seismic design of multistorey buildings

8/19/2019 Seismic Design of Multistorey Buildings

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REPORT ON TRAINING PROGRAMME “SEISMIC DESIGN OF MULTISTOREY BUILDING :

IS-1893 vs. Eur!"# 8$ %ELD FROM &' () ( &8 () M*+ &,1 *( N# D#/)0 r*20s#" +

IIT Rr4## 5 BMTPC *2" NORSAR.

By

Er. Prasanta Kumar Tripathy. A.E.E

Designs-II , Branch

O/O E.I.C (Civi!, "irmans#u$ha, Bhu%anes&ar

I2(r"u!(02 : A large number of reinforced concrete multi storied framed R.C buildings wereheavily damaged and many of them collapsed completely due to various earthquake. In Indiasevere damage had occurred due to Bhuj earthquake in !!". #ater on it is found that manybuildings in Ahmedabad $ which are situated at more than %! km away from the epicenter of the earthquake$ damaged or collapsed. &hese buildings were not designed and detailed for earthquake forces specified in I'(")*+ $ which was in e,istence from "*- and revised in

"*!$ "*- and "*)/.Recently earthquake of 0agnitude %.- of ritcher scale occurred in severalparts of 1disha and India. It is therefore very much essential to impart training to structuralengineers $ architects and field engineers for design $ planning and detailing of buildingsrespectively so that earthquake resistant buildings can be planned$ designed and constructedand there will be less damage to buildings and thereby saving valuable life and property. Atraining programme on seismic design of 0ultistory building $ I'(")*+ vs. 2urocode ) wasorgani3ed by II& Roorkee $ B0&4C and 51R'AR from -th to ) th 0ay !"/ at 5ew 6elhi.

D#(*0/s 6 (r*0202 7rr*# *2" 0(s *77/0!*(02 02 ()# 60#/" 6 s(ru!(ur*/ "#s02 *r#

2*rr*(#" #/:

1) Seismic hazard assessment was described by Dr. D.H.Lang. Senior Research

Engineer, NORSAR IS 1893 P*r( 1 ;202 *7 PGA5 I2(#2s0(+

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7 D#(#r020s(0!*//+ "#r0v#" 5 7ur#/+ *s#" 2 ()# *2# F*!(r 0.#.#66#!(0v# PGA:

Seismic

one

!ntensity

"e#e"

!ntensity

! $S%&'

one

(actor

II low 8I !."!

III moderate 8II !."-

I8 severe 8III !./

8 very severe I9 !.+-

7 D#6020(02 6 D#s02 B*s0s E*r()=u*4# DBE *2" M*

EPGA 6r DBE * )*;*r" r#"u!(02 6*!(r 6 ,. 0s *77/0#":

•

η η ⋅

⋅

⋅

=⋅

⋅

⋅

=

gS

RI

2EPGA

gS

RI

2ZA aMCEah

C*/!u/*(02 6 ()# "#s02

)r0;2(*/ s#0s0! !#660!0#2( Ah :

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( ) ( ) η η ⋅

⋅

⋅=⋅

⋅

⋅=

g

S

R

IEPGA

g

S

R

IZA aMCE

ah

7DD

with: Z , Z/2 - zoning factor (eecti!e PGA"#

Table 2

I - i$%ortance factor (f&nctiona' &e ofthe tr&ct&re"# Table 6

R - re%one re)&ction factor# Table 7

η - )a$%ing correction factor# Table 3

(for ξ * + η * 1.."

• Es(0*(02 6 6u2"*#2(*/ (0# 7#r0" T* :

use empirical e,pressions given in the Code I': ")*+;4art I !.!% h!.% for RC frame building&a > !.!)% h!.% for steel frame building=ithout brick infill panel&a, > !.!*h?@d along ,(a,is&a3 > !.!*h?@b$ along 3(a,iswhere h is the height of the building and d and b are the base dimensions of the buildingalong , and 3 a,is respectively.

•

C*/!u/*(02 6 ()# ((*/ *s# s)#*rB :

&he design value of base shear 8B8B > Ah == > 'eismic weight of the building

8ertical 6istribution of #ateral force at different floor levels can be obtained from the equation


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=here i > 6esign #ateral force at floor i =i > 'eismic weight of the floor i

hi > eight of floor i measured from base n > 5umber of 'toreysor design of the building and portions thereof$ the base shear corresponding to higher of Aha, and ADh,$similarly between Aha3 and ADh3 will be taken as minimum design lateral force.

PS%A !2(ur *7s 6r I2"0* NDMA &,11

+) Dr D.%. a-" Emerit-s (e""ow , Deartment o( earth/-a0e Engineering !! Roor0ee

described abo-t basic concet o( str-ct-ra" dynamics.

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) Dr D.H. Lang described abo-t site am"i(ication and roced-re (or (inding o-t

design resonse sectr-m as er ro#ision in !S 123 and E-rocode 2.

NE%RP s0/ !/*ss060!*(02

S0(# !/*ss D#s!r07(02

Av#r*# s0/ 7r7#r(0#s 6r ()# u77#rs( 3, m 6

s0/ /*+#rs

S)#*r-*v#

v#/!0(+ # s,4

S(*2"*r"

7#2#(r*(02 (#s(

v*/u# N

U2"r*02#" s

s(r#2() - s

A%*r"5 s(r25 02(*!( r!45

E*s(#r2 U.S. 2/+ 1,, s not a"icab"e not a"icab

BR!4 s( C*/06r20* r!4

!*s#s',H1,, s not a"icab"e not a"icab

C #r+ "#2s# s0/ *2" r!4 3',H', s , 1,,

D S(066 s0/s 18,H3', s 1 N , , N 1,,

E

S6( s0/s5 7r60/# 0() r#

()*2 3 m 6 s6( !/*+ "#602#" *s

s0/ 0() ! &,5 0s(ur#

!2(#2( w ,

J 18, s J 1 J ,

F

S0/s r#=u0r02 s0(#-s7#!060!

#v*/u*(02s5 #..5 7(#2(0*//+

/0=u#60*/# s*2" r 7#*(

not seci(ied not seci(ied not seci(ied

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Indian seismic code (IS 1893):

each ground !"e I# II# III is

conneced o a se o$ corner "eriods T B ,

T C

soil am"li%caion $acors S are nosoil am"li%caion $acors S are no

s"eci%ed&&&s"eci%ed&&&

S0/

(+7#D#s!r07(02 6 s(r*(0r*7)0! 7r60/# # s,4 5 s#! 6 s#!

I

R!4 r %*r" S0/:

#// r*"#" r*v#/ *2" s*2" r*v#/ 0


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Indian code (IS 1893):

sies"eci%c design res"onse s"ecra

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c&rrent'5 E&roco)e 3 i $ore a te$%'ate for a

co)e rather than a co$%'ete co)e (each

$e$6er co&ntr5 ha to %ro!i)e it own

7ationa' Anne8, e'ect 95%e 1 or 2 %ectra

an) an own ei$ic hazar) $a% * +um"s inhe le,el o$ seismic design loads

be-een neighboring counries are hus

auomaicall! "roduced"E7 1443 oi' c'aication: ite with )ee%

an)/or !er5 oft oi' )e%oit * "robabl! de

am"li%caion o$ ground moion IS 134 oi' c'aication !er5 o&t)ate) conce%t of contr&cting the horizonta' )eign

%ectr&$ i o&t)ate) for 6oth IS 134 an) E7

1443 (on'5 anchore) to PGA" * ./S/ conce"

much more inno,ai,e a);&t$ent of )a$%ing ratio higher than +

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∑=

= N

i i s

i

s

v

hv

1 ,

30,

30

0om"uaion o$ a,erage shear -a,e

,eloci! v s,30 :

#ayer ' EmF v s Em/sF

" +. "%

/.) +!

+ "+./ *

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/ %.) /-)

% . +-

- *.- /"

S)#*r *v# v#/!0(+ v s

'hear wave velocity vs is a parameter that is directly related to the dynamic properties of thematerial ;here: the soil


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- the amplitude of seismic waves increases so(called amplification ;de(amplification of groundmotion can also happen in certain circumstances


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&he collapse of RC frame buildings may be due to the following :

". 'oftness of base soil on which buildings are constructed.. 'oft first storey of the building if any

+. Bad structural system such as structural system using floating columns./. eavy water tanks on roof.%. #ack of earthquake resistant design.-. Improper detailing of reinforcement in beams and columns of the structure.. &ortional failure). 4ounding damage of adjacent buildings.*. #ack of stability of infill walls"!. 4oor construction quality

S6(2#ss 6 B*s# S0/

&he soft soil on which most buildings in Ahmedabad were founded would have affected theresponse of the buildings in three ways:

(i! Amplification of the ground motion at the base of the building(ii! Absence of foundation raft or piles(iii!Relative displacement between the individual column foundations vertically and laterally$ inthe absence of either the foundation struts as per I': /+- or the plinth beams

S6(-60rs( S(r#+:1pen ground storey ;stilt floor< used in most severely damaged or$ collapsed R.C. buildings$introduced Hsevere irregularity of sudden change of stiffnessD between the ground storey andupper storeys since they had infilled brick walls which increase the lateral stiffness of the frameby a factor of three to four times. 'uch a building is called a building with Hs#t gr#un$ st#rey $ inwhich the dynamic ductility demand during the probable earthquake gets concentrated in thesoft storey and the upper storeys tend to remain elastic. ence whereas the HsoftD storey isseverely strained causing its total collapse$ much smaller damages occurs in the upper storeys$if at all.

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Collapse of soft middle storey in a

building at BhujB*" S(ru!(ur*/ S+s(#:loating columns used in structural systems are very undesirable in earthquake 3ones of moderate to high intensity as in one III$ I8 J 8 since it will induce large vertical earthquakeforces even under hori3ontal earthquake ground motions due to overturning effects.

%#*v+ K*(#r T*24s 2 ()# R6:eavy water tanks add large lateral inertia forces on the building frames due to the so calledHwhippingD effect under seismic vibrations$ but remain unaccounted for in the design.

I7r7#r "#(*0/02 6 B#*s C/u2s:&he structural detailing of beams and columns was inadequate in terms of provisions in I':"+*!("**+ and also for proper installation of reinforcements in Beam(Column joints as per I':/%- and I': "+*!.

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<

I7r7#r D#(*0/02 6 R#026r!##2(:In detailing the stirrups in the columns$ no conformity appeared to satisfy lateral shear requirements in the concrete of the joint as required under I' /+-( "*- and I': "+*!("**+.&he shape and spacing of stirrups seen in collapsed and severely damaged columns withbuckled reinforcement was indicative of non(conformity even with the basic R.C. Code I': /%-("*).S)r( C/u2 D#(*0/02:In some situations the column is surrounded by walls on both sides such as upto the windowsills and then in the spandrel portion above the windows but it remains e,posed in the height of

the windows. 'uch a column behaves as a short column under lateral earthquake loading wherethe shear stresses become much higher than normal length columns and fail in shear.

6amage to buildings due to short column effect on columns

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Tr(02*/ F*0/ur#s:&ortional failures are seen to occur where the symmetry is not planned in the location of thelateral structural elements as for e,ample providing the lift cores at one end of the building or atone corner of the building or unsymmetrically planned buildings in # shape at the street corners.

#arge tortional shears are caused in the building columns causing there torsional shear failures.

Pu2"02 D**# 6 A"*!#2( Bu0/"02s:'evere damage even leading to collapse are seen due to severe impact between two adjacentbuildings under earthquake shaking if the adjacent blocks of a building or two adjacent buildingsare of different heights with floors at different levels and with inadequate separation. 'uchbuildings can vibrate out of phase with each other due to very different natural frequencies thushitting each other quite severely.

L*!4 6 S(*0/0(+ 6 I260// K*//s:&he infill walls were not properly attached either to the column or the top beams for stability

against out(of(plane bending under hori3ontal earthquake forces.

Pr C2s(ru!(02 u*/0(+:&he construction quality of the damaged R.C. buildings was found to be much below thatdesired$ as seen by the cover to reinforcement in the damaged members and the bad quality of concrete in the columns in "%! to +!! mm length just below the floor beams and within thebeam column joints.

L*!4 6 E*r()=u*4# R#s0s(*2( D#s02:

#ack of earthquake resistant design is the major factor due to which most of the high rise

buildings collapse. 0any buildings in Kujrat were not designed for the earthquake forcesspecified in I':")*+(!!. &he structural designers ignores the seismic forces in design. It mayalso be stated that most buildings are designed against lateral loads in the transverse direction$hence they collapse in longitudinal direction.

It is worst mentioning that earthquake resistant RC buildings should be designed to resistanticipated seismic forces. &he structure must satisfy safety and serviceability conditions inorder to resist e,pected loadings. It is necessary to understand the behaviour of materials likeconcrete and steel under seismic loading. In order to resist earthquake the structure must haveadequate ductility in order to dissipate the energy through inelastic deformation. It is alsoessential to provide adequate detailing of reinforcement in members of a structure so that thestructure can safely respond to strong ground motion. A general objective in design of RCmember is to so proportion such elements that they should posses adequate strength andstiffness. &he structure should posses adequate ductility. &he critical region should be capableof sustaining minimum number of deformation cycles of specified amplitude without loss of strength.

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In order to avoid the collapse of RC multi(storeyed frame buildings$ they should be wellconversant with the present codal design provisions. &he following are some important designcodal provisions related to earthquake resistant multi(storeyed frame buildings.

8) Dr 7.Singh described abo-t di((erent tyes o( b-i"ding con(ig-ration 9

Building Configurations :

Tuu/*r s+s(# : An efficient way to increase the lateral stiffness of tall buildings is to put themost of the lateral load resisting material along the perimeter of the building.&he resultingsystem is called a &ubular Building. 0ost of the tall buildings of world both in steel as well asconcrete have been constructed based on this concept.

Tu# S+s(#

&he basic idea is to make a rectangular tube out the the perimeter of the building.

&he tube is made up of closely spaced columns connected by stiff spandrel beams

creating very stiff moment frames.

rames parallel to direction of force act like webs to carry the shear.

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rames perpendicular to the direction of force act as

flanges. lange forces are not uniform.

Best applied to rectangular or circular plans.

'uitable for both steel and concrete.

Lse for buildings of /! stories or more.

rames are repetitive and easily constructed.

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Fig 1: Tube System

ig( ;&ube in tube<

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fIg a I5&2RI1R '&RLC&LR2'

fig b 29&2RI1R '&RLC&LR2'

2+


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&) Dr 7.Singh described abo-t the earth/-a0e resistant design and detai"ing o( R6

b-i"dings

Ductile Detailing

Concrete is known to be brittle material i.e. it fails suddenly when subjected to load . Butconcrete can be made ductile when confined by reinforcement. It can be seen that confinementnot only increases the strength of concrete$ but it tremendously increases the ductility of concrete. &he confinement is obtained by providing stirrups should be hooked at "+% ! in to coreconcrete otherwise these stirrups open up under force due to earthquake.

&he frame column(beam$ shear walls and foundation are designed by limit state theoryas per I': /%-:!!!$ then all details of longitudinal steel$ overlaps$ shear capacities$confining reinforcement requirements$ stirrups and ties etc. shall be worked out usingthe provisions of I': "+*!("**+.&he drawings should clearly show all the adopteddetails. 'ome typical detailing of reinforcement for e,ternal beam(column junction$detailing showing spacing of stirrups$ column and joint detailing and detailing showingtransverse reinforcement in column as per I':"+*!("**+ are indicated in igs. +$ /$ %and -$ respectively.

2

Fig. 4 Det!i"i#g s$%&i#g s'!(i#g sti))u's

*+S13920,1993 Fig 3: Det!i"i#g !t E-te)#!" Be!mC%"um# u#(ti%# *+S13920,1993


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2

Fig. 5 C%"um# !#/ %i#t /et!i"i#g*+S13920,1993


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Fig. T)!#se)se )ei#%)(eme#t i# (%"um#*+S13920,1993

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Fig 7 C!"(u"!ti%# % Desig# s$e!) %)(e %) (%"um#

Fig C!"(u"!ti%# % Desig# s$e!) %)(e %) (%"um#

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" &he spacing of hoops used as special confining reinforcement shall not e,ceed "?/ ofminimum member dimension but need not be less than % mm nor more than "!! mm.

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C2!/us02 :

'tructural ha3ard assessment$ basic concept of structural dynamics$ site amplification in varioustype of soil$ calculation of hori3ontal response spectrum for a particular soil $ structural system of

different types of multi storied buildings$ earthquake resistant design and detailing of multistoried building were discussed during this training programme. 6esign provision in I' aswell as 2urocode was discussed. ands on training was imparted for calculation of sitespecific design response spectra. &he procedure for dynamic as well as 'tatic analysis of multistoried buildings was discussed. Basic refreshing enriched knowledge was acquired in thefield of earthquake resistant design of multistoried buildings which can be applied in design of multistoried buildings of Kovernment in =orks 6epartment.

&he following conclusions are arrived at the end of the three days rigoroustraining session .

• &he multistoried framed building shall be analy3ed and designed as per provisions laid down in I':")*+(!!. 6ifferent methods of analysis like'tatic 'eismic coefficient method and 6ynamic response spectra methodmay be adopted for analysis of multistoried framed buildings in order tosatisfy the criteria of earthquake resistant design.

• 'uitable type of foundation may be adopted for multistoried framed buildingsfor different types of soil . &he soil e,ploration is very much essential. 8ariousI' codes like I':-/!+$ I':*""$ I':"*!/ and I':"))) etc may be referred for calculation of safe bearing capacity $ allowable bearing pressure andallowable settlement for design of isolated $ combined$ raft or pile foundations

for buildings.• After analysis and design detailing of column and beams and foundations

may be done as per I': "+*! for ensuring ductile performance duringearthquake. uality of construction in field shall be ensured and detailing of reinforcement shall be carried out as per structural drawings.

seismic design of multistorey buildings

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