Learning Earthquake Design and Construction 9. How to Make Buildings Ductile for Good Seismic Performance

8/10/2019 Learning Earthquake Design and Construction 9. How to Make Buildings Ductile for Good Seismic Performance

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CLASSROOM

Figu re 2 . Tens ion Test on

M a t e r i a l s d u c t il e v e rs u s

br i t t le mater ia ls .

Stee l i s used in maso nry and concre te bu i ld ings as re in forc em ent

ba rs o f d i a me t e r r a ng i ng f r om 6m m t o 40mm. R e i n f o r c i ng s te e l

can ca r ry both t ens il e and compress ive loads . More over , s t ee l i s

a ductile material.

T hi s i mpor t a n t p r ope r t y o f duc t i l i t y e nab l e s

s tee l ba rs to und ergo l a rge e longa t ion be fore breaking .

Concre te i s used in bu i ld ings a long wi th s t ee l re inforcement

bars . This composi te mater ia l i s ca l led

reinforced cement concrete

or s i mp l y

reinforced concrete

( P C ) . T he a moun t a nd l oc a t i on o f

s te e l i n a me m be r s hou l d be s uc h t ha t t he f a i l u re o f the m e m be r

i s by s t ee l reaching i t s s t rength in t ens ion be fore concre te

reaches it s s t rength in com press ion . Thi s type of fa ilu re i s

ductile

failure,

and henc e i s p re fe r re d over a fa i lu re whe re concre te fa il s

f i rs t in c ompr e ss i on. T he r e f o r e , c on t r a r y t o c om mo n t h i nk i ng ,

provid ing too m uch s t ee l in RC bui ld ing s can be ha rm ful even

Capacity Design Concept

Let us t ake two bars of s ame l eng th and

cross -sec t iona l a rea - one ma de o f a duc t i l e

mate r i a l and ano ther o f a br i t tl e m ate r i al .

Now, pul l these two bars un t i l th ey break

You wi l l no t i ce tha t the duc t i l e ba r e longa tes

by a l a rge amount be fore i t b reaks , whi le

t he b r i tt l e ba r b r e a ks s udde n l y on r e a c h i ng

i ts ma xi m um s t reng th a t a rel a t ive ly smal l

e longa t ion Figure 2). Am ong s t the mate r i a l s

used in bu i ld ing cons t ruc t ion , s t ee l is

ductile,

w hi l e ma s on r y a nd c onc r e t e ar e

brittle.

Now, l e t us mak e a cha in wi th l inks mad e of

brittle a nd ductile mater ia l s Figure 3). Each

of these l inks wi l l fa i l jus t l ike the bars

s how n i n

Figure

2. Now , hold th e last l ink a t

e i the r end of the cha in a nd ap ply a force F .

S ince the same force F is be ing t rans fe r re d

throu gh a ll the l inks , the force in each l ink

is the sam e, i.e. , F . As m ore an d m ore for ce

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CLASSROOM

Figure 3 Ductile chain

de

sign

is appl ied , eventua l ly the cha in wi l l b reak w hen the

weakest link

in i t breaks . I f the duct i le l ink is t he w e ak one i .e . , i t s capa ci ty to

take load is less) , the n th e chain wi l l show large f inal e longat io n.

Ins tead , i f the br i t t l e l ink i s the weak one , th en the ch a in wi l l fa i l

sudd enly and show smal l f ina l e longa t ion . There f ore , i f we want

to have such a

ductile

cha in , we have to make the duc t i l e l ink to

be the weakest l ink.

Earthquake Resistant Des ign of Buildings

Bui ld ings should be des igned l ike the duc t i l e cha in . For example ,

c ons i de r t he c om mo n u rba n r e s i de n ti a l a pa r t me n t c ons t ruc t i on

- t he mu l t i - s t o re y bu il d i ng m a de o f r e i n fo r c e d c onc ret e . I t

cons i st s of hor izonta l and ve r t i ca l memb ers , na me ly

beams

a n d

columns. The seismic iner t ia forces generated a t i t s f loor levels

a re t rans fe r red through the va r ious beams a nd columns to the

g round . T he c o r r e c t bu i l d i ng c ompone n t s ne e d t o be ma de

duc t il e . The fa i lure of a co lu mn can a f fec t the s t ab i l i ty of the

whole bui ld ing , but the fa i lure of a bea m causes loca l i zed e ffec t.

There fo re , i t is be t t e r to m ake beams to be the du c t i l e weak l inks

t ha n columns. T hi s me t h od o f de s i gn ing R C bu i l d i ngs i s c a ll e d

t he

strong-column weak-beam

d e si g n m e t h o d

Figure

4).

B y us i ng t he

routine

des ign codes me ant for des ign aga ins t non -

ea r thquake e f fec t s ) , des igners may not be ab le to achieve a

duct i le s t ructure . Specia l des ign provis ions are required to help

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des igners improve the d uc t i l i ty of

the s t ruc ture . S uch provis ions are

usua l ly pu t t oge the r in the f o r m o f

a special

se ismic

design code, e .g. ,

IS: 13920-1993 for RC str uc-t ures .

T h e s e c o d e s a l s o e n s u r e t h a t

adequa te duc t i l i ty i s provided in

the m e m be r s whe r e da m a ge i s

expected.

Q u a l i ty C o n t r o l i n C o n s t r u c t i o n

The capac i ty des ign concept in

e a r t h q u a k e - r e s i s t a n t d e s i g n o f

build ings wil l fa il i f the s trength s of the br i t t le l in ks fa ll below

t h e i r m i n i m u m a s s u re d v a lu e s. T h e s t r e n g t h o f b r i t t le

const ruc t ion mate r ia ls , l ike masonry and concre te , i s h ighly

sens i t ive to the qua l i ty of cons t ruc t ion mate ria ls , workm anship ,

superv ision, and con struc tion meth ods. Similar ly, specia l care is

needed in co ns t ruc t io n to ensure tha t the e lements mea nt to be

duc t i le a re indeed provided wi th fea tures tha t g ive adequa te

duc t i l i ty . Thus , s t r ic t adherence to presc r ibed s tandards of

con stru ctio n mater ia ls and con struct ion processes is essentia l in

assur ing an ea r thquake- res is tant bui ld ing . Regula r te s t ing of

con stru ctio n mater ia ls a t q ualif ied laborator ies (a t s i te or away),

pe r iodic t ra in ing of workm en a t profess ional t r a in ing houses ,

and on-s i te eva lua t ion o f the technica l work a re e lements of good

quali ty control .

S u g g e s t e d R e a d i n g

[1] T Paulay and M J N Priestley,

Seismic Design of Reinforced Concrete

Buildings and Masonry

John Wiley USA 1992.

[2] F M Mazzolani and V Piluso Theory

n d Design of Seismic.Resistant

Steel Frames

E FN Spon,UK,1996.

Figure 4. Reinforced Con

crete Bui ldin g Design:

t h e

b e a m s m u s t b e t h e w e ak

e s t l i n k s a n d n o t t h e c o l

u m n s t h is c a n b e a c h i

e v e d b y approp riately siz

ing t h e m e m b e r s and pro

viding c o r r e c t a m o u n t o f

s t e e l r e i n f o r c e m e n t

in

t h e m .

RESONANCE J Decem ber 2004

Suggest ions/comments may

be sent to: eqtips@iitk ac in

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Learning Earthquake Design and Construction 9. How to Make Buildings Ductile for Good Seismic Performance

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