Earthquake Load as per BNBC 2020

Earthquake load

Earthquake Load as per BNBC 2020

BNBC 2020 এ র earthquake load অংশটা আগের BNBC 2006 এর সাথে কিছুটা মিল থাকলেও অমিলের পরিমাণ বেশি। কোডের এই অংশটা সিভিল ইঞ্জিনিয়ারদের জন্য নিজে কোড পড়ে বুঝাটা খুব সহজ হবে না বলেই মনে হয়। এই শর্ট কোর্সটা অত্যন্ত চমৎকারভাবে earhquake load কে সহজ করা হয়েছে। ETABS model এ কিভাবে earthquake load parameters input দিতে হয় তাও এই কোর্সে আছে। BNBC did not directly adopted ASCE-7-05 earthquake load. It has done some modifications in it. this course will teach you how to calculate earthquake load manually and using software. Earthquake load as per BNBC 2020 short course will enhance your design capacity.

What you'll learn

  1.  Earthquake load calculation as per BNBC 2020
  2.  Earthquake resistant design concept of RCC structures
  3.  Serviceability limits
  1. Excel program for earthquake load calculation
  2.  Software inputs for earthquake load

Special Gift with The Course

Earthquake load calculation excel program

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
  • AMIEB students of IEB

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

Related Courses

  • Wind Load as per BNBC 2020
  • Seismic Detailing of RCC Structure as per BNBC 2020
  • Soils and Foundations as per BNBC 2020

Free Courses

  • Design Issues for Large Earth Structures: Stress, Deformation, FOS and Liquefaction
  • Webinar on Dynamic Analysis

Features of ourPROFESSORs.com

Frequently Asked Questions (FAQ)

Course Preview

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

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


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All documents of Earthquake Load as per 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
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Drift and deflection limits, Part-4
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Drift and deflection limits, Part-5
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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).  

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BNBC Quiz 4 : Earthquake load
44 questions

Earthquake load example

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Earthquake load input tutorial on Etabs as per BNBC 2020

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