earthquakes and their effects on structures

Seismic Design Provisions of BNBC-2020: Part 1

Date: 25 May 2021

URP S-09 Training Module S5

Dr. S. K. GhoshPresident, S. K. Ghosh Associates LLC

URP S-09 TrainingModule G4

Earthquakes and Their Effects on Structures

Date: 06 APR 2021

Dr. S. K. GhoshPresident, S. K. Ghosh Associates LLC

S. K. Ghosh Associates LLC International Code Council (ICC)

www.skghoshassociates.com 1

Inertia ForceInertia Force

u Roof

Column

Foundation

Soil

Acceleration

Inertia force and relative motion within a building

Inertia Force

üg(t)

müg(t)

=m m

FIXED BASE



Inertia Force

In some circumstances, dynamic amplification (due to near-resonance) can increase the building acceleration to a value two or more times that of the ground acceleration at the base.

Generally, buildings with higher natural frequencies or short natural periods tend to suffer higher accelerations but smaller displacements.

In the case of buildings with lower natural frequencies or long natural periods, this is reversed and the buildings experience lower accelerations but larger displacements.

Load Paths

Response of Concrete Buildings to Seismic Forces

Compression chord

Tension chord

Seismic forces concentrated at roof and floor diaphragms

Diaphragm shear forces at each end of diaphragm are transferred to the collector beam and the shear wall

Collector beam accumulates diaphragm shear and transfers it to the end of the shear wall

Compression Tension

Shear force

Bending moment

Passive pressure

Friction

Variable foundation contact pressure

Seismic forces tend to push the shear wall over causing an overturning moment

The overturning moment causes tension and compression boundary forces in the shear wall

Shear forces are transferred to the earth by friction on the bottom of footings and by passive pressure on the sides of footings

Forces from gravity loads and seismic overturning forces are transferred to the earth by vertical contact pressures beneath the footing



Load Paths

From point of load application or origin

down to soil underlying foundation:

Preferably,

Direct (simple configuration)

Multiple (redundancy)

Idealized Force-Displacement

Lateral Displacement

Fe

Fu

e u

U

Elastic

Elastic Inelastic



ASCE 7- 05

10



Important Reference to ASCE 7-022.5.7 Equivalent Static Analysis

2.5.7.1 Design base shear

Alternatively, for buildings with natural period less than or equal to 2.0 sec., the seismicdesign base shear can be calculated using ASCE 7 02 with seismic design parameters asgiven in Appendix C. However, the minimum value of should not be less than 0.044SDSI. The values of SDS are provided in Table 6.C.4 Appendix C.

Seismic Design ProvisionsBNBC-2020

Part 6, Chapter 2, Loads on Buildings and StructuresSection 2.5, Earthquake LoadsSection 2.7, Combinations of LoadsAppendix C, Seismic Design Parameters for Alternative Method of Base Shear Calculation

Chapters 11-23 0f ASCE 7-05 onwards

Section 1613 of 2006 IBC onwards



Detailing Rules in BNBC-2020

Part 6, Chapter 8, Detailing of Reinforcement in Concrete Structures

Section 8.3, Earthquake-Resistant Design Provisions

2.5.2 Earthquake Resistant Design –Basic Concepts

2.5.2.1 General principles

The purpose of earthquake resistant design provisions in this Code is to provide guidelines for the design and construction of new structures subject to earthquake ground motions in order to minimize the risk to life for all structures, to increase the expected performance of higher occupancy structures as compared to ordinary structures, and to improve the capability of essential structures to function after an earthquake. It is not economically feasible to design and construct buildings without any damage for a major earthquake event. The intent is therefore to allow inelastic deformation and structural damage at preferred locations in the structure without endangering structural integrity and to prevent structural collapse during a major earthquake.




2.5.2.2 Characteristics of Earthquake Resistant Buildings

The desirable characteristics of earthquake resistant buildings are described below:

Structural Simplicity, Uniformity and Symmetry:

Structural simplicity, uniformity and plan symmetry is characterized by an even distribution of mass and structural elements which allows short and direct transmission of the inertia forces created in the distributed masses of the building to its foundation. A building configuration with symmetrical layout of structural elements of the lateral force resisting system, and well-distributed in-plan, is desirable. Uniformity along the height of the building is also important, since it tends to eliminate the occurrence of sensitive zones where concentrations of stress or large ductility demands might cause premature collapse.


2.5.2.2 Characteristics of Earthquake Resistant Buildings Some basic guidelines are given below:(i) With respect to the lateral stiffness and mass distribution, the building structure shall be approximately symmetrical in plan with respect to two orthogonal axes. (ii) Both the lateral stiffness and the mass of the individual storeys shall remain constant or reduce gradually, without abrupt changes, from the base to the top of a particular building. (iii) All structural elements of the lateral load resisting systems, such as cores, structural walls, or frames shall run without interruption from the foundations to the top of the building.




2.5.2.2 Characteristics of Earthquake Resistant Buildings Some basic guidelines are given below:(iv) An irregular building may be subdivided into dynamically independent regular units well separated against pounding of the individual units to achieve uniformity. (v) The length to breadth ratio ( = / ) of the building in plan shall not be higher than 4, where and are respectively the larger and smaller in plan dimension of the building, measured in orthogonal directions.

SEISMIC INPUT PARAMETERSPART 1



ASCE 7-05, BNBC-2020

2.5.4 Earthquake Ground Motion

Appendix C Seismic Design Parameters for Alternative Method of Base Shear Calculation

Appendix C is closer to ASCE 7-05 than 2.5.4

Both use seismic zones and the same Seismic Zone Map (Fig. 6.2.24)

Both approaches yield the same design base shear.

ASCE 7-05 Figure 11.4-1: Design Response Spectrum



BNBC-2020 Fig. 6.2.25: MCE Response Spectrum

ASCE BNBC-7-05 2020T0 TBTS TCTL TD




Design Ground Motion (ASCE 7-05)

SS = mapped (MCE) spectral response acceleration at short periods for Site Class B

S1 = mapped (MCE) spectral response acceleration at 1.0-second period for Site Class B

ASCE 7-05 Figs. 22-1 through 22-14/ 2006 IBC Figs. 1613.5(1) through 1613.5(14) give contour maps for SSand S1, based on the 2002 edition of USGS seismic hazard maps

SS and S1 also available at http://eqhazmaps.usgs.gov



BNBC-2020 Table 6.C.1: Parameters SS and S1 for Different Seismic Zones

Parameters Zone 1 Zone 2 Zone 3 Zone 4

SS 0.3 0.5 0.7 0.9

S1 0.12 0.2 0.28 0.36

S1 = 0.4SS, not independent of SS, as in ASCE 7-05S1 = MCE-level PGA = Z, Seismic Zone Coefficient

Table 6.2.15: Seismic Zone Coefficient Z for Some Important Towns of Bangladesh



ASCE 7-05 Design Ground Motion

Site Class (Soil Type) Definitions (ASCE 7-05 Table 20.3-1, BNBC-2020 Table 6.2.13)

• Class A: Hard rock SA in BNBC “Benchmark” Soil Type in BNBC-2020

• Class B: Rock SB in BNBC “Benchmark” Site Class in ASCE 7-05 • Class C: Very dense soil and soft rock SC in BNBC• Class D: Stiff soil SD in BNBC• Class E: Soft soil SE in BNBC• Class F: Soils requiring site-specific evaluations S1, S2 in BNBC

2006 IBC 1613.5.2 “Default” Site Class

Site Class D must be used when the soil properties are not known in sufficient detail, unless the building official determines that Site Class E or F is likely to be present at the site.

Have not found equivalent statement in BNBC-2020.



ASCE 7-05 Table 11.4-1 Values of Site Coefficient Fa

a

Site ClassMapped Spectral Response Acc. at Short Periods

SS 0.25 SS = 0.5 SS = 0.75 SS = 1.00 SS 1.25

A 0.8 0.8 0.8 0.8 0.8

B 1.0 1.0 1.0 1.0 1.0

C 1.2 1.2 1.1 1.0 1.0

D 1.6 1.4 1.2 1.1 1.0

E 2.5 1.7 1.2 0.9 0.9

F Note b Note b Note b Note b Note b

ASCE 7-05 Table 11.4-1 Values of Site Coefficient Fa

a

a. Use straight line interpolation for intermediate values of mapped spectral acceleration at short period SS

b. Values shall be determined in accordance with Section 11.4.7 of ASCE 7



ASCE 7-05 Table 11.4-2 Values of Site Coefficient Fv

a

Site ClassMapped Spectral Response Acc. at 1 Second Period

S1 0.1 S1 = 0.2 S1 = 0.3 S1 = 0.4 S1 0.5

A 0.8 0.8 0.8 0.8 0.8

B 1.0 1.0 1.0 1.0 1.0

C 1.7 1.6 1.5 1.4 1.3

D 2.4 2.0 1.8 1.6 1.5

E 3.5 3.2 2.8 2.4 2.4

F Note b Note b Note b Note b Note b

ASCE 7-05 Table 11.4-2 Values of Site Coefficient Fv

a

a. Use straight line interpolation for intermediate values of mapped spectral acceleration at 1 second period S1

b. Values shall be determined in accordance with Section 11.4.7 of ASCE 7



BNBC-2020 Table 6.C.2: Coefficient Fa for Different Seismic Zones and Soil Types

Soil Type Zone-1 Zone-2 Zone-3 Zone-4

SA 1 1 1 1

SB 1.2 1.2 1.2 1.2

SC 1.15 1.15 1.15 1.15

SD 1.35 1.35 1.35 1.35

SE 1.4 1.4 1.4 1.4

Fa = S, dependent only on Site Class, no dependence on Zone or intensity of ground mtion

BNBC-2020 Table 6.C.3: Coefficient Fv for Different Seismic Zones and Soil Types

Soil Type Zone-1 Zone-2 Zone-3 Zone-4 Fv/Fa

SA 1 1 1 1 1SB 1.5 1.5 1.5 1.5 1.25

SC 1.725 1.725 1.725 1.725 1.5

SD 2.7 2.7 2.7 2.7 2

SE 1.75 1.75 1.75 1.75 1.25



Table 6.2.16: Site Dependent Soil Factor and Other Parameters Defining Elastic

Response Spectrum

ASCE 7-05 Design Ground Motion

SMS = soil-modified MCE spectral response acceleration at short periods

= FaSS

SM1 = soil-modified MCE spectral response acceleration at 1.0-second period

= FvS1



ASCE 7-05 Ground Motion

SDS = design spectral response acceleration at short periods

= (2/3) SMS

SD1 = design spectral response acceleration at 1.0-second period

= (2/3) SM1

BNBC-2020 Table 6.C.4: Parameter S for Different Seismic Zones and Soil Types


SA 0.2 0.333 0.466 0.6

SB 0.24 0.4 0.56 0.72

SC 0.23 0.383 0.536 0.69

SD 0.27 0.45 0.63 0.81

SE 0.28 0.466 0.653 0.84



BNBC-2020 Table 6.C.5: Parameter for Different Seismic Zones and Soil Types


SA 0.08 0.133 0.186 0.24

SB 0.12 0.2 0.28 0.36

SC 0.138 0.23 0.322 0.414

SD 0.216 0.36 0.504 0.648

SE 0.14 0.233 0.326 0.42

SD1 = SDS x 0.4 x Fv/Fa

1997 UBC Design Earthquake Ground Motion

Approximately 90% probability of non-exceedancein 50 years (approx. 475 yr. return period)



Design Basis: ASCE 7-05 vs. UBC

Design to avoid collapse in the Maximum Considered Earthquake, rather than to provide life safety in the 500-year return period earthquake


Maximum Considered Earthquake (MCE)

• Maximum level of earthquake ground shaking that is

considered reasonable to design buildings to resist




Maximum Considered Earthquake (MCE):

• Maximum Deterministic earthquakes (in coastal

California)- best estimate of ground motion from

maximum-magnitude earthquakes on seismic faults with

high probabilities of occurrence.

ACE 7-05 Ground Motion

Maximum Considered Earthquake (MCE)

• 2% probability of exceedance in 50 years

(approximately 2,500-year return period) where

deterministic approach is not used




TL Map of Contiguous USA




BNBC- ASCE2020 7-05TB T0TC TSTD TL

Tc = TS = SD1/SDS TD = 2s, fixed unlike TLTB (>)T0 = 0.20TS = 0.20TC;unclear how values were arrived at.





Response Spectrum

BNBC Design Response Spectrum

ASCE 7TS (TC) = SD1/SDS

T0 (TB) = 0.2TS

TL (TD) given on map

Soil Type (s) (s) (s)

SA 0.15 0.40 2.0

SB 0.15 0.50 2.0

SC 0.20 0.60 2.0

SD 0.20 0.80 2.0

SE 0.15 0.50 2.0



Figure 6.2.26 Normalized MCE Response Spectra for Different Site

Classes.



BNBC-2020 Table 6.C.1: Parameters SS and S1 for Different Seismic Zones

Parameters Zone 1 Zone 2 Zone 3 Zone 4

SS 0.3 0.5 0.7 0.9

S1 0.12 0.2 0.28 0.36

BNBC-2020 Table 6.C.2: Coefficient Fa for Different Seismic Zones and Soil Types


SA 1 1 1 1

SB 1.2 1.2 1.2 1.2

SC 1.15 1.15 1.15 1.15

SD 1.35 1.35 1.35 1.35

SE 1.4 1.4 1.4 1.4



BNBC-2020 Table 6.C.3: Coefficient Fv for Different Seismic Zones and Soil Types


SA 1 1 1 1

SB 1.5 1.5 1.5 1.5

SC 1.725 1.725 1.725 1.725

SD 2.7 2.7 2.7 2.7

SE 1.75 1.75 1.75 1.75

BNBC-2020 Table 6.C.4: Parameter S for Different Seismic Zones and Soil Types


SA 0.2 0.333 0.466 0.6

SB 0.24 0.4 0.56 0.72

SC 0.23 0.383 0.536 0.69

SD 0.27 0.45 0.63 0.81

SE 0.28 0.466 0.653 0.84



BNBC-2020 Table 6.C.5: Parameter for Different Seismic Zones and Soil Types


SA 0.08 0.133 0.186 0.24

SB 0.12 0.2 0.28 0.36

SC 0.138 0.23 0.322 0.414

SD 0.216 0.36 0.504 0.648

SE 0.14 0.233 0.326 0.42

SEISMIC ZONE VS.

SEISMIC DESIGN CATEGORY

PART 2



UBC Seismic Zones

Table 6.1.1 Building Occupancy Category

Occupancy Occupancy Category

Structures with low hazard to human life in the event of failure I

Standard occupancies IIStructures with substantial hazard to human life or economy in the event of failure. III

Designated essential facilities; utilities required for essential facilities; designated aviation-related structures; and structures containing highly toxic materials

IV

See BNBC 2020 Table 6.1.1 for detailed descriptions



SDC Based on Short-Period Response Acceleration – ASCE 7-05

Values of SDSOCCUPANCY CATEGORY

I or II III IV

SDS < 0.167 A A A

0.167 < SDS < 0.33 B B C

0.33 < SDS < 0.50 C C D

0.50 < SDS Da Da Da

No SDC A in Bangladesh

SDC Based on 1-sec Period Response Acceleration – ASCE 7-05

Values of SD1OCCUPANCY CATEGORY

I or II III IV

SD1 < 0.067 A A A

0.067 < SD1 < 0.133 B B C

0.133 < SD1 < 0.20 C C D

0.20 < SD1 Da Da Da

No SDC A in Bangladesh



SDC of IBC (Note a)

Value of S1OCCUPANCY CATEGORY

I or II III IV

S1 0.75g E E F

No SDC E, F in Bangladesh



SDC of BNBC-2020

Soil Type

OC I, II, and III OC IVZ1 Z2 Z3 Z4 Z1 Z2 Z3 Z4

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

SE,S1, S2

D D D D D D D D

BUILDING CONFIGURATIONPART 3



Building Configuration

Horizontal Irregularities (ASCE 7-05 Table 12.3-1)1a. Torsional irregularity 1b. Extreme torsional irregularity2. Re-entrant corners3. Diaphragm discontinuity4. Out-of-plane offsets5. Nonparallel systems

2.5.5.3 Building Irregularity

2.5.5.3.1 Plan irregularity: Following are the different types of irregularities that may exist in the plan of a building.

(i) Torsion irregularity

To be considered for rigid floor diaphragms, when the maximum storey drift ( ) as shown in Figure 6.2.27(a), computed including accidental torsion, at one end of the structure is more than 1.2 times the average [ =( + )/2] of the story drifts at the two ends of the structure. If >1.4 then the irregularity is termed as extreme torsional irregularity.

(ii) Re-entrant corners

Both projections of the structure beyond a re-entrant comer [Figure 6.2.27(b)] are greater than 15 percent of its plan dimension in the given direction.



2.5.5.3.1 Plan Irregularity(iii) Diaphragm Discontinuity

Diaphragms with abrupt discontinuities or variations in stiffness, including those havingcut out [Figure 6.2.27(c)] or open areas greater than 50 percent of the gross encloseddiaphragm area, or changes in effective diaphragm stiffness of more than 50 percentfrom one story to the next.

(iv) Out- of-Plane Offsets

Discontinuities in a lateral force resistance path, such as out of-plane offsets of vertical elements, as shown in Figure 6.2.27(d).

(v) Non-parallel Systems

The vertical elements resisting the lateral force are not parallel to or symmetric [Figure 6.2.27(e)] about the major orthogonal axes of the lateral force resisting elements.

Building ConfigurationVertical Irregularities (ASCE 7-05 Table 12.3-2)1a. Stiffness irregularity

– soft story 1b. Extreme soft story2. Weight (mass) irregularity3. Vertical geometric irregularity4. In-plane discontinuity in vertical

lateral-force-resisting elements5a. Discontinuity in lateral strength –

weak story5b. Extreme weak story



2.5.5.3 Building Irregularity2.5.5.3.2 Vertical Irregularity: Following are different types of irregularities that may exist along vertical elevations of a building.

(i) Stiffness Irregularity - Soft Storey

A soft storey is one in which the lateral stiffness is less than 70% of that in the storey above or less than 80% of the average lateral stiffness of the three storeysabove irregularity [Figure 6.2.28(a)]. An extreme soft storey is defined where its lateral stiffness is less than 60% of that in the storey above or less than 70% of the average lateral stiffness of the three storeys above.

(ii) Mass Irregularity

The seismic weight of any storey is more than twice of that of its adjacent storeys[Figure 6.2.28(b)]. This irregularity need not be considered in case of roofs.

2.5.5.3.2 Vertical Irregularity(iii) Vertical Geometric Irregularity

This irregularity exists for buildings with setbacks with dimensions given in Figure 6.2.28(c).

(iv) Vertical In-Plane Discontinuity in Vertical Elements Resisting Lateral Force

An in-plane offset of the lateral force resisting elements greater than the length of those elements Figure 6.2.28(d).

(v) Discontinuity in Capacity - Weak Story

A weak story is one in which the story lateral strength is less than 80% of that in the story above. The story lateral strength is the total strength of all seismic force resisting elements sharing the story shear in the considered direction [Figure 6.2.28(e)]. An extreme weak story is one where the storey lateral strength is less than 65% of that in the story above.



Building Configuration

Detailed discussion is included in Part 2 of this presentation

METHODS OF ANALYSISPART 4



ASCE 7-05 12.6 Analysis Procedure Selection

Simplified Analysis [12.14, not in BNBC-2020]

Equivalent Lateral Force Method [12.8, 2.5.7]

Modal Response Spectrum Analysis [12.9, 2.5.9]

Linear Response History Procedure [Ch. 16, 2.5.10]

Nonlinear Response History Procedure [Ch. 16, 2.5.11]

ASCE 7-05 Table 12.6-1 Permitted Analytical Procedures



Permitted Analytical Procedures – BNBC-2020 Section 2.5.6

ELF is allowed provided both of the following conditions are satisfies:

(a) The building period in the two main horizontal directions is smaller than both 4TC (TC is defined in Sec 2.5.4.3) and 2 seconds.

(b) The building does not possess irregularity in elevation as defined in Sec 2.5.5.3.

EQUIVALENT LATERAL FORCE

PROCEDURE

PART 5



WCV s

CS = Seismic Response CoefficientW = Effective Seismic Weight of Building

ASCE 7-05 12.8 Computation of Base Shear

ASCE 7-05 12.7.2 Effective Seismic Weight, W

W = Total Dead Load and Applicable Portions of Other Loads

25% of storage live load (2 exceptions)

Minimum 10 psf (0.5 kN/m2) partition load

Weight of permanent equipment

20% of uniform design snow load where flat roof snow load exceeds 30 psf (1.5 kN/m2)

Weight of landscaping and roof gardens



2.5.7.3 Seismic Weight, WSeismic 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) For live load up to and including 3 kN/m2, a minimum of 25 percent of the live load shall be applicable.

(b) For live load above 3 kN/m2, a minimum of 50 percent of the live load shall be applicable.

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

Where the probable imposed loads (mass) at the time of earthquake are more correctly assessed, the designer may go for higher percentage of live load.




ASCE 7-05 12.8.1.1 Design Base Shear






Response Spectrum



Figure 6.2.26 Normalized MCE Response Spectra for Different Site

Classes.

2.5.4.3 Design Response Spectrum

(6.2.34)



Example Calculation of Design Base Shear

An intermediate moment frame building in Dhaka on Soil Type ,

Residential apartment building, 1

1.35, Zone 2, 0.2

0.2 sec, 0.8 sec, 2.0 sec


1) sec

at sec,




2) sec sec

at sec,


0.8 sec sec

at sec,




sec sec

at sec,

Minimum Design Base Shear

, OK

Note: This check is not required by BNBC-2020



Calculation of Design Base Shear by ASCE 7-05

1) sec

For Zone 2, Soil Type , from BNBC-2020 Table 6.C.4


2) sec sec

For Zone 2, Soil Type ,

at sec,




3) sec (unspecified)

at sec,

min. = 0.79%, OK

Slightly different from 0.83% by BNBC-2020

ASCE 7-05 12.8.2, 2.5.7.2 Structure Period

Calculated by……

1. Approximate Formulas

2. Rational Analysis using structuralproperties and deformationalcharacteristics of resistingelements in a properlysubstantiated analysis



ASCE 7-05 12.8.2.1, 2.5.7.2(b)Approximate Fundamental Period

Ta = Ct hnx(m)

ASCE 7-05 Table 12.8-2, Table 6.2.20Values of Approximate Period Parameters Ct

and x(m)

Structure Type Ct X(m)Moment resisting frame systems of steel in which theframes resist 100 percent of the required seismic force andare not enclosed or adjoined by more rigid components thatwill prevent the frames from deflecting when subjected toseismic forces

0.0724 0.8

Moment resisting frame systems of reinforced concrete inwhich the frames resist 100 percent of the required seismicforce and are not enclosed or adjoined by more rigidcomponents that will prevent the frames from deflectingwhen subjected to seismic forces

0.0466 0.9



ASCE 7-05 Table 12.8-2, Table 6.2.20Values of Approximate Period Parameters Ct

and x(m) (cont’d)

Structure Type Ct X(m)

Eccentrically braced steel frames .0731 0.75

All other structural systems 0.0488 0.75

ASCE 7-05 12.8.2.1, 2.5.7.2(c)Approximate Fundamental Period



ASCE 7-05 12.8.2, 2.5.7.2 Upper Limit on Tby "Rational Analysis"

= 1.4 in BNBC-2020

Upper Limit on T by "Rational Analysis"Table 12.8 1 (ASCE 7 05)

Coefficient for Upper Limit on Calculated Period

Design Spectral Response Acceleration(SD1)

Coefficient Cu

0.40.30.20.150.10.05

1.41.41.51.61.71.7

Note: For drift analysis, upper limit on calculated T does not apply(Section 12.8.6.2)



Response Modification Coefficient, R

xe x E

Elastic Response of StructureVE

VY

V

Fully Yielded Strength

oR

Cd

Yielding

Lateral Deflection,

LateralSeism

icForce,V

Design Force Level

Response Modification Coefficient, R

ASCE 7-05 Table 12.2-1

R = 5 – Bearing wall system with special reinforced concrete or masonry shear walls

R = 8 – Special moment resisting frame system



Seismic Force-Resisting Structural Systems

EARTHQUAKE FORCE-RESISTING STRUCTURAL SYSTEMS OF CONCRETE —

ASCE 7-05, BNBC-2020 Table 6.2.19

BASIC SEISMIC FORCERESISTING SYSTEM

DETAILING REF.SECTION R 0 Cd

SYSTEM LIMITATIONS AND BUILDINGHEIGHT LIMITATIONS, ft (m), BYSEISMIC DESIGN CATEGORY

B C D EN/A FN/A

Moment Resisting Frame SystemsSpecial reinforced concrete momentframes

12.2.5.5 and14.2 8 3 51/2 NL NL NL NL NL

Intermediate reinforced concrete momentframes 14.2 5 3 41/2 NL NL NP NP NP

Ordinary reinforced concrete momentframes 14.2 3 3 21/2 NL NP NP NP NP

Dual Systems with Special Moment Frames

Special reinforced concrete shear walls 14.2 7 21/2 51/2 NL NL NL NL NL

Ordinary reinforced concrete shear walls 14.2 6 21/2 5 NL NL NP NP NP

Dual Systems with Intermediate Moment Frames

Special reinforced concrete shear walls 14.2 61/2 21/2 5 NLNL50 m

160NP

100 100

Ordinary reinforced concrete shear walls 14.2 51/2 21/2 41/2 NL NL NP NP NP




ASCE 7-05 , BNBC-2020 Table 6.2.19


DETAILING REF.SECTION R 0 Cd


B C D EN/A FN/A

Bearing Wall Systems

Special reinforced concrete shear walls 14.2 and14.2.3.6 5 21/2 5 NL NL

16050 m

160 100

Ordinary reinforced concrete shear walls 14.2 and14.2.3.4 4 21/2 4 NL NL NP NP NP

Detailed plain concrete shear walls N/A 14.2 and14.2.3.2 2 21/2 2 NL NP NP NP NP

Ordinary plain concrete shear walls N/A 14.2 and14.2.3.1 11/2 21/2 11/2 NL NP NP NP NP

Intermediate precast shear walls N/A 14.2 and14.2.3.5 4 21/2 4 NL NL 401 401 401

Ordinary precast shear walls N/A 14.2 and14.2.3.3 3 21/2 3 NL NP NP NP NP

1Increase in height to 45 ft is permitted for single-story storage warehouse facilities.


ASCE 7-05 , BNBC-2020 Table 6.2.19


DETAILINGREF. SECTION R 0 Cd


B C D EN/A FN/A

Building Frame Systems

Special reinforced concrete shear walls 14.2 and14.2.3.6 6 21/2 5 NL NL

16050 m

160 100

Ordinary reinforced concrete shear walls 14.2 and14.2.3.4 5 21/2 41/2 NL NL NP NP NP

Detailed plain concrete shear walls N/A 14.2 and14.2.3.2 2 21/2 2 NL NP NP NP NP

Ordinary plain concrete shear walls N/A 14.2 and14.2.3.1 11/2 21/2 11/2 NL NP NP NP NP

Intermediate precast shear walls N/A 14.2 and14.2.3.5 5 21/2 41/2 NL NL 401 401 401

Ordinary precast shear walls N/A 14.2 and14.2.3.3 4 21/2 4 NL NP NP NP NP

1Increase in height to 45 ft is permitted for single-story storage warehouse facilities.



2.5.5.2 Combinations (if not “Dual”)Same Direction

Horizontal (ASCE 7-05 12.2.3.2)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. The value of R used for design in that direction shall not be greater than the least value of R for any of the systems utilized in that direction. The deflection amplification factor, 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.

2.5.5.1 Combinations

Different Directions

Horizontal

Different seismic force–resisting systems are permitted to be used to resist seismic forces along each of the two orthogonal axes of the structure. Where different systems are used, the respective R and coefficients shall apply to each system, including the limitations on system use contained in Table 6.2.19.



2.5.5.1 CombinationsDifferent Directions (12.2.2)

• Use respective R, Cd, and 0 in each orthogonal direction

R = 8, Cd = 5 ½, o = 3

R = 5Cd = 5

o = 2½

RiskCategory

Importance Factor

I 1.0

II 1.0III 1.25

IV 1.5

ASCE 7-05 Table 11.5-1, Table 6.2.17Seismic Importance Factor, I



ASCE 7-05 12.8.3, 2.5.7.4 Determine FxOver the Height of the Building

Fx = CvxV

wx hxk

wi hikCvx =

k = 1 for T 0.5 seck = 2 for T 2.5 secLinear interpolation in between

2.5.7.5 Story Shear

(6.2.42)



ASCE 7-05 12.8.4, 2.5.7.5Horizontal Distribution of Forces

ASCE 7-05 12.8.4 , 2.5.7.5Horizontal Distribution of Forces



ASCE 7-05 12.8.4 , 2.5.7.5Horizontal Distribution of Forces

Torsion• Torsional moment due to difference in location of center

of mass and center of resistance must be consideredfor rigid diaphragms

Accidental torsion• For rigid diaphragms, must be included in addition to

the torsional moment• Displacement of center of mass = 5% building dimension

perpendicular to direction of applied forces

ASCE 7 12.8.4.3, 2.5.7.6.2Amplification of Accidental Torsional

Moments

For structures assigned to SDC C, D, E, or F, where Type 1a or 1b irregularity exists, ASCE 7-05 Section 12.8.4.3 requires that the torsional and accidental torsional moments be amplified by a torsional amplification factor of up to 3.

(6.2.44)



2.5.7.8 Overturning EffectsThe structure shall be designed to resist overturning effects caused by the seismic forces determined in Sec 2.5.7.4. At any story, the increment of overturning moment in the story under consideration shall be distributed to the various vertical force resisting elements in the same proportion as the distribution of the horizontal shears to those elements. The overturning moments at level , shall be determined as follows:

Where,

= Portion of the seismic base shear, induced at level

, = Height from the base to level or .

The foundations of structures, except inverted pendulum-type structures, shall be permitted to be designed for three-fourths of the foundation overturning design moment, determined using above equation.

(6.2.47)

ASCE 7-05 12.8.6, 2.5.7.7Story Drift Determination ( )

Lateral displacement of one level relative to the next level above or below

x



V

QE xe

Analysis of Structures under Code-Prescribed Seismic Forces


x = x - x-1 a

where….

x = Cd xe / I




Cd = displacement amplification factor

(ASCE 7-05 Table 12.2-1, Table 6.2.19)

xe = elastic analysis displacement

a = allowable story drift

(ASCE 7-05 Table 12.12-1, Table 6.2.21, Section 2.5.14)

I = seismic importance factor

(ASCE 7-05 Table 1.5-2, Table 6.2.17)

Allowable Story Drift ( a ) ASCE 7-05 Table 12.12-1, Table 6.2.21

BuildingOccupancy Category

I or II III IVBuildings 4 stories in height; other than masonry;Non-structural elements designed to accommodate story drift

0.025hsx 0.020hsx 0.015hsx

Masonry cantilever shear wall buildings

0.010hsx 0.010hsx 0.010hsx

Other masonry shear walls buildings0.007hsx 0.007hsx 0.007hsx

All other buildings 0.020hsx 0.015hsx 0.010hsx

= Story height below level



EARTHQUAKE LOAD EFFECTS AND

LOAD COMBINATIONS

PART 6

2.7.3 Strength Design Load Combinations

1. 1.4(D + F)

2. 1.2(D + F + T) + 1.6(L + H) + 0.5(Lr or R)

3. 1.2D + 1.6(Lr or R) + (L or 0.8W)

4. 1.2D + 1.6W + L + 0.5(Lr or R)

5. 1.2D + 1.0E + 1.0L

6. 0.9D + 1.6W + 1.6H

7. 0.9D + 1.0E + 1.6H



2.7.3 Strength Design Load Combinations (without F, H, T)

1. 1.4D

2. 1.2D + 1.6L + 0.5(Lr or R)

3. 1.2D + 1.6(Lr or R) + (L or 0.8W)

4. 1.2D + 1.6W + L + 0.5(Lr or R)

5. 1.2D + 1.0E + 1.0L

6. 0.9D + 1.6W

7. 0.9D + 1.0E

Seismic Strength Design Load Combinations

1.2D + 1.0E + 1.0L

0.9D + 1.0E

E = QE + 0.2SDSD ASCE 7-05 12.4.2

E = QE - 0.2SDSD ASCE 7-05 12.4.2

= 1 in Seismic Design Category (SDC) A, B, and C

= 1 or 1.3 in SDC D, E, and F



QE

xe

V

x

Analysis of Structures under Code-Prescribed Seismic Forces

2.5.13 Earthquake Load Effects and Load Combinations

In BNBC-2020

= 1 (not mentioned at all)

= (6.2.56)

=

=

=



Effect of Vertical Earthquake Ground Motion

Gravity and Earthquake Effects Additive

=

=

=

Dhaka, Zone-2, Soil Type SD: = 0.45

=

The load factor on live load L in combinations (3), (4), and (5) is permitted to be reduced to 0.5 for all occupancies in which minimum specified uniformly distributed live load is less than or equal to 5.0 kN/m2, with the exception of garages or areas occupied as places of public assembly.

Effect of Vertical Earthquake Ground Motion

Gravity and Earthquake Effects Counteractive

=

=

=

Dhaka, Zone-2, Soil Type SD: = 0.45

=



Load Combinations with OverstrengthFactor

Cantilever Column Systems 12.2.5.2SDC B F

Foundation and other elements used to provide overturning resistance at the base of cantilever column elements shall have the strength to resist the load combinations with over strength factor of Section 12.4.3.2.


Elements Supporting DiscontinuousWalls or Frames

12.3.3.3SDC B F

Elements supporting discontinuous walls or frames

SHEAR WALL

Elements supporting discontinuous walls or frames

SHEAR WALL




Collector Elements 12.10.2.1 (SDC C F)




2.5.13.4 Load Combinations with Overstrength Factor

Basic Combinations for Strength Design with Overstrength Factor

(1.2 + 0.2SDS)D + 0QE + L

(0.9 0.2SDS)D + 0QE

2.5.5.6 Provisions for Using System Overstrength Factor, o

2.5.5.6.1 Combinations of Elements Supporting Discontinuous Walls or Frames.

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 shall have the design strength to resist the maximum axial force that can develop in accordance with the load combinations with overstrength factor of Section 2.5.13.4. 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.




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

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 6-104 Vol. 2 and to collectors and for connections of collectors to the vertical elements. Collectors and their connections also shall be designed for these increased forces unless they are designed for the load combinations with overstrength factor of Section 2.5.5.4, in accordance with Section 2.5.13.4.


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

In structures assigned to Seismic Design Category C, D or E, collector elements, splices, and their connections to resisting elements shall resist the load combinations with overstrength of Section 2.5.13.4.




2.5.5.6.4 Batter Piles.

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. 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.

Questions?Thank you



earthquakes and their effects on structures

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