# 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. }*S*a = C_{s}

_{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, *C*_{d}

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 = *L*_{max }/ *L*_{min) }< 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, S_{1} AND S_{2} 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**

q **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

q **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

q **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 C_{s}.

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

*Sa = 2/3 ZI/R Cs*_{ }*V* = S_{a}*W*

Where,

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

· 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.

· *C*_{s} = 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/m^{2}, DL+0.25LL

b) LL > 3 kN/m^{2}, DL+0.50LL

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

** **

** **

** **

v *C*_{s }= 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 *S*and *T*_{B}, *T*_{C} and *T*_{D}, which are all functions of site class

Ø Constant *C*_{s} value between periods *T*_{B} and *T*_{C} represents constant spectral acceleration

Ø *S* = Soil factor which depends on site class

Ø *T *= Structure (building) period

Ø *T*_{B}= Lower limit of period of constant spectral acceleration branch as a function of site class

Ø *T*_{C} = Upper limit of period of constant spectral acceleration branch as a function of site class

Ø *T*_{D}= Lower limit of period of constant spectral displacement branch

Ø *η*= Damping correction factor

Where, ղ = √(10 /(5+ ζ) ) ≥ 0.55*ξ*

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* = *C*_{t}(h_{n})^{m }

*h*_{n} = 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 S_{1} and S_{2}

• 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: **

- Stiffness Irregularity - Soft Storey
- Mass Irregularity
- Vertical Geometric Irregularity
- Vertical In-Plane Discontinuity in Vertical Elements Resisting Lateral Force
- 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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