation in Soft Soil, Climate Resilient Concrete, Climate Resilient Road.

Professor Jahangir was involved in many consultancy projects where he designed/checked high rise RCC buildings, Communication towers, Jetty, Shore Pile, Embankment, Container Terminal, Ground Improvement, Bridge Foundation etc. He supervised many MSc and PhD students. He was involved in pile load testing, pile integrity testing, concrete mix design and development of laboratory and field-testing equipment.

Professor Jahangir actively involved in National and International professional bodies. He is life member and was Treasurer of Bangladesh Society for Geotechnical Engineering (BSGE), which is Bangladesh Chapter of ISSMGE. He is life fellow of Institute of Engineers Bangladesh (IEB). He is life member of Bangladesh Earthquake Society (BES). He was Treasurer of Bangladesh Earthquake Society (BES).

Certificate of Attendance

Certificate of attendance will be awarded after completion of all video lessons and quizzes

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Detail Course Outline

Definitions and general requirements

  • Definitions: Base shear, Basic wind speed, Bearing wall system, Braced frame, Building frame system, Concentric braced frame, Bearing wall system, collector, dead load, diaphragm, dual system, eccentric braced frame, horizontal bracing system, intermediate moment frame, live load, moment resisting frame, ordinary moment frame, primary framing system, shear wall, slender buildings and structures, soft storey, space frame, special moment frame, storey, storey, shear, strength, terrain, three-second gust speed, tower, three second gust, mean hourly, fastest mile wind, weak storey.
  • Basic Considerations: Building and Structure Occupancy Categories, Distribution of Horizontal Shear, Horizontal Torsional Moments, Stability Against Overturning and Sliding, Anchorage.
  • Basic Structural Systems: Special Moment Frames (SMF), Intermediate Moment Frames (IMF), Ordinary Moment Frames (OMF), Dual System.
  • Design for Lateral Loads: Design for wind load, Design for wind load

Drift and deflection limits

  • Basic considerations: Deflection limits, Control of deflections.
  • Serviceability for lateral loads: Drift and building separation, Load combination for serviceability, deflection due to creep, drift due earthquake loading.
  • Effective stiffness for determining lateral deflections
  • Cracked section and Input in analysis (ETABS)
  • Allowable storey drift limit

Basics of Earthquake Engineering

  • Response spectra
  • Frequencies of ground-motion
  • Spectral acceleration
  • Site Amplification
  • Ground Motion Deconvolution
  • Example of amplification
  • Double Resonance

Earthquake Load

  • Earthquake Resistant Design (Basic Concepts)
  • Characteristics of Earthquake Resistant Buildings: Structural Simplicity, Lateral stiffness, Structural Redundancy, Horizontal Bi-directional Resistance and Stiffness, Diaphragm Behavior.
  • Subsoil Investigation and Assessment of Site Conditions: Site investigation, Site classification.
  • Earthquake Ground Motion: Regional seismicity, Seismic zoning, Design response spectrum.
  • Vertical distribution of lateral forces
  • Site-Specific Design Spectrum
  • Seismic Design Category
  • Building irregularity
  • Load combinations
  • Earthquake Load Calculation

Wind Load

  • Wind load design procedures: Method 1: Simplified Procedure, Method 2: Analytical Procedure, Method 3: Wind Tunnel Procedure
  • Characteristics of Wind
  • Wind speed variation with height
  • Pressure co-efficient
  • Basic Wind Speed
  • Exposure
  • Wind Load Calculation

Detailing of Reinforcement in Concrete Structures

  • General requirements of detailing: Standard and seismic hook, Minimum bend diameter, Tolerance for placing reinforcement, Spacing of reinforcement, Bundled bar, Exposure condition and cover, Column detailing, Temperature and shrinkage bars, Structural integrity, Development length.
  • Seismic detailing: Occupancy Category, Importance factor, Seismic design category (OMRF, IMRF, SMRF), Flexural Members of Special Moment Frames (SMRF – beam), Special Moment Frame Members Subjected to Bending and Axial Load (SMRF – Column), Minimum flexural strength of columns , Special, Structural Walls and Coupling Beams, Requirements for Intermediate Moment Frames , Requirements for ordinary moment frame members, Comparison between OMRF, IMRF and SMRF.

Soils and Foundations

  • DIVISION A (Site Investigations, Soil Classifications, Materials and Foundation Types): Sub-Surface Survey, Methods of Exploration, Depth of Exploration, Geotechnical Investigation Report, USCS Classification, Organic soil, Expansive Soils, Collapsible Soils, Dispersive Soil, Exchangeable Sodium Percentage, Double Hydrometer Test, Soft Inorganic Soil.
  • DIVISION B (Service Load Design Method of Foundations): Bearing capacity of the soil, Dimension of footing, Depth of footing, Thickness of Footing, Scour, Mass Movement of Ground in Unstable Areas, Foundation Excavation, Design Considerations, Geotechnical design of shallow foundations, Settlement,
  • DIVISION C (Additional Considerations in Planning, Design and Construction of Building Foundations): Excavation, Dewatering, Slope stability of adjoining buildings.

BNBC Course content

1
BNBC intensive outline

CONTENTS

  • BNBC Part 6, Chapter 1 – Definition and General Requirement
  • BNBC Part 6, Chapter 2 – Load (wind and earthquake load)
  • BNBC Part 6, Chapter 3 – Soils and Foundations
  • BNBC Part 6, Chapter 8 – Seismic Detailing of RCC Structures
  • Software implementation of wind load and earthquake load


2
Bangladesh National Building Code (BNBC)-2017/2020 (pdf)

Volume 1

  • PART 1: SCOPE AND DEFINITION
  • PART 2: ADMINISTRATION AND ENFORCEMENT
  • PART 3: GENERAL BUILDING REQUIREMENTS, CONTROL AND REGULATION
  • PART 4: FIRE PROTECTION
  • PART 5: BUILDING MATERIALS


Volume 2

PART 6: STRUCTURAL DESIGN


Volume 3

  • PART 7: CONSTRUCTION PRACTICES AND SAFETY
  • PART 8: BUILDING SERVICES
  • PART 9: ADDITION, ALTERATION TO AND CHANGE OF USE OF EXISTING BUILDINGS
  • PART 10: SIGNS AND OUT-DOOR DISPLAY


3
All documents of BNBC 2020

Definitions and general requirements

1
Definitions and general requirements, Part-1

DEFINITIONS AND GENERAL REQUIREMENTS OF BNBC 2017

Table of contents

1.0 Definitions

2.0 Wind Speed Definitions

3.0 BNBC, 2017: Basic wind speed V = v3s

4.0 Basic considerations

5.0 Structural systems

5.1 Basic Structural Systems

6.0 Dual system

7.0 Design for lateral loads

7.1 Design for Wind Load

7.2 Design for Earthquake Forces

7.3 Overturning Requirements


1.0 Definitions

  • Base shear: Total design lateral force or shear at the base of a structure.
  • Basic wind speed: Three-second gust speed at 10m above the mean ground level in terrain Exposure-B and associated with an annual probability of occurrence of 0.02.
  • Bearing wall system: A structural system without a complete vertical load carrying space frame.
  • Braced frame: An essentially vertical truss system of the concentric or eccentric type which is provided to resist lateral forces.
  • Building frame system: An essentially complete space frame which provides support for loads.
  • Concentric braced frame (CBF): A steel braced frame.
  • Bearing wall system: A structural system without a complete vertical load carrying space frame.
  • Collector: A member or element used to transfer lateral forces from a portion of a structure to the vertical elements of the lateral force resisting elements.
  • Dead load: The load due to the weight of all permanent structural and non-structural components of a building or a structure, such as walls, floors, roofs and fixed service equipment.
  • Diaphragm: A horizontal or nearly horizontal system acting to transmit lateral forces to the vertical resisting elements. The term “diaphragm” includes horizontal bracing systems.
  • Dual system: A combination of Moment Resisting Frames and Shear Walls of Braced Frames to resist lateral load.
  • Eccentric braced frame (EBF): An eccentric braced frame is a structural system that is designed primarily to resist wind and earthquake forces and some members in the braced frame are eccentrically placed.
  • Horizontal bracing system: A horizontal truss system that serves the same function as a floor or roof diaphragm.
  • Intermediate moment frame (IMF): A concrete moment resisting frame.
  • Live load: The load superimposed by the use and occupancy of a building.
  • Moment resisting frame: A frame in which member and joints are capable of resisting forces primarily by flexure.
  • Ordinary moment frame (OMF): A moment resisting frame not meeting special detailing requirements for ductile behavior.
  • Primary framing system: That part of the structural system assigned to resist lateral forces.
  • Shear wall: A wall designed to resist lateral forces parallel to the plane of the wall (sometimes referred to as a vertical diaphragm or a structural wall).
  • Slender buildings and structures : Buildings and structures having a height exceeding five times the least horizontal dimension, or having a fundamental natural frequency less than 1Hz. For those cases where the horizontal dimensions vary with height, the least horizontal dimension at mid height shall be used.
  • T > 1 s = flexible structure
  • T < 1 s = rigid structure
  • Soft story : A soft story is one in which the lateral stiffness is less than 70 percent of that in the story above or less than 80 percent of the average stiffness of the three storeys above.
  • Space frame: A three-dimensional structural system without bearing walls composed of members interconnected so as to function as a complete self-contained unit with or without the aid of horizontal diaphragms or floor bracing systems.
  • Special moment frame (SMF): A moment resisting frame specially detailes to provide ductile behavior.
  • Storey: The space between any two floor levels including the roof of a building. Storey-x is the storey below level x.
  • Storey shear, vx: The summation of design lateral forces above the storey under consideration.
  • Strength: The usable capacity of an element or a member to resist the load as prescribed in these provisions.
  • Terrain: The ground surface roughness condition when considering the size and arrangement of obstructions to the wind.
  • Three-second gust speed: The highest average wind speed over a 3 second duration at a height of 10 m. The three-second gust speed is derived using Durst’s model in terms of the mean wind speed and turbulence intensity.
  • Tower: A tall, slim vertical structure.
  • Weak storey: Storey in which the lateral is less than 80 percent of that of the storey above.

2.0 Wind Speed Definitions

  1. Three Second Gust, V3s :
  2. Wind speed averaged over a period of three seconds.
  3. Codes: BS CP3, BNBC 2017, ASCE 7-05
  4. Mean hourly, Vmean :
  5. Wind speed averaged over a period of an hour.
  6. Codes: BS8100 (tower code).
  7. Fastest Mile Wind, VFM :
  8. Average speed of a one mile long sample of wind crossing a fixed point.
  9. Codes: BNBC 1993, TIA-EIA-F

3.0 BNBC, 2017: Basic wind speed V = v3s

3-sec gust wind at a height of 10m above ground in a terrain Exposure B having a return period of 50 years.

Figure 10: Wind speed vs. time graph of a typical storm


4.0 Basic considerations

1 Building and Structure Occupancy Categories:


Nature of Occupancy: Buildings and other structures that represent a low hazard to human life in the event of failure, including, but not limited to:

  1. Agricultural facilities.
  2. Certain temporary facilities.
  3. Minor storage facilities

Occupancy Category - i


Nature of Occupancy: All buildings and other structures except those listed in Occupancy Categories I, III and IV.

Occupancy Category - ii


Nature of Occupancy: Buildings and other structures that represent a substantial hazard to human life in the event of failure, including, but not limited to:

  1. Buildings with more than 300 people.
  2. Buildings with day care facilities with a capacity greater than 150.
  3. Buildings with elementary school or secondary school facilities with a capacity greater than 250.
  4. Buildings with a capacity greater than 500 for colleges or adult education facilities.
  5. Healthcare facilities with a capacity of 50 or more resident patients, but not having   surgery or emergency Treatment facilities.
  6. Jails and detention facilities.

Occupancy Category - iii


Nature of Occupancy: Buildings and other structures designated as essential facilities, including, but not limited to:

  1. Hospitals, Fire, rescue, ambulance, and police stations and emergency vehicle garages.
  2. Designated earthquake, hurricane, or other emergency shelters, emergency preparedness, communication.
  3. Power generating stations, Ancillary structures, Electrical substation structures, Aviation control towers, air traffic control centers.

Occupancy Category - iv


2 Distribution of Horizontal Shear: 

The total lateral force shall be distributed to the various elements of the lateral force-resisting system in proportion to their rigidities considering the rigidity of the horizontal bracing systems or diaphragms.


3 Horizontal Torsional Moments: 

Structural systems and components shall be designed to sustain additional forces resulting from torsion due to eccentricity between the centre of application of the lateral forces and the centre of rigidity of the lateral force resisting system. Forces shall not be decreased due to torsional effects.


4 Stability Against Overturning and Sliding

Every building or structure shall be designed to resist the overturning and sliding effects caused by the lateral forces specified in this Chapter.


5 Anchorage: 

Anchorage of the roof to wall and columns, and of walls and columns to foundations, shall be provided to resist the uplift and sliding forces resulting from the application of the prescribed loads. 


5. Structural systems

5.1 Basic Structural Systems:

Moment resisting frame system: 

Moment resisting frames also provide resistance to lateral load primarily by flexural action of members, and may be classified as one of the following types:

  • Special Moment Frames (SMF).
  • Intermediate Moment Frames (IMF).
  • Ordinary Moment Frames (OMF).

BEARING WALL SYSTEMS (no frame)

  • Special reinforced concrete shear walls.
  • Ordinary reinforced concrete shear walls.
  • Ordinary reinforced masonry shear walls.
  • Ordinary plain masonry shear walls.

BUILDING FRAME SYSTEMS (with bracing or shear wall)

  • Steel eccentrically braced frames, moment resisting connections at columns away from links.
  • Steel eccentrically braced frames, non-moment-resisting, connections at columns away from links.
  • Special steel concentrically braced frames.
  • Ordinary steel concentrically braced frames.
  • Special reinforced concrete shear walls.
  • Ordinary reinforced concrete shear walls.
  • Ordinary reinforced masonry shear walls.
  • Ordinary plain masonry shear walls.

MOMENT RESISTING FRAME SYSTEMS (no shear wall)

  • Special steel moment frames
  • Intermediate steel moment frames
  • Ordinary steel moment frames
  • Special reinforced concrete moment frames
  • Intermediate reinforced concrete moment frames
  • Ordinary reinforced concrete moment frames

DUAL SYSTEMS: SPECIAL MOMENT FRAMES CAPABLE OF RESISTING AT LEAST 25% OF PRESCRIBED SEISMIC FORCES (with bracing or shear wall) 

  • Steel eccentrically braced frames
  • Special steel concentrically braced frames
  • Special reinforced concrete shear walls
  • Ordinary reinforced concrete shear walls

DUAL SYSTEMS: INTERMEDIATE MOMENT FRAMES CAPABLE OF RESISTING AT LEAST 25% OF PRESCRIBED SEISMIC FORCES (with bracing or shear wall)

  • Special steel concentrically braced frames.
  • Special reinforced concrete shear walls.
  • Ordinary reinforced masonry shear walls.
  • Ordinary reinforced concrete shear walls.

DUAL SHEAR WALL-FRAME SYSTEM:

ORDINARY REINFORCED CONCRETE MOMENT FRAMES AND ORDINARY REINFORCED CONCRETE SHEAR WALLS.

STEEL SYSTEMS NOT SPECIFICALLY DETAILED FOR SEISMIC RESISTANCE.


6.0 Dual system

1.      An essentially complete space frame provides support for gravity loads.

2.      Resistance to lateral forces is provided by moment-resisting frames capable of resisting at least 25 percent of the design base shear and rest (less than 75%) by shear walls (ASCE 12.2.5.1).

3.      The two subsystems (moment-resisting frames and shear walls) are designed to resist the design base shear in proportion to their relative rigidities (ASCE 12.2.5.1).


7.0 Design for lateral loads

7.1 Design for Wind Load:

a)     Direction of wind: Structural design for wind forces shall be based on assumption that wind may blow from any horizontal direction.

b)     Design considerations: Design wind load on the primary framing systems and components of a building or structure shall be determined on the basis of the specific procedures considering the basic wind speed, shape and size of the building, and the terrain exposure condition of the site.

For slender buildings and structures, dynamic response characteristics, such as fundamental natural frequency, shall be determined to estimate gust response coefficient. Load effects, such as forces, moments, and deflections etc. on various components of building due to wind shall be determined from static analysis of the structure.

c)      Shielding effect: Reductions in wind pressure on buildings and structures due to apparent direct shielding effects of the up wind obstructions, such as man-made constructions or natural terrain features, shall not be permitted. *****

d)     Dynamic effects: Dynamic wind forces such as that from along-wind vibrations caused by the dynamic wind-structure interaction effects. For other dynamic effects such as cross-wind or torsional responses as may be experienced by buildings or structures having unusual geometrical shapes, response characteristics, or site locations, structural design.

e)      Wind tunnel test: Properly conducted wind-tunnel tests shall be required for those buildings or structures having unusual geometric shapes, response characteristics, or site locations for which cross-wind response such as vortex shedding, galloping etc.

f)      Wind loads during construction: Buildings, structures and portions thereof under construction, and construction structures such as formwork, staging etc. shall be provided with adequate temporary bracings or other lateral supports to resist the wind load on them during the erection and construction phase. *****

g)     Height limits: Unless otherwise specified elsewhere in this Code, no height limits shall be imposed, in general, on the design and construction of buildings or structures to resist wind-induced forces.


a)     Direction of wind: Structural design for wind forces shall be based on assumption that wind may blow from any horizontal direction.

b)     Design considerations: Design wind load on the primary framing systems and components of a building or structure shall be determined on the basis of the specific procedures considering the basic wind speed, shape and size of the building, and the terrain exposure condition of the site.

For slender buildings and structures, dynamic response characteristics, such as fundamental natural frequency, shall be determined to estimate gust response coefficient. Load effects, such as forces, moments, and deflections etc. on various components of building due to wind shall be determined from static analysis of the structure.

c)      Shielding effect: Reductions in wind pressure on buildings and structures due to apparent direct shielding effects of the up wind obstructions, such as man-made constructions or natural terrain features, shall not be permitted. *****

d)     Dynamic effects: Dynamic wind forces such as that from along-wind vibrations caused by the dynamic wind-structure interaction effects. For other dynamic effects such as cross-wind or torsional responses as may be experienced by buildings or structures having unusual geometrical shapes, response characteristics, or site locations, structural design.

e)      Wind tunnel test: Properly conducted wind-tunnel tests shall be required for those buildings or structures having unusual geometric shapes, response characteristics, or site locations for which cross-wind response such as vortex shedding, galloping etc.

f)      Wind loads during construction: Buildings, structures and portions thereof under construction, and construction structures such as formwork, staging etc. shall be provided with adequate temporary bracings or other lateral supports to resist the wind load on them during the erection and construction phase. *****

g)     Height limits: Unless otherwise specified elsewhere in this Code, no height limits shall be imposed, in general, on the design and construction of buildings or structures to resist wind-induced forces.


7.2 Design for Earthquake Forces:

The seismic forces on structures shall be determined considering seismic zoning, site soil 

characteristics, structure importance, structural systems and configurations, height and dynamic

properties of the structure.

a)     Requirements for directional effects: The directions of application of seismic forces used in the design shall be those which will produce the most critical load effects. Earthquake forces act in both principal directions of the building simultaneously. *****

b)     Structural system and configuration requirements : Seismic design provisions impose the following limitations on the use of structural systems and configurations:

  • The structural system used shall satisfy requirements of the Seismic Design Category and height limitations.
  • Structures assigned to Seismic Design Category D having vertical irregularity Type shall not be permitted. Structures with such vertical irregularity may be permitted for Seismic Design Category B or C but shall not be over two stories or 9 m in height. *****
  • Structures having irregular features shall be designed in compliance with the additional requirements.
  • Special Structural Systems may be permitted if it can be demonstrated by analytical and test data to be equivalent, with regard to dynamic characteristics, lateral force resistance and energy absorption, to one of the structural systems for obtaining an equivalent R and Cd value for seismic design.


7.3 Overturning Requirements: **

Every structure shall be designed to resist the overturning effects caused by wind or earthquake forces as well other lateral forces like earth pressure, tidal surge etc. The overturning moment Mx at any storey level-x of a building shall be determined as:

Where,

Mx = Summation of {Fi(hi-hx)}

  • Hi,hx,hn = Height in metres at level- I, -x or -n respectively.
  •  Fi = Lateral force applied at level- i, I = 1 to n

At any level, the increment of overturning moment shall be distributed to the various resisting elements in the same manner as the distribution of horizontal shear. Overturning effects on every element shall be carried down to the foundation level.

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Definitions and general requirements, Part-2
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Definitions and general requirements, Part-3
4
Definitions and general requirements, Part-4
5
Definitions and general requirements, Part-5
6
BNBC Quiz 1 : Definitions and general requirements
10 questions

Drift and deflection limits

1
Drift and deflection limits, Part-1

Drift and deflection limits


Table of Contents

1.0 Basic considerations

1.1 Serviceability: Deflection Limits a, b, c, h (Except earthquake load)

1.2 Control of Deflections

1.3 Serviceability:  Deflection limits

2.0 serviceability for lateral loads

2.1 Drift and Building Separation

2.2 Load Combination for Serviceability

2.3 Deflection due to creep

2.4 Drift due earthquake loading

2.5 Stiffness of frame members in analysis

2.6 Effective Stiffness for Determining Lateral Deflections

2.7 BNBC requirement of 1.4 times the cracked section for serviceability check

2.8 Input in Analysis (etabs)

2.9 Allowable Storey Drift Limit (Δa) for earthquake load

2.10 Separation between adjacent structures



1.0 Basic considerations

1.1 Serviceability: Deflection Limits a, b, c, h (Except earthquake load)

Construction

1 Roof members

  • Supporting plaster ceiling, L= L/360, Wf = L/360, Dg+Ld= L/240
  • Supporting non-plaster ceiling, L= L/240, Wf = L/240, Dg+Ld= L/180
  • Not supporting ceiling, L= L/180, Wf = L/180, Dg+Ld= L/120

2 Floor members , L= L/360, Dg+Ld= L/240

3 Exterior walls and interior partitions

  • With brittle finishes, Wf = L/240
  • With flexible finishes, Wf = L/120

4 Farm buildings, Dg+Ld= L/180

5 Greenhouses, Dg+Ld= L/120

Where, l,L,W and D stands for span of the member under consideration, live load, wind load and dead load respectively.


1.2 Control of Deflections

Minimum Thickness (h) of Nonprestressed beams or one-Way slabs Unless Deflections are calculated.

Member 1: Solid One- Way slabs (60 ksi)

  • Simply supported: L/20 , for fy>60 ksi multiply by (0.4+fy/700) MPa
  • One end continuous: L/24, for fy>60 ksi multiply by (0.4+fy/700) MPa
  • Both ends continuous: L/28, for fy>60 ksi multiply by (0.4+fy/100000) psi
  • Cantilever: L/10, for fy>60 ksi multiply by (0.4+fy/100000) psi

Member 2: Beams/Ribbed one-way slabs (60 ksi)

  • Simply supported: L/16, for fy>60 ksi multiply by (0.4+fy/700) MPa
  • One end continuous: L/18.5, for fy>60 ksi multiply by (0.4+fy/700) MPa
  • Both ends continuous: L/21, for fy>60 ksi multiply by (0.4+fy/100000) psi
  • Cantilever: L/8, for fy>60 ksi multiply by (0.4+fy/100000) psi


1.3 Serviceability:  Deflection limits :

1. Load deflecting :

  • For structural roofing and siding made of metal sheets, the total load deflection shall not exceed l/60.
  • For secondary roof structural members supporting formed metal roofing, the live load deflection shall not exceed l/150.
  • For secondary wall members supporting formed metal siding, the design wind load deflection shall not exceed l/90.
  • Interior partitions not exceeding 2m in height and flexible, folding and portable partitions are not governed by the provisions of this Section.

3. For cantilever members, I shall be taken as twice the length of the cantilever.******

4. The above deflections do not ensure against ponding. Roofs that do not have sufficient slope or camber to assure adequate drainage shall be investigated for ponding.

5. The wind load is permitted to be takes as 0.7 times the “component and cladding” loads for the purpose of determining deflection limits herein.*****

6. Deflection due to dead load shall include both instantaneous and long term effects.

7. For aluminum structural members or aluminum panels used in skylights and sloped glazing framing, roofs or walls of sunroom additions patio covers, not supporting edge of glass or aluminum sandwich panels, the total load deflection shall not exceed l/60.

8. For continuous aluminum structural members supporting edge of glass, the total load deflection shall not exceed l/175 for each glass lite or l/60 for for the entire length of the member, whichever is more stringent.

9. For aluminum sandwich panels used in roofs or walls of sunroom additions or patio covers, the total load deflection shall not exceed l/120.


2.0 Serviceability for lateral loads

2.1 Drift and Building Separation:

(a) Storey drift limitation :

Storey drift is the horizontal displacement of one level of a building or structure relative to the level above or below due to the design gravity (dead and live loads) or lateral forces (e.g. wind and earthquake loads). Calculated storey drift shall include both translational and torsional deflections and conform to the following requirements:

(I) Storey drift,  for lateral loads other than earthquake loads*, shall be limited as follows:

 < 0.005h   for T < 0.7 second

 < 0.004h   for T > 0.7 second *****

 < 0.0025h  for unreinforced masonry structures.

Where, h = height of the building or structure

 T = time period.

(ii) The drift limits set out in (i) above may be exceeded where it can be demonstrated that greater drift can be tolerated by both structural and nonstructural elements without affecting life safety.

(iii) For earthquake loads, the story drift,  shall be limited and given in ch. 2*


(b) Sway limitation:

The overall sway (horizontal deflection) at the top level of the building or structure due to wind loading shall be limited to 1/500 times of the total height of the building above ground.**


(c) Load combination

Load combination = 1.0 D + 0.5 L + 0.7 W

Lateral deflection at top level < h/500

This is not a criteria in ASCE-7-05

BNBC requirement of 1.4 times the cracked section for serviceability check


2.2 Load Combination for Serviceability

1. vertical deflection due to gravity load is: D+L [vertical and short term effect]

2. For serviceability limit states involving creep, settlement, or similar long-term or permanent effects, the suggested load combination is: D+0.5L [vertical and long term effect]

to account for long term creep effect, the immediate (e.g. elastic) deflection may be multiplied by a creep factor ranging from 1.5 to 2.0.

3. For serviceability limit state against lateral deflection of buildings and structures due to wind effect, the following combination shall be used: D+05L+0.7W [lateral and wind effect]

Due to its transient nature, wind load need not be considered in analyzing the effects of creep or other long-term actions. 


2.3 Deflection due to creep:

If the values are not obtained by a more comprehensive analysis, additional long-term deflection resulting from creep and shrinkage of flexural members (normal weight or lightweight concrete) shall be determined by multiplying the immediate deflection caused by the sustained load considered, by the factor λ△

where, ρ‘ shall be the value at midspan for simple and continuous spans, and at support for cantilevers. It shall be permitted to assume ξ, the time-dependent factor for sustained loads, to be equal to: 

λ△ =ξ/(1+50ρ‘)

ρ‘ is ratio of compression rebar As’/bd 

Table 3: Deflection due to creep (a)

5 years or more        2.0

12 months            1.4

6 months             1.2

3 months             1.0


Table 4:

1) Deflection due to creep in Sand and Hard Clay (b)

Isolated Foundations in Steel Structure:

  • Maximum Settlement : 50
  • Differential Settlement : 0.0033L
  • Angular Distortion : 1/300

Isolated Foundations in RCC Structure:

  • Maximum Settlement : 50
  • Differential Settlement : 0.0015L
  • Angular Distortion : 1/666

Notes: The values given in the Table may be taken only as a guide and the permissible total settlement, differential settlement and tilt (angular distortion) in each case should be decided as per requirements of the designer. 

L denotes the length of deflected part of wall/ raft or centre to centre distance between columns. 

H denotes the height of wall from foundation footing. 

* For intermediate ratios of L/H, the values can be interpolated


2.4 Drift due earthquake loading

δ x = Cd δxe / I

Δx = δx – δx-1


1.1 Stiffness of frame members in analysis

  1. Table 4-1 shows the range of values for the effective, cracked stiffness for each element based on the requirements of ACI 318 - 8.8.2. For beams cast monolithically with slabs, it is acceptable to include the effective flange width of ACI 318 - 8.12.
  2. More detailed analysis may be used to calculate the reduced stiffness based on the applied loading conditions.  For example, ASCE 41 recommends that the following (Table 4-2) Ie/Ig ratios be used with linear interpolation for intermediate axial loads.
  3. Note that for beams this produces Ie/Ig = 0.30. 
  4. When considering serviceability under wind loading, it is common to assume gross section properties for the beams, columns, and joints. *****

BNBC requirement of 1.4 times the cracked section for serviceability check


2.6 Effective Stiffness for Determining Lateral Deflections

Lateral deflections resulting from service lateral loads for reinforced concrete building systems shall be computed by either a linear analysis with member stiffness determined using 1.4 times the flexural stiffness defined in Sections 6.1.11.2 and 6.1.11.3 or by a more detailed analysis. Member properties shall not be taken greater than the gross section properties.


2.7 BNBC requirement of 1.4 times the cracked section for serviceability check

1.      Elastic second-order analysis

2.      Elastic second-order analysis shall consider section properties determined taking into account the influence of axial loads, the presence of cracked regions along the length of the member, and the effects of load duration

3.      It shall be permitted to use the following properties for the members in the structure:


Table 5: BNBC requirement of 1.4 times the cracked section for serviceability check

Value of I for:

  • Columns = 0.70 I
  • Shear Wall Uncracked = 0.70 I
  • Shear Wall Cracked = 0.35 I
  • Beams = 0.35 I
  • Flat plates & Flat Slabs = 0.25 I

Input in Analysis (ETABS) for Moment of Inertia I22 / I33

  • Columns = I22 = I33 = 0.70
  • Shear Wall Uncracked = modeled as shell – f11, f22 = 0.70
  • Shear Wall Cracked = modeled as shell – f11, f22 = 0.70
  • Beams = I22 = I33 = 0.35
  • Flat plates & Flat Slabs = modeled as membrane/shell – f11, f22, f12 = 0.25

BNBC requirement of 1.4 times the cracked section for serviceability check.


ACI 318 - 8.8.2

Table 7: Cracked stiffness modifiers.

Element le/lg

Beam 0.35-0.50

Column 0.50-0.70



ASCE 41

Table 8: Effective stiffness modifiers for columns.

Compression Due to Design Gravity Loads - le/lg

≥0.5Agf 'c 0.7

≥0.1Agf 'c 0.3



2.9 Allowable Storey Drift Limit (Δa) for earthquake load 

(a) Structures, other than masonry shear wall structures, 4 stories or less with interior walls, partitions, ceilings and exterior wall systems that have been designed to accommodate the story drifts.

  •  Occupancy Category i and ii = 0.025hsx
  •  Occupancy Category iii = 0.020hsx
  •  Occupancy Category iv = 0.015hsx

(b) Masonry cantilever shear wall structures :

  • Occupancy Category i and ii = 0.010hsx
  •  Occupancy Category iii = 0.010hsx
  •  Occupancy Category iv = 0.010hsx

(c) Other masonry shear wall structures :

  •  Occupancy Category i and ii = 0.007hsx
  •  Occupancy Category iii = 0.007hsx
  •  Occupancy Category iv = 0.007hsx

(d) All other structures :

  •   Occupancy Category i and ii = 0.020hsx
  •  Occupancy Category iii = 0.015hsx
  •  Occupancy Category iv = 0.010hsx

There shall be no drift limit for single-story structures with interior walls, partitions, ceilings, and exterior wall systems that have been designed to accommodate the storey drifts.


As per ASCE 7 Allowable story drift:

(a) Redundancy Factor ρ = 1.0

  •  Occupancy Category i and ii = 0.020hsx
  •  Occupancy Category iii = 0.015hsx
  •  Occupancy Category iv = 0.010hsx

(b) Redundancy Factor ρ = 1.3

  • Occupancy Category i and ii = 0.015hsx
  •  Occupancy Category iii = 0.012hsx
  •  Occupancy Category iv = 0.008hsx



2.10 Separation between adjacent structures

Buildings shall be protected from earthquake-induced pounding from adjacent structures or between structurally independent units of the same building maintaining safe distance between such structures as follows:

(i)        for buildings, or structurally independent units, that do not belong to the same property, the distance from the property line to the potential points of impact shall not be less than the computed maximum horizontal displacement (Sec 2.5.7.7) of the building at the corresponding level.

(ii)       for buildings, or structurally independent units, belonging to the same property, if the distance between them is not less than the square root of the sum- of the squares (SRSS) of the computed maximum horizontal displacements (Sec 2.5.7.7) of the two buildings or units at the corresponding level.

•      if the floor elevations of the building or independent unit under design are the same as those of the adjacent building or unit, the above referred minimum distance may be reduced by a factor of 0.7

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Drift and deflection limits, Part-2
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Drift and deflection limits, Part-3
4
Drift and deflection limits, Part-4
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Drift and deflection limits, Part-5
6
Drift and deflection limits, Part-6
7
BNBC Quiz 2 : Drift and deflection limits
10 questions

Basics of Earthquake Engineering

1
Basics of Earthquake Engineering, Part-1

Basics of Earthquake Engineering


Table of Contents

1.0 Geographical Layout

Section lines

1.1 Section ‘A-A’

1.2 Section ‘B-B’

2.0 EQ wave path

3.0 Typical Earthquake Records

3.1 Response spectra

4.0 Frequencies of ground-motion for engineering purposes

4.1 Frequency Response of Structures

5.0 Spectral acceleration/velocity/displacement

6.0 Site Amplification

6.1 Amplification of PGA as a function of VS30

7.0 1985 Mexican Earthquake

7.1 Example of amplification: 1985 Mexican Earthquake

8.0 Double Resonance



3 Response spectra

1.      Response means motion of structure in response to a base excitation (earthquake motion).

2.      For earthquake resistant design, the entire time history of response may not be required. Maximum value of response of a structure to particular base motion is important.

3.      The response spectrum describes the maximum response of a SDOF system to a particular input motion as function of natural frequency and damping ratio.


4 Frequencies of ground-motion for engineering purposes

1.      Up to 0.1 to 10 Hz (T=0.1 to 10 sec)

2.      Resonant period of typical N story structure ~ N/10 sec


5.0 Spectral acceleration/velocity/displacement

1.      The maximum value of response acceleration is called spectral acceleration (Sa)

2.      Thus, Sv, Sd

3.      Zero natural period = infinite natural frequency = rigid body è Sa= PGA


6.0 Site Amplification

1.      Ground shaking is amplified at “soft soil” (low velocity) sites

2.      Shear-wave velocity is commonly used to predict amplification

a.      VS30 ( time it takes for a shear wave to travel from a 30 m depth to the land surface, i.e., time-averaged 30-m velocity)


7.0 1985 Mexican Earthquake

1.      On September 19, 1985, a magnitude 8.1 earthquake struck the Pacific coast of Mexico, causing moderate damage in the epicentral area and extensive damage in Mexico City, 350 km from the epicenter.

2.      Hundreds of engineered multi-story buildings either collapsed or were irreparably damaged and over 8,000 lives lost in Mexico City.

3.      The duration of significant ground shaking is long, about a minute.



8.0 Double Resonance

1.      The damage was concentrated in the Lake Zone. This zone is underlain by very soft clay deposits with a shear velocity Vs ≅ 75 m/sec, and a total thickness of H ≅ 35~40 m supported by hard soil (dense sand and gravel)

T = 4H/Vs = (4(35))/75 = 2sec

  • The measured ground motions in the heavy damaged area had a predominant period of 2 sec.
  • The major damage occurred to structures with heights ranging from about 6 to 15 stories, i.e., having small-strain natural periods of vibration somewhat below 2 sec.

UNAM = Universidad Nacional Autónoma de México


2.      At the UNAM site, located on hard soil in the Hills zone of the city, the average peak ground acceleration (PGA) was in the order of 0.03g, and the spectral accelerations (for 5% damping) were about 0.10g for a range of period between 1~2 sec.

3.      On the other hand, at SCT site, located on soft clay in the heavy damaged zone, the average PGA is 0.14g, and there is a large peak at T ≅ 2 sec with spectral acceleration of 0.75g.

4.      Hence, the soil amplified the ground acceleration by a factor of 4 and it concentrated the energy of the ground motions at periods around 2 sec.

5.      As a result, the spectral accelerations, and thus the seismic forces acting on the buildings possessing this period were amplified 7~8 times on the average.

6.      The great contrast in stiffness at the boundary between the very soft clay and the hard soil under it, which essentially prevented any refraction back to the hard soil of the wave energy trapped in the soft clay, and thus contributed to the resonance of the soil profile.

7.      In the range of shear strains induced by the earthquake in the soil (≦0.3%), the Mexico city clay behaved almost linearly, with very little modulus reduction and small damping.

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Basics of Earthquake Engineering, Part-2
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Basics of Earthquake Engineering, Part-3
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Basics of Earthquake Engineering, Part-4
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Basics of Earthquake Engineering, Part-5
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BNBC Quiz 3 : Basics of Earthquake Engineering
15 questions

Earthquake load

1
Earthquake load, Part-1

Earthquake Resistant Design

1.0 Basic Concepts

1.1 General principles:

1.      To provide guidelines to minimize the risk to life for all structures,

2.      To increase expected performance of higher occupancy structures compared to ordinary structures ( by increasing importance factor).

3.      To improve the capability of essential structures to function after an earthquake (by increasing importance factor).

4.      Building design without any damage for a major earthquake event not economically feasible.

5.      To allow inelastic deformation & structural damage at preferred locations in structure.

6.      To prevent structural collapse during a major earthquake.

                                                               i.     Sa =   Cs

7.        Seismic zoning map divides country into four seismic zones.

8.      Design basis earthquake is taken as 2/3 of the maximum considered earthquake.

9.      Effects of earthquake ground motion expressed in terms of idealized elastic design acceleration response spectrum, depends on

10.  seismic zone coefficient and local soil condition defining ground motion

11.  importance factor (I) and response modification factor (R) representing building considerations

12.  Earthquake forces on the structure is reduced using the response modification factor R

13.  Importance factor I increases design forces for important structures.

14.  Elastic deformations calculated under these reduced design forces are multiplied by deflection amplification factor, Cd

15.  The soil supporting the structure will not liquefy, settle or slide during the earthquake.

16.  Premature failure due to shear or bond does not occur.

17.  Ductile detailing of reinforced concrete members is of prime importance

18.  In steel structures, high ductility should be obtained, avoiding premature failure due to elastic or inelastic buckling

19.  The building structure shall include complete lateral and vertical force-resisting systems

20.  Withstand the design ground motions within the prescribed limits of deformation and strength demand

21.  The design ground motions shall be assumed to occur along any horizontal direction

22.  The adequacy of the structural systems shall be demonstrated through the construction of a mathematical model



Characteristics of Earthquake Resistant Buildings

(i) Structural Simplicity, Uniformity and Symmetry: 

 Building structure shall be approximately symmetrical in plan 

• with respect to two orthogonal axes. 

• With respect to the lateral stiffness and mass distribution 


(ii) Lateral stiffness and mass of individual story shall remain

Constant or reduce gradually abrupt changes, from base to top


(iii) All structural elements of lateral load resisting systems, such as

cores, structural walls, or frames shall run without interruption from foundations to building top.


(iii) An irregular building may be subdivided into dynamically independent regular units well separated against pounding of the individual units to achieve uniformity


(iv)  Building lengts to breadth ratio should be less than 4

λ = Lmax / Lmin) < 4


(v) Structural Redundancy:

 High degree of redundancy accompanied by redistribution capacity through ductility is desirable: 

• Enabling more widely spread energy dissipation across entire structure and

• An increased total dissipated energy. 

 Use of evenly distributed structural elements increases redundancy. 

 Structural systems of higher static indeterminacy may result in higher response reduction factor R.  


#Redundancy means more than one path of resistance for lateral forces

Redundancy can be achieved by 

# providing a moment resisting frame with many columns and beams, all with ductile connections

OR

# Providing dual system (Moment Resisting Frame + Shear Wall) 



Characteristics of Earthquake Resistant Buildings

 Horizontal Bi-directional Resistance and Stiffness:

 Horizontal earthquake motion is a bi-directional phenomenon: 

• So the building structure needs to resist horizontal action in any direction 


 The structural elements of lateral force resisting system should be arranged in orthogonal (in plan) pattern 

  • Ensuring similar stiffness characteristics in both main directions 


 The stiffness characteristics of the structure should also limit the development of excessive displacements, which:  

  • Might lead to either instabilities due to second order effects or excessive damages 


 Diaphragm Behavior 

 In buildings, floors (including the roof) act as horizontal diaphragms 

• That collect -transmit inertia forces to vertical structural systems and

• Ensure those systems acting together in resisting the horizontal seismic action 


 Floor systems and the roof should be provided with 

• In-plane stiffness and resistance 

• Effective connection to the vertical structural systems 


 Particular care should be taken in cases of

• Non-compact or very elongated in-plan shapes 

• Large floor openings



 Foundation

 For buildings with individual foundation elements (footings or piles) The use of foundation Slab or Tie-beams is recommended 



Subsoil Investigation and Assessment of Site Conditions


Assessment of Site Conditions

Site investigation

For a structure belonging to Seismic Design Category C or D (Sec 2.5.5.2), subsoil investigation should also include determination of soil parameters for the assessment of the following:

  • Slope instability
  • Potential for Liquefaction and loss of soil strength
  • Differential settlement
  • Surface displacement due to faulting or lateral spreading
  • Lateral pressures on basement walls and retaining walls due to earthquake ground motion

Site class used to determine the effect of local soil conditions on the earthquake ground motion


Site classification

Ø Site will be classified as type SA, SB, SC, SD, SE, S1 AND S2 based on the provisions of this Section

Ø Classification will be done in accordance with Table 6.2.13 based on the soil properties of upper 30 meters of the site profile

Ø Average soil properties will be determined as given in the following equations

Ṽs = ∑_(i=1)^n▒di /∑_(i=1)^n▒di/Vsi

Ns = ∑_(i=1)^n▒di /∑_(i=1)^n▒di/Ni

Su = ∑(i=1)^k▒dci /∑(i=1)^k▒dci/Sui

Where,

n = Number of soil layers in upper 30m

di = Thickness of layer i

Vsi = Shear wave velocity of layer i

Ni = Field (uncorrected) Standard Penetration Value for layer i

K = Number of cohesive soil layers in upper 30 m

Dci = Thickness of cohesive layer i

Sui = Undrained shear strength of cohesive layer i


Ø Standard penetration value N measured without correction will be used

Ø Site classification should be done using average shear wave velocity Ṽs

  • If this can’t be estimated, the value of N may be used



Site Classification Based on Soil Properties

Site Class----soil profile------------,Vs (m/s)------SPT N ---------- shear Su (kPa)

SA---------------> 800----------------> 800 ----------------------------------------------

SB-------------360 – 800------------360 – 800--------> 50--------------> 250

SC-------------180 – 360 ----------180 – 360------15 – 50------------70 - 250

SD----------------< 180----------------< 180-----------< 15---------------< 70

*SE     --         --         --         --

S1 -------< 100 (indicative)----< 100 (indicative)-----------------------10 – 20

S2 --------------------------------------------------------------------------(Liquefiable) 


*SE = A soil profile consisting of a surface alluvium layer with Vs values of type SC or SD and thickness varying between about 5 m and 20 m, underlain by stiffer material with Vs > 800 m/s.


 For sites representing special soil type S1 or S2, site specific special studies for the ground motion should be done. 

 Soil type S1 (soft soil), having very low shear wave velocity and low material damping, can produce anomalous seismic site amplification and soil-structure interaction effects. 

 For S2 (liquefiable soil) soils, Liquefaction potential and possible consequences should be evaluated for design earthquake ground motions consistent with peak ground accelerations. 

 Any Settlement due to densification of loose granular soils under design earthquake motion should be studied. 

 The occurrence and consequences of geologic hazards such as slope instability or surface faulting should also be considered. 

 The dynamic lateral earth pressure on basement walls and retaining walls during earthquake ground shaking is to be considered as an earthquake load for use in design load combinations


Earthquake Ground Motion

Regional seismicity

Ø Bangladesh can be affected by moderate to strong earthquake for

•      Its proximity to collision boundary of

ü Northeast moving Indian plate and

ü Eurasian Plate

Ø Strong historical earthquakes with

•      Magnitude greater than 7.0

Affected parts of Bangladesh in last 150 years,

Ø Some of them had their epicenters within the country 



Seismic zoning

Ø To give an indication of MCE motion at different parts of country

Ø MCE motion considered 2% exceedance probability within period 50 years

Ø The country divided into four seismic zones with different levels of ground motion

Ø Each zone has a seismic zone coefficient (Z) which represents

•      The maximum considered peak ground acceleration (PGA)

•      On very stiff soil/rock (site class SA) in units of g (acceleration due to gravity)

Ø The zone coefficients (Z) of the four zones are:

q Z=0.12 (Zone 1)

q Z=0.20 (Zone 2)

q Z=0.28 (Zone 3)

q Z=0.36 (Zone 4)

Ø The most severe earthquake prone zone,

q  Zone 4 is in the northeast which includes Sylhet and has a maximum PGA value of 0.36g




Design response spectrum

Ø Earthquake ground motion is represented by design response spectrum

Ø Both static and dynamic analysis methods are based on response spectrum

Ø The spectrum is based on elastic analysis but Spectral accelerations are reduced by response modification factor R.

Ø For important structures,

•      Spectral accelerations are increased by importance factor I.

•      Design basis earthquake (DBE) ground motion is selected at ground shaking level that is 2/3 of the maximum considered earthquake (MCE) ground motion

Ø Effect of local soil conditions on response spectrum is incorporated in the normalized acceleration response spectrum Cs.

v The spectral acceleration for the design earthquake is given by the following equation:

Sa = 2/3 ZI/R Cs                     V = SaW

Where,

·        Sa = Design spectral acceleration (in units of g) shall not be less than 0.67ẞZIS

·        ẞ = Recommended value for ẞ is 0.11

·        Z = Seismic zone coefficient, as defined in Sec 2.5.5.2

·        I = Structure importance factor, as defined in Sec 2.5.5.1

·        R = Response reduction factor depends on type of structural system

The ratio  cannot be greater than one.

·        Cs = Normalized acceleration response spectrum



Seismic Weight, W


Seismic weight, W, is the total dead load of a building or a structure, including partition walls, and applicable portions of other imposed loads listed below:

a)        LL <= 3 kN/m2, DL+0.25LL

b)        LL >  3 kN/m2, DL+0.50LL

c)        Total weight (100%) of permanent heavy equipment or retained liquid or any imposed load sustained in nature shall be included.

 

 

 

Cs = Normalized acceleration response spectrum

Cs = S(1+T/TB(2.5ƞ – 1))     --   for 0 ≤ T ≤ TB

Cs = 2.5Sƞ    --  for TB ≤ T ≤ TC

Cs = 2.5Sƞ(Tc/T)      --   for Tc ≤ T ≤ TD

Cs = 2.5Sƞ(TcTD/T2) -- for TD ≤ T ≤ 4sec



Minimum Sa = 0.67βZIS

Sa = 2/3 ZI/R Cs    ----------      V = SaW


Ø       depends on Sand TB, TC and TD, which are all functions of site class

Ø Constant Cs value between periods TB and TC represents constant spectral acceleration

Ø S = Soil factor which depends on site class

Ø T = Structure (building) period

Ø TB= Lower limit of period of constant spectral acceleration branch as a function of   site class

Ø TC = Upper limit of period of constant spectral acceleration branch as a function of  site class

Ø TD= Lower limit of period of constant spectral displacement branch

Ø η= Damping correction factor

Where, ղ = √(10 /(5+ ζ) ) ≥ 0.55ξ

is the viscous damping ratio of the structure, expressed as a percentage of critical damping

The value of η cannot be smaller than 0.55




Site Dependent Soil Factor and Other Parameters Defining Elastic Response Spectrum

Soil type---------S--------TB (s)------TC (s)---------TD (s)

SA---------------1.00-------0.15-------0.40-----------2.0

SB---------------1.20-------0.15-------0.50-----------2.0

SC---------------1.15-------0.20-------0.20-----------2.0

SD---------------1.35-------0.20-------0.20-----------2.0

SE----------------1.40------0.15-------0.50-----------2.0

 


Approx. Period of structure, T

Structure type------------------------------------Ct-----------m

Concrete moment-resisting frames---------0.0466 -------0.9

Steel moment-resisting frames--------------0.0724 -------0.8

Eccentrically braced steel frame ------------0.0731------0.75

All other structural systems------------------0.0488------0.75


NOTE:

Consider moment resisting frames as frames which resist 100% of seismic force and are not enclosed or adjoined by components that are more rigid and will prevent the frames from deflecting under seismic forces.

           

T = Ct(hn)m              

hn        =         Height of building in metres from foundation or from top of rigid basement.

This excludes the basement storeys, where basement walls are connected with the ground floor deck or fitted between the building columns. But it includes the basement storeys, when they are not so connected.



Approx. T for masonry or concrete shear wall structures

 

T = 0.0062/√Cwhn 

Cw = 100/AB ∑_(i=1)^x (hn/hi)2        Ai/([1+0.83(hi/Di)2)

VERTICAL DISTRIBUTION OF LATERAL FORCES 

Fx = V (Wx hx ^k)/(∑_(i=1)^n wi hi ^k )

Fx = Part of base shear force induced at level x

wi  and wx = Part of the total effective seismic weight of the structure (W) assigned to level i or x

hi  and hx = the height from the base to level i or x

K = 1 For structure period £ 0.5s

           = 2 for structure period ≥ 2.5s

           = linear interpolation between 1 and 2 for other periods.

n = number of stories



Site-Specific Design Spectrum


Ø For site class S1 and S2

•      Site-specific studies needed to obtain design response spectrum

Ø For important projects

•      Site-specific studies carried out to determine spectrum

Ø The objective of such site-specific ground-motion analysis is

•      To determine ground motions for local seismic and site conditions with higher confidence than is possible using simplified equations.


Site Class---Occupancy Category I---------III-Occupancy Category IV

----------Zone 1-Zone 2-Zone 3-Zone 4--Zone 1-Zone 2-Zone 3-Zone 4

SA---------B--------C--------C-------D--------C--------D-------D-------D

SB---------B--------C--------D-------D--------C--------D-------D-------D

SC---------B--------C--------D-------D--------C--------D-------D-------D

SD---------C--------D--------D------D---------D-------D-------D-------D

S1, S2-----D-------D--------D-------D--------D--------D-------D-------D


Importance Factor

Occupancy Category----Importance factor I

I, II---------------------------------1.00

III---------------------------------1.25

IV---------------------------------1.50


Building irregularity:

 Plan irregularity

a. Torsion irregularity

b. Re-entrant corners

c. Out-of-Plane Offsets

d. Non-parallel Systems

e. Diaphragm Discontinuity 


Ø Vertical Irregularity:

  1. Stiffness Irregularity - Soft Storey
  2. Mass Irregularity
  3. Vertical Geometric Irregularity
  4. Vertical In-Plane Discontinuity in Vertical Elements Resisting Lateral Force
  5. Discontinuity in Capacity - Weak Storey



System Overstrength Factor, Ω_o

  • Higher R-factors represent more ductile systems and, therefore, yield a lower seismic design force. 
  • Similarly, the System Overstrength Factor, Ωo, is an amplification factor that is applied to the elastic design forces to estimate the maximum expected force that will develop.


Response Reduction Factor, Deflection Amplification Factor and Height Limitations for Different Structural Systems

Seismic Force–Resisting System Response Reduction Factor, R System Overstrength Factor, Ω_o Deflection Amplification

Factor, C_d Seismic Design Category B Seismic Design Category C Seismic Design Category D

A. BEARING WALL SYSTEMS (no frame)

B. BUILDING FRAME SYSTEMS (with bracing or shear wall)

1. Special reinforced concrete shear walls 

2. Ordinary reinforced concrete shear walls

3. Ordinary reinforced masonry shear walls

4. Ordinary plain masonry shear walls

1. Steel eccentricallybraced frames, moment resisting connections at columns away from links

2. Steel eccentricallybraced frames, non-moment-resisting connections at columns away from links

3. Special steel concentrically braced frames

4. Ordinary steel concentrically braced frames

B. BUILDING FRAME SYSTEMS (with bracing or shear wall) [frame resist 0-24% lateral load]

6. Ordinary reinforced concrete shear walls

7. Ordinary reinforced masonry shear walls

8. Ordinary plain masonry shear walls

C. MOMENT RESISTING FRAME SYSTEMS (no shear wall)

1. Special steel moment frames

2. Intermediate steel moment frames

3. Ordinary steel moment frames

4.Special reinforced concrete moment frames

5. Intermediate reinforced concrete moment frames

6. Ordinary reinforced concrete moment frames

D. DUAL SYSTEMS: SPECIAL MOMENT FRAMES CAPABLE OF RESISTING AT LEAST 25% OF PRESCRIBED SEISMIC FORCES (with bracing or shear wall) [framecapable to resist 25%+ lateral load]

1. Steel eccentrically braced frames

2. Special steel concentrically braced frames

2. Special steel concentrically braced frames

4. Ordinary reinforced concrete shear walls

E. DUAL SYSTEMS: INTERMEDIATE MOMENT FRAMES CAPABLE OF RESISTING AT LEAST 25% OF PRESCRIBED SEISMIC FORCES (with bracing or shear wall)

1. Special steel concentrically braced frames

2. Special reinforced concrete shear walls

3. Ordinary reinforced masonry shear walls

4. Ordinary reinforced concrete shear walls

F.DUAL SHEAR WALL-FRAME SYSTEM: ORDINARY REINFORCED CONCRETE MOMENT FRAMES AND ORDINARY REINFORCED CONCRETE SHEAR WALLS

G.STEEL SYSTEMS NOT SPECIFICALLY DETAILED FOR SEISMIC RESISTANCE


Load combinations for SMRF (SDC – D)

1. 1.4 D

2. 1.2 D +1.6 L 

3. (1.2 D + EV) + 1.0 E + 1.0 L

4. (0.9 D - EV) + 1.0 E

5. (1.2 D + EV) + 1.0 L + 1.0 E(X) + 0.3 E(Y)

6. (1.2 D + EV) + 1.0 L + 1.0 E(Y) + 0.3 E(X)

7. (0.9 D - EV) + 1.0 E(X) + 0.3 E(Y)

8. (0.9 D - EV) + 1.0 E(Y) + 0.3 E(X)


SDC-C + seismic plan irregularity V (non parallel system) = 100%Ex+30%Ey

SDC-D          = 100%Ex+30%Ey



Ø Combination of structural systems  

Combinations of Structural Systems in Different Directions:

ü Different seismic force–resisting systems are permitted to be used to resist seismic forces along each of the two orthogonal axes of the structure.

ü     Where different systems are used, the respective R and coefficients apply to each system, including the limitations on system use contained in Table 6.2.19


Combinations of Structural Systems in the Same Direction:

ü Where different seismic force–resisting systems are used in combination to resist seismic forces in the same direction of structural response, other than those combinations considered as dual systems, the more stringent system limitation contained in Table 6.2.19 shall apply

ü value of R used for design in that direction not greater than the least value of R for any of the systems utilized in that direction

ü       in the direction under consideration at any story shall not be less than the largest value of this factor for the R factor used in the same direction being considered


Ø     Provisions for Using System Overstrength Factor,

Ø Combinations of Elements Supporting Discontinuous Walls or Frames

ü Columns, beams, trusses, or slabs supporting discontinuous walls or frames of structures having horizontal irregularity Type IV of Table 6.1.5 or vertical irregularity Type IV of Table 6.1.4 have the design strength to resist the maximum axial force that can develop in accordance with the load combinations with overstrength factor.

ü The connections of such discontinuous elements to the supporting members shall be adequate to transmit the forces for which the discontinuous elements were required to be designed

Ø Increase in Forces Due to Irregularities for Seismic Design Categories D through E.

ü For structures assigned to Seismic Design Category D or E and having a horizontal structural irregularity of Type I.a, I.b, II, III, or IV in Table 6.1.5 or a vertical structural irregularity of Type IV in Table 6.1.4, the design forces determined from Section 2.5.7 shall be increased 25 percent for connections of diaphragms to vertical elements and to collectors and for connections of collectors to the vertical elements

ü Collector Elements Requiring Load Combinations with Overstrength Factor for Seismic Design Categories C through E


ü Batter Piles

ü Batter piles and their connections shall be capable of resisting forces and moments from the load combinations with overstrength factor of Section 2.5.13.4

ü Where vertical and batter piles act jointly to resist foundation forces as a group, these forces shall be distributed to the individual piles in accordance with their relative horizontal and vertical rigidities and the geometric distribution of the piles within the group



Vertical component of earthquake motion

The maximum vertical ground acceleration shall be taken as 50 percent of the expected horizontal peak ground acceleration (PGA).


The vertical seismic load effect  may be determined as:

                                              (6.2.56)

Where,

 = expected horizontal peak ground acceleration (in g) for design

 = effect of dead load, S = site dependent soil factor (see Table 6.2.16).  

2
Earthquake load, Part-2
3
Earthquake load, Part-3
4
Earthquake load, Part-4
5
Earthquake load, Part-5
6
Earthquake load, Part-6
7
Earthquake load, Part-7
8
Earthquake load, Part-8
9
Earthquake load, Part-9
10
Earthquake load, Part-10
11
Earthquake load, Part-11
12
Earthquake load, Part-12
13
BNBC Quiz 4 : Earthquake load
44 questions

Earthquake load example

1
Earthquake load example, Part-1
2
Earthquake load example, Part-2
3
Earthquake load example, Part-3
4
Earthquake load example, Part-4
5
Earthquake load example, Part-5
6
Earthquake load example, Part-6

Earthquake load input tutorial on Etabs as per BNBC 2020

1
Earthquake load input tutorial, Part-1
2
Earthquake load input tutorial, Part-2
3
Earthquake load input tutorial, Part-3
4
Earthquake load input tutorial, Part-4

Wind load

1
Wind load, Part-1

“WIND LOAD ON BUILDINGS & STRUCTURES”

( As per BNBC 2017 )




2.4 Wind Load


q 2.4.1 design wind loads determined using one of following procedures: 

Ø Method 1:       Simplified Procedure

Ø Method 2:       Analytical Procedure*****

Ø Method 3:       Wind Tunnel Procedure


Shielding

There shall be no reductions in velocity

pressure due to apparent shielding afforded by

buildings and other structures or terrain features.



Minimum Design Wind Loading

q For Main Wind-Force Resisting System: The design wind load

Ø     For enclosed or partially enclosed building   

ü  Ps ≥ 0.5 kN/m2

ü projected onto vertical plane normal to wind direction.

           

Ø For open buildings

ü Ps ≥ 0.5 kN/m2


q For Components and Cladding: The design wind load

•     Ps ≥ 0.5 kN/m2

•     acting in either direction normal to surface.



2.4.3 Method 2: Analytical Procedure


q Conditions:

Ø Building is regular-shaped

Ø Building does not have response characteristics making it subject to

•     across-wind loading

•     vortex shedding

•     instability due to galloping or flutter

•     does not have a site location for channeling effects



Laminar flow Vs.Turbulent flow


Wind induced oscillations


There are three forms of wind induced motion as follows:-

·        Galloping - Galloping is transverse oscillations of some structures due to the development of aerodynamic forces which are in phase with the motion.

·        Flutter - Flutter is unstable oscillatory motion of a structure due to coupling between aerodynamic force and elastic deformation of the structure. Perhaps the most common form is oscillatory motion due to combined bending and torsion. Long span suspension bridge decks or any member of a structure with large values of d/t ( where d is the depth of a structure or structural member parallel to wind stream and t is the least lateral dimension of a member ) are prone to low speed flutter.

·        3) Ovalling : This walled structures with open ends at one or both ends such as oil storage tanks and natural draught cooling towers in which the ratio of the diameter of minimum lateral dimension to the wall thickness is of the order of 100 or more, are prone to ovalling oscillations. These oscillations are characterized by periodic radial deformation of the hollow structure.



Method 2: Analytical Procedure


1.     basic wind speed (V) & wind directionality factor, Kd

2.     importance factor, I

3.     exposure category and velocity pressure exposure coefficient, Kz or Kh

4.     topographic factor, Kzt

5.     gust effect factor, G or Kf

6.     enclosure classification

7.     Internal pressure coefficient, G Cpi  

8.     External pressure coefficients, Cp  or GCpf , or force coefficients Cf ,

9.     Velocity pressure, qz or qh

10. Design wind load (P or F) determination



q  2.4.9.5 Velocity pressure  evaluated at height z:

q_z=0.000613*K_z K_zt K_d V^2 I ;  (kN/m2),V in m/s

Where

           K_d, wind directionality factor,

            K_z, velocity pressure exposure coefficient

            K_zt, topographic factor

           q_z ,velocity pressure at mean roof height h



Velocity Pressure:

           q_z=0.000613*K_z K_zt K_d V^2 I ;  (kN/m2)


Design Wind Load for:

Rigid Buildings of All Heights,    p=qGC_p-q_i (GC_pi )                (k N⁄m^2 )

Low-Rise Building (Rigid),          p=q_h [(GC_pf )-(GC_pi )]         (kN⁄m^2 )

Flexible Buildings,                          p=qG_f C_p-q_i (GC_pi )                (k N⁄m^2 )

Parapets,                                  p_p=q_p GC_pn                                   (kN⁄m^2 )

Components and cladding, p=q_h [(GC_p )-(GC_pi )] (k N⁄m^2 )    h≤18.3m

p=q(GC_p )-q_i (GC_pi ) (kN/m^2 )      h>18.3 m

 Design pressure for the MWFRSs of monoslope, pitched, or troughed roofs,

p=q_h GC_N



7. Design wind force for other structures, F=q_z GC_f A_f   (kN)            

  for windward walls evaluated at height above the ground

 for leeward walls, side walls, and roofs, evaluated at height

 for windward walls, side walls, leeward walls, and roofs of enclosed buildings and for negative internal pressure evaluation in partially enclosed buildings



Low-rise building?


Enclosed or partially enclosed buildings that comply with the following conditions

•     Mean roof height h less than or equal to 18.3 m.

•     Mean roof height h does not exceed least horizontal dimension.



Three Second Gust, V3s:

Wind speed averaged over a period of three seconds.

Codes: BS CP3, BNBC 2017, ASCE 7-05


Mean hourly, Vmean:

Wind speed averaged over a period of an hour.

Codes: BS8100 (tower code).


Fastest Mile Wind, VFM:

Average speed of a one mile long sample of wind crossing a fixed point.

Codes: BNBC 1993, TIA-EIA-F


BNBC, 2017: Basic wind speed V = V3s

3-sec gust wind at a height of 10m above ground in a terrain Exposure B having a return period of 50 years

Basic Wind Speed (V) map


Velocity Pressure:

q_z=0.000613*K_z K_zt K_d V^2 I (kN/m2)


* Tornadoes have not been considered in

developing the basic wind-speed distributions



Table 6.2.8: Wind Speed for some selected region in Bangladesh


Location         Basic Wind Speed (m/s)

Angarpota      47.8

Bagerhat         77.5

Bandarban      62.5

Barguna          80.0

Barisal 78.7

Bhola  69.5

Bogra 63.9

Brahmanbaria 56.7

Chandpur       50.6

Chapai Nawabganj     41.4

Chittagong     80.0

Chuadanga     61.9

Lalmonirhat    63.7

Madaripur      68.1

Magura           65.0

Manikganj      58.2

Meherpur        58.2

Maheshkhali   80.0

Moulavibazar 53.0

Munshiganj    57.1

Mymensingh  67.4

Naogaon         55.2

Narail 68.6

Narayanganj   61.1



Table 6.2.12: Wind directionality factor, Kd


Structure Type ------Directionality Factor K_d*

Buildings

 Main Wind Force Resisting System 0.85

 Components and Cladding   0.85

Arched Roofs 0.85

Chimneys, Tanks, and Similar   Structures  

  Square         0.90

  Hexagonal   0.95

  Round         0.95




Table: Importance Factor (accordingly)


Occupancy Category or Importance Class = --- i---ii---iii---iv

Non-Cyclone Prone Regions and Cyclone Prone Regions with V = 38-44 m/s = 0.87---1.0---1.15----1.15

Cyclone Prone Regions with V > 44 m/s = 0.77----1.00----1.15-----1.15



Exposure

q Surface roughness categories

Ø ground surface roughness within each 450 sector determined



q Surface Roughness A:

Ø  Urban and suburban areas,

Ø  wooded areas, or

Ø  closely spaced obstruction terrain

Ø  Mostly single family dwelling  



Exposure


q Surface Roughness B:

·        Open terrain with scattered obstructions

·        having heights generally less than 9.1 m.

·        includes flat open country,

·        grasslands and

·        all water surfaces in cyclone prone regions.




Exposure


q Surface Roughness C:

  •  Flat, unobstructed areas and

o  water surfaces outside cyclone prone regions

  •  smooth mud flats and salt flats



q Exposure categories:

q Exposure A:

·        Surface Roughness A

·        upwind direction prevail distance at least 792 m

·        20 times the height of the building



Exposure A:


q Exception: For buildings

·        mean roof height h ≤ 9.1 m

·        upwind distance may be reduced to 457 m




When H ≤ 9.1 m

Exposure


q Exposure B:

Ø Exposure B shall apply for all cases

•     where Exposures A or C do not apply




Exposure


q Exposure C:

Ø Surface Roughness C

Ø prevails in upwind direction for

•      distance ≥ 1,524 m 

•     20 times building height



Exposure


q Exposure C extend into downwind areas of Surface Roughness A or B

Ø for a distance of 200 m or

Ø 20 times height of the building,



q For site located in transition zone between exposure categories,

Ø category resulting in largest wind forces


q Exception: An intermediate exposure between preceding categories is:

Ø permitted in transition zone provided

Ø determined by rational analysis method

Ø defined in recognized literature.



q  Notes: Topographic factor       

                                               

o   For values of H/Lh, x/Lh and z/Lh other than those shown (previous slide)

·        linear interpolation is permitted


o   If H/Lh > 0.5 then assume H/Lh = 0.5 for

·        evaluating K1 and

·         substitute 2H for Lh for evaluating K2 and K3


o   Multipliers are based on assumption that

·        wind approaches hill or escarpment along the direction of maximum slope   



Gust Effect Factor


q Rigid structure ( T < 1 s)

Ø For rigid structures gust-effect factor shall be taken as 0.85

Ø or calculated by the formula:

G=0.925 (1+1.7g_Q I_z ̅ Q)/(1+1.7g_v I_z ̅ )         

I_z ̅ =c(10/z ̅ )^(1⁄6)   

Where,

I_z ̅ = the intensity of turbulence at height z ̅

z ̅ = equivalent height of structure as 0.6h, not less than z_min for heightsh.

g_Q and the value of g_v shall be taken as 3.4.



q Rigid structure ( T < 1 s)

Ø The background response Q is given by

Q=√(1/(1+0.63((B+h)/L_z ̅ )^0.63 ))"          


Where,

B, h are defined in Sec 2.1.4; and

L_z ̅ = integral length scale of turbulence at equivalent height L_z ̅ =l(z ̅/10)^ϵ ̅    

In which l and ∈ ̅ are constants



q Flexible or dynamically sensitive structures ( T > 1 s)


Ø For flexible or dynamically sensitive structures as

·        natural period greater than 1.0 second


the gust-effect factor shall be calculated by

G_f=0.925((1+1.7I_z ̅ √(g_Q^2 Q^2+g_R^2 R^2 ))/(1+1.7g_v I_z ̅ ))

The value of both  and   shall be taken as 3.4 and

g_R=√(2 ln(3600n_1) )+0.577/√(2 ln(3600n_1) )


           R, the resonant response factor, is given by                                                 

           R=√(1/β R_n R_h R_B (0.53+0.47R_L))                                     R_n=(7.47N_1)/(1+10.3N_1)^(5⁄3)

           N_1=(n_1 L_z ̅ )/V ̅_z ̅                                                                                 R_l=1/η-1/(2η^2 ) (1-e^(-2η) ) for η>0

R_l=1  for  η=0


·        n_1 = building natural frequency

·        R_l=R_h setting η=4.6 n_1 h/V ̅_z ̅

·        R_l=R_B setting η=4.6 n_1 B/V ̅_z ̅

·        R_l=R_L setting η=15.4  n_1 L/V ̅_z ̅

·        β = damping ratio, percent of critical

·        V ̅_z ̅ = mean hourly wind speed at height z ̅ (=0.6h) determined from Eq. 6.2.16.

·        V ̅_z ̅ =b ̅(z ̅/10)^∝ ̅ V [table 6.2.10]


Enclosure Classifications


q For purpose of determining internal pressure coefficients


Ø all buildings shall be classified as :

·        enclosed

·        partially enclosed

·        open



Enclosed building


o  A_o≤1.10A_oi

o  A_o≤0.37m^2  ≤ 0.01A_g 

o  A_oi/A_gi>0.20

o  A_o = total area of openings in a wall that receives positive external pressure (m2)

o  A_oi = sum of areas of openings in building envelope 

o  A_gi = sum of gross surface areas of building envelope not including A_g

o  A_g = the gross area of that wall in which A_o is identified (m2).



Partially enclosed building


  • A_o>1.10A_oi
  •  A_o>0.37m^2  >0.01A_g
  • A_oi/A_gi≤0.20
  • A_o = total area of openings in a wall that receives positive external pressure (m2)
  • A_oi = sum of areas of openings in building envelope 
  • A_gi = sum of gross surface areas of building envelope not including A_g



v Building envelope:

•     Cladding, roofing, exterior walls, glazing

•     door assemblies, window assemblies, skylight assemblies

•     other components enclosing the building.




Open building

           A_o≥0.8A_g 

where,

·         = total area of openings in wall that receives positive external pressure (m2)

·         = gross area of that wall in which  is identified (m2)



Velocity Pressure Exposure Coefficients, Kh  and Kz

For 4.57 m ≤ z zg        Kz = 2.01 (z/zg)2/α

For z < 4.57          Kz = 2.01 (4.57/zg)2/α

Note: z shall not be taken less than 9.1 m for rigid building in exposure A



Exposure----α--------z_g (m)

A------------7.0--------365.76

B------------9.5--------274.32

C------------11.5------213.36

Kz and Kh may be taken from Table 6.2.11 directly



Velocity Pressure:

q_z=0.000613*K_z K_zt K_d V^2 I ;  (kN/m2)


Design Wind Load for:


·       Rigid Buildings of All Heights,    p=qGC_p-q_i (GC_pi )                (k N⁄m^2 )

·       Low-Rise Building (Rigid),          p=q_h [(GC_pf )-(GC_pi )]         (kN⁄m^2 )

·       Flexible Buildings,                         p=qG_f C_p-q_i (GC_pi )                (k N⁄m^2 )

·       Parapets,                                 p_p=q_p GC_pn                                   (kN⁄m^2 )

·       Components and cladding, p=q_h [(GC_p )-(GC_pi )] (k N⁄m^2 )    h≤18.3m                        

p=q(GC_p )-q_i (GC_pi ) (kN/m^2 )      h>18.3 m

·       Design pressure for the MWFRS of monoslope, pitched, or troughed roof    p=q_h GC_N        

·       Design wind force for other structures, F=q_z GC_f A_f   (kN)            



Internal pressure coefficient, GCpi

Enclosure Classification---------------GCpi

Open Building ---------------------------0.00

Partially Enclosed Building------+0.55, -0.55

Enclosed Building------------------+0.18, -0.18

Notes:

1. Plus and minus signs signify pressures acting toward and away

   from the internal surfaces, respectively.

2. Values of GCpi shall be used with qz or qh as specified in Sec

    2.4.11.

3. Two cases shall be considered to determine the critical load

    requirements for the appropriate condition:

    (i) a positive value of GCpi applied to all internal surfaces

    (ii) a negative value of GCpi applied to all internal surfaces.



q  Reduction Factor for Large Volume Buildings,:

·        For a partially enclosed building

·        containing a single, unpartitioned large volume,

 

** internal pressure coefficient,  multiplied by reduction factor, :

(R_i=1.0    or,    R)_i=0.5(1+1/√(1+V_i/(6951A_og )))≤1.0


External Pressure Coefficient, Cp for wall


Roof Pressure Coefficients, Cp, for use with qh



Design Wind Loads on Enclosed and Partially Enclosed Buildings


·        Case 1. 

o  Full design wind pressure acting on the projected area perpendicular to each principal axis of the structure, considered separately along each principal axis.

·        Case 2. 

o  Three quarters of the design wind pressure acting on the projected area perpendicular to each principal axis of the structure in conjunction with a torsional moment as shown, considered separately for each principal axis. 

·        Case 3. 

§ Wind loading as defined in Case 1, but considered to act simultaneously at 75% of the specified value.

·        Case 4.  

o  Wind loading as defined in Case 2, but considered to act simultaneously at 75% of the specified value.


Notes:

·        Design wind pressures for windward and leeward faces shall be determined in accordance with the provisions of Sec 2.4.11 as applicable for building of all heights.

·        Diagrams show plan views of building.

·        Notation:

·        Pwx, PwY :  Windward face design pressure acting in the x, y principal axis, respectively.

·        PLX, PLY :              Leeward face design pressure acting in the x, y principal axis, respectively.

·        e(ex, ey):     Eccentricity for the x, y principal axis of the structure, respectively.

·        MT :          Torsional moment per unit height acting about a vertical axis of the building.




Low-rise shed (MWFRS)

H < 18.3 m

a = 10 percent of least horizontal dimension or 0.4h, whichever is smaller, but not less than either 4% of least horizontal dimension or 0.9 m.

h = Mean roof height, in meters, except that eave height shall be used for Θ  ≤ 10°.

Θ = Angle of plane of roof from horizontal, in degrees.



Enclosed, Partially Enclosed Buildings: Low-rise Walls & Roofs

combined gust &

external pressure

effect, GCpf



Enclosed, Partially Enclosed Buildings: Low-rise Walls & Roofs

combined gust &

external pressure

effect, GCpf




Enclosed, Partially Enclosed Buildings: Low-rise Walls & Roofs GCpf


Notes

·       For the design of the MWFRS providing lateral resistance in a direction parallel to a ridge line or for flat roofs, use θ = 0° and locate the zone 2/3 boundary at the mid-length of the building.

·       The roof pressure coefficient GCpf, when negative in Zone 2 or 2E, shall be applied in Zone 2/2E for a distance from the edge of roof equal to 0.5 times the horizontal dimension of the building parallel to the direction of the MWFRS being designed or 2.5 times the eave height, he, at the windward wall, whichever is less; the remainder of Zone 2/2E extending to the ridge line shall use the pressure coefficient GCpf for Zone 3/3E.


Notation:

·       a: 10 percent of least horizontal dimension or 0.4h, whichever is smaller, but not less than either 4% of least horizontal dimension or 0.9 m.

·       h: Mean roof height, in meters, except that eave height shall be used for θ ≤ 10°.

·       θ: Angle of plane of roof from horizontal, in degrees.



COMPONENT AND CLADDING FOR LOW RISE SHED

H < 18.3 M

Component and Cladding (h<18.3)

Internal pressure coefficient from figure 6.2.5



Note:

q Values of GCP for walls shall be reduced by 10% when θ ≤ 100.

Notations:

a: 10 percent of least horizontal dimension or 0.4h, whichever is smaller, but not less than either 4% of least horizontal dimension or 0.9m.

h: Mean roof height, in meters, except that eave height shall be used for Θ  ≤ 100.

Θ : Angle of plane of roof from horizontal, in degrees.



Special notes

v Critical Load Condition: Values of external and internal pressures shall be combined algebraically to determine the most critical load.

v Tributary Areas Greater than 65 m2: Component and cladding elements with tributary areas greater than 65 m2 shall be permitted to be designed using the provisions for MWFRSs.



Explanation of equations


Design wind pressure for MWFRS for rigid buildings of all height


q Main wind-force resisting systems (MWFRS)

Rigid Buildings of All Heights: Design wind pressures                       

p=qGC_p-q_i (GC_pi )  (k N⁄m^2 )                                

Where,

q=q_z  for windward walls evaluated at height z above ground

q=q_h  for leeward walls, side walls, roofs, evaluated at height h

q_i=q_h for windward walls, side walls, leeward walls, roofs etc

q_i=q_z for positive internal pressure in partially enclosed buildings

For positive internal pressure evaluation, q_i at height h=(q_i=q_h )

G= gust effect factor

C_p= external pressure coefficient

GC_pi= internal pressure coefficient

q and q_i shall be evaluated using exposure



Design wind pressure for MWFRS for low rise building (h<18.3m)


q Low-Rise Building: design wind pressures for the MWFRS


shall be determined by :

p=q_h [(GC_pf )-(GC_pi )] (kN⁄m^2 )                



Design wind pressure for MWFRS for flexible buildings of all height


q Flexible Buildings: Design wind pressures for MWFRS   

Ø shall be determined by:

     p=qG_f C_p-q_i (GC_pi ) (k N⁄m^2 )          

 Where,

 q, q_i, C_p, and GC_pi shown in previous section

G_f= gust effect factor



Design wind pressure at parapet


q Parapets: design wind pressure for effect of parapets:

p_p=q_p GC_pn  (kN⁄m^2 )

 Where,

p_p= Combined net pressure front and back parapet surfaces.

(Plus and minus signs signify net pressure direction)

q_p= Velocity pressure evaluated at top of parapet

GC_pn                        

= Combined net pressure coefficient

= +1.5 for windward parapet

= −1.0 for leeward parapet



Components and claddings for buildings with h<18.3 m


Components and claddings for buildings with h>18.3 m



FLOW CHART FOR WIND LOAD CALCULATION (MWFRS)



Summary


  • for building main structure, internal pressure is not needed
  •  for component and cladding design, internal and external both pressures are need to be considered
  •  for shed main structure, both external and internal pressure have to be applied
  •  for shed component and cladding, both external and internal pressure have to be applied


2
Wind load, Part-2
3
Wind load, Part-3
4
Wind load, Part-4
5
Wind load, Part-5
6
Wind load, Part-6
7
Wind load, Part-7
8
Wind load, Part-8
9
Wind load, Part-9
10
Wind load, Part-10
11
BNBC Quiz 5 : Wind load
31 questions

Wind load Examples

1
Wind load example-1, Part-1
2
Wind load example-1, Part-2
3
Wind load example-1, Part-3
4
Wind load example-1, Part-4
5
Wind load example-2, Part-1
6
Wind load example-2, Part-2
7
Wind load example-2, Part-3
8
Wind load example-2, Part-4
9
Wind load example-2, Part-5
10
Wind load example-2, Part-6
11
Wind load example-3, Part-1
12
Wind load example-3, Part-2
13
Wind load example-3, Part-3

Wind load input tutorial on Etabs as per BNBC 2020

1
Wind load input tutorial, Part-1
2
Wind load input tutorial, Part-2
3
Wind load input tutorial, Part-3
4
Wind load input tutorial, Part-4

Seismic Detailing

1
General requirement of seismic detailing, Part-1

DETAILING OF REINFORCEMENT IN CONCRETE STRUCTURES

( According to BNBC 2017 )


Contents of this presentation

·        General requirements of detailing

•      Standard and seismic hook

•      Minimum bend diameter

•      Tolerance for placing reinforcement

•      Spacing of reinforcement

•      Bundled bar

•      Exposure condition and cover

•      Column detailing

•      Temperature and shrinkage bars

•      Structural integrity

•      Development length



Standard hooks


For free end of bar,

  • 180o bend plus an extension≥4db 65 mm 
  • 90o bend plus an extension ≥ 12db 

§ For stirrup and tie anchorage:

  •   For db≤16mm, 90o bend plus an extension ≥ 6db 
  •  For 19≤db≤25mm, 90o bend plus an extension≥12db
  •   For db≤25mm, 135o bend plus ≥ 6db   
  •  For closed ties and continuously wound ties, 135o bend plus an extension ≥      4db 65 mm 



For free For stirrup and tie anchorage of bar

Seismic Hook

  • Seismic hook bend should be ≥ 1350
  •  Circular hoops bend should be ≥ 900 

·        Seismic hook extension ≥ 6db, 75mm


Minimum bend diameters

The minimum diameter of bend for standard hook and for tie and stirrup hooks if db ≥16mm

Bar Size------------------------------Minimum Diameter of Bend

10 mm ≤ d_b ≤ 25 mm---------6d_b

25 mm < d_b ≤ 40 mm----------8d_b

40 mm < d_b ≤ 57 mm----------10d_b


·        For stirrups and tie hooks, if db≤16mm, inside diameter of bend ≥4 db


Spacing of Reinforcement

·        The minimum clear spacing between parallel bars of slab ≥1db , 25 mm

·        1.33* maximum nominal size of coarse aggregate

·        Where parallel reinforcement is placed in two or more layers, clear distance between layers ≥25 mm

·        For compression members, the clear distance between longitudinal bars ≥1.5 db , 40 mm

·        1.33* maximum nominal size of coarse aggregate

·        In walls and one-way slabs the maximum bar spacing ≤3*wall or slab thickness h, 450 mm

·        For two-way slabs, maximum spacing of bars ≤ 2* slab thickness h, 450 mm

·        For temperature steel, maximum spacing ≤ 5* slab thickness h , 450 mm



Bundled bars

·        Groups of parallel reinforcing bars bundled in contact to act as a single unit shall be limited to four.

·        Bundled bars shall be enclosed within stirrups or ties.

·        Bars larger than 32 mm diameter shall not be bundled in beams.              

·        Individual bars within a bundle terminated within the span of flexural members shall terminate at different points with at least 40db stagger.

·        Where spacing limitations and minimum concrete cover are based on bar diameter db, a unit of bundled bars shall be treated as a single bar of a diameter derived from the equivalent total area

·        Exposure Condition and Minimum Cover



Exposure Condition and Recommended Cover

·        Cast-in-place concrete :

  • Minimum concrete cover (permanently exposed to earth)= 75 mm.
  • Concrete exposed to earth or weather, the minimum clear cover shall be as under

    19 mm to 57 mm bar diameter:                                  50 mm

    16 mm diameter bar and smaller:                                               40 mm

 

 

Concrete in Corrosive Environments

  1. A specified concrete cover for reinforcement not less than 50 mm for walls and slabs and not less than 65 mm for other members may be used. For precast concrete members a specified concrete cover not less than 40 mm for walls and slabs and not less than 50 mm for other members may be used.
  2. Minimum compressive strength of concrete  for the corrosive environment or other severe exposure conditions shall be 25 MPa with minimum cement of 400 kg per cubic meter. Coarse aggregate shall be 20 mm down well-graded stone chips and fine aggregate shall be coarse sand of minimum FM 2.20.
  3. For any non-structural member like drop wall, railing, fins etc., 12 mm down well graded stone chips may be used as coarse aggregate.
  4. Use of brick chips (khoa) as coarse aggregate is strictly prohibited for the corrosive environment or other severe exposure conditions.
  5. Water cement ratio shall be between 0.4-0.45. Potable water shall be used for all concreting

8.1.9 Lateral Reinforcement for Columns



Spiral:

·        Size of spirals for cast in situ construction ≥ 10 mm diameter

·        Clear spacing between spirals 25 mm ≤ s ≤ 75 mm

·        Anchorage of spiral reinforcement shall be provided by 1.5 extra turns of spiral bar or wire at each end of a spiral unit.

·        Lap Splices ≥      48 spiral diameter for deformed uncoated bar or wire, 72 spiral diameter for other cases, 300 mm

 

Ties:

·        Vertical spacing of ties ≤ 16 db   ,  48 dtie,  Least dimension of column

 

 

8.1.8 Reinforcement Details for Columns

Offset Bars: Offset bent longitudinal bars shall conform to the following:

(a)   The maximum slope of inclined portion of an offset bar with axis of column shall not exceed 1 in 6.

(b)   Portions of bar above and below an offset shall be parallel to the axis of column.

(c)    Horizontal support at offset bends shall be provided by lateral ties, spirals, or parts of the floor construction. Horizontal support provided shall be designed to resist 1.5 times the horizontal component of the computed force in the inclined portion of the offset bars. Lateral ties or spirals, if used, shall be placed not more than 150 mm away from points of bend.

(d)   Offset bars shall be bent before placement in the forms (see Sec 8.1.3).

(e)   Where the face of the column above is offset 75 mm or more from the face of the column below, longitudinal bars shall not be permitted to be offset bent. The longitudinal bars adjacent to the offset column faces shall be lap spliced using separate dowels. Lap splices shall conform to Sec 8.2.14.

(f)     The lowest tie in any storey shall be placed within one-half the required tie spacing from the top most horizontal reinforcement in the slab or footing below.

(g)   The uppermost tie in any storey shall be within one-half the required tie spacing from the lowest horizontal reinforcement in the slab or drop panel above.

(h)   Where beams or brackets provide concrete confinement at the top of the column on all (four) sides, top tie shall be within 75 mm of the lowest horizontal reinforcement

(i)     Ties shall be arranged such that every corner and alternate longitudinal bar shall have lateral support provided by the corner of a tie with an included angle not more than 135o.

(j)     No vertical bar shall be farther than 150 mm clear on each side along the tie from such a laterally supported bar



Shrinkage and Temperature Reinforcement

·        Area of shrinkage and temperature reinforcement shall provide at least the following ratios of reinforcement area to gross concrete area:

·        In any case, the reinforcement ratio shall not be less than 0.0014.

·        Area of shrinkage and temperature reinforcement for brick aggregate concrete shall be at least 1.5 times that provided in above.

·        Spacing of shrinkage and temperature reinforcement ≤ 5*slab thickness, 450 mm



Brick aggregate concrete

Area of shrinkage and temperature reinforcement for brick aggregate concrete shall be at least 1.5 times that provided in above.



Development of Deformed Bars and Deformed Wires in Tension

·        Development length for deformed bars and deformed wire in tension, ld ≥300 mm

·        For deformed bars or deformed wire, ld shall be as follows:


Clear spacing of bars or wires being developed or spliced not less thand_b, clear cover not less than d_b, and stirrups or ties throughout l_d not less than the Code minimum

Or, Clear spacing of bars or wires being developed or spliced not less than 2d_b and clear cover not less than d_b *****

((f_y ψ_t ψ_e)/(2.1λ√(f'_c ))) d_b

Where

·        ld=development length

·        fy=yield strength of the tension rebars (Mpa)

·        fc’=compressive strength of concrete (Mpa)

·        db=bar diameter (mm)

·        ψt=rebar location factor that accounts for the position of rebars in freshly placed concrete

·        ψe=rebar coating factor reflecting the effects of epoxy coating

·        ψs= rebar size factor

·        λ= lightweight aggregate concrete factor


The factors used in the expressions for development of deformed bars and deformed wires in tension are as follows:

·        Where horizontal reinforcement is placed such that more than 300 mm of fresh concrete is cast below the development length or splice, ψt=1.3. For other cases, ψt=1.0.

·        For epoxy-coated bars or wires with cover less than 3db, or clear spacing less than 6db, ψe=1.5. For all other epoxy-coated bars or wires, ψe=1.2. For uncoated and zinc-coated (galvanized) reinforcement, ψe=1.0. However, the product ψt ψe need not be greater than 1.7.

·        For 19 mm diameter and smaller bars, and deformed wires, ψs=0.8. For 20 mm diameter and larger bars, ψs=1.0.

·        Where lightweight concrete is used, λ shall not exceed 0.75 unless fct is specified. Where normal weight concrete is used, λ=1.0.

o  Development of Deformed Bars and Deformed Wires in Compression

·        Development length for deformed bars and deformed wire in compression, ldc  ≥ 200 mm

·        Development length for deformed bars and deformed wire in

     compression, ldc≥ (0.24f_y d_b)/(λ√(f_c^' )) 0.043f_y d_b

Where the constant 0.043 carries the unit of mm2/N



Lap splices in bundled bar

·        Lap splices of bundled bars shall be based on the lap splice length required for individual bars within the bundle.

·        Individual bar splices within a bundle shall not overlap. Entire bundles shall not be lap spliced.




Splices of Deformed Bars and Deformed Wire in Tension

·        The minimum length of lap for tension splices shall be as required for Class A or B splice, but not less than 300 mm, where the classification shall be as follows:

Class - A splice: 1.0 L

Class - B splice: 1.3 L


·        Lap splices of deformed bars and deformed wire in tension shall be class B splices except that Class A splices are allowed when the area of reinforcement provided is at least twice that required by analysis over the entire length of the splice, and one-half or less of total reinforcement is spliced within the required lap length.



Minimum Splices in Compression

·        The minimum length of lap for compression splice shall be 0.071f_y d_b for f_y =420 N/mm2 or less or (0.13f_y-24)d_b for f_y greater than 420 N/mm2, but not less than 300 mm. For f_c^' less than 21 N/mm2, length of lap shall be increased by one-third.

·        Development length and lap length will increase 33% for brick aggregate concrete

·        Development length and lap length will increase 30 - 50% for epoxy coated rebar




2
General requirement of seismic detailing, Part-2
3
General requirement of seismic detailing, Part-3
4
General requirement of seismic detailing, Part-4
5
General requirement of seismic detailing, Part-5
6
General requirement of seismic detailing, Part-6
7
General requirement of seismic detailing, Part-7
8
How to determine seismic detailing category

Special Seismic Detailing

1
Seismic detailing principles

EARTHQUAKE-RESISTANT DESIGN PROVISIONS

(According to BNBC 2017)


Contents

  1. Occupancy Category
  2. Importance factor
  3. Seismic design category (OMRF, IMRF, SMRF)
  4. Flexural Members of Special Moment Frames (SMRF – beam)
  5. Special Moment Frame Members Subjected to Bending and Axial Load (SMRF – Column)
  6. Minimum flexural strength of columns
  7. Special Structural Walls and Coupling Beams
  8. Requirements for Intermediate Moment Frames
  9. Requirements for ordinary moment frame members
  10. Comparison between OMRF, IMRF and SMRF








Seismic zone


1.      Southwestern part including Barisal, Khulna, Jessore, Rajshahi,

·        Seismic Intensity= Low,

·        Seismic Zone Coefficient, Z =0.12


2.      Lower Central and Southwestern part including Noakhali, Dhaka, Pabna, Dinajpur, as well as Southwestern corner including Sundarbans

·        Seismic Intensity= Moderate,

·        Seismic Zone Coefficient, Z =0.20


3.      Upper Central and Northwestern part including Brahmanbaria, Sirajganj, Rangpur

·        Seismic Intensity= Severe,

·        Seismic Zone Coefficient, Z =0.28


4.      Northeastern part including Sylhet, Mymensingh, Kurigram

·        Seismic Intensity= Very Severe,

·        Seismic Zone Coefficient, Z =0.36



 Building and Structure Occupancy Categories:


Nature of Occupancy: Buildings and other structures that represent a low hazard to human life in the event of failure, including, but not limited to:

  1. Agricultural facilities.
  2. Certain temporary facilities.
  3. Minor storage facilities

Occupancy Category - i


Nature of Occupancy: All buildings and other structures except those listed in Occupancy Categories I, III and IV.

Occupancy Category - ii


Nature of Occupancy: Buildings and other structures that represent a substantial hazard to human life in the event of failure, including, but not limited to:

  1. Buildings with more than 300 people.
  2. Buildings with day care facilities with a capacity greater than 150.
  3. Buildings with elementary school or secondary school facilities with a capacity greater than 250.
  4. Buildings with a capacity greater than 500 for colleges or adult education facilities.
  5. Healthcare facilities with a capacity of 50 or more resident patients, but not having   surgery or emergency Treatment facilities.
  6. Jails and detention facilities.

Occupancy Category - iii


Nature of Occupancy: Buildings and other structures designated as essential facilities, including, but not limited to:

  1. Hospitals, Fire, rescue, ambulance, and police stations and emergency vehicle garages.
  2. Designated earthquake, hurricane, or other emergency shelters, emergency preparedness, communication.
  3. Power generating stations, Ancillary structures, Electrical substation structures, Aviation control towers, air traffic control centers.

Occupancy Category - iv


Site Dependent Soil Factor and Other Parameters Defining Elastic Response Spectrum

Soil type---------S--------TB (s)------TC (s)---------TD (s)

SA---------------1.00-------0.15-------0.40-----------2.0

SB---------------1.20-------0.15-------0.50-----------2.0

SC---------------1.15-------0.20-------0.20-----------2.0

SD---------------1.35-------0.20-------0.20-----------2.0

SE----------------1.40------0.15-------0.50-----------2.0

 

Site Class---Occupancy Category I---------III-Occupancy Category IV

----------Zone 1-Zone 2-Zone 3-Zone 4--Zone 1-Zone 2-Zone 3-Zone 4

SA---------B--------C--------C-------D--------C--------D-------D-------D

SB---------B--------C--------D-------D--------C--------D-------D-------D

SC---------B--------C--------D-------D--------C--------D-------D-------D

SD---------C--------D--------D------D---------D-------D-------D-------D

S1, S2-----D-------D--------D-------D--------D--------D-------D-------D




2
Special detailing of Beam longitudinal rebar
3
Special detailing of Beam stirrup
4
Strong Column – Weak Beam concept, Part-1
5
Strong Column – Weak Beam concept, Part-2
6
Special detailing of transverse rebar of Column, Part-1
7
Special detailing of transverse rebar of Column, Part-2
8
Special joint detailing

Intermediate Seismic Detailing

1
Intermediate detailing of Beam
2
Intermediate detailing of Column

Other Seismic Detailing

1
General requirement of Flat plate, Part-1
2
General requirement of Flat plate, Part-2
3
Intermediate detailing of Flat plate
4
Ordinary detailing of Beam and Column
5
Special detailing of Foundation
6
BNBC Quiz 6 : Seismic Detailing
10 questions

Soils and foundations

1
Soils and foundations, Part-1

SOILS AND FOUNDATIONS


Contents of Chapter 3


The Soils and Foundations Chapter of the Code is divided into the following three distinct Divisions :


•     Division A: Site Investigations, Soil Classifications, Materials and Foundation Types

•     Division B: Service Load Design Method of Foundations

•     Division C: Additional Considerations in Planning, Design and Construction of Building Foundations






Ultimate // Safe // Allowable Bearing Capacity

1.      Ultimate bearing capacity = soil pressure at which failure occur

2.      Safe bearing capacity = ultimate/FS; so safety margin exist, no failure; but may have excessive settlement

3.      Allowable bearing capacity = soil pressure at which settlement of footing will be within permitted limit

4.      settlement governs for most of the soils

5.      Ultimate bearing capacity is determined using any formula = 600 kPa

6.      Safe Bearing Capacity = Ultimate / FS = 200 kPa

7.      Say footing size is 3 m x 3 m using safe bearing capacity and calculated settlement is 150 mm

8.      To limit the settlement within allowable limit (say 50 mm), footing size need to be increased. Say, 4 m x 4 m is ok at 100 kPa

9.      So, Allowable bearing capacity = 100 kPa



Bearing capacity vs. bearing pressure

Bearing capacity

1.     Ultimate bearing capacity is determined using any formula = 600 kPa

2.     Safe Bearing Capacity = Ultimate / FS = 200 kPa

3.     Say footing size is 3 m x 3 m using safe bearing capacity and calculated settlement is 150 mm

4.     To limit the settlement within allowable limit (say 50 mm), footing size need to be increased. Say, 4 m x 4 m is ok at 100 kPa

5.     So, Allowable bearing capacity = 100 kPa


Bearing pressure

1.      Bearing pressure = pressure applied on soil at the bottom of footing = soil pressure

2.      Applied load divided by footing area

3.      Bearing pressure may be less, equal or more than Allowable bearing capacity

4.      Bearing pressure should be less than Allowable bearing capacity





DIVISION A

Site Investigations, Soil Classifications, Materials and Foundation Types



3.4.1 Sub-Surface Survey


Depending on the type of project thorough investigations has to be carried out for identification, location, alignment and depth of various utilities, e.g., pipelines, cables, sewerage lines, water mains etc. below the surface of existing ground level. Detailed survey may also be conducted to ascertain the topography of existing ground.

  1. identification, location, alignment and depth of various utilities
  2. the topography of existing ground


3.4.2 Sub-Soil Investigations


  1. describing the character, nature, load bearing capacity and settlement capacity of the soil
  2. establish the soil, rock and groundwater conditions,
  3. determine the properties of the soil and rock
  4. gather additional relevant knowledge about the site.
  5. Careful collection, recording and interpretation of geotechnical information shall be made including ground conditions, geology, geomorphology, seismicity and hydrology, as relevant.
  6. Indications of the variability of the ground shall be taken into account.
  7. An engineering geological study may be an important consideration to establish the physiographic setting and stratigraphic sequences of soil strata of the area. Geological and agricultural soil maps of the area may give valuable information of site conditions.
  8. a competent graduate engineer having experiences in supervising sub-soil exploration works shall be employed by the drilling contractor.



3.4.3 Methods of Exploration


  1. Reconnaissance

a)     Geophysical measurement

b)     Sounding or probing (eg. DCP)

  1. Exploration and detail investigation

a)     Drilling and/or excavations for sampling

b)     Groundwater measurements

c)     Field tests (CPT, SPT etc)

d)     Laboratory tests



3.4.4 Number and Location of Investigation Points:

When selecting the locations of investigation points, it should be :

       i.           Arranged in such a pattern that the stratification can be assessed across the site.

     ii.           Placed at critical points relative to the shape, structural behavior and expected load distribution.

   iii.           Arranged at adequate offsets to the centre line for linear structures (road, railway).

   iv.           Arranged outside the project area for structures on or near slopes and steps in the terrain.

     v.           The locations and spacing of sounding, pits and boreholes shall be such that the soil profiles obtained will permit a reasonably accurate estimate of the extent and character of the intervening soil or rock mass.



For building structures, the following guidelines shall be followed:

On uniform soils, at least three borings, not in one line, should be made for small buildings and at least five borings one at each corner and one at the middle should be made for large buildings.

As far as possible the boreholes should be drilled closed to the proposed foundations but outside their outlines.

Spacing of exploration depends upon nature and condition of soil, nature and size of the project. In uniform soil, spacing of exploration (boring) may be 30 m to 100 m apart or more and in very erratic soil conditions, spacing of 10 m or less may be required.

Spacing of exploration depends upon nature and condition of soil, nature and size of the project. In uniform soil, spacing of exploration (boring) may be 30 m to 100 m apart or more and in very erratic soil conditions, spacing of 10 m or less may be required.



Number and Location of Investigation Points:

For large areas covering industrial and residential colonies, the whole area may be divided into grid pattern with Cone Penetration Tests performed at every 100 m grid points. The number of boreholes or trial pits shall be decided by examining the variation in penetration curves. At least 67% of the required number of borings or trial pits shall be located within the area under the building.


3.4.5 Depth of Exploration

The site investigation should be carried to such a depth that the entire zone of soil or rock affected by the changes caused by the building or the construction Change of effective stress is less than 10% of the average contact pressure of foundation or Less than 5% of the effective stress in the soil at that depth

At least 1 BH upto 30 m depth to define site class




  1. Where substructure units will be supported on spread footings, the minimum depth boring should extend below the anticipated bearing level a minimum of two footing widths for isolated, individual footings where length £ 2 times of width, and four footing widths for footings where length > 5 times of width. For intermediate footing lengths, the minimum depth of boring may be estimated by linear interpolation as a function of length between depths of two times width and five times width below the bearing level. Greater depth may be required where warranted by local conditions.
  2. For more heavily loaded structures, such as multistoried structures and for framed structures, at least 50% of the borings should be extended to a depth equal to 1.5 times the width of the building below the lowest part of the foundation.
  3. Normally the depth of exploration shall be 1.5 times the estimated width or the least dimension of the footing below the foundation level. If the pressure bulbs for a number of loaded areas overlap, the whole area may be considered as loaded and exploration shall be carried down to one and a half times the least dimension. In weak soils, the exploration shall be continued to a depth at which the loads can be carried by the stratum in question without undesirable settlement or shear failure.
  4. Where substructure units will be supported on deep foundations, the depth boring should extend a minimum of 6 m below the anticipated pile of shaft tip elevation. Where pile or shaft groups will be used, the boring should extend at least two times the maximum pile or shaft group dimension below the anticipated tip elevation, unless the foundation will be end bearing on or in rock.
  5. For piles bearing on rock, a minimum of 1.5 m of rock core should be obtained at each boring location to ensure the boring has not been terminated in a boulder.
  6. For shafts supported on or extending into rock, a minimum of 1.5 m of rock core, or a length of rock core equal to at least three times the shaft diameter for isolated shafts or two times the maximum shaft group dimension for a shaft group, whichever is greater, should be obtained to ensure that the boring had not been terminated in a boulder and to determine the physical properties of rock within the zone of foundation influence for design.
  7. The depth, to which weathering process affects the deposit, shall be regarded as the minimum depth of exploration for a site. However, in no case shall this depth be less than 2 m, but where industrial processes affect the soil characteristics, this depth may be more.
  8. At least one boring should be carried out to bedrock, or to well below the anticipated level of influence of the building. Bedrock should be ascertained by coring into it to a minimum depth of 3 m.




For structures supported on spread footings:

In weak soils, the exploration shall be continued to a depth at which the loads can be carried by the stratum in question without undesirable settlement or shear failure.

a)     For Deep foundations : D ≥ 6 m below the anticipated pile shaft tip elevation.

 

For piles bearing on rock :

 A minimum of 1.5 m of rock core should be obtained at each boring location to  ensure the boring has not been terminated in a boulder

a)     For shafts supported on or extending into rock,

b)     The depth, to which weathering process affects the deposit, shall be regarded as the minimum depth of exploration for a site. However, in no case shall this depth be less than

c)     At least one boring should be carried out to bed rock, or to well below the anticipated level of influence of the building. Bedrock should be ascertained by coring into it to a minimum depth of 3 m



3.4.7 Geotechnical Investigation Report


The results of a geotechnical investigation shall be compiled in the Geotechnical Investigation Report which shall form a part of the Geotechnical Design Report. The Geotechnical Investigation Report shall consist of the following:

1.     A presentation of all appropriate geotechnical information on field and laboratory tests including geological features and relevant data;

2.     A geotechnical evaluation of the information, stating the assumptions made in the interpretation of the test results.

3.     The names of all consultants and contractors;

4.     The dates between which field and laboratory investigations were performed;

5.     Evidence of groundwater;

6.     Behaviour of neighbouring structures;

7.     Exposures in quarries and borrow areas;

8.     Areas of instability;

9.     Difficulties during excavation;

10. History of the site;

11. Geology of the site,

12. Survey data with plans showing the structure and the location of all investigation points;

13. Local experience in the area;

14. Desk studies;

15. Field investigations

16. Laboratory tests and test standard followed


USCS Classification is adopted by BNBC 2017


3.5.4 Organic Soil


Table 6.3.3: Classification and Description of Organic Soils (after Edil, 1997) (ASTM D2974-07a)


Organic Content< 5 %=Little effect on behavior; considered inorganic soil.

Organic Content 6 ~ 20 %=Effects properties but behavior is still like mineral soils; organic silts and clays.

Organic Content 21 ~ 74 %=Organic matter governs properties; traditional soil mechanics may be applicable; silty or clayey organic soils.

Organic Content > 75 %=Displays behavior distinct from traditional soil mechanics especially at low stress.




Problems of organic soil


  1. Low bearing capacity
  2. High consolidation and creep settlement
  3. Swelling and shrinkage potential
  4. Vertical and lateral capacity of pile is low
  5. If it is PEAT>>> extremely high natural moisture content, high compressibility including significant secondary and even tertiary compression and very low undrained shear strength at natural moisture content.




How to determine organic content?


  1. Loss on ignition
  2. Mass loss on treatment with hydrogen peroxide (H2O2)



3.5.5 Expansive Soils


  1. Expansive soils are those which swell considerably on absorption of water and shrink on the removal of water.
  2. In monsoon seasons, expansive soils imbibe water, become soft and swell. In drier seasons, these soils shrink or reduce in volume due to evaporation of water and become harder.
  3. As such, the seasonal moisture variation in such soil deposits around and beneath the structure results into subsequent upward and downward movements of structures leading to structural damage, in the form of wide cracks in the wall and distortion of floors.
  4. For identification and classification of expansive soils parameters like
  5. liquid limit, plasticity index, shrinkage limit, free swell, free swell index, linear shrinkage, swelling potential, swelling pressure and volume change from air dry to saturate condition
  6. Various recommended criteria for identification and classification of expansive soils are presented in Appendix E.



Based on the values of plasticity index and shrinkage limit, United States Bureau of Reclamation (USBR) suggests the following classification criteria for expansive soil:


Plasticity Index------Shrinkage Limit-----Degree of Expansion

>35------------------------------<10---------------------Very High

25-41-------------------------------6-12-------------------High

15-28------------------------------8-18-----------------Medium

<18--------------------------------->13-----------------------Low




On the basis of previous data for linear shrinkage of Bangladesh soils, criteria for the degree of expansion proposed by Hossain (1983) is as follows:

Linear Shrinkage (%) ---------------Degree of Expansion

> 14--------------------------------------High 

10-14------------------------------------Medium

0-10-------------------------------------Low



On the basis of the values of free swell, Indian standard (IS: 1948, 1970) recommends criteria of expansion is as follows:



Based on the value of free swell index, Indian Standard (IS: 2911, Part III, 1980) suggests the following criteria for the degree of expansion of soils:



Based on the values of liquid limit, plasticity index and shrinkage limit, Indian Standard (IS: 2911, Part 3, 1980) suggests the following criteria for the degree of expansiveness of soils:



Based on the values of swelling potential, Seed et al. (1962) proposed the following four categories of expansion characteristics:



Based on the values of swelling pressure, Chen (1965) proposed the following criteria for degree of expansion:



Based on the values of volume change from air dry to saturated condition, Seed et al. (1962) proposed the following four categories of expansion characteristics:



Look (2007) reports that the plasticity index by itself can be misleading, as the test is carried out on the percent passing the 425 micron sieve, i.e. any sizes greater than 425 µm is discarded. There have been cases when a predominantly “rocky/granular” site has a high PI test results with over 75 percent of the material discarded. The weighted plasticity index (WPI) considers the percent of material used in the test, where WPI=PI×% passing the 425 micron sieve. Degree of expansion with weighted plasticity index is presented as under. 




Collapsible Soils

Unsaturated soils which collapsed upon wetting


Soil deposits most likely to collapse are; 

(i) loose fills, 

(ii) altered wind-blown sands, 

(iii) hill wash of loose consistency and 

(iv) decomposed granite or other acid igneous rocks.




sausage test for cohesive soil


Two undisturbed cylindrical samples (sausages) of the same diameter and length (volume) are carved from the soil. One sample is then wetted and kneaded to form a cylinder of the original diameter. A decrease in length as compared to the original, undisturbed cylinder will confirm a collapsible grain structure. 

Collapse is probable when the natural void ratio, e_i is higher than a critical void ratio, e_c that depends on void ratios e_L and e_P at liquid limit and plastic limits respectively. The following formula should be used to estimate the critical void ratio. 

e_c=0.85e_L+015e_P (6.3.1)


Collapsible soils (with a degree of saturation, S_r  0.6) should satisfy the following condition:

(e_L-e_i)/(1+e_i )≤0.10 (6.3.2)



Consolidation test to identify Collapsible Soil (clay or sand)


A consolidation test is to be performed on an undisturbed specimen at natural moisture content and to record the thickness, “H” on consolidation under a pressure “p” equal to overburden pressure plus the external pressure likely to be exerted on the soil. The specimen is then submerged under the same pressure and the final thickness H’ recorded. Relative subsidence, I_subs is found as:

I_subs =(H-H^')/H (6.3.3)

Soils having Isubs  0.02 are considered to be collapsible. 



Collapsible soil (sand)

Collapse is probable when the natural void ratio, e_i is higher than a critical void ratio

Critical void ratio can be determined by direct shear test



Dispersive Soil

dispersive soil can lead to catastrophic failures of earth embankment dams as well as severe distress of road embankments.

Clay soil which easily disperse in water

It is one of the major causes of soil erosion in Bangladesh

pinhole test

The pinhole test was developed to directly measure dispersive potential of compacted fine grained soils in which water is made to flow through a small hole in a soil specimen, where water flow through the pinhole simulates water flow through a crack or other concentrated leakage channel in the impervious core of a dam or other structure. The test is run under 50, 180, 380 and 1020 mm heads and the soil is classified as follows in Table 6.3.4.



Exchangeable Sodium Percentage (ESP)

Another method of identification is to first determine the pH of a 1:2.5 soil/water suspension. If the pH is above 7.8, the soil may contain enough sodium to disperse the mass. Then determine: (i) total excahangable bases, that is, K^+, Ca^(2+), Mg^(2+)and Na+ (milliequivalent per 100g of air dried soil) and (ii) cation exchange capacity (CEC) of soil (milliequivalent per 100g of air dried soil). The Exchangeable Sodium Percentage ESP is calculated from the relation:

ESP=N_a/CEC×100(%) (6.3.4)

EM_g P is given by: 

EM_g P=Mg/CEC×100(%) (6.3.5)

If ESP > 8% and ESP + EM_g P > 15, dispersion will take place.

ESP =7 to 10 are moderately dispersive

ESP > 15 have serious piping potential



Table 6.3.4: Classification of Dispersive Soil on the Basis of Pinhole Test (Sherard et. al. 1976)

Test Observation------------------------------------------------------Type of Soil---------------Class of Soil

Fails rapidly under 50 mm head------------------------------Dispersive soils---------------D1 and D2

Erode slowly under 50 mm or 180 mm head-------------Intermediate soils-----------ND4 and ND3

No colloidal erosion under 380 mm or 1020 mm head----Non-dispersive soils-----ND2 and ND1



Double Hydrometer Test

The test evaluates the dispersibility of a soil by measuring the natural tendency of the clay fraction to go into suspension in water. The procedure involves the determination of the percentage of particles in the soil that are finer than 0.005 mm using the standard hydrometer test.



Soft Inorganic Soil

Low shear strength, 

High compressibility and 

Severe time related settlement problems *****

Soft clays have undrained shear strengths between about 10kPa and 40kPa,


N-value---------------Consistency--------------Undrained Shear Strength (kN/m2)

Below 2----------------Very soft---------------------Less than 20

2 – 4------------------------Soft-------------------------20 - 40

However, SPT is not a good testing method for soft soil.

FVT and CPT are most suited for soft soil characterization




MATERIALS


Concrete :

All concrete materials and steel reinforcement used in foundations shall conform to the requirements specified in this chapter. For different types of foundation the recommended concrete properties are given in this table: 



Steel :

All steel reinforcement and steel materials used in foundations shall conform to the requirements specified in Chapter. Corrosion is the main dangerous thing for steel. For the purpose of calculations, a maximum corrosion rate of 0.015 mm per side per year may be used. In recent-fill soils or industrial waste soils, where corrosion rates may be higher, following protection systems should be considered.



Timber: 


Timber may be used only for foundation of temporary structure but not for concrete concrete structure.

Where timber is exposed to soil or used as load bearing pile above ground water level, it shall be treated in accordance with BDS 819:1975 (Code of practice for preservation of timber).



TYPES OF FOUNDATION

Shallow Foundations

Footing

Raft/Mat

Deep Foundations

Driven Piles

Bored Piles/Cast-in-Situ Piles

Drilled Pier/Drilled Shafts

Caisson/Well










DIVISION B

DESIGN OF FOUNDATIONS 



ULTIMATE, SAFE AND ALLOWABLE BEARING CAPACITY OF THE SOIL


Ultimate bearing capacity : Ultimate bearing capacity is the maximum pressure that a foundation soil can withstand without undergoing shear failure. Again the Ultimate Bearing Capacity of the soil is the max load which can be applied. It is also defined as the ultimate pressure per unit area of the foundation that can be supported by the soil in excess of the pressure caused by the surrounding soil at the foundation level.

Safe bearing capacity : Safe bearing capacity is the safe extra load the foundation soil is subjected to in addition to initial overburden pressure. Again Safe bearing capacity is the maximum Pressure that a soil bears without shear failure.

Allowable bearing pressure : Allowable bearing pressure is the maximum pressure where the foundation soil is subjected to considering both shear failure and settlement. Again Allowable bearing pressure is the net load intensity at which no failure occurs.



3.8.2 DIMENSION OF FOOTING


Dimension of Footings :

Footings shall generally be proportioned from the allowable bearing pressure and stress limitations imposed by limiting settlement. 

The angle of spread of the load from the wall base to outer edge of the ground bearing shall not exceed the following:



3.8.2 Depth of footing


Dimension of Footings :

A footing shall be placed to depth so that: 

Adequate bearing capacity is achieved,

In case of clayey soil , shrinkage and swelling due to seasonal weather change is not significant, 

It is below possible excavation close by, and

It is at least 500 mm below natural 

ground level unless rock or other 

weather resistant material is at the surface.

So, Df > 1.5 m



3.8.3 Thickness of Footing


The minimum thickness for different types of footing for light structures, shall be as follows :

Type of Footing Minimum Thickness Remark

Masonry 250 mm; twice the maximum projection from the face of the wall Greater of the two values shall be selected

Plain concrete 200 mm, or twice the maximum offset in a stepped footing

Reinforced concrete (depth above bottom reinforcement) 150 mm 300 mm Resting on soil Resting on pile



3.8.6 Minimum Depth of Foundation


The minimum depth of foundation shall be :

For permanent structures 1.5 m for exterior footing in cohesive soils and 2 m in cohesionless soils. 

For temporary structures the minimum depth of exterior footing shall be 400mm. 



3.8.7 Scour


Footings supported on soil shall be embedded sufficiently below the maximum computed scour depth or protected with a scour countermeasure



3.8.8 Mass Movement of Ground in Unstable Areas


In certain areas mass movement of ground may occur from causes independent of the loads applied to the foundation. These include 

Mining subsidence

Landslides on unstable slopes

Creep on clay slopes.




3.8.9 Foundation Excavation

Foundation excavation below ground water table shall be made such that the hydraulic gradient at the bottom of the excavation is not increased to a magnitude that would case the foundation soils to loosen due to upward flow of water. 

Footing excavations shall be made such that hydraulic gradients and material removal do not adversely affect adjacent structures

Seepage forces and gradients may be evaluated by standard flow net procedures. 

Dewatering or cutoff methods to control seepage shall be used when necessary. 



3.8.10 Design Considerations for Raft foundation


For raft supports structure consisting of several parts with varying loads and height, it is advisable to provide separate joints between these parts. ***** 

Joints shall also be provided wherever there is a change in the direction of the raft.

The minimum depth of foundation shall generally be not less than 1.5 m in cohesive soil and 2 m in cohesionless soils.



Geotechnical Design of Shallow Foundation


3.9.1 General : 

The location of the resultant pressure due to seismic and dynamic loads on the base of the footings should be maintained preferably within B/6 of the centre of the footing.



Design Load : 

Shallow foundation design considering bearing capacity due to shear strength shall consider the most unfavourable effect of the following combinations of loading:

D + L

D + L + E or W

0.9 D + Buoyancy Pressure


Shallow foundation design considering settlement shall consider the most unfavourable effect of the following combinations of loading:


SAND


D + L

D + L + E or W

 

CLAY


D + 0.5 L 



Bearing Capacity of Shallow Foundations :


Established bearing capacity equations shall be used for calculating bearing capacity. A factor of safety of between 2.0 to 3.0 shall be adopted to obtain allowable bearing pressure when dead load and normal live load is used.

Allowable increase of bearing pressure due to wind and earthquake forces : The allowable bearing pressure of the soil determined in accordance with this Section may be increased by 33 percent when lateral forces due to wind or earthquake act simultaneously with gravity loads. 

Presumptive bearing capacity for preliminary design : For lightly loaded and small sized structures and for preliminary design of any structure, the presumptive bearing values (allowable) as given in next slide may be assumed for uniform soil in the absence of test results.

Soil Type Soil Description Safe Bearing Capacity, kPa

1 Soft Rock or Shale 440


2 Gravel, sandy gravel, silty sandy gravel; very dense and offer high resistance to penetration during excavation (soil shall include the groups GW, GP, GM, GC)

400**

3 Sand (other than fine sand), gravelly sand, silty sand; dry (soil shall include the groups SW, SP, SM, SC) 200**

4 Fine sand; loose & dry (soil shall include the groups SW, SP) 100**


5 Silt, clayey silt, clayey sand; dry lumps which can be easily crushed by finger (soil shall include the groups ML,, SC, & MH)

150

6 Clay, sandy clay; can be indented with strong thumb pressure (soil shall include the groups CL, & CH) 150

7 Soft clay; can be indented with modest thumb pressure (soil shall include the groups CL, & CH) 100

8 Very soft clay; can be penetrated several centimeters with thumb pressure (soil shall include the groups CL & CH) 50

9 Organic clay & Peat (soil shall include the groups OH, OL, Pt) To be determined after investigation.

10 Fills To be determined after investigation.

Two stories or less (Occupancy category A, B, C and D)


** 50% of these values shall be used where water table is above the base, or below it within a

   distance equal to the least dimension of foundation


Settlement of Shallow Foundation : Foundations can settle in various ways and each affects the performance of the structure


a) Total settlement : Total settlement (ձ) is the absolute vertical movement of the foundation from its as-constructed position to its loaded position.


Secondary consolidation is due to particle reorientation, creep, and decomposition of organic materials.

Secondary compression is always time-dependent and can be significant in highly plastic clays, organic soils, and sanitary landfills, but it is negligible in sands and gravels.



Differential settlement :

Differential settlement is the difference in total settlement between two foundations or two points in the same foundation. This kind of settlement can occur due to the following circumstances: 


Non-uniformity in subsoil.*****

Non-uniform pressure distribution.*****

Ground water condition during and after construction.

Loading influence of adjacent structures.

Uneven expansion and contraction due to moisture migration, uneven drying, wetting or softening.


Notes: The values given in the Table may be taken only as a guide and the permissible total settlement, differential settlement and tilt (angular distortion) in each case should be decided as per requirements of the designer. 

L denotes the length of deflected part of wall/ raft or centre to centre distance between columns. 

H denotes the height of wall from foundation footing. 

* For intermediate ratios of L/H, the values can be interpolated



3.9.5 Liquefaction Potential


Soil liquefaction is a phenomenon in which a saturated soil deposit loses most, if not all, of its strength and stiffness due to the generation of excess pore water pressure during earthquake-induced ground shaking.


Sandy and silty soils tend to liquefy; clay soils do not undergo liquefaction except the sensitive clays.

Resistance to liquefaction of sandy soil depends on fines content. Higher the fines content lower is the liquefaction potential. ???

As a rule of thumb, any soil that has a SPT value higher than 30 will not liquefy. ???



Raft foundation reactions : 

For determining the distribution of contact pressure below a raft both analytical and numerical methods require values of the modulus of subgrade reaction (k) of the soil. 

       

          k=0.65×((E_s B^4)/EI)^(1⁄12) E_s/((1-μ^2 ) ) 1/B         

Where,  

E_s = Modulus of elasticity of soil 

EI = Flexural rigidity of foundation

B = Width of foundation

μ = Poisson’s ratio of soil



Raft foundation reactions :

 For use in preliminary design, indicative values of the modulus of subgrade reaction (k) for cohesionless soils are given below :

Stiff

Very Stiff

Hard

The values apply to a square plate 300 mm x 300 mm. The above values are based on the assumption that the average loading intensity does not exceed half the ultimate bearing capacity



Critical section for moment :

 External moment on any section of a footing shall be determined by passing a vertical plane through the footing and computing the moment of the forces acting over the entire area of the footing on one side of that vertical plane.


Reinforcement in band width/Total reinforcement in short direction=2/((β+1) 




DIVISION C

ADDITIONAL CONSIDERATIONS IN PLANNING, DESIGN AND CONSTRUCTION OF BUILDING FOUNDATIONS 



3.12 EXCAVATION:


Excavation for building foundation or for other purpose shall be done in a safe manner so that no danger to life and property prevails at any stage of the work or after completion. 

Permanent excavations shall have retaining walls of sufficient strength made of steel, masonry, or reinforced concrete to retain the embankment, together with any surcharge load. 

Excavations for any purpose shall not extend within 300 mm under any footing or foundation, unless such footing or foundation is properly underpinned or protected against settlement, beforehand. 

The design and construction of deep excavation work more than 6 m depth or excavation in soft soil or erratic soil must be checked by a competent Geotechnical Engineer.


Excavations for any purpose shall not extend within 300 mm under any footing or foundation, unless such footing or foundation is properly underpinned or protected against settlement, beforehand



Notice to Adjoining Property :

Prior to any excavation close to an adjoining building in another property, a written notice shall be given to the owner of the adjoining property at least 10 days ahead of the date of excavation. 

The protective measures shall incorporate the following: 


Where the level of the foundations of the adjoining structure is at or above the level of the bottom of the proposed excavation, the vertical load of the adjoining structure shall be supported by proper foundations, underpinning, or other equivalent means. 

Where the level of the foundations of the adjoining structure is below the level of the bottom of the proposed excavation, provision shall be made to support any increased vertical or lateral load on the existing adjoining structure caused by the new construction.



Excavation Work :


a) Method of Protection :

Shoring, Bracing and Sheeting 


With the exception of rock cuts, the sides of all excavations, including related or resulting embankments, 1.5 m or greater in depth or height measured from the level of the adjacent ground surface to the deepest point of excavation, shall be protected and maintained by shoring, bracing and sheeting, sheet piling, or other retaining structures. Alternatively, excavated slopes may be inclined not steeper than 1:1, or stepped so that the average slope is not steeper than forty five degrees with no step more than 1.5 m high, provided such slope does not endanger any structure, including subsurface structures. All sides or slopes of excavations or embankments shall be inspected after rainstorms, or any other hazard increasing event, and safe conditions shall be restored. Sheet piling and bracing needed in trench excavations shall have adequate strength to resist possible forces resulting from earth or surcharge pressure. Design of Protection system shall be checked by a qualified Geotechnical Engineer. 



Guard Rail :


A guard rail or a solid enclosure at least 1 m high shall be provided along the open sides of excavations, except that such guard rail or solid enclosure may be omitted from a side or sides when access to the adjoining area is precluded, or where side slopes are one vertical to three horizontal or flatter


Every excavation shall be provided with safe means of entry and exit kept available at all times.

When an excavation has been completed, or partly completed and discontinued, abandoned or interrupted, or the required permits have expired, the lot shall be filled and graded to eliminate all steep slopes, holes, obstructions or similar sources of hazard. 

Fill material shall consist of clean, noncombustible substances. 

The final surface shall be graded in such a manner as to drain the lot, eliminate pockets, prevent accumulation of water, and preclude any threat of damage to the foundations on the premises or on the adjoining property.


So that workers may escape if they see any danger or guess any failure of protection



Placing of construction material : 

Excavated materials and superimposed loads such as equipment, trucks, etc. shall not be placed closer to the edge of the excavation than a distance equal to one and one-half times the depth of such excavation, unless the excavation is in rock or the sides have been sloped or sheet piled (or sheeted) and shored to withstand the lateral force imposed by such superimposed load. When sheet piling is used, it shall extend at least 150 mm above the natural level of the ground. In the case of open excavations with side slopes, the edge of excavation shall be taken as the toe of the slope.



Safety regulations :

Whenever subsurface operations are conducted that may impose loads or movement on adjoining property, such as driving of piles, compaction of soils, or soil densification, the effects of such operations on adjoining property and structures shall be considered. The owner of the property that may be affected shall be given 48 hours written notice of the intention to perform such operations. Where construction operations will cause changes in the ground water level under adjacent buildings, the effects of such changes on the stability and settlement of the adjacent foundation shall be investigated and provision made to prevent damage to such buildings. When a potential hazard exists, elevations of the adjacent buildings shall be recorded at intervals of twenty four hours or less to ascertain if movement has occurred. If so, necessary remedial action shall be undertaken immediately. 

On excavation, the soil material directly underlying footings, piers, and walls shall be inspected by an engineer/architect prior to construction of the footing.


Except in cases where a proposed excavation will extend less than 1.5 m below grade, all underpinning operations and the construction and excavation of temporary or permanent cofferdams, caissons, braced excavation surfaces, or other constructions or excavations required for or affecting the support of adjacent properties or buildings shall be subject to controlled inspection. The details of underpinning, and construction of cofferdams, caissons, bracing or other constructions required for the support of adjacent properties or buildings shall be shown on the plans or prepared in the form of shop or detail drawings and shall be approved by the engineer who prepared the plans.



3.13 DEWATERING


All excavations shall be drained and the drainage maintained as long as the excavation continues or remains. Where necessary, pumping shall be used. No condition shall be created as a result of construction operations that will interfere with natural surface drainage. Water courses, drainage ditches, etc. shall not be obstructed by refuse, waste building materials, earth, stones, tree stumps, branches, or other debris that may interfere with surface drainage or cause the impoundment of surface water.



3.14 SLOPE STABILITY OF ADJOINING BUILDINGS


Overturning :

The possibility of overturning and sliding of the building shall be considered. 

The minimum factor of safety against overturning of the structure as a whole shall be 1.5. 

Stability against overturning shall be provided by the dead load of the building, the allowable uplift capacity of piling, anchors, weight of the soil directly overlying footings provided that such soil cannot be excavated without recourse to major modification of the building, or by any combination of these factors. 



Sliding : 

The minimum factor of safety against sliding of the structure under lateral load shall be 1.5. 



SLOPE STABILITY OF ADJOINING BUILDINGS Continue


Sliding : Resistance to lateral loads shall be provided by 

friction between the foundation and the underlying soil, 

passive earth pressure, 

batter piles or by plumb piles, 


But note that: 

The resistance to lateral loads due to passive earth pressure shall not be taken into consideration where the abutting soil could be removed inadvertently by excavation.

In case of pile supported structures, frictional resistance between the foundation and the underlying soil shall be discounted. 

The available resistance to friction between the foundation and the underlying soil shall be predicted on an assumed friction factor of 0.5. A greater value of the coefficient of friction may be used subject to verification by analysis and test.



Missing information for excavation in BNBC 2017


Allowable limits of lateral movement shore pile

Allowable limit of vertical subsidence of retained soil near excavation

Instrumentation options for deep excavation



2
Soils and foundations, Part-2
3
Soils and foundations, Part-3
4
Soils and foundations, Part-4
5
Soils and foundations, Part-5
6
Soils and foundations, Part-6
7
Soils and foundations, Part-7
8
Soils and foundations, Part-8
9
Soils and foundations, Part-9
10
Soils and foundations, Part-10
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Soils and foundations, Part-11
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Soils and foundations, Part-12
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Soils and foundations, Part-13
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Soils and foundations, Part-14
15
BNBC Quiz 7 : Soils and foundations
35 questions
5
5 out of 5
5 Ratings

Detailed Rating

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5
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