A Comparative Study of the Design Spectra Defined by EC8, UBC, IBC, Turkish Code on RC Sample...

8/17/2019 A Comparative Study of the Design Spectra Defined by EC8, UBC, IBC, Turkish Code on RC Sample Buildings Vvgd

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J Seismol (2006) 10:335–351DOI 10.1007/s10950-006-9020-4

O R I G I N A L A RT I C L E

A comparative study of the design spectra dened

by Eurocode 8, UBC, IBC and Turkish EarthquakeCode on R/C sample buildingsAdem Do˘gang ün · Ramazan Livao˘ glu

Received: 8 November 2004 / Accepted: 20 April 2006C Springer Science + Business Media B.V. 2006

Abstract Earthquake codes have been revised and up-dated depending on the improvements in the represen-tation of ground motions, soils and structures. Theserevisions have been more frequently seen in recentyears. One of the key changes in earthquake codeshas been performed on the design spectra. In this pa-per, the designspectra recommended by TurkishEarth-quakeCode andthreeother well knowncodes (UniformBuilding Code, Eurocode 8, and International BuildingCode) are considered for comparison. The main pur-

pose of this studyis to investigatethe differencescausedby the use of different codes in the dynamic analysisand seismic verication of given types of buildings lo-cated at code dened different sites. The differencesin expressions and some important points for elasticand inelastic spectra dened by the codes are brieyillustrated in tables and gures. Periods, base shears,lateral displacements and interstory drifts for the ana-lyzed buildings located at code dened ground type arecomparatively presented.

Keywords Design spectra . Eurocode 8 . Groundtypes . IBC . Seismic codes . Seismic response of building . Turkish earthquake code . UBC

A. Doğang ün ( ) · R. Livao ğluKaradeniz Technical University, Department of CivilEngineering, 61080 Trabzon, Turkey

Introduction

Earthquake codes are periodically revised and updateddependingon theimprovements in therepresentationof ground motions, soils and structures. Moreover, theserevisions have been made more frequently in recentyears. The Turkish Earthquake Code (TEC, 1998) wasalso revised in 1997 and has been in effect since 1998.Unfortunately, two destructive earthquakes [Kocaeliand D üzce] occurred in Turkey in 1999 one year after

the enforcement of TEC. These earthquakes resulted inmore than 18,000 recorded deaths and 50,000 seriousinjuries. More than 51,000 buildings were either heav-ily damaged or totally collapsed. Some other 110,000buildings moderately or lightly suffered damage. Thedamage inicted by the Kocaeli earthquake on struc-tures in general, and on modern RC buildings in par-ticular, is perhaps the worst seen in many years, andcertainly the worst in Europe in recent history. Aver-age total loss may be in the range of 16 Billion USD,about 7% of theNation’s GDP(Erdik, 2004).A number

of separate teams from different disciplines conducteddamage surveysand reported someconclusions follow-ing the two major earthquakes as briey summarizedbelow.

Early reports stressed that quality of constructionmaterial were poor and that there were many structuralmistakes and deciencies due to the non-compliancewith the earthquake code. It was concluded that thenature of the strong-motion was also a major con-tributing factor to the level of damage. Some of the

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336 J Seismol (2006) 10:335–351

teams indicated that soil effects played an importantrole on damage with different forms such as lique-faction, bearing capacity loss, subsidence and lateralspread. In some studies, response spectra computedfrom recorded accelerograms were compared with de-sign spectra derived from TEC. It wasconcluded that in

a period range of about 1.0–1.5 sec, which correspondsto the pulse periods of the Bolu and D ¨ uzce records thecapacity demand is considerably in excess of the de-sign spectrum curve (Akkar and G¨ ulkan, 2002). It wasalso concluded that the acceleration response spectrafor the D üzce motion exceeded the UBC (1997) designspectrumat periods less than 0.5 s and greater than 3.0 s(Erdik, 2004). Another spectra-related study in whichthe observed and simulated displacement spectra werecompared with the Eurocode 8 design displacementspectrum for the Bolu expressway viaduct was carried

out by Faccioli et al. (2002). One another conclusionwasthat peak acceleration formost near-source recordswere not sohigh asexpectedand seemed tobecomesat-urated for increasing magnitudes. However, the peak velocities and corresponding drift demands were usu-ally considerable, and this was conrmed by structuraldamage (Akkar and G¨ ulkan, 2002).

Under the light of the observations and lessons fromthe 1999 earthquakes, many studies concerning TEChave been carried out up to now and a number of im-provements in TEC might be brought up to the agenda

in the near future. As known, one of the key changesin the earthquake codes has been generally performedon the design spectra. The requirements recommendedby EC8 are also very important for the Turkish com-munity; so it will be helpful to establish a comparisonbetween the design spectra recommended by the EC8(2004) and TEC. The inuence of local ground con-ditions on the seismic action is generally considereddepending on the ground types described by the strati-graphic proles and parameters. That is why the em-phasis has been placed on the differences caused by the

use of spectra given in the TEC and other well knowncodes such as UBC, EC8 and IBC(2003) in the seismicanalysis of sample buildings.

The effects of ground types dened in the codes ontheseismic responseof buildings werealso investigatedfor both different building congurations and differentsupporting soils. Base shears and interstory drifts forconsidered ground types and buildings are comparedto reveal the differences. As design requirements givenin the earthquake codes are signicantly different, the

seismic design of buildings is out of this paper scope.However,authors triedto consider basicdesign require-ments recommended in the codes for selection of thebuilding systems.

Seismic hazard is expressed in TEC and EC8 by asingle parameter, namely the reference peak ground

acceleration agR at the surface on rock for a refer-ence mean return period. The reference return periodrecommended for the non-collapse performance levelis the 475 year, corresponding to 10% probability of exceedance in 50 years in a Poisson occurrence pro-cess both TEC and EC8. A comparative study of theseismic hazard assessments in European National Seis-mic Codes including TEC was carried out by Garcia-Mayordomo et al. (2004). In EC8 the design groundacceleration ( a g ) is equal to a gR times the importancefactor γ I . A singleparameter, however, is insufcientto

characterize thegroundmotionhazard, because the fre-quency content as well as the amplitude of the groundmotion affect structural response. The standard methodas in TEC and in EC8 is to describe the frequencycontent by a response spectrum. Two inuences onfrequency content are recognized by TEC and EC8,namely the type of ground present at the site underconsideration and the magnitude of earthquake. Theformer is accounted for by describing various groundtypes as mentioned below:

Code dened ground types

The site conditions have been classied into differentcategories in earthquake codes. These categories arenamed groundtypes, soil prole types, local site classesor subsoil classes. In this paper, thetermofgroundtypesis selected in accordance with EC8. Table 1 presentsground types and shear wave velocities given in thecodes including FEMA 368 (2001). As seen from thistable, TEC gives more information about ground types

depending on the topmost layer thickness of soil ( h1).Four and six ground types are dened in TEC and UScodes, respectively. It should be noted that in the 1998edition of EC8 only three types of A, B and C weredened. However, ve main ground types as to be A,B, C, D, E and two special ground types S 1 and S 2 havebeen described in the nal version of EC8.

The site classication system is based on denitionsof site classes in terms of a representative average shearwave velocity, Standard Penetration Test blow-count,

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J Seismol (2006) 10:335–351 337

T a

b l e 1

G r o u n d t y p e s d e n e d i n t h e T E C

, U B C

, I B C

, F E M A 3 6 8 a n d E C 8

T E C

U B C

, I B C & F E M A 3 6 8

E C 8

G r o u n d t y p e s

D e s c r i p t i o n





Z 1

–

M a s s i v e v o l c a n i c r o c k s , u n w e a t h e r e d

s o u n d m e t a m o r p h i c r o c k s , s t i f f

c e m e n t e d s e d i m e n t a r y r o c k s V

s >

1 0 0 0 m / s ; V e r y d e n s e s a n d , g

r a v e l V

s

> 7 0 0 m / s ; H a r d c l a y , s

i l t y c l a y V

s >

7 0 0 m / s

S ∗ A

H a r d r o c k

V s

> 1 5 0 0 m / s

h + 1 ≤ 1 5 m

S o f t v o l c a n i c r o c k s s u c h a s t u f f a n d

a g g l o m e r a t e , w e a t h e r e d c e m e n t e d

s e d i m e n t a r y r o c k s w i t h p l a n e s o f

d i s c o n t i n u i t y V

s ≈ 7 0 0 –

1 0 0 0 ; D e n s e

s a n d , g r a v e l V

s ≈ 4 0 0 –

7 0 0 ; V e r y s t i f f

c l a y , s

i l t y c l a y V

s ≈ 3 0 0 – 7 0 0

S B

R o c k V

s ≈ 7 6 0

– 1 5 0 0

A

R o c k o r r o c k - l

i k e g e o l o g i c a l

f o r m a t i o n i n c l u d i n g m o s t 5 m

w e a k e r m a t e r i a l a t t h e s u r f a c e

V s , 3 0 > 8 0 0 m / s

Z 2

h 1 > 1 5 m

S o f t v o l c a n i c r o c k s s u c h a s t u f f a n d

a g g l o m e r a t e , w e a t h e r e d c e m e n t e d

s e d i m e n t a r y r o c k s w i t h p l a n e s o f

d i s c o n t i n u i t y V

s ≈ 7 0 0 –

1 0 0 0 ; D e n s e

s a n d , g r a v e l V

s ≈ 4 0 0 –

7 0 0 V e r y s t i f f

c l a y , s

i l t y c l a y V

s ≈ 3 0 0 –

7 0 0

S C

V e r y d e n s e s o i l

o r s o f t r o c k V

s

≈ 3 6 0 –

7 6 0

B

D e p o s i t o f v e r y d e n s e s a n d , g

r a v e l

o r v e r y s t i f f c l a y , a

t l e a s t s e v e r a l

t e n s o f m i n t h i c k n e s s e s ,

c h a r a c t e r i z e d b y a g r a d u a l

i n c r e a s e o f m e c h a n i c a l

p r o p e r t i e s w i t h d e p t h V s

, 3 0

≈

3 6 0 –

8 0 0

h 1

≤ 1 5 m

H i g h l y w e a t h e r e d s o f t m e t a m o r p h i c r o c k s

a n d c e m e n t e d s e d i m e n t a r y r o c k s w i t h

p l a n e s o f d i s c o n t i n u i t y V

s ≈ 4 0 0 –

7 0 0 ;

M e d i u m d e n s e s a n d a n d g r a v e l V

s ≈

2 0 0 –

4 0 0 ; S t i f f c l a y , s

i l t y c l a y V

s ≈

2 0 0 –

3 0 0

Z 3

1 5 m <

h 1

≤ 5 0 m




s ≈ 4 0 0 –

7 0 0 ;


s ≈

2 0 0 –

4 0 0 ; S t i f f c l a y , s

i l t y c l a y V

s ≈

2 0 0 –

3 0 0

S D

S t i f f s o i l V

s , 3

0 ≈

1 8 0 –

3 6 0

C

D e e p d e p o s i t s o f d e n s e o r

m e d i u m - d e n s e s a n d , g

r a v e l o r

s t i f f c l a y w i t h t h i c k n e s s f r o m

s e v e r a l t e n s t o m a n y h u n d r e d s o f

m V

s ≈ 1 8 0 –

3 6 0

h 1

≤ 1 0 m

S o f t , d e e p a l l u v i a l l a y e r s w i t h h i g h w a t e r

t a b l e V s

<

2 0 0 ; L o o s e s a n d V

s ≈ 2 0 0 ;

S o f t c l a y , s i l t y c l a y V

s <

2 0 0

( C o n t i n u e d o n n e x t p a g e )

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338 J Seismol (2006) 10:335–351

T a

b l e 1

( C o n t i n u e d )

Z 4

h 1 > 5 0 m




s ≈ 4 0 0 –

7 0 0 ;


s ≈

2 0 0 –

4 0 0 ; S t i f f c l a y , s i l t y c l a y V

s ≈

2 0 0 –

3 0 0

S E S o f t s o i l V

s <

1 8 0

D

D e p o s i t s o f l o o s e - t o - m e d i u m

c o h e s i o n l e s s s o i l ( w i t h o r

w i t h o u t s o m e s o f t c o h e s i v e

l a y e r s ) , o r o f p r e d o m i n a n t l y

s o f t - t o - r m c o h e s i v e s o i l . V

s , 3

0

<

1 8 0

H 1 >

1 0 m

S o f t

, d e e p a l l u v i a l l a y e r s w i t h h i g h w a t e r

t a b l e V

s <

2 0 0 ; L o o s e s a n d V

s <

2 0 0 ;

S o f t c l a y , s i l t y c l a y V

s <

2 0 0

-

I n a l l s e i s m i c z o n e s , s o f t

, d e e p a l l u v i a l

l a y e r s w i t h

h i g h w a t e r t a b l e V s

<

2 0 0 ,

l o o s e s a n d V

s <

2 0 0 a n d s o f t c l a y , s

i l t y

c l a y V

s < 2 0 0 w i t h w a t e r t a b l e l e s s t h a n

1 0 m f r o m t h e s o i l s u r f a c e s h a l l b e

i n v e s t i g a t e d a n d t h e r e s u l t s s h a l l b e

d o c u m e n t e d t o i d e n t i f y w h e t h e r t h e

L i q u e f a c t i o n P o t e n t i a l e x i s t s

, b y u s i n g

a p p r o p r i a t e a n a l y t i c a l m e t h o d s b a s e d

o n i n - s

i t u a n d l a b o r a t o r y t e s t s .

S F S o i l r e q u i r i n g

s i t e s p e c i c

e v a l u a t i o n . I t i s

m o r e d e t a i l e d

d e n e d i n t h e

I B C

E

A s o i l p r o l e c o n s i s t i n g o f a

s u r f a c e a l l u v i u m l a y e r w i t h V

s , 3

0

v a l u e s o f c l a s s C o r D a n d

t h i c k - n e s s v a r y i n g b e t w e e n

a b o u t 5 a n d 2 0 m , u

n d e r l a i n b y

s t i f f e r m a t e r i a l w i t h V

s , 3

0 >

8 0 0 m / s

S 1

D e p o s i t s c o n s i s t i n g o r c o n t a i n i n g a

l a y e r a t l e a s t 1 0 m t h i c k o f s o f t

c l a y s / s i l t s w i t h h i g h p l a s t i c i t y

i n d e x ( P I > 4 0 ) a n d h e i g h t w a t e r

c o n t e n t , V

s , 3 0

<

1 0 0 m / s

S 2

D e p o s i t s o f l i q u e a b l e s o i l s

, o f

s e n s i t i v e c l a y s , o

r a n y o t h e r s o i l

p r o l e n o t i n c l u d e d i n t y p e s A – E

o r S 1

∗ S A

, S B , S

C , S

D , S

E a n d S F g i v e n

i n t h e U B C a r e s y m b o l i z e d w i t h A

, B , C

, D , E , a

n d F , r e s p e c t i v e l y , i n t h e I B C a n d F E M A 3 6 8

+ h

1 i s t h e t o p m o s t l a y e r t h i c k n e s s f o r s u b s o i l

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unconned compression strength, relative density, etc.in some earthquake codes like TEC. However,based onempirical studies by Borcherdt (1994), recommendedshear wave velocity V S − 30 as a means of classifyingsites for building codes, and similar site categorieswere selected for the FEMA seismic design provisions

for new buildings (Dobry et al., 2000). Boore et al.(1994) indicate that the ideal parameter would be theaverage shear-wave velocity to a depth of one-quarterwavelength for the period of interest, as was used byJoyner and Fumal (1984). By the quarter-wavelengthrule, 30m is the appropriate depth for period of 0.16 sforstiff soil andperiodvalues tend to increase as thesoilgets softer (Boore et al., 1994). It should be noted thatcode dened spectra depending on ground types areprovided only for cases where the 30 m of soil imme-diately below the site dominates the frequency content

of the design motions. The average shear wave velocityof the upper 30 m of soil ( V S− 30 ) is also considered inEC8 and the velocityboundsof360 m/s and 180 m/s fortypes B, C and D, make the velocity values consistentwith the N SPT values (Sabetta and Bommer, 2002).

Elastic and inelastic response and designspectra

Various seismological and geophysical parameters af-fect the shape of response spectra. Ambraseys et al.(1996) and Bommer and Acevedo (2004) presentedand discussed the effects of earthquake magnitude,source-to-site distance, site classication, and style-of-faulting on the strong-motion accelerograms and con-sequently response spectra. As known, the dampingratio and structural vibration period are other param-eters affecting the response spectra. The earthquake-induced ground shaking is generally represented in theform of acceleration response spectra or displacementresponse spectra.

Acceleration response spectra

In all current seismic codes, the earthquake actionsare represented in the form of a spectrum of abso-lute acceleration. But code acceleration spectra tendto be conservative at longer periods with the result thatthe long-period ordinates of the displacement spectraare unnecessarily high (Bommer et al., 2000). This has

Fig. 1 Typical shape of elastic design spectra.

been shown to be case for EC8 by Tolis and Faccioli(1999).

A typical shapeof horizontal elastic designspectrumcan be drawn as seen in Fig. 1. In this gure, T showsperiods of structure, S e A and S e B show the ordinatevalues atpointsA and B of the elastic designspectra, T Band T C show thelowerandthe upper limitsof theperiodof the constant spectral acceleration branch, and T Dshows the value dening the beginning of the constantdisplacement response range of the spectrum. Thereare some differences in spectral shapes recommendedby the earthquake codes. Therefore, thedifferences andsimilarities in the spectra used by considered seismic

codes are mentioned below:EC8 has a notestartingthat, ifdeepgeology isnotac-

counted for, the recommended choice is the use of twotypes of spectra: Type 1 and Type 2. If the earthquakesthat contribute most to the seismic hazard dened forthe site for the purpose of probabilistic hazard assess-ment have a surface-wave magnitude, M s , not greaterthan 5.5, it is recommended that the Type 2 spectrumis adopted.

To show differences and similarities, importantpoints of elastic design spectra shown in Fig. 1 and re-

quirements related to these points dened in the TEC,UBC and EC8 are shown comparatively in Table 2. Inthis table, S is the soil factor dened in EC8 depend-ing on ground types and η is the damping correctionfactor with a reference value of η = 1 for 5% viscousdamping.

The ordinates of elastic design spectra S e and in-elastic design spectra S d for the reference return perioddened by the earthquake codes except IBC can bedetermined using the expressions given in Table 3. In

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T a

b l e 2

C o m m e n t o n t h e i m p o r t a n t p e r i o d s o f e l a s t i c d e s i g n s p e c t r a d e n e d i n t h e T E C

, U B C a n d E C 8

E C 8

T w o t y p e s o f s p e c t r u m ( T y p e - 1

a n d T y p e - 2

) d e n e d d e p e n d i n g o n t h e

T E C

U B C

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

T B

D e n e d d e p e n d i n g o n l y s u b s o i l c l a s s e s .

T h e v a l u e s o f T

B c o r r e s p o n d i n g t o Z 1 ,

Z 2 , Z 3 a n d Z 4 g r o u n d t y p e s c a n b e

t a k e n 0 . 1 0

, 0 . 1

5 , 0 . 1 5 a n d 0 . 2 0

,

r e s p e c t i v e l y .

D e n e d d e p e n d i n g o n g r o u n d t y p e s a n d

e a r t h q u a k e z o n e . I

n a d d i t i o n , f o r

s e i s m i c z o n e 4 i t a l s o d e p e n d s o n t h e

n e a r s o u r c e f a c t o r s .

• F o r T y p e - 1 t h e

v a l u e s o f T

B

c o r r e s p o n d - d i n g t o A

, B , C , D

a n d E g r o u n d

t y p e s c a n b e t a k e n 0 . 1 5

, 0 . 1

5 , 0 . 2 0

, 0 . 2

0 a n d

0 . 1 5

, r e s p e c t i v e l y .

T B =

0 . 2

C v

2 . 5 · C a

T h e v a l u e o f T

B i s c h a n g e d f r o m 0 . 1 s t o

0 . 2 2 s

• F o r T y p e - 2 t h e

v a l u e s o f T

B

c o r r e s p o n d - d i n g t o A

, B , C , D

a n d E g r o u n d

t y p e s c a n b e t a k e n 0 . 0 5

, 0 . 0

5 , 0 . 1 0

, 0 . 1

0 a n d

0 . 0 5

, r e s p e c t i v e l y .

T C

D e n e d d e p e n d i n g o n l y s u b s o i l c l a s s e s .

T h e v a l u e s o f T C c o r r e s p o n d i n g t o Z 1 ,

Z 2 , Z 3 a n d Z 4 g r o u n d t y p e s c a n b e

t a k e n 0 . 3 , 0 . 4 , 0 . 6 a n d 0

. 9 , r

e s p e c t i v e l y .

D e n e d d e p e n d i n g o n g r o u n d t y p e s a n d

e a r t h q u a k e z o n e . I

n a d d i t i o n , f o r

s e i s m i c z o n e 4 i t d e p e n d s o n t h e n e a r

s o u r c e f a c t o r s .

• , F

o r T y p e - 1

T h e v a l u e s o f T C c o r r e s p o n d i n g

t o A

, B , C , D

a n d E g r o u n d t y p e s c a n b e

t a k e n 0 . 4 , 0 . 5 , 0 . 6 , 0 . 8 a n d 0 . 5 , r e s p e c t i v e l y .

T C =

C v

2 . 5 · C a

T h e v a l u e o f T C i s c h a n g e d f r o m 0 . 4 s t o

1 . 0 7 s

• F o r T y p e - 2 T h e v a l u e s o f T C c o r r e s p o n d i n g

t o A

, B , C , D

a n d E g r o u n d t y p e s c a n b e

t a k e n 0 . 2 5

, 0 . 2 5 , 0 . 2 5

, 0 . 3

a n d 0 . 2 5

,

r e s p e c t i v e l y .

T D

– ( n o t d e n e d )

– ( n o t d e n e d )

• I t i s e q u a l t o 2 f o r T y p e - 1

I t i s e q u a l t o 1 . 2 f o r T y p e - 2

S e A

1 . 0

· a g

1 . 0

· C a · g

S a g

S e B

2 . 5

· a g

2 . 5

· C a · g

2 . 5

· S g · η a g

W h e r e C

a a n d C

v

a r e s o i l m o d i e d

g r o u n d m o t i o n p a r a m e t e r s

• F o r T y p e - 1 c o n s i d e r i n g 5 % d a m p i n g r a t i o

,

S e c o r r e s p o n d i n g t o A

, B , C , D

a n d E

g r o u n d t y p e s c a n b e t a k e n a g

t i m e s 2 . 5 , 3 ,

2 . 8 7 5 , 3 . 3 7 5 a n d 3 . 5 , r e s p e c t i v e l y .

• F o r T y p e - 2 c o n s i d e r i n g 5 % d a m p i n g r a t i o

,

S e c o r r e s p o n d i n g t o A

, B , C , D

a n d E

g r o u n d t y p e s c a n b e t a k e n a g

t i m e s 2 . 5 ,

3 . 3 7 5 , 3 . 7 5

, 4 . 5 a n d 4 , r e s p e c t i v e l y .

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Table 3 Ordinates of elastic and inelastic design spectra ( S e and S d ) for TEC, UBC and EC8

T ≤ T B T B ≤ T ≤ T C T ≥ T C

TEC S e = a gR [1 + 1.5 T T B ] S e = 2.5 · a gR S e = 2.5 · a gR [T C T ]

0.8

S d = ag Ra

[1 + 1.5 T T B ] S d = 2.5·a g

RaS d =

2.5a g Ra

[ T C T ]0.8

UBC S e = [C a + 1.5·C a ·T

T B] · g S e = 2.5 · C a · g S e = C vT g

S d = [C a + 1.5·C a ·T T B ] · g · γ I R S d = 2.5 · C a · g ·

γ I R S d =

C vT g ·

γ I R

EC8 S e = a g · S [1 + T T B (η2.5 − 1)] S e = 2.5 · a g · S · η T C ≤ T ≤ T D → S e = 2.5a g · S · η · [T C T ]

S d = a g S [ 23 + T T B

( 2.5q − 23 )] S d =

2.5q · a g ·S T C ≤ T ≤ T D → S d

= 2.5q ag · S · [T C T ]

≥ β · a gT D ≤ T ≤ 4s → S e = 2.5a g · S · η · [ T C T DT 2 ]

T ≥ T D → S d = 2.5q ag · S · [T C T D

T 2 ] ≥ β · a g

this table, β shows lower bound factor for the hori-zontal design spectrum, recommended value for β is

0.2 and γ I shows importance factor. The design spec-tra dened by IBC are not directly compared with theothers (TEC, UBC, EC8) due to signicant differencesfor seismic design provisions. The IBC does not useseismic zones to establish design earthquake groundmotion or to impose additional design requirementsand structural limitations. Rather than seismic zone,the IBC uses a parameter called Seismic Design Cat-egory as the mechanism for imposing design restric-tions, detailingrequirements, andstructurallimitations.The seismic design category assigned to a building

is important in that it affects the permissible analy-sis procedures, applicability of structural redundancy,methodof lateral load distribution, limitations on struc-tural systems, applicability of special load combina-tions, and ductile detailing requirements. The designground motion parameters fall into are now S DS andS D1 rather than seismic zone factor. S DS and S D1 arethe ordinate values that equal to ve-percent-dampeddesign spectral response accelerations at short periodsand 1 second period respectively. S DS determines theupper-bound design base shear (the “at-top” of the

design spectrum) used in seismic design. S

D1 denesthe descending branch or the period-dependent partof the design spectrum. The values of S DS and S D1are estimated depending on the mapped spectral re-sponse accelerations prepared for USA as explainedbelow.

S DS and S D1 are two-thirds of S M S and S M 1 , whichare the soil modied spectral response accelerationsat short period and 1.0 second period, respectively.S M S is obtained by multiplying the mapped spectral

acceleration S S by F a , the acceleration-related soilfactor. S M 1 is similarly obtained by multiplying the

mapped spectral acceleration S 1 by F v , the velocity-related soil factor. The specic factor F a is dened overa low-period range ( T = 0.1–0.5 sec) and F v whichis dened over a mid-period range ( T = 0.4–2 sec).These site factors derived using both observationaland analysis-based approaches (Dobry et al., 2000) areanalogous to C a /Z and C v /Z of the UBC 97. Responseacceleration at any period in the high frequency rangeis equal to the design spectral response accelerationat short periods and any point in the constant velocityrange can be obtained from the relationship of S D1 / T.

Elastic design spectra were drawn as shown in Fig. 2using theexpressionsshown in Table 1–3forallgroundtypes dened in the codes. As seen from Fig. 2, onlyTEC considers the same peak values for all groundtypes. EC8 gives the maximum peak values for groundtypes other than ground type A. The shapes of the elas-tic response spectrum of Type-2 are more peaking forshort period structures except for ground type A.

The concept of dividing the elastic response spec-tra by a single factor to arrive at the inelastic designspectra is a practical one and has been adopted by most

earthquake codes.Thefactorused forreducing theelas-tic response spectrum is called behaviour factor ( q) inEC8, response modication coefcient ( R) in FEMA368 (2001), R coefcient in UBC and the seismic loadreduction factor Ra in the TEC .

Earthquake codes describe different behaviour fac-tors. The values of the maximum allowable behaviourfactor are taken considering the type of structural sys-tem, regularity in elevation and prevailing failure modein thesystem with walls inEC8, whereas, TECspecies

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Fig. 2 Normalized elastic design spectra drawn for ground types described in TEC, UBC and EC8 (they normalized by the designground acceleration)

periods ( T and T B) dependent values of behaviour fac-tor in addition to structural system.

Figure 3 illustrates inelastic design spectra that wereobtained considering the structure importance factorequal to 1, the behaviour factor equal to 4 and the refer-

ence peak groundaccelerationequal to 0.4 g for samplestructures and soil conditions. As seen from this gure,the trend of the graph for UBC with the period valuesfrom zero to T B, the rst limit of the constant spectralacceleration branch, is different from those drawn forTEC and EC8. Eurocode 8 gives the maximum peak values for ground types other than ground type A. Thevalues of S d dramatically decrease from 0 to T B forEC8. The maximum peak values obtained for the EC8Type-2 spectrum and the large values obtained for this

spectrum are within the small period range. The ordi-nates of Type-1 and Type-2 are constant after periodT D , whereas there is not a lower bound constant valuefor the other codes.

Displacement response spectra

Sözen and his associates developed the substructureconcept that enabled the use of an elastic displacementspectrum in design by using the displacement capac-ity of an inelastic system in 1970s (G¨ ulkan and S¨ozen,1974). But force-based seismic design remains, in spiteof its shortcomings, the method most widely used incodes up to recent years. However, the recognition of the poor correlation between transient inertial forces

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Fig. 3 Inelastic design spectra drawn for ground types described in TEC, UBC and EC 8 for a reference peak ground acceleration of 4m/s 2

induced by earthquake shaking and damage to struc-tures has led to the development of displacement-basedapproaches, which utilize displacement spectra. Therst and most obvious way to obtain elastic displace-ment spectra ( S De ) for design would be to convert theelastic design spectrum of absolute acceleration ( S e )dened in the code, via the pseudo-spectral relation-ship:

S De (T ) =S e(T )

ω2 (1)

The resulting displacement spectra obtained in thisway are used inmorethan20 seismic designcodes fromaround the world (Bommer et al., 2001). The transfor-mation of the acceleration spectra in current seismiccodes to displacement spectra will generally not pro-duce reliable displacement ordinates at the longer pe-riods relevant to displacement-based design (BommerandElnashai, 1999; Faccioli et al., 2004).This transfor-mation shall normally be applied for vibration periodsnot exceeding 4seconds. For structures with vibrationperiods greater than 4 sec, a more complete denitionof the elastic spectrum is presented in EC8. There iscurrently a signicant level of disagreement regarding

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appropriate values for the control periods of the dis-placement spectra (Bommer and Mendis, 2005).

Faccioli et al. (1998) derived relationship betweendampingandductilityfordisplacement spectra depend-ing on European earthquakes and Borzi et al. (2001)described derivation of inelastic displacement spectra

for displacement-based design in detail. In seismic de-sign codes such as EC8, the displacement spectra fordamping ratios other than 5% are obtained by apply-ing scaling factor to the ordinates of the 5% dampedspectrum as below;

η = 105 + ξ (2)where ξ is the viscous damping ratio of the structure,expressed as a percentage. Bommer and Mendis (2005)have recently reviewed several different proposals forthese scaling factors. In the EC8, the displacementspectra were dened for various damping ratios. Sincesuch displacement spectra are not included in the TEC,it isnotpossible tomake a comparison for these spectra.

Structural data

Sample buildings describedherein wereselectedas typ-ical (not a template project) 6 and 12 story reinforcedconcrete buildings. The buildings have three differentoor plans that are symmetric (SB), monosymmetric(MB), and unsymmetric (UB). Six buildings are con-sidered and they are henceforth referred to as; 6-SB,6-MB, 6-UB, 12-SB, 12-MB and 12-UB. The plan di-mensions of buildings, typical at all oors, are 22.7 mby 13.75 m, with a story height of 3 m (Fig. 4). Thestructural systems of thebuildings are selected as struc-tural systems consisting of structural walls andmoment

resisting frames in both directions. It is assumed thatthe structural systems have nominal ductility level. Inthis case, the value of 4 is recommended by TEC forstructural behaviour factor ( R). Seismic load reductionfactor ( Ra ) can be determined in terms of R accordingto TEC. If natural vibration period ( T ) is smaller orequal to the lower limit of the period of the constantspectral acceleration branch ( T B), Ra will be equal to1.5 + ( R-1.5) T/T B . If T is greater than T B , Ra will beequal to R. As the fundamental periods obtained for

sample buildings considered in this study are greaterthan T B, Ra is taken equal to R.

Columns, beams, structural walls andslabs are sizedconsidering the requirements given in TEC. The di-mensions of columns and structural walls for x and ydirections, the thickness of slabs, the width and height

of beams are given in Table 4. As seen from this table,the cross-sections of columns have been changed afterthe 3rd story for 6-story buildings, and changed afterthe 4th and 8th story for 12-story buildings.

Flexural rigidities for longitudinal and transverse di-rections are different for each building. Total momentsof inertia of vertical structural elements can be deter-mined using dimensions given in Table 4 for x and ydirections. It shouldbe noted that values used for rigidi-tiesaregrossvaluesandtheyarenotreducedtoconsidercracking. Minimum and maximum values of Torsional

Irregularity Factors ( ηb ) (which are dened for any of the two orthogonal earthquake directions as the ratioof the maximum storey drift at any story to the averagestory drift at the same story in the same direction) forsample buildings are also estimated. This factor reachas its maximum values as; 1.12 for 6-SB, 1.55 for 6-MB, 1,39 for 6-UB, 1,22 for 12-SB, 1.34 for 12-MBand 1,89 for 12-UB. According to TEC, torsional irreg-ularity occurs in buildings when ηb is greater than 1.2.No other structural irregularities occurred for samplebuildings.

Finite element modeling of buildings andanalysis results

To evaluate the seismic response of the buildings, elas-tic analyses were performed by the response spectrummethod using the computer program SAP2000 (2003).The seismic analyses of the buildings are carried outseparately in the longitudinal and the transverse direc-tions. However, seismic responses only for y direction

are comparatively presented with graphs and tables inthis paper for the sake of brevity. Sample nite elementmodels of the six and twelve story buildings are showninFig.5. Degrees offreedom atthe basenodes are xed,for other nodes are left free. Therefore, there is no niteelement model for subsoil to consider soil-structure in-teraction. Columns andbeams are modelled with frameelements, slabs and structural walls are modelled withshell elements. Slabs also have been considered as arigid diaphragmin each story level. Themasses of inll

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Fig. 4 Floor plan for sixand twelve story buildings(lengths of spans are in m)

walls are also taken into account in the model. In theanalysis, Young’s modulus and unit weight of concreteare taken to be 28000 MPa and 25 kN/m 3 , respectively.The damping ratio is assumed as 5% in all modes.

The reference peak ground acceleration is taken tobe 0.4 g that is recommended as seismic zone 1 in TECand 4 in UBC. Thus, it is assumed that the buildingsare sited in high seismicity zone. Seismic analysis of the buildings accounting for the inuence of the lo-cal ground conditions is carried out with the help of the design spectra given in Fig. 3 for TEC, UBC andEC 8.

Periods for the analyzed buildings

The mode numbers taken into account for six andtwelve-story buildings are 10 and 20, respectively. Therst seven or eightmodeswith periods andparticipatingmass ratios for the buildings are presented in Table 5.As seen from this table, the fundamental periods are in

the range between 0.463 s and 1.043 s. In the rst modethe 6-SB, 6-MB, 6-UB and 12-MB vibrate dominantlyin the y direction; whereas 12-SB and 12-UB vibrate inthe x direction. The third mode takes place as torsionalmodes for all buildings considered.

Base shears for the analyzed buildings

The base shear expressions dened in the codes are

given in Table 6. The base shears of the buildings wereacquired from seismic analysis using the design spec-tra corresponding to 5% critical damping and consider-ing xed base condition. Seismic analysis of buildingswere carried out for four ground types dened in TEC,ve ground types in UBC and in EC8. Therefore, four-teen ground types in total are considered for the site.Figs. 6 and 7 present the base shears and maximumdifferences obtained for 6 and 12-story buildings, re-spectively.

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Table 5 First seven/eight periods (s) and modal properties for six different buildings considered

Horizontal modes for the buildings

Buildings x -direction y-direction Torsional mode

6-SB Mode, period 2nd, 0.440 5th, 0.126 – 1st, 0.463 4th, 0.139 7th, 0.068 3rd, 0.371 6th 0.111Mass ratio 0.000 0.000 0.748 0.135 0.053 0.000 0.000

6-MB Mode, period 2nd, 0.418 5th, 0.119 – 1st, 0.546 4th, 0.175 7th, 0.093 3rd, 0.388 6th, 0.117Mass ratio 0.000 0.003 0.658 0.103 0.040 0.104 0.021

6-UB Mode, period 2nd, 0.430 5th, 0.123 – 1st, 0.493 4th, 0.151 7th, 0.076 3rd, 0.382 6th, 0.115Mass ratio 0.004 0.001 0.714 0.123 0.048 0.035 0.081

12-SB Mode, period 1st, 0.871 4th, 0.298 7th, 0.156 2nd, 0.825 5th, 0.226 8th, 0.100 3rd, 0.622 6th, 0.174Mass ratio 0.000 0.000 0.000 0.687 0.157 0.000 0.000 0.000

12-MB Mode, period 2nd, 0.769 5th, 0.229 8th, 0.115 1st, 1.043 4th, 0.348 6th, 0.182 3rd, 0.534 7th, 0.157Mass ratio 0.000 0.000 0.021 0.543 0.091 0.046 0.188 0.034

12-UB Mode, period 1st, 0.924 4th, 0.273 7th, 0.130 2nd, 0.813 5th, 0.254 8th, 0.117 3rd, 0.648 6th, 0.180Mass ratio 0.013 0.049 0.016 0.634 0.088 0.016 0.022 0.061

Table 6 Base shear denedin the TEC, UBC and EC8 Codes Base shear

TEC V t = S d (T ) · W g ≥ 0.1 Ao · γ I · W

UBC V s = S d (T ) W g ≥0.11 C a · γ I · W 0.8 Z N v γ I

R W (for zone4 )2.5C a γ I

R W ≥ V s

T = min ( T a , 1.3T e ) for zone 4

EC8 F b = S d (T ) · W g · λ where λ = 0.85 if T 1 ≤ 2 T C or λ = 1.00 otherwise

Note : The effective modal mass mk ,

corresponding to a mode k , is determined so that

the base shear force F bk , acting in the direction of

application of the seismic action, may be expressedas F bk = S d (T k ) mk

Note . For S d (T ) given inthis table, differentexpressions introduced inTable 4 depending on soil

and structure propertiesincluding structuralbehaviour factor

is no a dominant ground type like D in TEC and UBC.TEC presents the maximum base shear for ground typeof Z4 and maximum difference reaches 128% betweenZ4 and Z1 for 12-UB. UBC gives the maximum baseshear for ground type S E and maximum differencereaches 146%between S E and S A for 12-UB.EC8givesthe maximum base shear for ground type D and maxi-mum difference reaches 154% between classes D andA for 12-UB.

The story number of story in which the maximumshear force occurred was investigated. Maximum shearforce occurs at the 1st story for 6-SB and 6-MB, 12-SBand 12-UB, whereas it occurs at the 4th story for 6-UBand 12-MB.

As seen from Figs. 6 and 7, very different base shearvalues were obtained for different ground types and

building structural systems. Although all ground typesdened in the codes are included in this study, a generalconclusion cannot be reached due to the limited num-ber (6) of buildings. For a general conclusion, morebuildings having different structural systems and pe-riod values should be investigated taking into accountvarious design ground acceleration. Even now, it maybe said mainly two things for considered buildings:Firstly differences for maximum base shears are ten-dency to increase for unsymmetrical buildings. Sec-ondly when the number of story increase, in this casemass of the structure is also increase, the differencesbetween base shears obtained for various ground typesbecome larger. These increases for six to twelve storybuildings are meanly 69% for TEC, 60% for UBC and75% for EC 8.

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Fig. 6 Base shears for 6-story buildings considering fourteen ground types dened in the codes

Fig. 7 Base shears for 12-story buildings considering fourteen ground types dened in the codes

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Table 7 Maximum and minimum displacements obtained for the buildings considering each code

TEC UBC EC8

Buildings Min. Max. % Min. Max. % Min. Max. %

6-SB Displacement 13 mm 18 mm 38 12 mm 20 mm 67 16 mm 25 mm 119Ground type Z1 Z4 S A S D A E

6-MB Displacement 21 mm 33 mm 57 19 mm 36 mm 89 24 mm 43 mm 79Ground type Z1 Z4 S A S D A D

6-UB Displacement 17 mm 26 mm 53 17 mm 28 mm 65 21 mm 35 mm 67Ground type Z1 Z4 S A S D A E

12-SB Displacement 28 mm 62 mm 121 24 mm 55 mm 129 30 mm 80 mm 167Ground type Z1 Z4 S A S E A D

12-MB Displacement 47 mm 113 mm 140 39 mm 112 mm 187 48 mm 129 mm 169Ground type Z1 Z4 S A S E A D

12-UB Displacement 40 mm 96 mm 140 33 mm 86 mm 161 42 mm 112 mm 167Ground type Z1 Z4 S A S E A D

withina story, ( i )max , forcolumns andstructural wallsof the i th story of a building for each earthquake di-rection satises the conditions given by ( i )max / hi ≤0.0035 (dened in the TEC) for the 6-story building.However, as seen from Fig. 9, the maximum valuesof the story drift exceed the condition for 12-MB sup-ported on ground types of D, Z4, S E , and for 12-UBsupported on ground type of D.

All maximum and minimum displacement valuesdetermined for each code are given in Table 7. As seen

from this table, the smallest differences between maxi-mum andminimum displacementvalues for the6-storybuildings are obtained as to be 38% between Z1 andZ4 in TEC, 65% between S A and S D in UBC and 67%between A and E in EC8. The largest differences be-tweenmaximumandminimumdisplacementvalues forthe 12-story buildings are obtained as to be 140% be-tween Z1 and Z4 in TEC, 187% between S A and S E inUBC and 169% between A and D in EC8. It should benoted here that when the soil gets softer, as mentionedabove, the lateral displacements are increase.

Conclusions

The differences among the code dened response spec-tra may be summarized as: (a) The near source factorsare considered only in the UBC. Such near source fac-tors are not dened in EC8 and TEC. EC8 has a foot-note such that the Type 2 Spectrum is adopted when asurface-wave magnitude is notgreater than 5.5. (b) The

ordinate value of design spectra increases with T forUBC, decreases for the TEC and EC8 for small valuesof vibration period ( T < T B). (c) The TEC species thesame peak values for all ground types, whereas UBCand EC8 specify different peak values. (d) EC8 spec-ies the values of the maximum allowable behaviourfactor depending on type of structural system, regu-larity in elevation and prevailing failure mode in thesystem with walls, whereas TEC species periods forstructure and ground class ( T and T B) dependent values

of behaviour factor in addition to structural system. (e)Although all domains of the response spectrum are de-ned differently in the EC8, the constant displacementand constant velocity domains are not dened differ-ently in UBC and TEC.

There are signicant differences between the threecodes (TEC, UBC, EC8) and IBC&FEMA seismic de-sign provisions. The biggest change related to the de-sign spectra from the codes to the IBC is in the designground motion parameters, now S DS and S D1 , ratherthan seismic zone factor.

EC8presentsan annexforelastic displacementspec-trum for periods of long vibration period. As the cur-rent trend in seismic design is displacement-based, it isexpected that the displacement design spectra and fordifferent peak values of the separate ground types arealso included in the new versions of the TEC.

For thebuildings, EC8gives themaximum andUBCtheminimumdisplacement values.EC8 generallygivesthe larger base shear for similar ground types denedin the other codes. The maximum base shears occurred

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for ground types of D or E dened in EC8. The numberof the story in which maximum shear force is occurringchanges depending on the ground types.

Some engineers share the view that internal forcesdecrease and lateral displacements increase from therst ground type to the last ground type code dened,

that is, as if the groundbecomes softer. Thisviewis ver-ied in the analysis carried out considering all groundtypes and the requirements dened in TEC, while itis valid only for the rst two ground types dened inEC8 and UBC when analyzing the sample buildings.However, the view loses its meaning especially for thelast two ground types given in EC8 and UBC, becauselarger internal forces are obtained for the 4th groundtypes (D in EC8, S D in UBC) than that for the 5thground type (E in EC8, S E in UBC). Therefore, thismay lead to some mistakes for design for soft soils. It

should be noted that ground class E does not alwaysidentify a soil prole softer than class D.

Acknowledgement Authors wish to tank Ezio Faccioli for hishelpful comments and suggestions on earlier versions of thismanuscript.

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