32

Architectural and Structural Considerations

Building Configuration:

In recent years, there has been increased emphasis on the importance of a building’s

configuration in resisting seismic forces. Early decisions concerning size, shape,

arrangement, and location of major elements can have a significant influence on the

performance of a structure. Since the design professional plays a large role in these

early decisions, it is imperative that the architect thoroughly understand the concepts

involved.

Building configuration refers to the overall building size and shape and the size and

arrangement of the primary structural frame, as well as the size and location of the

nonstructural components of the building that may aspect its structural performance.

Significant nonstructural components include such things as heavy nonbearing

partitions, exterior cladding, and large weights like equipment or swimming pools.

In the current UBC, elements that constitute both horizontal and vertical irregularities

are specifically defined, so it is clear which structures must be designed with the

dynamic method and which structures may be designed using the static analysis

method. The code states that all buildings must be classified as either regular or

irregular. Whether a building is regular or not helps determine if the static method may

be used. Irregular structures generally require design by the dynamic method, and

additional detailed design requirements are imposed depending on what type of

irregularity exists.

The following sections describe some of the important aspects of building

configuration.

• Torsion

Lateral forces on a portion of a building are assumed to be uniformly distributed and

can be resolved into a single line of action acting on a building. In a similar way, the

shear reac

single line

rigidity, th

If the she

rigidity, t

coincide

directions

lateral loa

When the

direction

The UBC

torsion be

occupied

each leve

ction force

e of action

hese lines

ear walls

the resulta

with the

s with an

ad alone (F

e force on

as that cau

C requires

e planned

building

el is assum

es produce

n. For sym

of action

or other

ant of the

applied

eccentrici

Figure 2.8

Figu

n a vertic

used by th

that even

d for. Thi

cannot be

med to be

ed by the v

mmetric bu

pass throu

vertical e

eir shear r

lateral fo

ty, torsion

8).

ure (2.8): D

cal elemen

he lateral l

n in symm

s account

e known f

e displace

vertical re

uildings w

ugh the sa

elements

resisting f

orce. Sinc

n force is

Developm

nt caused

oad direct

metrical bu

ts for the

for certain

ed from t

esisting ele

with vertic

ame point.

are not s

forces, the

ce the fo

developed

ment of tor

by the e

tly, they m

uildings a

fact that

n. The co

the calcul

ements ca

al resistin

ymmetric

e center o

rces are

d, which i

rsion

eccentricit

must be ad

certain am

the posit

ode requir

ated cente

an be resol

ng element

c or are o

of rigidity

acting in

is in addit

ty acts in

dded.

mount of

tion of lo

res that th

er of mas

33

lved into a

ts of equa

of unequa

, does no

n opposite

tion to the

the same

accidenta

oads in an

he mass a

ss in each

3

a

l

l

t

e

e

e

l

n

t

h

direction

perpendic

by a dist

cular to the

tance equ

e direction

Figure (2

ual to 5 p

n of the fo

.9): Torsio

percent of

orce under

on’s analo

f the build

r considera

ogous simp

ding dime

ation.

plification

ension at

n

34

that leve

4

l

The impo

following

• Plan S

Irregularit

which sh

troublesom

During an

that stress

center of

establishe

section.

Of cours

requireme

shapes are

building c

connectio

ortance of

g sections (

Shape & D

ties in pla

hould be

me plan sh

n earthqua

s concentr

f mass and

ed that res

e, buildin

ents beyon

e unavoid

can be se

on, or the i

f understan

(Figure 2.

Dimension

an shape

avoided

hapes is th

ake, the gr

rations ar

d the cen

ults in a tw

ng shape

nd the con

dable, ther

parated w

inside corn

Fig

nding the

9).

ns:

can creat

wheneve

he re-entra

round mot

re develop

nter of rig

wisting of

is often

ntrol of th

re are way

with a seis

ner can be

gure (2.10)

concept o

te torsion

er possibl

ant corner

tion cause

ped at the

gidity do

f the entire

dictated

he architec

ys to mini

smic joint

e splayed (

): Problem

of torsion

and conc

le. One

.

es the stru

e inside co

not coinc

e structure

by the

ct or engin

imize the

t, they can

(Figures 2

m plan sha

will beco

centrations

of the m

ucture to m

orners. In

cide, there

e as discus

site, the

neer. In th

problem.

n be tied

2.10 and 2.

apes

ome appar

s of stres

most com

move in su

addition,

e is an ec

ssed in th

program,

he cases w

The porti

together

.11).

35

rent in the

s, both of

mmon and

uch a way

since the

ccentricity

e previous

or other

where such

ons of the

across the

5

e

f

d

y

e

y

s

r

h

e

e

A second

stiffness a

d common

and streng

n problem

gth of the p

Figure

Figure (2

m that ari

perimeter.

(2.11): So

2.12): Var

ises with

olution to r

riation in p

building

re-entrant

perimeter

plans is

corners

stiffness

a variati

36

ion in the

6

e

37

Even though a building may be symmetric, the distribution of mass and lateral

resisting elements may place the centers of mass and rigidity in such a way that torsion

is developed.

During an earthquake, the open end of the building acts as a cantilevered beam

causing lateral displacement and torsion. There are four possible ways to alleviate the

problem. In the first instance, a rigid frame can be constructed with symmetric rigidity

and then the cladding can be made nonstructural. Secondly, a strong, moment-resisting

or braced frame can be added that has stiffness similar to the other walls. Third, shear

walls can be added to the front if this does not compromise the function of the

building. Finally, for small buildings, the structure can simply be designed to resist the

expected torsion forces.

The ratio of plan dimensions should not be inordinately large to prevent different types

of forces acting on different plan sections. If this cannot be achieved, then seismic

joint should be provided in such a building.

Elevation Design

The ideal elevation from a seismic design standpoint is one that is regular,

symmetrical, continuous, and that matches the other elevations in configuration and

seismic resistance. Setbacks and offsets should be avoided for the same reason as re-

entrant corners in plan should be avoided; that is, to avoid areas of stress

concentration. Of course, perfect symmetry is not always possible due to the

functional and aesthetic requirements of the building, but there are two basic

configurations that should (and can) be avoided by the architect early in the design

process.

The first problem configuration is a discontinuous shear wall. This is a major mistake

and should never happen. Discontinuities can occur when large openings are placed in

shear walls, when they are stopped short of the foundation, or when they are altered in

some other way. Since the entire purpose of a shear wall is to carry lateral loads to the

foundatio

this is cou

be placed

Two com

wall is sto

to open u

great that

2.13).

The secon

floors abo

shear wal

load path

transfer o

In all case

continuou

n and act

unterprodu

in shear w

mmon exam

opped at th

up the first

t even ext

nd exampl

ove are ca

l continue

for the la

f forces fr

es of disco

usly to the

as a beam

uctive. Of

walls if pr

mples of d

he second

t floor, bu

tra reinfor

Figure

le is also

antilevered

es, the offs

ateral load

rom one sh

ontinuous

foundatio

m cantilev

f course, sm

oper reinf

discontinuo

d floor lev

ut it create

rcing cann

e (2.13): D

a common

d slightly

fset also cr

ds is inter

hear wall t

shear wal

on.

vered out

mall open

forcement

ous shear

vel and sup

es a situat

not alway

Discontinu

n design f

from the

reates an u

rrupted, an

to the nex

lls, the sol

of the fou

nings like d

is provide

walls are

pported by

tion where

ys resist th

uous shear

feature wh

first floor

undesirabl

nd the flo

xt.

lution is si

undation,

doors and

ed.

shown. In

y columns

e stress co

he build-u

r walls

here the s

r shear wa

le situation

oor structu

imple: she

any inter

d small win

n the first

s. This is o

oncentrati

up of stre

econd flo

all. Even t

n because

ure has to

ear walls s

38

rruption of

ndows can

, the shear

often done

ons are so

ss (Figure

or and the

though the

e the direc

o carry the

should run

8

f

n

r

e

o

e

e

e

t

e

n

Another s

when the

at any flo

the greate

case of th

or when t

2.14).

A soft sto

story and

these situ

floors abo

When ear

weak floo

members.

There are

eliminate

serious pr

ground flo

or, it is m

est. The di

he soft stor

the first st

ory can als

the groun

ations to o

ove for the

rthquake

or instead

.

e several w

it and try

roblem w

oor is wea

most seriou

scontinuo

ry. Others

tory is hig

F

so be crea

nd level is

occur. For

e guest roo

loads occ

of being

ways to so

y to work

ith buildin

aker than t

us at grade

ous shear w

s can occu

gh compar

Figure (2.

ated when

open. Of

r example

oms.

cur, the fo

uniformly

olve the pr

k the arch

ng config

the floors

e level bec

wall discu

ur when al

red with th

14) Soft f

n there is h

f course, th

e, a hotel m

orces and

y distribut

roblem of

hitectural

guration is

above. Al

cause this

ssed in the

ll columns

he other f

first storie

heavy ext

here are u

may need

deformat

ted among

f a soft sto

solution a

s the soft

lthough a

is where

e previous

s do not e

floors of th

s

erior cladd

sually val

a high fir

tions are

g all the f

ory. The f

around th

t story. Th

soft story

the lateral

s section i

extend to t

he structu

dding abov

lid reasons

rst story, b

concentra

floors and

first, of co

he extra co

39

his occurs

can occur

l loads are

s a specia

the ground

ure (Figure

ve the firs

s for all of

but shorter

ated at the

d structura

ourse, is to

olumns or

9

s

r

e

l

d

e

t

f

r

e

l

o

r

40

lower height. If height is critical, extra columns can be added at the first floor. Another

solution is to add extra horizontal and diagonal bracing. Finally, the framing of the

upper stories can be made the same as the first story. The entire structure then has a

uniform stiffness. Lighter, intermediate floors can be added above the first between

the larger bays so they do not aspect the behavior of the primary structural system.

Lightweight Construction:

The greater the structural mass, the greater the seismic forces. In contrast to wind

design, seismic design calls for lighter construction with a high strength-to-weight

ratio to minimize the internal forces.

Ductility:

The ductility of the structure can be considered as a measure of its ability to sustain

large deformations without endangering its load-carrying capacity. Therefore, in

addition to seismic strength, the ductility of the structure should be given serious

consideration.

• The required ductility can be achieved by proper choice of framing and

connection details.

• Ductility is improved by limiting the ratio of reinforcement on the tension

side of beams.

• Using compression reinforcement in beams enhances ductility.

• Using adequate shear reinforcement enhances ductility.

• Provision of spiral reinforcement or closely spaced ties improved ductility.

Adequate Foundations:

Differential settlement of buildings is to be minimized through proper design of

footings. Earthquake oscillations can cause liquefaction of loose soils, resulting in an

uneven settlement. Stabilization of the soil prior to building construction and the use

of deep footings are some remedial measures needed to overcome such a problem.

Short Co

Frequentl

in design,

due to the

Even if ve

collapses

concept; e

following

Separatio

The mutu

caused sig

sufficient

Joints an

Joints are

strong ho

concrete f

olumn Eff

y a colum

, such as

e short len

ery strong

have been

eliminatin

g relation V

on of Stru

ual hamme

gnificant

clearance

nd Connec

e often th

orizontal c

frames are

fects:

mn is short

the partia

ngth of the

g stirrups a

n frequent

ng such pa

V = 2 M (p

Figure (2.

uctures:

ering rece

damage. T

e so that th

ctions:

he weakes

confining

e often res

ened by el

al-height i

e column

are used it

t. The onl

rtial heigh

plastic) / L

.15): Failu

eived by b

The simpl

he free mo

t link in

reinforcem

ponsible f

lements, w

infill walls

when subj

t is difficu

ly possibl

hts of infil

L (Figure

ure due to

buildings

lest metho

otion of th

a structur

ment with

for collaps

which hav

s. This cr

bjected to v

ult to save

e solution

ll walls. T

2.15).

short colu

in close p

od of prev

e two stru

ral system

hin the joi

ses in eart

ve not been

eates very

very large

such colu

n is to use

he shear f

umn effect

proximity

venting da

uctures can

m. It is ne

int zone. J

hquakes.

n taken int

y large sh

e bending

umns, ther

e different

force is giv

t

of one an

amage is t

n occur.

ecessary t

Joints in

41

to accoun

hear forces

moments

efore such

t structura

ven by the

nother has

to provide

to provide

reinforced

1

t

s

.

h

l

e

s

e

e

d

42

Inadequate Shear Strength:

To enhance shear capacity one should first use suitable amount of stirrups and ties to

prevent the brittle type of failure associated with shear. Diagonal reinforcement is

recommended for deep members to resist diagonal tension.

Materials and Workmanship:

It is obvious that no design can save the structure if bad materials are used or if

workmanship is not good. The best available quality design codes are deemed useless

unless quality control is kept starting from the design process and ending up with the

site execution.

Bond, Anchorage, and Splices:

Bond, when effectively developed, enables the concrete and reinforcement to form a

composite structure. If the area of concrete surrounding the bar is small, splitting is the

common mode of failure. One should avoid splices and anchorage at the location

where the surrounding concrete is extensively cracked (i.e., plastic hinges).

Detailing of Structural Elements:

Closely spaced stirrups and ties are used in columns and walls, to hold the

reinforcement in place and to prevent buckling of longitudinal bars. Closely spaced

stirrups and ties are used in potential hinge regions of beams, to ensure strength

retention during cyclic loading. Detailing of special transverse steel through beam-

column joints in ductile frames to maintain the integrity of the joints during adjacent

beam hinge plastic deformation is required.

Detailing of Non-Structural Elements:

The tendency of non-structural elements to be damaged, as the building sways need to

be addressed. To overcome such problems, either separation is kept between structural

and non-structural members, or the forces resulting from the attachment of structural

elements need to be taken into consideration.

The Effec

When an

the inertia

the groun

the buildi

opposite d

vibrate ba

Theoretic

states that

by the giv

acting on

the structu

If a buildi

from side

one full s

of the bui

ct of Eart

earthquak

a of the st

nd causes t

ing and a

direction.

ack and fo

Fi

ally, the f

t force eq

ven earth

it. Howev

ure- its na

ing is defl

to side. T

ide-to-sid

lding.

thquakes

ke occurs,

tructure m

the buildin

shear forc

As the dir

rth.

igure (2.16

force on t

quals mass

quake, the

ver, the ac

atural perio

lected by a

The period

e oscillati

on Buildi

the first re

mass. Almo

ng to mov

ce at the b

rection of

6): Buildin

the buildin

s times ac

e greater

cceleration

od (Figure

a lateral fo

d is the tim

on. The p

ings

esponse o

ost instant

ve sideway

base, as th

f the accel

ng motion

ng can be

cceleration

the mass

n of the bu

e 2.16).

orce such

me in seco

period is d

of a buildin

taneously,

ys at the b

hough forc

eration ch

n during an

found by

n. Since th

of the bu

uilding de

as the win

onds it tak

dependent

ng is not to

however,

base causi

ces were b

hanges, the

n earthqua

y using Ne

he acceler

uilding, th

epends on

nd or an e

kes for a b

on the ma

to move at

, the accel

ing a later

being app

e building

ake

ewton’s la

ration is e

he greater

another p

earthquake

building to

ass and th

43

t all due to

leration of

ral load on

lied in the

g begins to

aw, which

established

r the force

property of

e, it moves

o complete

he stiffness

3

o

f

n

e

o

h

d

e

f

s

e

s

In a theor

is zero. Th

When the

accelerati

force on t

induced, a

Natural p

cabinet to

retical, com

he acceler

e building

on decrea

the buildin

and stiff, s

periods va

o about 0.1

mpletely s

ration of s

g is mor

ases. As m

ng. Theref

short-perio

ary from a

1 sec. for a

Fig

stiff buildi

such an in

e flexible

mentioned

fore, flexib

od buildin

about 0.05

a one-story

gure (2.18

ing, there

nfinitely ri

e, its per

above, as

ble, long-

ngs have m

5 sec. for

y building

8): Fundam

is no mov

gid buildi

riod incre

the accele

period bu

more latera

r a piece

g.

ments perio

vement, an

ng is the s

ases and

eration de

ildings ha

al force ind

of furnitu

ods

nd the natu

same as th

the corr

ecreases, s

ave less la

nduced.

ure such a

44

ural period

he ground

responding

o does the

teral force

as a filing

4

d

d.

g

e

e

g

45

A rule of thumb is that the building period equals the number of stories divided by 10.

As the building moves, the forces applied to it are either transmitted through the

structure to the foundation, absorbed by the building components, or released in other

ways such as collapse of structural elements.

The goal of seismic design is to build a structure that can safely transfer the loads to

the foundation and back to the ground and absorb some of the energy present rather

than suffering damage.

The ability of a structure to absorb some of the energy is known as ductility, which

occurs when the building deflects in the inelastic range without failing or collapsing.

The elastic limit is the limit beyond which the structure sustains permanent

deformation. The greater the ductility of a building, the greater is its capacity to absorb

energy.

Ductility varies with the material. Steel is a very ductile material because of its ability

to deform under a load above the elastic limit without collapsing. Concrete and

masonry, on the other hand, are brittle materials. When they are stressed beyond the

elastic limit, they break suddenly and without warning. Concrete can be made more

ductile with reinforcement, but at a higher cost.

Resonance

The ground vibrates at its natural period. The natural period of ground varies from

about 0.4 sec. to 2 sec. depending generally on the hardness of the ground.

The terrible destruction in Mexico City in the earthquake of 1985 was primarily the

result of response amplification caused by the coincidence of building and ground

motion periods. Mexico City was some 400 km from the earthquake focus, and the

earthquake caused the soft ground under downtown buildings to vibrate for over 90

seconds at its long natural period of around 2 seconds. This caused tall buildings

around 20 stories tall to resonate at a similar period, greatly increasing the

accelerations within them. This amplification in building vibration is undesirable. The

possibility of it happening can be reduced by trying to ensure that the building period

46

will not coincide with that of the ground. Other buildings, of different heights and with

different vibrational characteristics, were often found undamaged even though they

were located right next to the damaged 20 story buildings. Thus, on soft (long period)

ground, it would be best to design a short stiff (short period) building.

General Goals in Seismic-Resistant Design and Construction

• If basic, well-understood principles are ignored and short cuts taken, disaster can

occur.

• Many tall buildings that survived major earthquakes show that adherence to these

principles can produce structures out of which people can be sure of walking alive,

even if some structural damage has occurred.

The philosophy of earthquake design for structures other than essential facilities has

been well established and proposed as follows.

• To prevent non-structural damage in frequent minor ground shaking.

• To prevent structural damage and minimize non-structural damage in occasional

moderate ground shaking.

• To avoid collapse or serious damage in rare major ground shaking.

Structura

The Unifo

structural

1- Be

2- Bu

3- Mo

4- Du

1- Bearin

wall lines

used to re

not contai

support fl

2- Buildin

vertical lo

building f

3- Momen

frame thro

frame elem

al System

form Build

systems:

aring Wa

uilding Fr

oment Re

ual System

g wall sys

s and at in

esist latera

in comple

loor and ro

ng frame

oads, but

frame syst

nt-resistin

oughout th

ments to r

Earms Defined

ding Code

all System

ame Syste

esisting Fr

ms

stems con

nterior loc

al forces a

te vertical

oof vertica

systems u

use either

tem with s

Figu

ng frame s

he buildin

resist later

rthquaked:

e (UBC)

ms

ems

rame Syst

nsist of ve

ations as n

and are th

l load carr

al loads.

use a com

r shear w

shear walls

ure (2.19)

ystems, sh

ng to carry

ral forces.

e-Resista

earthquak

tems

ertical load

necessary

hen called

rying spac

mplete thre

walls or br

s is shown

Building

hown in F

y vertical l

ant Syst

ke provisio

d carrying

y. Many of

d shear wa

ce frames b

ee dimens

raced fram

n in Figure

Frame Sy

Figure (2.2

loads, and

ems

ons recog

g walls loc

f these be

alls. Bearin

but may u

ional spac

mes to resi

e (2.19).

stem

20), provid

they use

gnize these

cated alon

earing wal

ing wall s

use some c

ce frame t

ist lateral

de a comp

some of th

47

e building

ng exterior

ls are also

ystems do

columns to

to suppor

forces. A

plete space

hose same

7

g

r

o

o

o

rt

A

e

e

4. A dua

provides

specially

moment-r

shear, an

proportion

This syste

buildings

Lateral-F

Lateral-fo

wind and

walls, bra

Shear Wa

A shear w

wall throu

foundatio

(2.21) sho

another in

al system

support f

detailed

resisting f

nd the tw

n to their r

em, which

where per

Force-Res

orce-resist

d seismic

aced frame

alls:

wall is a ve

ugh shear

n, and, ju

ows two e

n a multist

Figure (2

is a stru

for gravity

moment-r

frame mus

wo system

relative rig

h provide

rimeter fra

sisting Ele

ting eleme

forces. T

es, and mo

ertical stru

r and bend

st as with

examples

tory buildi

2.20): Mo

uctural sy

y loads, a

resisting f

st be capa

ms must b

gidities.

es good re

ames are u

ements

ents must b

he three

oment- res

uctural ele

ding. Such

a beam, p

of a shea

ing.

oment resis

ystem in w

and resista

frame and

able of re

be designe

edundancy

used in co

be provide

principal

sisting fram

ement that

h a wall a

part of its

ar wall, on

sting fram

which an

ance to la

d shear w

sisting at

ed to res

y, is suita

onjunction

ed in ever

types of

mes.

resists lat

acts as a b

strength d

ne in a si

me system

essential

ateral load

walls or b

least 25 p

sist the to

able for m

with cent

ry structur

resisting

teral force

beam cant

derives fro

imple one

lly compl

ds is prov

braced fra

percent o

otal latera

medium-to

tral shear w

re to brace

elements

es in the pl

ntilevered

om its dep

e-story bui

48

lete frame

vided by a

ames. The

f the base

al load in

o-high rise

wall core.

e it agains

are shear

lane of the

out of the

pth. Figure

ilding and

8

e

a

e

e

n

e

t

r

e

e

e

d

49

Figure (2.21): Shear walls

In Figure (2.21.a), the shear walls are oriented in one direction, so only lateral forces

in this direction can be resisted. The roof serves as the horizontal diaphragm and must

also be designed to resist the lateral loads and transfer them to the shear walls. Figure

(2.21.a) also shows an important aspect of shear walls in particular and vertical

elements in general. This is the aspect of symmetry that has a bearing on whether

torsional effects will be produced. The shear walls in Fig. (2.21.a) are symmetrical in

the plane of loading.

Figure (2.21.b) illustrates a common use of shear walls at the interior of a multistory

building. Because walls enclosing stairways, elevator shafts, and mechanical shafts are

mostly solid and run the entire height of the building, they are often used for shear

walls. Although not as efficient from a strictly structural point of view, interior shear

walls do leave the exterior of the building open for windows. Notice that in Figure

(2.21.b) there are shear walls in both directions, which is a more realistic situation

because both wind and earthquake forces need to be resisted in both directions. In this

diagram, the two shear walls are symmetrical in one direction, but the single shear

wall produces a nonsymmetrical condition in the other since it is off center. Shear

walls do n

torsional e

Shear wal

high.

Shear wa

their abili

What is a

Reinforce

Walls (Fi

start at fo

thickness

walls are

like verti

foundatio

Advantag

Properly

performan

not need t

effects.

lls, when

lls may h

ity to resis

a Shear W

ed concret

igure 2.22

foundation

can be as

usually p

cally-orien

n.

ges and D

designed

nce in pas

to be symm

used alon

have openi

st lateral lo

Wall Build

te buildin

2) in addit

n level an

s low as 1

provided a

nted wide

Figure (

Disadvanta

and deta

t earthqua

metrical in

ne, are suit

ings in th

oads is red

ding?

ngs often

tion to sla

nd are con

50mm, or

along both

e beams t

2.22): Rei

ages of Sh

ailed build

akes.

n a buildi

table for m

hem, but t

duced dep

have vert

abs, beam

ntinuous t

r as high a

h length an

that carry

inforced c

hear Wall

dings with

ng, but sy

medium r

the calcula

ending on

tical plate

ms and col

throughou

as 400mm

nd width

y earthqua

concrete sh

ls in Rein

h shear w

ymmetry i

ise buildin

ations are

n the perce

e-like RC

umns. Th

ut the bui

m in high r

of buildin

ake loads

hear wall

nforced Co

walls have

is preferre

ngs up to

e more dif

entage of o

walls cal

hese walls

ilding heig

rise buildin

ngs. Shear

downwar

oncrete B

e shown v

50

d to avoid

20 stories

fficult and

open area.

lled Shear

generally

ght. Their

ngs. Shear

r walls are

rds to the

Buildings:

very good

0

d

s

d

.

r

y

r

r

e

e

:

d

51

Shear walls in high seismic regions require special detailing. However, in past

earthquakes, even buildings with sufficient amount of walls that were not specially

detailed for seismic performance (but had enough well-distributed reinforcement)

were saved from collapse. Shear wall buildings are a popular choice in many

earthquake prone countries, like Chile, New Zealand and USA. Shear walls are easy to

construct, because reinforcement detailing of walls is relatively straightforward and

therefore easily implemented at site. Shear walls are efficient, both in terms of

construction cost and effectiveness in minimizing earthquake damage in structural and

nonstructural elements (like glass windows and building contents).

On the other hand, shear walls present barriers, which may interfere with architectural

and services requirement. Added to this, lateral load resistance in shear wall buildings

is usually concentrated on a few walls rather than on large number of columns.

Architectural Aspects of Shear Walls:

Most RC buildings with shear walls also have columns; these columns primarily carry

gravity loads (i.e., those due to self-weight and contents of building). Shear walls

provide large strength and stiffness to buildings in the direction of their orientation,

which significantly reduces lateral sway of the building and thereby reduces damage

to the structure and its contents.

Since shear walls carry large horizontal earthquake forces, the overturning effects on

them are large. Thus, design of their foundations requires special attention. Shear

walls should be provided along preferably both length and width. However, if they are

provided along only one direction, a proper grid of beams and columns in the vertical

plane (called a moment-resistant frame) must be provided along the other direction to

resist strong earthquake effects.

Door or window openings can be provided in shear walls, but their size must be small

to ensure least interruption to force flow through walls. Moreover, openings should be

symmetrically located. Special design checks are required to ensure that the net cross-

sectional

force.

Shear wal

twist in bu

directions

perimeter

Ductile D

Just like r

perform m

wall, type

the buildin

Overall G

Shear wal

much larg

shaped se

shafts aro

taken adv

area of a

lls in build

uildings (F

s in plan

r of the bu

Design of S

reinforced

much bett

es and am

ng help in

Geometry

lls are rec

ger than t

ections are

ound the e

vantage of

wall at an

dings mus

Figure 2.2

. Shear w

ilding–suc

F

Shear Wa

d concrete

ter if desig

mount of re

n improvin

y of Walls

tangular i

the other.

e also use

elevator c

to resist e

n opening

st be symm

23). They c

walls are

ch a layou

Figure (2.2

alls:

beams an

gned to b

einforcem

ng the duc

:

n cross-se

While re

ed (Figure

core of bu

earthquake

g is suffici

metrically

could be p

more ef

ut increase

23): Shear

nd column

be ductile.

ment, and c

ctility of w

ection, i.e.

ectangular

e 2.24). Th

uildings a

e forces.

ient to car

y located i

placed sym

ffective w

es resistanc

wall layo

ns, reinfor

Overall

connection

walls.

., one dim

r cross-sec

hin-walled

also act as

rry the ho

n plan to

mmetricall

when loca

ce of the b

ut

ced concr

geometric

n with rem

mension of

ction is co

d hollow

s shear w

orizontal e

reduce ill

ly along o

ated along

building to

rete shear

c proportio

maining el

f the cross

ommon, L

reinforced

walls, and

52

earthquake

effects of

one or both

g exterior

o twisting

walls also

ons of the

lements in

-section is

L- and U-

d concrete

should be

2

e

f

h

r

.

o

e

n

s

-

e

e

Braced F

A braced

lateral for

the brace

forces fro

one-story

other end

uses com

compressi

Figure 2.2

compressi

from eith

same resu

direction.

Braced fr

placed in

problems

resisting s

Frames:

frame is

rces are re

d frame d

om each b

braced fr

d only one

mpression b

ion, depen

25.b) show

ion memb

her directio

ult, but the

raming ca

one struc

for windo

system.

Fig

a truss s

esisted thr

depends o

building el

frame. At

e bay is b

braces be

nding on w

ws two me

ber in one

on. Altern

ey must be

n be plac

ctural bay

ows and d

gure (2.24)

system of

rough axia

on diagon

lement to

one end o

braced. Th

cause the

which way

ethods of

e bay can

nately, ten

e run both

ed on the

or several

doorways,

): Shear w

f the conc

al stresses

nal membe

the found

of the bui

his buildin

e diagonal

y the force

bracing a

n be used

nsion diag

ways to a

e exterior

l. Obviou

, but it is

wall geome

centric or

s in the m

ers to pro

dation. Fig

ilding two

ng is only

l member

e is applied

multistor

to brace

gonals can

account fo

or interio

sly, a brac

a very ef

etry

eccentric

members. J

ovide a lo

gure (2.25

o bays are

braced in

may be e

d.

y building

against la

n be used

r the load

or of a bu

ced frame

fficient and

c type in

Just as wi

oad path

5.a) shows

e braced a

n one dire

either in

g. A single

ateral load

d to accom

coming fr

uilding, an

e can pres

d rigid lat

53

which the

ith a truss

for latera

s a simple

and at the

ection and

tension or

e diagona

ds coming

mplish the

from either

nd may be

ent design

teral force

3

e

,

l

e

e

d

r

l

g

e

r

e

n

e

j

Moment-

Moment-r

joints. Joi

and theref

and beam

The UBC

is the spe

ductile be

The secon

with less

intermedi

The third

frame doe

concrete f

Moment-r

frames; th

become m

other, and

which inc

Two type

-Resisting

resisting f

ints are de

fore any l

ms. They ar

C differenti

ecial mom

ehavior an

nd type is

restrictiv

ate frames

type is th

es not mee

frames can

resisting

he horizon

more prob

d special

creases the

s of mome

g Frames:

frames car

esigned an

lateral def

re used in

iates betw

ment-resist

d comply

the interm

ve require

s cannot b

e ordinary

et the spe

nnot be us

frames ar

ntal deflec

blematic.

attention

e column b

ent-resisti

Figure (2

:

rry lateral

nd constru

flection of

low-to-m

ween three

ting frame

with the p

mediate mo

ements tha

be used in

y moment-

cial detail

sed in zone

re more

ction, or d

Adjacent

must be

bending st

ng frames

2.25) Brac

l loads pri

ucted so t

f the fram

medium rise

types of m

e that mu

provisions

oment-res

an specia

seismic zo

-resisting

ling requir

es 3 or 4.

flexible t

drift, is gre

buildings

paid to th

tresses.

s are show

ed frames

imarily by

they are th

me occurs f

e building

moment re

ust be spe

s of the UB

sisting fram

l moment

ones 3 or

frame. Th

rements fo

than shea

eater, and

s cannot b

he eccentr

wn in Figur

y flexure i

heoreticall

from the b

gs.

esisting fra

ecifically

BC.

me, which

t-resisting

4.

his concret

for ductile

ar wall st

thus non-

be located

ricity dev

re (2.26)

in the mem

ly comple

bending o

rames. The

detailed t

h is a conc

g frames.

te momen

behavior

tructures

-structura

d too clos

veloped in

54

mbers and

etely rigid

of columns

e first type

to provide

rete frame

However

nt-resisting

. Ordinary

or braced

l elements

se to each

n columns

4

d

d,

s

e

e

e

r,

g

y

d

s

h

,

Advantag

- Pro

whi

ext

- The

from

Disadvan

- Poo

cata

fail

- Bea

con

- Req

Horizont

In all late

the vertic

most com

A diaphra

There are

ges:

ovide a po

ich can a

ernal clad

eir flexibi

m the forc

ntages:

orly desi

astrophica

lures aroun

am colum

nsiderable

quires goo

tal Elemen

eral force-

cal resistin

mmon way

agm acts a

two types

Figur

otentially

allow free

dding.

ility and a

cing motio

gned mo

ally in ea

nd beam-c

mn joints re

skill to de

od fixing s

nts (Diaph

resisting s

ng elemen

used is th

as a horizo

s of diaphr

re (2.26) M

high-duct

edom in

associated

ons on stif

oment res

arthquakes

column jo

epresent a

esign succ

skills and c

hragms):

systems, t

nts. This i

he diaphra

ontal beam

ragms: fle

Moment r

tile system

architectu

d long per

ff soil or ro

sisting fr

s, mainly

oints.

an area of

cessfully.

concreting

there must

s done wi

agm.

m resisting

exible and

esisting fr

m with a

ural plann

riod may

ock sites.

rames ha

y by form

high stres

g.

t be a way

ith severa

forces wi

d rigid.

rames

good deg

ning of in

serve to d

ave been

mation of

ss concent

y to transm

al types of

ith shear a

gree of re

nternal sp

detune the

observed

f weak st

tration, wh

mit latera

f structure

and bendin

55

dundancy

paces and

e structure

d to fai

tories and

hich needs

l forces to

es, but the

ng action.

5

y,

d

e

l

d

s

o

e

Although

between t

distributed

A flexible

times the

comparing

adjoining

distributed

With a r

vertical el

(assuming

diagram a

to each e

between t

The illust

However,

unequal.

Concrete

deck cons

of their co

no horiz

the two ty

d.

e diaphrag

average

g the midp

vertical re

d accordin

rigid diaph

lements w

g there is

are twice a

end wall a

these two.

tration sho

, if the ve

floors are

struction.

onstruction

ontal elem

ypes beca

gm is one

story drif

point in-p

esisting el

ng to tribu

hragm, th

will be in p

s no torsio

as stiff as

and one-th

ows symm

ertical res

e consider

Steel deck

n. Wood d

Figure

ment is co

ause the ty

e that has

ft of that

lane defle

lements un

utary areas

he shear f

proportion

on), as sh

the interio

hird to th

metrically

sisting ele

red rigid d

ks may be

decks are c

e (2.27) Di

ompletely

ype affect

a maximu

story. Th

ection of th

nder equiv

s as shown

forces tra

n to the re

hown in F

or walls, t

he two int

placed sh

ements are

diaphragm

e either fle

considere

iaphragm

y flexible

ts the way

um lateral

his deform

he diaphra

valent trib

n in Figure

ansmitted

lative stiff

Fig, (2.27

then one-t

terior wal

hear walls

e asymme

ms, as are

exible or ri

d flexible

load distr

or rigid,

y in whic

l deforma

mation can

agm with t

utary load

e (2.27.a).

from the

ffness of th

7.b). If th

third of th

lls, which

, so the d

etric, the

steel and

igid, depe

diaphragm

ibution

distinction

ch lateral

ation more

n be deter

the story d

d. The late

.

e diaphrag

he vertica

he end wa

he load is d

h is equall

distribution

shearing

concrete

ending on

ms.

56

n is made

forces are

e than two

rmined by

drift of the

eral load is

gm to the

l elements

alls in the

distributed

ly divided

n is equal

forces are

composite

the details

6

e

e

o

y

e

s

e

s

e

d

d

.

e

e

s

57

Load Path:

The structure shall contain one complete load path for Life Safety for seismic force

effects from any horizontal direction that serves to transfer the inertial forces from the

mass to the foundation.

There must be a complete lateral-force-resisting system that forms a continuous load

path between the foundation, all diaphragm levels, and all portions of the building for

proper seismic performance.

The general load path is as follows: seismic forces originating throughout the building

are delivered through structural connections to horizontal diaphragms; the diaphragms

distribute these forces to vertical lateral-force-resisting elements such as shear walls

and frames; the vertical elements transfer the forces into the foundation; and the

foundation transfers the forces into the supporting soil.

If there is a discontinuity in the load path, the building is unable to resist seismic

forces regardless of the strength of the existing elements. Mitigation with elements or

connections needed to complete the load path is necessary to achieve the selected

performance level. The design professional should be watchful for gaps in the load

path. Examples would include a shear wall that does not extend to the foundation, a

missing shear transfer connection between a diaphragm and vertical element, a

discontinuous chord at a diaphragm notch, or a missing collector.

In cases where there is a structural discontinuity, a load path may exist but it may be a

very undesirable one. At a discontinuous shear walls, for example, the diaphragm may

transfer the forces to frames not intended to be part of the lateral-force-resisting

system. While not ideal, it may be possible to show that the load path is acceptable.

Primary Load-Path Elements:

Within every building, there are multiple elements that are used to transmit and resist

lateral forces. These transmitting and resisting elements define the building’s lateral-

load path.

and conne

An appre

everyone

resist eart

There are

such as sh

horizontal

The roof a

force-tran

stories at

immediate

method of

depends o

Shear wa

perform f

an upper-

therefore

a shear w

elements t

. This path

ection, to t

eciation of

involved

thquakes.

two orien

hear walls

l, such as

and floor

nsmitting o

and abo

ely below

f distribut

on that cla

alls and fr

force-trans

-story inte

must tran

wall, forces

that partic

Fi

h extends

the founda

f the criti

in the de

ntations of

s, braced f

the roof, f

elements

or force-d

ve their l

w. Diaphra

ting earthq

assification

rames are

smitting fu

erior shear

nsmit its fo

s are trans

cipate in th

igure (2.2

from the u

ation.

ical impor

esign, cons

f primary

frames, an

floors, and

are known

distributing

level and

agms are

quake forc

n. Concre

primarily

unctions. F

r wall ma

orces to a

smitted in

he earthqu

8): Primar

uppermos

rtance of

struction,

elements

nd momen

d foundati

n as diaph

g element

deliver t

classified

ces from t

ete diaphra

y lateral f

For examp

ay not con

floor diap

nto a found

uake load p

ry structur

t roof or p

a comple

and inspe

in the load

nt frames,

ion.

hragms. D

ts that tak

them to w

d as eithe

the diaphr

agms are c

force- res

ple and wh

ntinue to

phragm. A

dation ele

path are sh

ral load pa

parapet, th

ete load p

ection of

d path: tho

and those

Diaphragm

ke horizon

walls or f

er flexible

agm to th

considered

isting elem

hile not ne

the base o

Also, at the

ement. The

hown in F

ath elemen

hrough eac

path is ess

buildings

ose that ar

e that are e

ms serve pr

ntal forces

frames in

e or rigid

he resisting

d rigid.

ments but

ecessarily

of the bui

e base of a

e primary

Figure (2.2

nts

58

ch elemen

sential for

that mus

re vertical

essentially

rimarily as

s from the

the story

d, and the

g elements

t can also

desirable

ilding and

a frame or

y structura

28).

8

t

r

t

,

y

s

e

y

e

s

o

,

d

r

l

Foundatio

transmitti

of friction

of soil in w

Foundatio

forces fro

Secondar

Within th

needed to

forces ar

between h

Two impo

member a

forces. A

walls or fr

In the cas

because th

perimeter

ons form

ng it to th

nal resista

which the

ons must

om shear w

ry Load-P

he primar

o resist sp

e transmi

horizontal

ortant sec

along the

collector

frames. Fig

Figu

se of floor

hey form

r is typic

the final

he ground

ance along

ey are emb

also supp

walls and f

Path Elem

ry load-pa

pecific for

itted. Par

seismic e

ondary el

e boundary

is a struc

gure (2.29

ure (2.29):

rs and roof

the interfa

ally the

link in th

d. Foundat

g their low

bedded.

port addit

frame colu

ments:

ath eleme

rces or to

ticular at

elements (d

ements ar

y of a di

ctural mem

9) depicts t

: Function

fs, the per

face betwe

location f

he load p

tions resis

wer surfac

tional vert

umns.

ents, ther

provide

ttention m

diaphragm

re chords

iaphragm

mber that

the overal

n of diagra

rimeter ed

een the dia

for vertic

path by co

st lateral f

e and late

tical load

re are ind

specific p

must be g

ms) and ve

and colle

that resis

transmits

ll function

am chords

dges or bou

aphragms

cal seismi

ollecting

forces thro

eral bearin

s caused

dividual s

pathways a

given to

ertical seis

ctors. A c

sts tension

diaphragm

n of chords

and collec

undaries a

and the p

ic elemen

the base

ough a co

ng against

by the ov

secondary

along wh

transmitti

smic elem

chord is a

n and co

gm forces

s and colle

ctors

are critical

perimeter w

nts, althou

59

shear and

ombination

t the depth

verturning

elements

ich latera

ing forces

ents.

structura

mpression

into shear

ectors.

l locations

walls. The

ugh many

9

d

n

h

g

s

l

s

l

n

r

s

e

y

buildings

resistance

Boundary

depending

As shown

tend to be

and comp

greatest b

vertical re

which the

side is in

forces re

compressi

In concret

plane ben

frame in

diaphragm

walls are

frames ar

boundary

also hav

e also crea

y element

g on the ax

n in Figur

end the di

pression. S

bending str

esisting s

e forces ar

tension. T

everse. Th

ion.

te walls, r

nding in t

the story

m boundar

often inte

re normal

.

Figure

e shear w

ates a diaph

ts in diap

xis along w

re (2.30), t

iaphragm

Similar to

ress and l

eismic ele

re being a

These tens

herefore,

reinforcing

the wall. C

y immedia

ry (See Fig

errupted b

lly located

e (2.30): U

walls or fr

hragm bou

phragms u

which late

the forces

and the c

o a uniform

argest def

ements. T

applied is

sion and c

each cho

g steel is p

Collectors

ately belo

gure 3.12)

y opening

d in only

Use of coll

frames at

undary.

usually s

eral loads

s acting pe

chord mem

mly loade

flection at

The chord

in compr

compressi

ord must

placed at t

s are need

ow the di

). This is a

gs for win

y a few o

lector elem

interior lo

serve as b

are consid

erpendicu

mber must

ed beam,

t or near th

d on the s

ression, an

on forces

be desig

the diaphra

ded when

aphragm

a very com

ndows and

of the fram

ment at int

ocations.

both chor

dered to b

ular to the

t resist the

a diaphrag

he center

side of th

nd the cho

reverse w

gned for

agm level

an indiv

is not co

mmon situ

d doors, an

me bays

terior shea

An interi

rds and

e applied.

boundary

e associate

gm exper

of its span

he diaphra

ord on the

when the e

both ten

l to resist t

vidual shea

ontinuous

uation bec

nd becaus

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61

The following statements contained in the 1997 UBC clearly require that a complete

load path be provided throughout a building to resist lateral forces. “All parts of a

structure shall be interconnected and connections shall be capable of transmitting the

seismic force induced by the parts being connected.”

“Any system or method of construction shall be based on a rational analysis... Such

analysis shall result in a system that provides a complete load path capable of

transferring all loads and forces from their point of origin to the load-resisting

elements.”

To fulfill these requirements, connections must be provided between every element in

the load path. When a building is shaken by an earthquake, every connection in the

lateral-force load path is tested. If one or more connections fail because they were not

properly designed or constructed, those remaining in parallel paths receive additional

force, which may cause them to become overstressed and to fail. If this progression of

individual connection failures continues, it can result in the failure of a complete

resisting seismic element and, potentially, the entire lateral-force-resisting system.

Consequently, connections are essential for providing adequate resistance to

earthquakes and must be given special attention by both designers and inspectors.

Connections are details of construction that perform the work of force transfer

between the individual primary and secondary structural elements discussed above.

They include a vast array of materials, products, and methods of construction.

In concrete construction, diaphragm-reinforcing steel resists forces in the diaphragm

and chord tension stresses, and reinforcing dowels are generally used to transfer forces

from the diaphragm boundaries to concrete walls or frames.