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SEISMIC RESISTANT DESIGN STANDARD FOR

BUILDING STRUCTURES

SNI17262002

By :

Wiratman WangsadinataEmeritus Professor, Tarumanagara University

President Director, Wiratman & Associates

Chairman SNI-1726-2002 Committee

ABSTRACT

In this summary paper, the main principles of the Indonesian Seismic Resistant Design Standard for

Building Structures SNI-1726-2002 are explained. The summary covers seismic design provisions on

basic requirements for building design and material strengths. The Design Earthquake considered

has a return period of 500 years (10 % probability of exceedance in 50 years) and the resulting peakbase rock acceleration forms the basis for establishing the Indonesian Seismic Zoning Map. The peak

ground acceleration depends on the soil category (site-class) present on top of the base rock. With

this acceleration the response spectra of the Design Earthquake are defined for determining the effect

of the Design Earthquake upon building structures. Under the effect of the Design Earthquake the

building structure is at its state of near collapse with a maximum deflection, assumed to be the same

for any ductility level of the structure. With this assumption and known overstrength in the structure,

for a certain level of ductility, a simple formulation is established regarding the effect of the Design

Earthquake upon a building structure, such as elastic load, maximum load on the structure at its state

of near collapse, first yield load and nominal load for design. Against the effect of the Design

Earthquake, a building structure is in general analysed dynamically using response spectrum modal

analysis method. However, regular building structures, having their first and second mode motion

dominantly in translation, may be analysed statically using equivalent static seismic loads. The

substructure (basement and foundation) may be analysed as a separate structure subjected to theeffect of the Design Earthquake originating from the superstructure, from own inertial forces and

from the surrounding soil. Finally the strength design of the substructure based on the Load and

Resistance Factor Design method is discussed.

Key words: Standard, earthquake, dynamic response, structure, building, ductility.

1. INTRODUCTION

This standard has taken into account as far as possible the latest development

of earthquake engineering in the world, particularly what has been reported by the

National Earthquake Hazards Reduction Program (NEHRP), USA, in its report titled

NEHRP Recommended Provisions for Seismic Regulations for New Buildings and

Other Structures (February 1998), but on the other hand maintains as close as

possible the format of the previous Indonesian standard Rules for Earthquake

Resistant Design of Houses and Buildings (SNI 03-1726-1989).

In general this standard is sufficient to be used as the basis for the modern

design of seismic resistant building structures, particularly highrise buildings.

In order that the building engineering community understands what the basic

principles are of this standard, in this paper their background are explained. More

detailed explanations can be found in the commentary of the respective clauses,

which is an integral part of the standard.

2. DESIGN EARTHQUAKE AND SEISMIC ZONING MAP OF

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O

0 9 5O

1 0 0

O

1 0 5

1 1 0

O

1 1 5

O

15OS

O10 S

SO

5

0O

N5O

10ON

O

1 2 0

1 2 5

O

1 3 0

O

1 3 5

O

1 4 0

O

0.03 g1 0.15 g3

0.20g

4 0.25g

5 0.30g6

0.10 g2

Kilometer

800 200 400

INDIA N

O C E

A N

Figure 1. The Seismic Zoning Map of Indonesia with peak base acceleration with a

return period of 500 years.

Cornell, 1968), (2) the annual total probability, (3) the annual event probability

(Poissons function), (4) the return period (which is the inverse of the annual

probability), and (5) the peak base accelerations with a mean return period of 500

years, obtained through interpolation (logarithmic).

On the Seismic Zoning Map of Indonesia (Figure 1) it can be seen, that

Indonesia is divided into 6 seismic zones, Seismic Zone 1 being the least and

Seismic Zone 6 the most severe seismic zone. The mean peak base acceleration for

each zone starting from Seismic Zone 1 to 6 are respectively as follows : 0.03 g, 0.10

g, 0.15 g, 0.20 g, 0.25 g and 0.30 g (see Figure 1 and Table 2).

It should be noted, that the peak base acceleration for Seismic Zone 1 is the

minimum value to be considered in the design of building structures, to provide a

minimum robustness to the structure. Therefore, this peak base acceleration has a

rather longer return period than 500 years (conservative).

3. LOCAL SOIL CATEGORY AND PEAK GROUND

ACCELERATION

From the previous discussion it follows, that the peak ground acceleration may

be obtained from the result of a seismic wave propagation analysis, whereby the

waves are propagating from the base rock to the ground surface. However, this

standard provides conveniently the value of the peak ground acceleration for every

seismic zone for 3 categories of soil present on top of the base rock, namely Hard

Soil, Medium Soil and Soft Soil.

According to this standard, the differentiation of the soil category is defined by

the following 3 parameters : shear wave velocity vs, Standard Penetration Test (SPT)

or N-value and undrained shear strength (Su). The base rock for example is defined

as the soil layer below the ground surface having shear wave velocities reaching 750

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m/sec, with no other deeper layers having lower shear wave velocity values.

According to another definition, the base rock is the soil layer below the ground

surface having Standard Penetration Test values of at least 60, with no other deeper

layers having lower N-values.

The soil on top of the base rock generally consists of several layers, each withdifferent values of the soil parameters. Therefore, to determine the category of the

soil, the weighted average of the soil parameter must be computed using the

thickness of each soil layer as the weighing factor. The weighted average shear wave

velocity s , Standard Penetration Test value N and undrained shear strength uS ,

can be computed from the following equations :

sii

m

1i

i

m

1is

v/t

t

v

=

== .. (1)

ii

m

1i

i

m

1i

N/t

t

N

=

== .. (2)

uii

m

1i

i

m

1iu

S/t

t

S

=

== ..... (3)

where ti is the thickness of layer i, vsi the shear wave velocity of layer i, Ni the

Standard Penetration Test value of layer i, Sui the undrained shear strength of layer i

and m is the number of soil layers present in the considered soil.

Due to the fact that the amplification of waves propagating from the base rock

to the gound surface is determined only by the soil parameters up to a certain depth

from the ground surface, in using eqs.(1), (2) and (3) the total depth of the considered

soil must not be taken more than 30 m. To consider soil depths of more than this is

not allowed, as the weighted average of the soil strength tends to increase with depth,

whereas soil layers below 30 m do not contribute in amplifying the waves. So, using

the weighted average of soil parameters according to eqs.(1), (2) and (3) for a total

depth of not more than 30 m, the definition of Hard Soil, Medium Soil and Soft Soilis shown in Table 1.

In Table 1, PI is the plasticity index and wn the natural water content.

Furthermore, what is meant by Special Soils are soils having high liquefaction

potentials, very sensitive clays, soft clays with a total thickness of 3 m or more,

loosely cemented sands, peat, soils containing organic materials with a thickness of

more than 3 m, and very soft clays with a plasticity index of more than 75 and a

thickness of more than 30 m. For these Special Soils the peak ground acceleration

must be obtained from the result of a seismic wave propagation analysis.

For the soil categories defined in Table 1, the peak ground acceleration Ao for

each seismic zone is shown in Table 2.

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Table 1. Soil Categories

Soil Category

Average shear wave

velocity

s (m/sec)

Average Standard

Penetration

N

Average undrained

shear strength

uS (kPa)

Hard Soils > 350 N > 50 uS > 100

Medium Soil 175 < s < 350 15 < N < 50 50 < uS < 100

s < 175 N < 15 uS < 50

Soft Soil Or, any soil profile with more than 3m of soft clays with PI > 20, wn

> 40% and Su < 25 kPa.

Special Soil Site specific evaluation required.

Table 2. Peak Base Acceleration and Peak Ground Acceleration Ao

Peak Ground Acceleration Ao (g)SeismicZone

Peak Base

Acceleration

(g) Hard Soil Medium Soil Soft Soil Special Soil

1

2

3

4

5

6

0.03

0.10

0.15

0.20

0.25

0.30

0.04

0.12

0.18

0.24

0.28

0.33

0.05

0.15

0.23

0.28

0.32

0.36

0.08

0.20

0.30

0.34

0.36

0.38

Site specific

evaluation

required.

4. RESPONSE SPECTRA OF THE DESIGN EARTHQUAKE AND

MODAL ANALYSIS

In general a response spectrum is a diagram representing the maximum

response acceleration of a Single Degree of Freedom (SDOF) system to the input

earthquake ground motion, as a function of the damping factor (fraction of critical

damping) h and the natural vibration period T of the SDOF system. Thus, a responsespectrum may be computed analytically and its diagram plotted for any input

earthquake ground motion with given accelerogram. For T=0 the SDOF system is

very stiff, so that it follows almost completely the ground motion. Therefore, for T=0

the maximum response acceleration becomes identical with the peak ground

acceleration Ao. For a certain damping factor h, the maximum response acceleration

follows a random function. Taking the T-axis in horizontal direction and the

maximum response acceleration axis in vertical direction, that random function starts

with an initial value Ao at T=0, then goes upwards until it reaches a certain

maximum value, after which it goes downwards again approaching the T-axis

asymptotically.

In this standard the maximum response acceleration of the SDOF system due

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to the Design Earthquake is expressed in the gravity acceleration (g) and is called the

Seismic Response Factor C (non-dimensional). Furthermore, the C-T function is

simplified into a smooth curve, consisting of 3 branches, namely : for 0 < T < 0.2 sec

the C value increases linearly from Ao till Am; for 0.2 sec < T < Tc the C value is

constant and equal to Am; for T > Tc the C value decreases following a hyperbolicfunction C = Ar/T. In this case, Tc is called the natural corner period, while the

SDOF system considered has a damping factor of h = 5 %. For the short range of

natural period 0 < T < 0.2 sec, the C value contains various uncertainties in relation

to the ground motion as well as to the ductility of the SDOF system considered.

Therefore, in this range the C value should be taken equal to Am. It can then be stated

that for T < Tc the response spectrum is associated with a constant maximum

response acceleration, while for T > Tc it is associated with a constant maximum

response velocity, as a consequence of the hyperbolic function in this range.

Table 3. Response Spectra of the Design Earthquake

Hard Soil

Tc = 0.5 sec.

Medium Soil

Tc = 0.6 sec.

Soft Soil

Tc = 1.0 sec.SeismicZone

Ao Am Ar Ao Am Ar Ao Am Ar

1

2

3

4

5

6

0.04

0.12

0.18

0.24

0.28

0.33

0.10

0.30

0.45

0.60

0.70

0.83

0.05

0.15

0.23

0.30

0.35

0.42

0.05

0.15

0.23

0.28

0.32

0.36

0.13

0.38

0.55

0.70

0.83

0.90

0.08

0.23

0.33

0.42

0.50

0.54

0.08

0.20

0.30

0.34

0.36

0.38

0.20

0.50

0.75

0.85

0.90

0.95

0.20

0.50

0.75

0.85

0.90

0.95

According to this standard Am is defined as 2.5 Ao, which is an average

condition found in response spectra in general. It is also defined that Tc = 0.5 sec for

Hard Soil, Tc = 0.6 sec for Medium Soil and Tc = 1.0 sec for Soft Soil, all of which

being approximate values. Based on these conditions for each seismic zone, the

values of Ao, Am and Ar of the response spectra of the Design Earthquake are as

listed in Table 3.

For easy application, the response spectra of the Design Earthquake for Hard

Soil, Medium Soil and Soft Soil for each seismic zone of Indonesia are shown on

Figure 2.The response spectra C-T of the Design Earthquake are used as input data for

the dynamic response analysis of building structures in the elastic range, using the

response spectrum modal analysis method. In this method the building structure is

modelled as a Multi Degree of Freedom (MDOF) system, being excited at its

foundation by the Design Earthquake. Applying the modal analysis method,

whereby a coordinate transformation is performed, the equations of motion of the

MDOF system, which in the original coordinates constitute of coupled second order

differential equations, become uncoupled in the new coordinates. Each free equation

has the form of the equation of motion of an SDOF system. The transformation

soil)(SoftT

0.50C =

soil)(MediumT

0.23C =0.38

0.20

soil)(SoftT

0.20C =

soil)(MediumT

0.08C =

soil)(Hard0.05

C =

0.30 soil)(HardT

0.15C =

0.50Seismic Zone 1

C

Seismic Zone 2

0.20

.1

C

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Figure 2. Response Spectra of the Design Earthquake.

matrix involved is the eigenvector matrix, which contains orthogonality properties

among its modes, causing the uncoupling of the equations. Furthermore, the

expression of the total dynamic response of the MDOF system takes the form of a

superposition of the dynamic response of each single mode, whereby the higher the

mode is the smaller its participation in producing the total response. This fact creates

the possibility of using the response spectra of the Design Earthquake as a basis for

determining the maximum dynamic responses of those single modes. It should be

recognized however, that the dynamic responses of the single mode, determined from

the response spectra of the Design Earthquake, are maximum responses, whereas in

general each mode reaches its maximum response at different times. Therefore, the

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superposition of the maximum dynamic responses must be modified. According to

this standard, which is based on various studies, if the MDOF system possesses

sparsely spaced natural periods, the superposition of the maximum dynamic

responses may be performed using the method known as Square Root of the Sum of

Squares (SRSS), while if those natural periods are closely spaced, the superpositionmust be performed using the method known as Complete Quadratic Combination

(CQC). Natural periods must be considered closely spaced, if their difference is less

than 15%. The number of modes considered in the superposition may be limited, as

long as the total mass participation in producing the total response is at least 90%.

The vertical effect of earthquakes shall be considered in balconies, canopies,

long cantilevers, transfer beams, long-span prestressed beams, simultaneously with

their horizontal effect. The vertical acceleration induced by the Design Earthquake to

the building is expressed as Ao I, where Ao is the peak ground acceleration, I is the

importance factor (see section 6.1) and is a coefficient depending on the seismic

zone as listed in Table 4. It is obvious that the value of is increasing withincreasing seismicity of the seismic zone, as the epicenters become closer.

Table 4. Coefficient to compute the vertical acceleration

of the Design Earthquake

Seismic Zone Coefficient

1

2

3

4

5

6

0.5

0.5

0.5

0.6

0.7

0.8

5. DUCTILITY, OVERSTRENGTH AND THE EFFECT OF THE

DESIGN EARTHQUAKE ON THE BUILDING STRUCTURE

According to this standard, against the effect of the Design Earthquake any

building structure must be designed to remain standing, although it may have

reached a state of near collapse. The load-deflection history of a building structure

until reaching its state of near collapse, depends on the level of ductility of the

structure. However, whatever the level of ductility, the maximum deflection reached

by the building structure at its state of near collapse, is assumed to be the same

according to this standard. This is known as the constant maximum displacement

concept, a phenomena shown by many elasto-plastic systems (Veletsos, Newmark,

1960).

The load-deflection diagram of a building structure designed to remain elastic

and designed to possess a certain level of ductility, based on the constant maximum

displacement concept may be visualized as shown in Figure 3, whereby m =

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constant. In this figure the load is represented by the base shear load V resisted by

the structure, and the deflection is represented by the top floor deflection of the building structure. Furthermore, the level of ductility according to this standard is

expressed by a factor called ductility factor , which is the ratio between the

maximum deflection m and the deflection at first yield y (at which the first plastichinge develops), so that :

m

y

m

1 = ............. (4)

where = 1 is the ductility factor of a building structure designed to remain elastic

up to its state of near collapse (m = y), while m is the maximum ductility factorwhich can be mobilized by the structure. For various structural systems this standard

provides the values ofm. The largest m value is of a full ductile structure, namely

m = 5.3. The higher the value of possessed by a structure (the more ductile the

structure) the lower the value of the first yield load Vy and also the lower the value ofthe maximum seismic load Vm absorbed by the structure at its state of near collapse.

In the process of load increase from Vy to Vm the V- diagram follows a paraboliccurve, during which more and more plastic hinges are developed in the highly

redundant structure, accompanied by continuous redistribution of moments, until a

condition is reached at which the structure is at its state of near collapse. The higher

the value of, the longer the V- curve will be.

Figure 3. Load deflection diagram (V- diagram) of a building structure.

V

Fi

zi

V

Ve

Vy

f Vn

f

n m 0

f2

Vn

Vm

f1

R Vn

y

R

ductile

elastic

If the elastic load Ve of a building structure in its elastic condition is known,

for example from the result of a response spectrum modal analysis as described in

section 4, and the building structure is to be designed to have a certain ductility

factor , which according to this standard may be chosen by the designer or thebuilding owner, then from Figure 3 it can be seen, that the seismic load producing

first yield is:

= ey

VV .... (5)

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At the seismic load level Vy, the first plastic hinge begins to develop at the most

critical section of the structure. To design the strength of that critical section based

on the Load and Resistance Factor Design method as required by this standard, the

seismic load to be considered, called the nominal seismic load Vn, must be taken

lower that Vy, to accommodate the strength margin required to cope with overload onthe structure and understrength of the material. The nominal seismic load Vn is

obtained by reducing Vy by a certain overstrength factor f1, so that the following

expression applies :

R

V

f

VV e

1

y

n == ... (6)

in which

1fR = .... (7)

where R is called the seismic reduction factor (see Figure 3).Theoretically the minimum value of f1 is the product of the load factor and the

material factor used in the Load and Resistance Factor Design, namely f1 = 1.05 x

1.15 = 1.2. The material factor is the inverse of the capacity reduction factor (= 1/).In reality there will always be oversized steel sections or excessive concrete

reinforcements in structural members, so that in general f1 > 1,2. According to this

standard the overstrength factor is assumed to be constant namely f1 = 1.6. Therefore,

eqs. (6) and (7) becomes :

R

V

1.6

VV e

y

n == ........ (8)

in which

mR1.6R61. = . (9)

where Rm is the maximum seismic reduction factor that can be mobilized by the

building structure, its value being given in the standard together with its related mvalue for various structural systems. The largest Rm value is therefore that of a full

ductile structure, namely Rm = 1.6 x 5.3 = 8.5.

The ratio between Vm and Vy is another factor called overstrength factor f2,

which is mobilized because of the redundancy of the structure. Hence, the following

relationship can be written (see figure 3) :

Vm = f2 Vy ................................................. (10)

The higher the redundancy of the building structure, the higher the value of f2 that

can be mobilized by the building structure. The largest value of f2 is of a full ductile

structure ( = 5.3), namely f2 = 1.75. The smallest value of f2 is of a full elastic

building structure ( = 1.0), where no plastic hinges have developed norredistribution of moments has occurred, namely f2 = 1.00. Applying the equal initial

slope principle of a parabola, based on the above boundary conditions, the

relationship between and f2 may be expressed as follows :

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f2 = 0.83 + 0.17 ...................................... (11)

The relationship between Vm and Vn can now be expressed as follows :

....(12)nm VfV =

where

f = f1 f2 = 1.6 f2 ....(13)

For the whole spectrum of ductility of building structures, from the full elastic

( = 1) up to the full ductile ( = 5.3), in Table 5 the values of the parameters R, f2and f are listed. All ductility levels between full elastic and full ductile is referred to

as partially ductile.

Table 5. Ductility parameters of building structures

Performance

level R

eq.(7)

f2eq.(11)

f

eq.(13)

Full elastic

Partially ductile

Full ductile

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.3

1.6

2.4

3.2

4.0

4.8

5.6

6.4

7.2

8.0

8.5

1.00

1.09

1.17

1.26

1.35

1.44

1.51

1.61

1.70

1.75

1.6

1.7

1.9

2.0

2.2

2.3

2.4

2.6

2.7

2.8

In the implementation of the seismic resistant design of building structures in

practice, the process starts with an analysis of the building structure under the effect

of the nominal seismic load Vn, for example by performing a response spectrum

modal analysis as described in section 4, using the response spectra of the Design

Earthquake. The ordinates are firstly multiplied by the importance factor I (see

section 6.1) and then divided by the seismic reduction factor R for the selected value. The whole result can thus be used for the strength design of the structure

based on the Load and Resistance Factor Design method.

The deflection of the building structure n due to the nominal seismic load Vn,can also be used to calculate deflections of the building structure at various

conditions under the effect of the Design Earthquake, such as the deflection at first

yielding :

y = f1 n = 1.6 n .... (14)

and the deflection at its state of near collapse :

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m = R n .... (15)

The deflection at first yielding y is used as a measure of its serviceability limit stateperformance, in preventing excessive yielding of the steel or excessive cracking of

the concrete, excessive non-structural damage and inconvenience to the occupants.This deflection is oftenly said to have been caused by a small to moderate earthquake,

which occurs only once in the life time of the building, thus with a probability of

occurrence of about 60% in the life time of the building according to the probability

theorem. The deflection of the building structure at its state of near collapse m isused as a measure of its ultimate limit state performance, in limiting the possibility of

structural collapse that may cause loss of human lives and limiting the possibility of

dangerous pounding between buildings or between structural components separated

by separation joints. The detailed provisions can be found in the respective clauses in

the standard.

6. THE ANALISIS OF 3D STRUCTURES

6.1. GENERAL

If soil-structure interaction is not considered, for the analysis of the upper part

of a building structure (the superstructure), it may be assumed that it has its lateral

restraint (fixity) at the level of the ground floor, if there is a basement, and at the

level of the top of the pile cap of a pile foundation or at the level of the bearing plane

of a footing or a raft foundation, if there is no basement.

Based on the structural layout of the building, the most critical direction of theearthquake action must be determined, which is parallel to the most dominant

direction of the structural subsystems (open frames, shear walls). Usually this

direction is also the most suitable one to be use as the direction of one of the

coordinate axis (x-axis or y-axis) of the global coordinate system for the structural

analysis. For highly irregular structural layouts, the critical direction of the

earthquake action must be determined through a trial and error procedure.

In reality the earthquake action will have an arbitrary direction, so that in

general there will always be 2 components of earthquake action on the structure,

each parallel to the orthogonal coordinate axes. Biaxial earthquake loading may have

a more detrimental effect on the structure than the full uniaxial one. This condition is

simulated by the requirement in this standard to always consider 100% earthquakeaction in one direction, in combination with 30% earthquake action in its

perpendicular direction.

Stiffness reduction due to concrete cracking must be taken into account, by

assigning proper flexural and torsional stiffness modifiers in the analysis of the

structure.

If in the direction of a coordinate axis the R value is not known yet, its value

must be computed as the weighted average of the R value of all structural subsystems

present in that direction, using the seismic base shear Vs resisted by each subsystem

as the weighing factor. In this case the R value of each subsystem in that direction

must be known, for example R = 8.5 for an open frame and R = 5.3 for a shear wall,

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which are their maximum values according to this standard. For the x-axis direction,

the weighted average R value may be computed as follows :

xsxs

o

x

xsxs

xs

x /RV

V

/RV

VR

==....(16)

and for the y-axis direction :

ysys

o

y

ysys

ys

yR/V

V

R/V

VR

=

= ....(17)

To be able to apply eqs. (16) and (17), a common way is to carry out a response

spectrum modal analysis as described in section 4 due to the elastic Design

Earthquake (R = 1 and I = 1) for each of the direction of the coordinate axis, to

determine Vs of each structural subsystem. The representative value of the overallseismic reduction factor R of the 3D building structure, is then computed as the

weighted average of Rx and Ry, using and as the weighing factors :oxV

oyV

y

o

yx

o

x

o

y

o

x

R/VR/V

VVR

+

+= ....(18)

The R value according to eq.(18) is a maximum value that can be used, so that a

lower value may be considered if desired in accordance with the chosen value.In the analyses of the 3D building structure, the P-Delta Effect must be

considered, if the building height is more than 10 stories or 40 m. The P-Delta Effect

is a phenomena occurring in flexible building structures, where due to the large

lateral displacements of the floors, additional lateral loads are generated as the result

of the overturning moments produced by the laterally displaced gravity loads.

The 3D character of the building structure is reflected by the requirement in the

standard to consider a design eccentricity ed between the Center of Mass and the

Center of Rotation of each floor, each floor being considered as horizontally rigid

diaphragms. This is to cope with the effect of the rotational component of the ground

motion, the possible change of the position of the Center of Mass due to change in

gravity loads and the possible change of the Center of Rotation due to post-elastic

plastification. The detailed provisions can be found in the respective clauses in thestandard.

Before proceeding with the seismic response analysis of the structure, the

fundamental vibration period T1 of the building structure must be examined. For

irregular building structures, T1 is obtained directly from the result of a 3D free

vibration analysis (with due consideration of the P-Delta Effect and the design

eccentricity ed), while for regular building structures behaving almost as 2 D

structures in each principle direction (see section 6.3) T1 can be obtained from the

static deflections of the structure as the result of a static load 3D analysis (again with

due consideration of the P-Delta Effect and the design eccentricity ed), by

substituting those deflections into the well-known Rayleighs formula (see further

section 6.3). The fundamental vibration period must satisfy the following

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requirement :

T1 < n .... (19)

where n is the number of stories of the building and is a coefficient depending onthe seismic zone where the building is located according to Table 6.

Table 6. The coefficient for the limitation of T1

Seismic Zone

1

2

3

45

6

0.20

0.19

0.18

0.170.16

0.15

Before proceeding with the seismic response analysis of the structure, also the

category of the building must be determined, by assigning the value of its importance

factor I. This factor is intended to adjust the return period of the Design Earthquake,

whether it is longer or shorter than the return period of 500 years. A return period

longer then 500 years (I > 1) must be considered, if the 2 following cases are

encountered : (1) the probability of occurrence of the Design Earthquake in the 50

years life time of the building must be taken lower than 10% (for example forhospitals), or (2) the life time of the building is much longer than 50 years (for

example for monuments and very tall buildings), while the 10% probability of

occurrencre of the Design Earthquake in the longer life time is maintained. For both

cases the return period of the Design Earthquake is longer than 500 years. A return

period shorter than 500 years (I < 1) may be considered, if the life time of the

building is shorter than 50 years (for example for low-rise buildings), so that with a

10% probability that in the shorter life time of the building the Design Earthquake

will occur, the return period of that earthquake is shorter than 500 years. For various

categories of buildings, the importance factor I according to this standard is

formulated as follows :

I = I1 I2 .... (20)

where I1 is the importance factor to adjust the return period of the Design Earthquake

related to the adjustment of its occurrence probability and I2 is the importance factor

to adjust the return period of the Design Earthquake related to the adjustment of the

life time of the building. The factors I1 and I2 are given in Table 7.

Table 7. Importance Factor for several building categories

Importance FactorBuilding Category

I1 I2 I

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General buildings such as for residential,

commercial and office use

1.0 1.0 1.0

Monuments and monumental buildings 1.0 1.6 1.6

Post earthquake important buildings such as

hospital, clean water installation, power plant,emergency and rescue center, radio and

television facilities

1.4 1.0 1.4

Buildings for storing dangerous goods such as

gas, oil products, acid, toxic materials

1.6 1.0 1.6

Chimneys, elevated tanks 1.5 1.0 1.5

Note:

For all building structures, which usage permit is issued prior to the enforcement

date of this standard, the importance factor I may be multiplied by 0.8.

6.2. THE IRREGULAR BUILDING STRUCTURE

After the fundamental period T1 satisfies eq.(19), its modal motions must

further be examined. According to this standard, the fundamental mode motion must

be dominant in translation, in order that the building structure doesnt respond

dominantly in torsion to the seismic loading, which is disturbing the convenience of

the occupants. If this requirement is not satisfied, the structural system must be

rearranged by placing more rigid structural elements at the periphery of the building

to increase its overall torsional stiffness.

Based on the fundamental period T1, the nominal static equivalent base shear

due to the Design Earthquake is computed as follows :

t1

1 WR

ICV = .... (21)

where C1 is the Seismic Response Factor obtained from the response spectra of the

Design Earthquake shown on Figure 2 for the first natural period T1, I the importance

factor of the building, R the representative seismic reduction factor of the building

structure (Table 5) and Wt the total weight of the building, including an appropriate

portion of the live load (see section 6.3). The base shear V1 is a reference quantity

for the total nominal base shear Vt obtained from the result of a response spectrum

modal analysis as described in section 4, whereby the response spectrum used is thatof the Design Earthquake shown on Figure 2, its ordinates being multiplied by I/R.

The following requirement must be satisfied :

Vt > 0.8 V1 .... (22)

to ensure that a certain minimum effect of the Design Earthquake is guaranteed in

cases where the total response is smaller than the static equivalent base shear.

To satisfy the requirement expressed by eq.(22), the nominal story shears

obtained from the result of the response spectrum modal analysis, must be multiplied

by a scaling factor as follows :

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1V

V0,8FactorScaling

t

1 = .... (23)

Visually the result of the above described scaling, is shown on Figure 4, where the

CQC curve is the nominal story shear distribution obtained from the result of theresponse spectrum modal analysis.

Story

Nominal Story Shear

CQC (total response)

Modified design curve

0.8V1Vt

0

First mode resp onse

V1

0.8V1

Vt

CQC (design curve)

Story

Nominal Story Shear

CQC (total response)

Modified design curve

0.8V1Vt

0

First mode resp onse

V1

0.8V1

Vt

CQC (design curve)0.8V

1

Vt

CQC (design curve)

Figure 4. The nominal story shear diagrams along the height of

the building structure.

The example shown on Figure 4 represents a case, where the nominal story

shear curve is showing an inward turn. For such a case, if desired the curve may be

modified conservatively as shown by the broken line. From the final nominal storyshear curve, the nominal static equivalent seismic load at each floor level can be

obtained by subtracting story shears of two adjoining stories. With these nominal

static equivalent seismic loads, a 3D static analysis is carried out to obtain the

internal forces in the building structure.

For the commonly used structural system, consisting of a combination of open

frames and shear walls, it is required by this standard, that the story shears resisted

by the open frames are not less than 25% of the total story shears. If this requirement

is not met, additional lateral loads must be applied in such a way, that the above

requirement is fulfilled, keeping the story shears resisted by the shear walls

unchanged. The objective of this requirement is to give the open frames extra

strength, to cope with a possible redistribution of lateral loads if cracking occurs inthe shear walls.

If desired, the dynamic response analysis of the irregular building structure

may be performed using 3D time history dynamic response analysis, using a

digitized accelerogram as the input earthquake motion. The detailed provisions can

be found in the respective clauses in the standard.

6.3. THE REGULAR BUILDING STRUCTURE

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A building structure is catagorized as regular, if it meets the following criteria:

- The height of the building structure, measured from its ground floor, is not more

than 10 stories or 40 m.

- The building structure in plan has a rectangular layout without any protrusions;

where this is not met, the length of the protrusion is not more than 25% of thelargest plan dimension of the structural layout plan in that direction.

- The building structure in plan has no re-entrant angles at the corners; where this is

not met, the length of the wings is not more than 15% of the largest plan

dimension of the structural layout plan in that direction.

- The structural system of the building is composed of orthogonally arranged lateral

load resisting subsystems, each parallel to the orthogonal principle axes of the

building structure as a whole.

- The building structure has no setbacks; where this is not met, the plan dimension

of the upper portion in each direction is at least 75% of the largest corresponding

plan dimension of the lower portion of the building structure. In this case, a

penthouse structure of not more than 2 stories in height need not be considered as

a setback.

- The structural system has a uniform stiffness distribution along its height, without

having any soft story. A soft story is one having lateral stiffness less than 70% of

that of the story above it, or less than 80% of the average stiffness of 3 stories

above it. In this case, stiffness of a story is defined as the shear applied at that

particular story, causing unit interstory drift of that story.

- The structural system has a uniform floor weight distribution along its height,

meaning that the weight of any floor does not exceed 150% of the weight of the

floor above it. The weight of a penthouse need not be considered to comply with

this requirement.- The structural system has continuous vertical structural elements of its lateral load

resisting subsystems without any offset of their vertical axis; where this is not met,

these offsets must not be more than half of the dimension of that element in the

offset direction.

- The structural system has continuous floor slabs without any opening larger than

50% of the area of the whole floor slab. If floor slabs with openings meeting this

requirement are present, their number must not exceed 20% of the total number of

floors of the building.

Regular building structures meeting the above criteria, has a typical dynamic

characteristic. If a 3D free vibration analysis is conducted on a regular building

structure, its first mode motion will be dominant in translation in the direction of oneof its principle axes, while its second mode motion will be dominant in translation in

the direction of the other principle axis. Therefore, the regular 3D building structure

behaves almost like a 2D structure in the direction of its principle axes.

With the above dynamic characteristic, the response spectrum modal analysis

described in section 4 may be modified, because the following 2 assumptions can be

made :

- the total dynamic response of the structure is dominantly determined by that of the

first mode, so that the contribution of the response of the other modes may be

neglected (as in the case of a 2D structure);

- the shape of the first mode may be simplified into a straight line (instead of a

curve), as the structure is not very high (less than 10 stories or 40 m).

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With the above assumptions the effect of the Design Earthquake in the direction of

the principle axis of a regular building structure, will appear as an equivalent static

seismic loading according to the 2 following provisions :

- The nominal static equivalent base shear V at the base of the building structure

induced by the effect of the Design Earthquake is

t1 WR

ICV = .... (24)

- The nominal base shear according to eq.(24) may be distributed along the height

of the building structure into nominal static equivalent seismic loads Fi acting at

the center of mass of floor i according to the following expression :

V

zW

zWF

ii

n

1i

ii1

=

= ....(25)

where Wi is the weight of floor i, including an appropriate portion of the live load;

zi is the height of floor i measured from the level of its lateral restraint at the base;

while n is the top floor number.

Hence, for regular building structures dynamic analyses are not at all necessary.

Even to compute its fundamentral period T1 no free vibration analysis is necessary,

because as mentioned in section 6.1, to determine its value the well-known

Rayleighs formula of a 2D structure may be used :

=

==n

1i

ii

n

1i

2

ii

1

dFg

dW

2T ....(26)

where Wi and Fi have the same meaning as described previously, while di is the

horizontal static deflection of floor i from the result of a static analysis and g is the

gravity acceleration. This static analysis may be performed using static equivalent

seismic loads Fi based on an arbitrary base shear V.

7. SUBSTRUCTURE

7.1. SEISMIC LOADING ON THE SUBSTRUCTURE

What is meant by substructure is that portion of the building structure, which is

below the ground surface, consisting of the basement, if any, and the foundation. To

simplify the analysis, this standard allows the substructure to be considered as a

separate underground structure, isolated from the superstructure. The substructure is

then considered to be exerted by seismic loading originated from the superstructure,

from inertial forces at the basement floor levels and from the surrounding soil.

During any strong seismic event, it is not possible for the superstructure to

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perform well, if the substructure has failed earlier. Therefore, this standard requires

the substructure to be designed to remain full elastic at any time, including under the

effect of the Design Earthquake, at which the superstructure is in its state of near

collapse. This means that the nominal maximum seismic load exerted by the

superstructure on the substructure is:

n2

1

Vff

Vff

R

VV n21

m

mn=== ....(27)

In line with eq.(27), the support reactions of all vertical elements (columns and shear

walls) at the ground floor level due to the nominal seismic loading on the

superstructure, multiplied by the overstrength factor f2, become the nominal seismic

loading on the substructure.

Inertial forces acting at the basement floors are generated by the interaction of

soil and structure under the effect of the Design Earthquake, so that masses on the basement floors undergo accelerations. If it is not determined through other more

rational methods, the effect of the Design Earthquake on the basement floor masses

may be assumed to appear as static equivalent seismic loads. Its nominal value for

the strength design of the substructure based on the Load and Resistance Factor

Design method is to be computed according to the following empirical formula :

Fbn = 0.10 Ao I Wb ....(28)

where Ao is the peak ground acceleration due to the effect of the Design Earthquake

(Table 2), I the importance factor of the building and Wb the weight of the basement

floor, including an appropriate portion of the live load.The last effect of the Design Earthquake upon the substructure, is the lateral

soil pressure from the front soil, the value of which may be assumed to have reached

its maximum possible value, that is equal to its yield stress (identical with the passive

soil pressure) along the height of the substructure and other components of the

substructure. As the substructure must be fully elastic under any circumstances, for

its strength design based on the Load and Resistance Factor Design method, the said

soil yield stress must be transformed into its nominal value by reducing it by the

required seismic reduction factor R = f1 = 1.6 (full elastic).

In the analysis of the 3D substructure, the existence of the back soil must be

considered by modelling it as compression springs, while the side and bottom soil

may be modelled as shear springs. The properties of the compression and shear

springs must be derived rationally from the existing soil data.

7.2. THE LOAD AND RESISTANCE FACTOR DESIGN FOR THE

SUBSTRUCTURE

The strength of the basement structure, similar to that of the superstructure,

must be designed based on the Load and Resistance Factor Design method according

to this standard. Therefore, it is but logical that the strength of the foundation is

designed based on the same principles, like it is recommended by this standard. By

so doing a consistency is reached in the strength design of the building structure as a

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whole.

The Load and Resistance Factor Design method for the foundation is in

principle the same as that for the superstructure and the basement structure, namely

that at the ultimate condition, the ultimate bearing capacity of the foundation Ru is at

least equal to the ultimate loading Qu on it, according to the following expression:

Ru > Qu ....(29)

where Ru, being the ultimate bearing capacity of the foundation, is the multiplication

of the nominal bearing capacity Rn and the capacity reduction factor, as follows :

Ru = Rn ....(30)

and Qu, being the ultimate loading on the foundation, is the multiplication of the

nominal loading Qn

and the load factor, summed up for all loadings, as follows :

Qu = Qn ....(31)

On the load-settlement curve, the nominal bearing capacity Rn is at a point,

where the foundations behaviour is still elastic, with an ample margin to the point

where any increase in load will produce continuing larger settlements. Therefore, the

nominal bearing capacity of a foundation is most accurate, if it is obtained from the

result of a loading test until failure. However, this standard allows its value to be

determined analytically using rational methods, based on the available soil data. As

an approximation, the nominal bearing capacity of a foundation is twice the

allowable bearing capacity computed in a conventional way.

The capacity reduction factor for a foundation is given in Table 8 for footingsand rafts, and in Table 9 for driven piles and bored piles.

Table 8. Capacity reduction factor for footings and rafts

Soil category

Sand

Clay

Rock

0.35 0.55

0.50 0.60

0.60

Table 9. Capacity reduction factor for driven piles and bored piles.

Foundation

typeSource of resistance Type of loading

Driven piles Friction + end bearing

pure friction

pure end bearing

0.55 0.75

0.55 0.70

0.55 0.70

Axial compression

Axial compression/tension

Axial compression

Bored piles friction + end bearing

pure friction

0.50 0.70

0.55 0.75

Axial compression

Axial compression/tension

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pure end bearing 0.45 0.55 Axial compression

In Table 8 and 9, the lowest value of in its range is used, if the nominal bearing capacity of the foundation Rn is computed analytically using soil data

derived from Standard Penetration Test result (N-SPT). The average value of in itsrange is used, if the nominal bearing capacity of the foundation Rn is computed

analytically using soil data derived from Cone Penetration Test result (CPT). The

highest value of in its range is used, if the nominal bearing capacity of thefoundation Rn is determined from the result of a loading test until failure.

8. REFERENCE

SNI-1726-2002, Indonesian Seismic Resistant Design Standard for Building

Structures.

25 indonesia code

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