Comparison of code specific nonlinear seismic performance

Comparison of code specificnonlinear seismic performance&1 Amir Kiani PhD

Project Scientist, International Institute of Earthquake Engineering andSeismology, Tehran, Iran

&2 Salar Manie PhDAssistant Professor, Department of Civil Engineering, Islamic AzadUniversity, Sanandaj Branch, Sanandaj, Iran

&3 Babak Mansouri PhDAssistant Professor, Head of Emergency Management Dept.,International Institute of Earthquake Engineering and Seismology,Tehran, Iran (Corresponding author: [email protected])

1 2 3

This paper reports a comparative study conducted on the non-linear performance of steel moment-resisting frame

structures designed with the Iranian, European and Japanese seismic design codes of practice. To this end, typical

three-, six-, nine- and 12-storey steel moment-resisting frames of the high-ductility category are designed based on

the requirements adopted in the codes. The structural performance of each frame is then evaluated extensively based

on Fema 356 provisions for life safety performance level utilising non-linear static procedure. Comparisons are made

regarding the elastic stiffness, peak strength, target displacement, plastic hinge rotations of elements at the target

displacement, sequences of hinging in various elements, global lateral response, as well as the total amount of

materials used in each structure. Results reveal differences in global and local performances of the structures designed

according to regulations relevant to each code. Moreover, in some cases, the requirements for life safety performance

level are violated. Finally, recommendations are provided based on the results of the study.

NotationA peak ground acceleration (PGA) normalised by

acceleration of gravity in IRC2800Ai distribution factor in BCJag,R reference PGA in Eurocode 8B 5% damped elastic pseudo-acceleration design

spectrum in IRC2800C storey shear factor in IRC2800Ci storey shear factor at ith storey in BCJC0 modification factor to relate spectral displacement

of an equivalent single-degree-of-freedom (SDOF)system

C0 shear factor for level 2 intensity in BCJC1 modification factor to relate expected maximum

inelastic displacementC2 modification factor to represent effect of pinched

hysteretic shape, stiffness degradation and strengthdeterioration

C3 modification factor to represent increaseddisplacements due to dynamic P–Δ effects

DS structural characteristic factor in BCJFb seismic base shear force in Eurocode 8Fb total design base shear in Eurocode 8

Fes,i shape factor in BCJFi design lateral force for ith storey in IRC2800Fi distribution of horizontal seismic forces in

Eurocode 8Ft whipping force in IRC2800Fy lower-bound strength valueFye upper-bound strength valueH total height of buildingHi depth of shear wave velocity of ith soil layerhi height of ith storey measured from base in IRC2800hi thickness of ith soil layerKe effective lateral stiffness of buildingKi elastic lateral stiffness of buildingL number of soil layersm total mass above top of basement in Eurocode 8N number of storeysR response modification factor (behaviour factor)

dependent on structural system and prescribed levelof design ductility in IRC2800

Rt design spectrum in BCJRt spectral ordinate at building fundamental period

in BCJS soil modification factor

1

Structures and Buildings

Comparison of code specific nonlinearseismic performanceKiani, Manie and Mansouri

Proceedings of the Institution of Civil Engineers

http://dx.doi.org/10.1680/jstbu.14.00028Paper 1400028Received 09/03/2014 Accepted 09/09/2015Keywords: buildings, structures & design/codes of practice &standards/steel structures

ICE Publishing: All rights reserved

mailto:[email protected]

Sa response spectrum accelerationSd(T1) ordinate of design spectrum at period T1 in

Eurocode 8Se 5% damped elastic pseudo-acceleration design

spectrum in Eurocode 8T, T1 building natural periodT0, Ts corner periods dependent on soil type in IRC2800TB, TC, TD corner periods of the spectrum depending on soil

type in Eurocode 8Te effective fundamental periodTg fundamental period at foundation soilTi elastic fundamental periodTJ corner period determined according to soil type

in BCJV static design base shear in IRC2800Vi elastic shear force demand in BCJVi shear wave velocity of ith soil layerVi strong seismic shear (level 2) of ith level in BCJVrequired,i design (required) storey shear in BCJVS,30 average velocity of shear waves in the top 30 m

of soilW effective weight of structure in IRC2800Wi effective weight of ith storey in IRC2800wj effective (seismic) weight of jth floor in BCJZ seismic zone factor in BCJzi height of mass above level of application of seismic

action in Eurocode 8β equivalent viscous dampingβ lower bound factor for horizontal design spectrum

in Eurocode 8β0 hysteretic dampingδ target displacementγI importance factor in Eurocode 8η damping correction factor for different values of

viscous damping in Eurocode 8κ knowledge factorλ correction factor in Eurocode 8

1. IntroductionIn past decades, various seismic codes of practice have beendeveloped in earthquake-prone regions of the world. A normalpart of the design process in such regions is to ensure that cer-tain requirements of the adopted seismic codes are met. Earlycodes appeared primarily based on observations and lessonslearned from earthquakes (Booth and Key, 2014). For instance,following destructive earthquakes in Italy (Messina in 1909),Japan (Tokyo in 1923), USA (Long Beach in 1933) (Boothand Key, 2014) and Iran (Booein-Zahra in 1962) (BHRC,2015), early seismic design regulations evolved in thosecountries as part of structural design requirements. Since theemergence of early regulations, the codes have experienced sub-stantial and continuous changes as more research findings havebecome available in the field. Such a trend promises theimprovement of the building design process, which eventuallyresults in enhancing public safety.

The USA, Japan, New Zealand and Europe, among others,have made significant contributions to the evolution ofmodern seismic design rules during past decades (e.g. Boothand Key, 2014; Bozorgnia and Bertero, 2004). However, thereare differences among these model codes regarding the funda-mental design concepts and philosophies, and the adoptedanalysis procedures.

The Iranian seismic code, referred to hereinafter as IRC2800,was first introduced in 1987. Recently, the fourth edition ofIRC2800 was released (BHRC, 2015). The provisions adoptedin the latest edition are very similar to those adopted in theUS seismic design code (ASCE, 2010). In this study, the thirdedition (BHRC, 2005) is considered. Seismic analysis anddesign requirements of the third edition of IRC2800 are verysimilar to those of the UBC-97 (ICBO, 1997) code.

The European design code of practice, referred to hereinafteras Eurocode 8 (CEN, 2003), covers a wide variety of practicaldesign issues such as requirements for structures with irregula-rities in height and/or plan, structures with masonry infills,and so on.

The Japanese seismic design code, referred to hereinafteras BCJ (BCJ, 1997), was adopted in 1981 (Aoyama, 1981).Prior to 1981, seismic design of structures was based on auniform and vertical lateral load on the structure with the totalload equal to 20% of the sum of dead and live loads imposedon the structure (Tremblay et al., 1996). In 2001, new pro-visions were implemented in the code (BSL, 2000). In compari-son with the older 1981 edition, the 2001 edition of BCJadopts a revised procedure for the determination of designseismic forces.

Steel buildings in Iran and Japan have been affected by twolarge earthquakes, namely, the Mw 7·4 (Manjil, Iran, 1990)and the Mw 7·2 (Kobe, Japan, 1995). The Kobe earthquakewas the first event to cause widespread damage to steel build-ings (Tremblay et al., 1996). Also, many buildings designedwith BCJ (BCJ, 1997) have experienced the large Tohoku 2011(Mw 9·0) earthquake.

Comparative study of different design codes of practice is cer-tainly beneficial in improving our knowledge in the field.Comparisons may deal with the adopted intensities of groundshaking, backgrounds to the general design concepts andphilosophies, adopted analysis and design procedures, amongother features. Such studies will certainly enable both the engi-neering community and academia to obtain a better insightinto issues relevant to structural seismic design, which even-tually will lead to the development of more robust and efficientcodes of practice.

The present paper is the result of a comparative studyof seismic performance of different steel frame designs with

2

Structures and Buildings Comparison of code specific nonlinearseismic performanceKiani, Manie and Mansouri

Offprint provided courtesy of www.icevirtuallibrary.comAuthor copy for personal use, not for distribution

three model codes, namely, the IRC2800, Eurocode 8 andBCJ. For the purpose of this study, the performance of eachdesigned frame was then verified utilising performance evalu-ation procedures according to Fema 356 (Fema, 2002) andATC 40 (ATC, 1996) guidelines. Comparisons have been madebased on the performance of all frames in terms of overalllateral response, yield displacements, pre- and post-peak re-sponses and sequences of elements’ plastic hinge formation.Also, the overall performance of the structures was evaluatedaccording to the provisions adopted in Fema 356 for thedesired ground motion intensity. Results generally reveal sig-nificant differences in seismic performance of the consideredmodels. Discussions will be provided for various aspects ofstructural seismic performance under strong ground motions.

2. MethodologyFor the purposes of this study, a set of two-dimensional steelmoment-resisting frames composed of three-, six-, nine- and12-storey residential buildings designed according to the speci-fications outlined in IRC2800, Eurocode 8 and BCJ (BCJ,1997) seismic design codes for high-ductility steel momentframes were considered. Details of the models and theirloading and design characteristics will be provided in Section2.5. As will be discussed in the following sections, all modelshave been analysed and designed under similar conditions interms of gravity loading, seismic intensity, soil type and thestructural design method. In the following sections, the deter-mination of seismic design forces relevant to the three codes isreviewed briefly.

2.1 Ground motion intensitiesAll the three codes define two design load levels. Generally,one is defined for the serviceability conditions under moderateground shaking and the other is for critical conditions understrong seismic attacks. In IRC2800, structures are checkedfor peak ground acceleration (PGA) ranging between 0·033gto 0·058g for the serviceability level corresponding to 99·5%probability of exceedance in 50 years and 0·2g to 0·35g forthe strong seismic demand corresponding to 10% probabilityof exceedance in 50 years. In Eurocode 8, a two-tier seismicdemand, namely the ‘no-collapse’ and ‘damage limitation’, arechecked for structures based on two different reference PGAvalues (denoted by agR) corresponding to (as recommendedtherein) 10% probability of exceedance in 50 years and 10%probability of exceedance in 10 years, respectively. The refer-ence PGA (agR) for use in a country or parts of the countryin the Euro zone are derived from zonation maps found in theNational Annex specific to that country. In BCJ, level I seismicdesign load is applied for small to moderate ground shakingscorresponding to 50% probability of exceedance in 30 yearswith PGA ranging from 0·8 m/s2 to 1·0 m/s2, whereas thelevel II design load is considered as a strong shakingcorresponding to 10% probability of exceedance in50 years with PGA ranging from 3·0 m/s2 to 4·0 m/s2. Basedon these acceleration values, BCJ stipulates maximum design

base shear for PGA ranging from 0·2 g to 1·0 g. It appearsthat the IRC2800 adopts the lowest and the Eurocode 8 adoptsthe highest level of ground shaking for the serviceabilitylimit state.

The reference PGA was selected to be 0·4g for this study. Thisvalue is a reference value for most regions of Japan as well asfor some regions of high seismicity in Europe. Althoughthe maximum PGA in IRC2800 is 0·35g for regions with highseismicity rates, available site-specific seismic hazard analysesshow that for many sites located in such regions (for examplethe capital city of Tehran) a PGA value equal to 0·4g may bemore rational (Mansouri et al., 2014). Thus, it is believed thatthis value could represent a strong ground motion intensityappropriate for seismic design for areas of high seismicity inIran, the Euro zone and Japan.

2.2 Soil typesThe IRC2800 defines four soil type (types I to IV from veryhard rock to soft soil profiles), while Eurocode 8 and BCJemploy five (types A to E from rock formations to soft soilprofiles) and three (type I to III from hard to soft profiles) soiltypes, respectively. In IRC2800 and Eurocode 8, soil types aredefined primarily on the basis of average shear wave velocity insoil layers to the depth of 30 m beneath the foundation level(VS,30) as calculated using the following equation

1: VS;30 ¼ LPni¼1ðhi=ViÞ

in which L is the number of soil layers and hi and Vi denotesthe thickness and the shear wave velocity of the ith layer,respectively.

The soil classification in BCJ is made based on soil typesdefined according to the fundamental period at the foundationsoil (Tg) calculated as in Equation 2 below. The comparison ofsoil types in these three codes is tabulated in Table 1. Notethat in order to be able to make comparison, a soil layer of30-m thickness over a rock formation has been assumed. Forcomparing soil types in Eurocode 8 and IRC2800 with thosein BCJ, the corresponding Tg as calculated by Equation 2 withthe same Vi in the two former codes is considered (Marinoet al., 2005). The calculated Tg values have been representedfor each soil type in separate rows, each relevant to one code

2: Tg ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi32

XLi¼1

hiðHi�1 þHi=2ÞV2

i

vuut

where L denotes the number of soil layers between the base ofthe foundation and the rock soil, and hi, Hi and Vi are thethickness, depth and shear wave velocity of the ith soil layer,respectively.

3



For this study, a medium-dense soil layer on rock basementhas been assumed. Thus, the soil is classified as type II, type Cand type III according to IRC2800, Eurocode 8 and BCJclassification criteria, respectively.

2.3 Response spectraAs mentioned previously, the three model codes specify elasticdesign spectra for seismic demand determination.

In IRC2800, 5% damped elastic pseudo-acceleration designspectrum (normalised by the acceleration of gravity, g) isdefined as a function of the building natural period ofvibration (T ) and soil type. The code recognises three distinctregions in the spectrum, ordinates of which are calculatedusing Equation 3 below

3a: 0 � T � T0 B ¼ 1þ STT0

� �

3b: T0 � T � Ts B ¼ S þ 1

3c: T � Ts B ¼ ðS þ 1Þ Ts

T

� �2=3

where the corner periods T0 and Ts are dependent on the soiltype. T0 equals 0·1 for soil types I and II, and equals 0·15 forsoil types III and IV. Ts equals 0·4, 0·5, 0·7 and 1·0 for soiltypes I, II, III and IV, respectively. Also, the S parameter isdependent on the soil type as well as the level of seismic riskspecific to the region of interest. For regions with high or veryhigh rates of seismicity (areas with PGA values more than0·3g), the parameter equals 1·5 for soil types I and II, andequals 1·75 for soil types III and IV. Recall from the previoussection that, in this study, the soil type is considered as type IIIas per the IRC2800 classification criteria.

Eurocode 8 defines basic elastic spectrum Se (in the form ofthe pseudo-acceleration normalised by the reference PGA forground type ‘A’ determined in the National Annexes accordingto results of seismic hazard analyses, ag, R) as a function ofthe building natural period (T ) using Equation 4

4a: 0 � T � TB Se ¼ γIag;RS 1þ TTB

ð2�5η� 1Þ� �

4b: TB � T � TC Se ¼ 2�5γIag;RSη

Cod

eSo

iltype

IRC28

00I

IIIII

IVVs,30

Vs≥75

037

5≤Vs≤75

017

5≤Vs≤37

517

5≥Vs

Equivalent

T gpe

rBC

J0·16

0·32

0·32

0·69

Eurocode

8A

BC

DE

Vs,30

Vs≥80

036

0≤Vs≤80

018

0≤Vs≤36

018

0≥Vs

alluvium

layerwith

Vsvalues

oftype

Cor

Dan

dthickn

essvaryingbe

tweenab

out5m

and20

m,

unde

rlain

bystiffer

materialw

ithVs>80

0m/s

Equivalent

T gpe

rBC

J0·15

0·33

0·33

0·67

III

IIIBC

JVs,30

Vs≥60

015

0≤Vs≤60

015

0≤Vs

—

Equivalent

T gpe

rBC

J0·20

0·20

0·75

Table

1.Classificatio

nof

soils

accordingto

IRC28

00,Eu

rocode

8

andBC

J

4



4c: TC � T � TD Se ¼ 2�5γIag;RSηTC

T

4d: TD � T � 4�0 Se ¼ 2�5γIag;RSηTCTD

T2

In Equations 4a–4d, γI is the importance factor equal to 1·0for ordinary class (residential) buildings considered in thisstudy and ag,R is the reference PGA for ground type ‘A’ asdefined above. As stated previously, for this study, ag,R wasassumed to be 0·4g. Also, η is the damping correction factorfor different values of viscous damping as a function of T, S isthe soil amplification factor, and TB, TC and TD are the cornerperiods of the spectrum depending on the soil type. Forground type ‘C’ considered in this study, S, TB, TC and TD are1·15, 0·2, 0·6 and 2·0, respectively. Moreover, the design spec-trum of Eurocode 8 was calculated assuming 5% damping. Inmost seismic design practices, a 5% damped response spectrumis typically utilised (Bozorgnia and Bertero, 2004).

In BCJ, the following equations are utilised for calculating thedesign spectrum for different ranges of period of vibration.The design spectrum values (Rt) normalised to the accelerationof gravity are calculated as

5a: 0 � T � TJ Rt ¼ 1

5b: TJ � T � 2TJ Rt ¼ 1� 0�2 TTJ

� 1� �2

5c: 2TJ , T Rt ¼ 1�6TJ

T

where the TJ is a corner period determined according to thesoil type and is equal to 0·4, 0·6 and 0·8 s for soil types I, IIand III, respectively.

Figure 1 depicts the design spectra of the three codes for thesoil type and other conditions assumed in this study. It isobserved that the spectral ordinates in the short periods (sayT < 0·6 s) are close in all the three codes. However, in themedium and long period ranges, significant differences areevident between the Eurocode 8 and the other two codes.

2.4 Calculation of storey design forcesUnder certain conditions, all the three codes allow the use ofthree linear seismic analysis procedures: (a) equivalent static;

(b) modal response spectrum; and (c) dynamic time history.Since all models are low rise and regular in height frames (interms of both the stiffness and strength), the equivalent staticprocedure is generally allowed in all the three codes. Thus, onlythis method is reviewed briefly as adopted in the three codes.

In IRC2800, the ‘static design base shear’ is calculated usingEquation 6a

6a: V ¼ CW

in Equation 6a, C is calculated from Equation 6b and W is theeffective weight of the structure composed of total dead loadof the structure and a portion of the live load (equal to 20%for the residential buildings considered in this study)

6b: C ¼ ABIR

where A is the PGA normalised by the acceleration of gravity(g) derived from seismic hazard maps (set to be 0·4 in thisstudy); B is the ordinates of the design spectrum as calculatedusing Equation 6 above; I is the important factor (equal to 1·0for residential buildings considered in this study) and R is theresponse modification factor (behaviour factor) dependent onthe structural system and a prescribed level of design ductility(equal to 10 at allowable stress level for special moment framesin IRC2800). Upon calculating the total base shear (V ) fromEquation 6a, storey forces can be calculated using Equation 6cbelow

6c: Fi ¼ WihiPnj¼1 Wjhj

ðV � FtÞ

where Fi is the design lateral force for the ith storey, Wi is theeffective weight of the ith storey and hi is the height of the ith

00

0·2

0·4

0·6

0·8

1·0

1·2

1·4

IRC2800 (Soil type II)

Eurocode 8 (Soil type C)

BCJ (Soil type III)

1 2

Period: s

B, S

e, R

t: /g

3 4

Figure 1. Elastic response spectra according to IRC2800,

Eurocode 8 and BCJ

5



storey measured from the base. Also, Ft is the whipping force,which equals 0 for structures with fundamental period ofvibration (T ) under 0·7 s and equals 0·07 TV for structures withT > 0·7 s. The code applies the whipping force in order to incor-porate the effects of higher modes in the static loading pro-cedure. For steel moment frames, T is calculated by the formula:T=0·08H3/4 (H is the total height of the building in metres). Italso can be calculated using analytical procedures but shall notbe 1·25 times the value calculated by the above formulae.

In Eurocode 8, the seismic base shear force (Fb) is determinedby Equation 7a

7a: Fb ¼ SdðT1Þmλ

In Equation 7a, m is the total mass above the top of the base-ment, T1 is the fundamental period, Sd(T1) is the ordinate of thedesign spectrum at the period T1, as calculated from Equations7b–7e incorporating inelastic response reduction by way of thebehaviour factor (q), which equals 6·5 for steel moment-resistingframes of ‘ductility class high’ (DCH), and finally λ is a correc-tion factor, the value of which is 0·85 if T1≤ 2Tc and the buildinghas more than two storeys, or 1·0 otherwise. All other par-ameters have been defined in previous paragraphs

7b: 0 � T � TB Sd ¼ γIag;RS23þ TTB

2�5ηq

� 23

� ��

7c: TB � T � TC Sd ¼ 2�5γIag;RSη=q

7d: TC � T � TD Sd ¼ 2�5γIag;RSηðTc=TÞq

� βγIag;R

7e: TD � T Sd ¼ 2�5γIag;RSηðTCTD=T2Þq

� βγIag;R

In the above equations, β is the lower bound factor for thehorizontal design spectrum, which can be found in NationalAnnexes, with recommended value of 0·2.

In Eurocode 8, the fundamental period T may be calculatedby the formulae T = CtH

3/4 (Ct is 0·085 for moment-resistantspace steel frames and H is the total height of the buildingfrom the foundation or from the top of a rigid basement inmetres). It also can be calculated using other formulas rep-resented in the code. In this study, T has been calculated basedon the above formula.

Eurocode 8 stipulated the distribution of the horizontal seismicforces in regular structures according to Equation 7f below

7f: Fi ¼ FbzimiPnj¼1 zjmj

In Equation 7f, Fb is the total design base shear as determinedfrom Equation 7a, zi is the height of the mass mi above thelevel of application of the seismic action (foundation or top ofa rigid basement).

In BCJ seismic design code, strong seismic shear (level 2) ofthe ith level is calculated using Equation 8a in the elastic range

8a: Vi ¼ Ci

XNj¼1

wj

where Ci is the level 2 storey shear factor at the ith storey, wj isthe effective (seismic) weight of the jth floor and N is thenumber of storeys. The storey shear factor (Ci) is calculatedusing Equation 8b below

8b: Ci ¼ ZRtAiC0

in which Z is the seismic zone factor equal to 1·0 for highseismic zones (the case assumed in this study), Rt is the spec-tral ordinate at the fundamental period of vibration of thestructure (T1), Ai is the high distribution factor and finally C0

is the shear factor for the level 2 intensity equal to 1·0 asrequired by BCJ. The distribution factor Aiwhich takes intoaccount the higher mode effects is given by Equation 8c

8c: Ai ¼ 1þ 1ffiffiffiffiαi

p � αi

� �2T1

1þ 3T1

The αi parameter is calculated as

8d: αi ¼XNj¼i

wj

W

where W is the total weight of the buildings. In Figure 2, vari-ation of Ai with αi is depicted for six values of fundamentalperiods of vibration.

Design (required) storey shear is then calculated usingEquation 8e below

8e: Vrequired;i ¼ DSFes;iVi

where DS is the structural characteristic factor (or inverse ofthe behaviour factor as in IRC2800 or Eurocode 8) equal to

6



0·25 for ductile moment-resisting frames (FA frames accordingto the BCJ classification and the case considered in this study),Fes,i is the shape factor, which is set dependent on the storeystiffness and eccentricity of the plan (equal to 1·0 for two-dimensional frames with no irregularity in height), and Vi isthe elastic shear force demand as calculated by Equation 8a.

In Figure 3, behaviour factors of the frames (R-value inIRC2800, q-value in Eurocode 8, 1/DS in BCJ) are compared.Note that in the third edition of IRC2800, the R-values havebeen provided at allowable stress level. In order for the allow-able stress level behaviour factor of IRC2800 to be comparablewith the ultimate stress level factors of Eurocode 8 and BCJ,the original behaviour factor of 10 for moment-resistingframes in IRC2800 has been multiplied by 0·7 (i.e. it is equalto 7). It is observed that IRC2800 allows a larger reduction of

the elastic seismic forces in comparison to Eurocode 8 andBCJ for steel moment-resisting frames of the high-ductilityclass.

2.5 Storey drift requirementsAll the IRC2800, Eurocode 8 and BCJ codes stipulate that themaximum inter-storey drift value of any building be limited to0·5% for ‘damage control’ level under moderate earthquakes incases where brittle masonry infills are used in construction.In cases of ductile infill walls, the codes, respectively, stipulate0·8%, 0·75% and 0·83% as the maximum allowable inter-storeydrift. In IRC2800, under strong earthquakes, the maximuminter-storey drift value of the buildings shall be limited to 2·5%or 2% (for T < 0·7 s and T > 0·7 s, respectively).

In the design of all models considered in this study, storey driftrequirements have been checked at all storeys under the pre-scribed ground motions assuming that non-ductile (brittle)infill panels are used in construction.

2.6 Geometrical properties of the models – gravity andlateral loadings – structural design

2.6.1 Geometrical properties of the modelsFor the aims of this study, four three-bay steel moment frameswith three, six, nine and 12 storeys and of the high-ductilitycategory have been considered. Storey height and span of allframes are equal to 3·0 and 5·0 m, respectively. Column basesare fixed.

2.6.2 Gravity loadingIn modelling the structure, uniform dead and live loadson each beam of all storeys, except the roof beams, have beenconsidered to be 3000 kgf/m and 1000 kgf/m, respectively.For roof beams the corresponding values are 3500 kgf/mand 750 kgf/m. It is noted that all frames are representativeof an internal frame of a three-dimensional (space) structure.In Figure 4, a schematic layout of the frames with the tributarygravity loads on beams are displayed.

2.6.3 Lateral loadingBased on the provision of the three model codes discussed inSections 2.4 and 2.5, and structural and gravity loadingassumptions as outlined in Sections 2.6.1 and 2.6.2, lateralloads have been calculated.

In Table 2, the calculated effective masses of all the designedmodels according to the three codes are represented.Figures 5(a)–5(d) depict the storey lateral forces as calculatedby the three codes. It is observed that for low-rise structures(say the three-storey model), storey design forces are almostidentical for the three codes, whereas for the six-, nine- and12-storey models, Eurocode 8 generally shows different storeyforces compared with IRC2800 and BCJ. Design lateral storeyforces are very close for models designed with the IRC2800

01

2

3

4

5

6

7

8

9

0·1 0·2 0·3 0·4 0·5αi

Ai

T1 = 2

T1 = 1·5

T1 = 0·5T1 = 0·2T1 = 1

0·6 0·7 0·8 0·9 1·0

Figure 2. Variation of Ai with αi for six values of fundamental

periods of vibration

IRC2800

Beha

viou

r fa

ctor

0

1

2

3

4

5

6

7

8

Eurocode 8 BCJ

Figure 3. Comparison of behaviour factors for steel moment-

resisting frames adopted in the codes

7



and BCJ codes. Table 3 shows the analytical periods ofvibration of the first three modes of all models. Global (P–Δ)and local (P–δ) effects were accounted for in the analysis anddesign processes as required by the AISC (AISC, 1999) code.The former was performed under the gravity load combi-nation of ‘1·2 dead load+ 0·5 live load’ which affected thelateral drifts and member sizes extensively. The second(local P–δ) effects were considered in the design process ofcolumns by way of the moment magnification factors asrequired by American Institute of Steel Construction (AISC,1999).

2.6.4 Design of the modelsBased on the assumptions and loading conditions described inprevious sections, all models were designed using the load andresistant factor design (LRFD) method based on the AISCcode of practice (AISC, 1999). In particular, the followingpoints have been accounted for in the design process

& required strength of all structural elements under the AISCload combinations

& elements’ width-to-thickness ratio provision as required byAISC code (seismic design provisions) for high-ductilitymoment-resisting steel frames

& serviceability limit states under gravity loads as requiredby AISC

& special requirements for seismic-resistant elements withhigh ductility, and finally

& beam-to-column capacity ratios as dictated by AISC toensure the strong column/weak beam design philosophy.

In Table 4, the final beams and column sizes of all frames areshown for each code. In the table, HEBx denotes wide flange‘H’ section with total height equal to ‘x’ Also, IPEx stands foruniversal ‘I’ sections with total height of ‘x’. All sections arestandard European sections.

2.7 Seismic performance assessmentFor a precise seismic performance evaluation of structuralsystems, it is generally necessary to assess their responses inthe inelastic range under lateral loads. According to the per-formance evaluation strategies adopted in Fema 356 (Fema,2002), non-linear static (pushover) analysis is deemed satisfac-tory for the cases considered in this study.

To perform the inelastic analyses, non-linear models were builtutilising the concentrated plasticity approach (Bozorgnia andBertero, 2004). All models have been created and analysed inthe SAP2000 computer program (CSI, 2008). Since all gravityloads are distributed uniformly on the beams, potential hingeswere considered to form at both ends of columns and beams.Beams’ depth-to-span ratios as well as their gravity loadingconditions are such that all beams would be regarded asflexure-dominant elements; thus, for beams elements, only flex-ural hinges were assigned. For columns, on the other hand,hinges with coupled axial force-bending moment (P–M ) be-haviour were utilised. Non-linear force–deformation curvesof all elements were defined based on the provisions adoptedin Fema 356 (Fema, 2002) relevant to steel moment framesand according to the design sections. In this study, only thelife safety performance level was checked. This is generally thereference performance level for residential building structuresin IRC2800, Eurocode 8 and BCJ. Figure 6 shows a typicalcolumn and beam hinge force–deformation back-bone curve asadopted in Fema 356 for steel elements.

In the evaluation process, a deformation-controlled behaviourwas assumed for flexural actions in beams, while force-controlled behaviour was considered for shear in beams as wellas for axial force in columns. It should also be noted thatall beams and columns were considered as ‘primary elements’in the performance evaluation process.

DL = 3500 kgf/mLL = 750 kgf/m




Foundation

5·00

3·00

Thre

e/si

x/ni

ne a

nd 1

2 st

orey

s

3·00

3·00

5·00 5·00

Figure 4. Schematic layout of the frames with the tributary

gravity loads on beams (all dimensions are in metres; DL: dead

load; LL: live load). See Table 4 for beams and columns sections

No. of storeys IRC2800 Eurocode 8 BCJ

Three storeys 17·98a 17·91 17·93Six storeys 36·31 36·13 36·33Nine storeys 55·03 54·20 54·75Twelve storeys 74·11 72·91 74·28

aAll values are in tonf.s2/m

Table 2. Total seismic masses of all models designed according to

each code

8



Material strengths were defined separately for deformation-and force-controlled hinges. A lower-bound strength valueequal to Fy = 2400 kgf/cm2 and an upper-bound strength valueequal to Fye = 1·10 (Fy) = 2640 kgf/cm2 were utilised as

material characteristics for force- and deformation-controlledelements, respectively. A knowledge factor (κ) equal to 1 hasbeen assumed for the acceptance criteria (as commonly usedfor new structures), as per Fema 356 (Fema, 2002).

00 0

1

2

3

4

5

6

1

IRC2800

Eurocode 8

BCJ

IRC2800

Eurocode 8

BCJ

IRC2800

Eurocode 8

BCJ

IRC2800

Eurocode 8

BCJ

2

3

0·5

(a) (b)

(c) (d)

Stor

eySt

orey

Stor

eySt

orey

Fi/Ftop storey

1·0 0 0·5Fi/Ftop storey

1·0

0 0·5

Fi/Ftop storey

1·000

1

0

1

2

3

4

5

6

7

8

9

10

11

12

2

3

4

5

6

7

8

9

0·5

Fi/Ftop storey

1·0

Figure 5. Normalised lateral force distribution as calculated using

IRC2800, BCJ and Eurocode 8 codes: (a) three-storey model;

(b) six-storey model; (c) nine-storey model; (d) 12-storey model

9



For the purpose of performing pushover analyses, four lateralload patterns including the square root of the sum of squaresof modal responses of modes (sum of modal masses equal90% of the total mass of the structure) (denoted by ‘Spec.comb.’ in the relevant tables and figures), uniformly distributedloading (denoted by ‘Uniform’), triangular distributed loadingbased on the code-specified lateral load distribution (denotedby ‘Design static’) and finally a distribution based on the firstmode shape of the structures (denoted by ‘Mode 1’) were con-sidered. Although studies (e.g. Fema, 2005) have shown thatthere may be no remarkable advantage in using multiple loadpatterns, comparisons were made among the performance ofthe structures under all of the aforementioned lateral load

patterns. In performing the analyses, the control node was con-sidered to be located at the roof. Target displacements of thestructures were calculated according to the coefficient methodof Fema 356 (Fema, 2002) as well as the capacity spectrummethod of ATC 40 (ATC, 1996). Two gravity load com-binations including ‘1·10 dead load+ 1·10 live load’ (termed‘Gravity load 1’) and ‘0·9 dead load’ (termed as ‘Gravityload 2’) were introduced as the initial states before pushing thestructure.

3. Results and discussionIn this section, the results of the analyses are presented. Resultinclude the force–deformation (pushover) curves of the

No. of storeys IRC2800 Eurocode 8 BCJ

T1 T2 T3 T1 T2 T3 T1 T2 T3

Three 0·687 0·207 0·103 0·745 0·234 0·119 0·732 0·232 0·118Six 1·045 0·387 0·219 1·189 0·439 0·236 1·030 0·385 0·207Nine 1·329 0·489 0·284 1·796 0·677 0·386 1·412 0·546 0·307Twelve 1·657 0·607 0·359 1·954 0·733 0·436 1·527 0·587 0·340

Table 3. The first three periods of vibrations of the models

No. of storeys Storey IRC2800 Eurocode 8 BCJ

BeamsInteriorcolumns

Exteriorcolumns Beams

Interiorcolumns

Exteriorcolumns Beams

Interiorcolumns

Exteriorcolumns

Three storeys 1 IPE400 HEB400 HEB400 IPE400 HEB360 HEB280 IPE400 HEB360 HEB3602 IPE400 HEB300 HEB300 IPE400 HEB300 HEB280 IPE400 HEB280 HEB2803 IPE360 HEB280 HEB280 IPE330 HEB280 HEB220 IPE360 HEB240 HEB240

Six storeys 1 IPE550 HEB500 HEB450 IPE500 HEB450 HEB400 IPE500 HEB600 HEB6002 IPE550 HEB450 HEB360 IPE500 HEB450 HEB360 IPE500 HEB500 HEB5003 IPE550 HEB400 HEB300 IPE450 HEB450 HEB300 IPE500 HEB400 HEB4004 IPE500 HEB360 HEB300 IPE400 HEB400 HEB280 IPE450 HEB400 HEB4005 IPE500 HEB300 HEB280 IPE360 HEB300 HEB280 IPE400 HEB300 HEB3006 IPE360 HEB220 HEB220 IPE330 HEB220 HEB220 IPE360 HEB240 HEB240

Nine storeys 1, 2 IPE600 HEB550 HEB600 IPE500 HEB500 HEB450 IPE550 HEB600 HEB6003, 4 IPE600 HEB550 HEB450 IPE500 HEB500 HEB400 IPE550 HEB500 HEB5005, 6 IPE600 HEB500 HEB360 IPE400 HEB400 HEB300 IPE500 HEB450 HEB4507, 8 IPE550 HEB450 HEB300 IPE360 HEB330 HEB280 IPE450 HEB330 HEB3309 IPE360 HEB360 HEB280 IPE330 HEB220 HEB220 IPE360 HEB240 HEB240

Twelve storeys 1, 2 IPE600 HEB600 HEB600 IPE550 HEB600 HEB550 IPE550 HEB650 HEB6503, 4 IPE600 HEB600 HEB600 IPE550 HEB550 HEB500 IPE550 HEB650 HEB6505, 6 IPE600 HEB500 HEB500 IPE500 HEB500 HEB400 IPE550 HEB600 HEB6007, 8 IPE600 HEB400 HEB400 IPE500 HEB450 HEB360 IPE550 HEB500 HEB5009, 10 IPE550 HEB360 HEB300 IPE400 HEB400 HEB300 IPE500 HEB400 HEB400

11, 12 IPE360 HEB360 HEB280 IPE360 HEB300 HEB240 IPE360 HEB280 HEB280

Table 4. Summary of design results of the models for each code

10



structures, sequences of plastic hinge formation and rotationof the plastic hinges of the structural elements, target dis-placements (using the above-mentioned methods) and perform-ance assessment of each model based on the provisions andaccepted criteria adopted in Fema 356 (Fema, 2002) for defor-mation-controlled and force-controlled elements of the frames.

3.1 Pushover curvesFigures 7(a)–7(d) represent the capacity (pushover) curves ofthe three-, six-, nine- and 12-storey frames designed withIRC2800. As can be observed, the capacity curves under all ofthe lateral load patterns are very close, except the one associ-ated with ‘Uniform’ loading pattern. This is primarily becausethe models are essentially first-mode dominant. In fact, the‘Uniform’ loading pattern resulted in the highest elastic stiff-ness and peak strength in all structures. Such behaviour wasalso observed for all frames designed with Eurocode 8 andBCJ. Note that according to Fema 356 (Fema, 2002), the mostcritical lateral load pattern should be considered in the evalu-ation process of all elements. This implied that, for example,the critical load pattern for evaluation of a column in a storeyof the structure may not be the one which governs the evalu-ation of the beams in that storey.

Capacity curves of all structures under the initial conditionof ‘Gravity load 2’ for IRC2800, Eurocode 8 and BCJ areshown in Figures 8(a)–8(d) under ‘Uniform’ lateral loading.Except for the three-storey model, the curves demonstrate sig-nificant differences among the lateral behaviour of thedesigned models, as discussed next. Similar results were alsoobtained from analyses under the ‘Gravity load 1’ combi-nation, which owing to space limitations are not reproducedhere.

Pushover curves of the three-storey model are close for allthree codes. However, the yield displacement for the framedesigned with IRC2800 is smaller than the two other codes.Thus, for a specified ultimate roof displacement, a little higherductility may be achieved for low-rise steel moment framesdesigned with IRC2800. For all other cases, many more

differences are observed among the capacity curves. Suchdifferences are remarkable for the nine- and 12-storey modelsand are generally attributed to the way that the codes distributethe strength (and hence stiffness) among the structuralmembers of the structure. The capacity curve for the six-storeymodel designed with IRC2800 stopped at a roof displacementvalue approximately equal to 40 cm, while the capacity curvescontinued (not shown) to higher displacements for the framesdesigned with Eurocode 8 and BCJ.

Q/Q

y

1·0

b

a

A

BC

D E c

θ or ∆

Figure 6. Generalised force–deformation curve for steel columns

and beams (courtesy of Fema 356)

00

50100150200250300

Initial cond.: gravity load 1

Spec. comb.

Design static

Uniform

Mode 1

10 20 30Displacement: cm

Base

she

ar: t

onf

40 50 60

00

50100150200250300


Spec. comb.

Design static

Uniform

Mode 1


Base

she

ar: t

onf

40 50

00

50

100

150

200

250

Initial cond.: gravity load 1Spec. comb.

Design static

Uniform

Mode 1


Base

she

ar: t

onf

40

00

20406080

100120140160


Spec. comb.

Design static

Uniform

Mode 1

10 20

(a)

(b)

(c)

(d)

30Displacement: cm

Base

she

ar: t

onf

40

Figure 7. Pushover curves of frames designed with IRC2800

using four lateral load patterns under gravity load 1 as initial

condition: (a) three-storey model; (b) six-storey model;

(C) nine-storey model; (d) 12-storey model

11



Thus, models designed with Eurocode 8 and BCJ couldbe regarded as being more stable up to onset of lateral col-lapse, which is attributable to the way in which strengthand stiffness are distributed among the structural members.Post-peak responses show better structural stability forthe six-storey frame designed with Eurocode 8 and BCJ,while early softening effects have lowered the displacementcapacity of this frame designed using IRC2800. Note that thesix-storey structure (with the first-mode period being around

1·05 s – see Table 3) would be considered as a medium-periodstructure.

For the nine- and 12-storey long period frames (see Table 3),the differences among pushover curves are much more pro-nounced. Particularly, as is obvious from Figure 8(c), there arenotable differences among the peak strengths attained bythe nine-storey model designed with IRC2800 and BCJ,as compared with the corresponding model designed withEurocode 8. The peak strength of the models designed withIRC2800 and BCJ are approximately 250 tonf comparedwith the lower 200 tonf value for the model designed withEurocode 8. However, as will be shown later, the lower peakstrength capacity of structures designed with Eurocode 8 doesnot have severe adverse effects on their overall performance.It is worth noting that, although the pushover curves forsix- and 12-storey models designed with IRC2800 stop at alower roof displacement compared with the models designedwith Eurocode 8 and BCJ, they still pass the performance pro-visions calculated according to the Fema 356 criteria discussedlater.

3.2 Target displacementAs part of the performance evaluation procedures adoptedin both Fema 356 and ATC 40, a ‘target displacement’ hasto be determined for checking the structure at that displace-ment. Two methods including the ‘coefficient method’ andthe ‘capacity spectrum method (CSM)’ are adopted inFema 356 and ATC 40 for estimating the target displace-ment, respectively. The former seems to be more appealingfor practical purposes due to its simplicity, and recent evalu-ation guidelines (e.g. ASCE 41-13 (ASCE, 2014)) generallyregarded the ‘coefficient method’ as the primary method ofcalculating the target displacement. The theoretical back-ground of these methods can be found in the literature (Chenand Lui, 2006; Chopra and Goel, 1999; Fema, 2005; Freeman,1998).

In Fema 356, the following equation is used for estimating thetarget displacement

9: δ ¼ C0C1C2C3SaT2e

4π2

� �

where C0 is the modification factor to relate spectral displace-ment of an equivalent single-degree-of-freedom (SDOF)system to the roof displacement of the building multi-degree-of-freedom (MDOF) system; C1 is the modification factor torelate expected maximum inelastic displacement to displace-ments calculated for linear elastic response; C2 is the modifi-cation factor to represent the effect of pinched hysteretic shape,stiffness degradation and strength deterioration on maximumdisplacement response; C3 is the modification factor to rep-resent increased displacements due to dynamic P–Δ effects;

00

050

100150200250300

20406080

100

140120

160180


Uniform–IRC2800

Uniform–Eurocode 8

Uniform–BCJ


Uniform–IRC2800


Uniform–BCJ

10 20

(a)

(b)

(c)

(d)

30Displacement: cm

Base

she

ar: t

onf

Base

she

ar: t

onf

40

0 10 20 30Displacement: cm

40

050

100150200250300


Uniform–IRC2800


Uniform–BCJ

Base

she

ar: t

onf

Base

she

ar: t

onf


40 50

0

100

200

300

400Initial cond.: gravity load 2

Uniform–IRC2800


Uniform–BCJ


40 50 60

Figure 8. Pushover curves of the frames designed with Eurocode

8 and BCJ under gravity load 2 as initial condition: (a) three-storey

model; (b) six-storey model; (c) nine-storey model; (d) 12-storey

model

12



and finally, Sa is the response spectrum acceleration at theeffective fundamental period and damping ratio of the build-ing. The effective fundamental period of the building (Te) iscalculated as

10: Te ¼ Ti

ffiffiffiffiffiffiKi

Ke

r

where Ti, the elastic fundamental period (in seconds), is calcu-lated by elastic dynamic analysis, Ki is the elastic lateral stiff-ness of the building and Ke is the effective lateral stiffness ofthe building.

In Fema 356, two methods have been proposed for calculatingthe factor C0. The first method simply takes the C0 valuedirectly from table 3·2 of the FEMA guideline, while thesecond method estimates the C0 value based on the ‘modalparticipation factor’ of the first mode. Analysis conducted bythe authors showed that in almost all cases, except for thethree-storey model, target displacements calculated based onC0 values read from table 3·2 of the guideline give themaximum value for the target displacements, while target dis-placements determined based on C0 values as calculated usingthe modal participation factor of the first mode of responseprovide the minimum estimate.

ATC 40 utilises the CSM for determining the target displace-ment (performance point) and performance evaluation ofstructures. The iterative process adopted therein needs morecomputational effort compared with the ‘coefficient method’of Fema 356 for the determination of target displacement.

Analysis conducted by the authors shows that, in almost allcases, target displacement values calculated using the CSM ofATC 40 lie between two bounds as given by Fema 356 usingthe above-mentioned C0 values. Thus, the authors decided tocheck and evaluate all models based on target displacementsderived from the CSM.

In performing the CSM, the inherent viscous damping valuewas considered to be 5% of critical value for all structures, asrecommended in ATC 40. Also, the structural type was con-sidered to be ‘type A’ (essentially new buildings) based on theclassification criteria of ATC 40. The equivalent viscousdamping, which accounts for both inherent viscous dampingand hysteretic damping due to inelastic response (β0), is thencalculated as

11: β ¼ 0�05þ β0

The hysteretic damping (β0) value is calculated based on a bi-linear representation of the original capacity curve of the struc-ture (in acceleration–displacement response spectrum (ADRS)Lo

addistrib

ution

IRC28

00Eu

rocode

8BC

J

Three-storey

Six-storey

Nine-storey

12-storey

Three-storey

Six-storey

Nine-storey

12-storey

3-storey

6-storey

9-storey

12-storey

Spec.comb.

V:tonf

116·87

165·93

235·20

233·66

101·89

129·78

114·32

164·60

116·95

193·16

214·30

314·94

Dt:cm

15·3

29·5

40·10

55·1

14·9

26·9

41·2

46·8

14·5

25·8

37·2

43·9

Mod

e1

V:tonf

117·11

164·87

234·69

231·06

101·20

127·16

108·28

157·15

109·80

172·60

195·19

292·04

Dt:cm

15·2

29·2

34·40

55·0

15·0

26·8

39·8

44·5

16·1

28·8

41·6

49·9

Designstatic

V:tonf

120·11

167·16

235·63

231·84

103·28

134·56

113·64

157·84

113·34

182·40

203·00

289·78

Dt:cm

14·9

28·7

39·40

57·2

14·7

26·2

39·2

46·5

15·3

27·7

40·9

51·6

Uniform

V:tonf

132·35

183·82

252·80

254·66

114·4

165·90

142·88

199·59

124·68

218·60

242·48

341·04

Dt:cm

12·4

24·0

32·10

44·8

13·3

22·6

33·7

37·9

12·8

22·5

34·4

39·9

Note:

V,ba

seshear;Dt,target

displacemen

t.

Table

5.Target

displacemen

ts(in

centim

etres)forallthe

four

lateralloa

dpa

tterns

ofallm

odelsas

calculated

basedon

the

capa

city

spectrum

metho

d

13



format) as well as based on yield and performance displace-ments and accelerations derived from the bi-linear curve. Thedesign standard (5% damped) spectrum is then reduced basedon the calculated β value above to intersect the capacity curve(in ADRS format). The procedure is often iterative and thefinal intersection point is the target displacement of the struc-ture at which the performance of the structure needs to bechecked.

Table 5 shows target displacements calculated for allfour lateral load patterns in all models and the correspondingbase shears. It is interesting to note that, in all cases, targetdisplacements based on the ‘Uniform’ load pattern arethe least compared with other load patterns. No general trendis recognised, however, for maximum values of targetdisplacement.

3.3 Performance evaluationThe evaluation of each frame model was carried out at thetarget displacement based on acceptance criteria for defor-mation-controlled as well as force-controlled elements’ actionsin the non-linear models adopted in Fema 356 for steelmoment frames. The performance evaluations were carried outfor all four lateral load patterns introduced previously underthe effect of both ‘Gravity load 1’ and ‘Gravity load 2’.

Figures 9–12 display the state of plastic hinges of all modelsdesigned according to different codes for the critical initialconditions and under the ‘Spec. comb.’ lateral load pattern atthe target displacement. Results will be discussed next.

Analyses (not reproduced here for space limitations) showedthat utilising the uniform lateral load pattern has the effect of

z

(a) (b)

(c)

x

B I0 LS CP C D E B I0 LS CP C D E

B I0 LS CP C D E

z

x

z

x

Figure 9. State of the plastic hinges of the three-storey model

designed according to different codes for the critical initial

conditions and under the ‘Spec. comb.’ lateral load pattern at the

target displacement: (a) IRC2800; (b) Eurocode 8; (c) BCJ (B, C, D

and E correspond to the points depicted in Figure 6 and IO, LS

and CP limits correspond to immediate occupancy, life safety and

collapse prevention performance limits)

14



moving the critical hinges from upper storeys to the bottomlevels. The structures designed based on BCJ are exceptions,wherein under all load patterns, plastic hinges were typicallyformed at the lower part of the structures. Also, in most cases,plastic hinging was initiated from the first-storey columnshinges (at the base) when a uniform lateral load patternwas applied to the structure. The above-mentioned trends werealso observed for all models with different numbers of storeys.

Although few plastic hinges have formed in the columns(Figures 9–12), generally for all structures, the beam-sway

mechanism was the dominant mechanism rather than thecolumn-sway mechanism. Thus, it is argued that the intendedstrong-column/weak-beam provision has been met in almostall cases. Plastic hinges located at column bases were found tobe more critical for all structures designed with BCJ than withthe other two codes.

In Figure 13 and Figure 14, performance evaluations of allframes designed with the three codes are provided. Thesefigures represent the number of plastic hinges that fall withinspecific performance limits; namely, fully elastic (A to B in the

z

x

z

x

z

x

(a)

B I0 LS CP C D E

(c)

B I0 LS CP C D E

(b)

B I0 LS CP C D E

Figure 10. State of the plastic hinges of the six-storey model




and E and IO, LS and CP limits are as described in caption to

Figure 9)

15



figures), IO (B to IO), LS (IO to LS), CP (LS to CP) andbeyond collapse (CP to C or C to E), according to the defi-nitions shown in Figure 6. Also shown is the total number ofhinges formed in the model at the target displacement. Notethat only the critical cases between the two assumed initialconditions are reproduced in the figures. As implied fromFigure 7 and Figure 8, capacity curves of each model are veryclose for the ‘Spec. combo.’, ‘Mode 1’ and ‘Design static’ loadpatterns. Thus, for simplicity, only results of ‘Uniform’ and‘Spec. comb.’ load patterns are displayed and compared inFigure 13 and Figure 14, respectively.

Assessment of the results reproduced in Figure 13 andFigure 14 reveals the following findings.

(a) Except in negligible cases, all hinges of structuresdesigned with IRC2800 have satisfied the life safetyperformance level. In rare cases, typically at columnbases and the lower part of the structure, hinges havepassed the life safety limit. This should not causeany concern, since a very small portion of hinges(less than 2% of all) failed to satisfy the limit, andalso plastic rotations and force demands respectively

zz

x

z

x

x

(a) (b)

(c)

B I0 LS CP C D E B I0 LS CP C D E

B I0 LS CP C D E

Figure 11. State of the plastic hinges of the nine-storey model


conditions and under the ‘Spec. Comb.’ lateral load pattern at the


and E and IO, LS and CP limits are as described in caption to

Figure 9)

16



in displacement- and force-based elements (notreproduced here), were only slightly more than thespecified limits of Fema 356. For structures designedwith IRC2800, generally, plastic hinging in beamsand columns of the lower levels has been the dominantcase. Such behaviour is directly attributed to the waythat IRC2800 assigns strength (and stiffness) to themembers of the structures, as noted previously. Alsofor structures designed with this code, in almost all cases,the ‘Uniform’ lateral load pattern is the governingpattern.

(b) For structures designed with Eurocode 8, generally,all have satisfied the life safety performance level.In all cases, Eurocode 8 has provided the best designsin terms of strength, stiffness and ductility assignment,which has apparently resulted from the very uniformdistribution of required strength and stiffness throughoutthe structures. In almost all cases, with few exceptions,beam-sway mechanism is the dominant plasticmechanism. Distribution of strength and stiffness instructures design with Eurocode 8 is such that, typically,plastic hinges appear in upper storeys. In most cases,

z

x

B I0 LS CP C D E

(a)

zx

(b)

B I0 LS CP C D E

(c)

B I0 LS CP C D E

z

x

Figure 12. State of the plastic hinges of the 12-storey model



target displacement: (a) IRC2800; (b) Eurocode 8; (c) BCJ (in the

keys, B, C, D and E and IO, LS and CP limits are as described in

caption to Figure 9)

17



01020304050607080

IRC2800

Eurocode 8

BCJ

(a) (b)

(c) (d)

Performance

Perc

ent

plas

tic h

inge

s fo

rmed

: %

> E0

102030405060708090

IRC2800

Eurocode 8

BCJ

Performance

Perc

ent

plas

tic h

inge

s fo

rmed

: %

> E

0102030405060708090

IRC2800

Eurocode 8

BCJ

Performance

Perc

ent

plas

tic h

inge

s fo

rmed

: %

> E0

102030405060708090

IRC2800

Eurocode 8

BCJ

Performance

Perc

ent

plas

tic h

inge

s fo

rmed

: %

> E

AtoB

BtoIO

IOtoLS

LStoCP

CPtoC

CtoD

DtoE

AtoB

BtoIO

IOtoLS

LStoCP

CPtoC

CtoD

DtoE

AtoB

BtoIO

IOtoLS

LStoCP

CPtoC

CtoD

DtoE

AtoB

BtoIO

IOtoLS

LStoCP

CPtoC

CtoD

DtoE

Figure 13. Total number of plastic hinges (%) formed (as a

fraction of total existing hinges in the model within specific

performance limits under ‘Uniform’ lateral load pattern:

(a) three-storey model; (b) six-storey model; (c): nine-storey model;

(d): 12-storey model

AtoB

01020304050607080

(a)

Performance

Perc

ent

plas

tic h

inge

sfo

rmed

: %

BtoIO

IOtoLS

LStoCP

CPtoC

CtoD

DtoE

> E

IRC2800

Eurocode 8

BCJ

(b)

100

2030405060708090

IRC2800

Eurocode 8

BCJ

Performance

Perc

ent

plas

tic h

inge

s fo

rmed

: %

> E

(c)

100

2030405060708090

IRC2800

Eurocode 8

BCJ

Performance

Perc

ent

plas

tic h

inge

s fo

rmed

: %

> E

(d)

100

2030405060708090

IRC2800

Eurocode 8

BCJ

Performance

Perc

ent

plas

tic h

inge

s fo

rmed

: %

> E

AtoB

BtoIO

IOtoLS

LStoCP

CPtoC

CtoD

DtoE

AtoB

BtoIO

IOtoLS

LStoCP

CPtoC

CtoD

DtoE

AtoB

BtoIO

IOtoLS

LStoCP

CPtoC

CtoD

DtoE

Figure 14. Total number of plastic hinges (%) formed (as a

fraction of total existing hinges in the model within specific

performance limits under ‘Spec. comb.’ lateral load pattern:

(a) three-storey model; (b) six-storey model; (c): nine-storey model;

(d): 12-storey model

18



unlike IRC2800 and BCJ, column bases have not beencritical.

(c) For structures designed with BCJ, except in negligiblecases, the structures meet the intended life safetyperformance criteria. In assessing the performancesof the models, similarities were observed among thegeneral performances of structures designed with theIRC2800 and BCJ. However, structures designed withBCJ generally performed better than those designedwith IRC2800. Typically, for the nine- and 12-storey(long period) structures designed with BCJ, first-storeycolumn-base hinges did not meet the requirementsof the life safety performance level. Recall that instructures designed with Eurocode 8, the beam-swaymechanism was the dominant plastic mechanismin almost all models. In addition, as with IRC2800,distribution of strength and stiffness in structuresdesigned with Eurocode 8 is such that, typically,plastic hinges tend to be more critical in lower storeys.

Analyses indicated that the above trends are also valid in force-controlled (column) members.

Another interesting comparison was made in terms of thetotal weight and amount of steel required for all buildingsdesigned with the codes (Table 6). The table shows thatEurocode 8 has provided the most economical designs. Thereduction of the total amount of steel used is more noticeablein the nine- and 12-storey (medium-rise) buildings. Generally,more steel was needed for structures designed according toIRC2800.

4. Concluding remarksThis research investigated and compared the performanceof steel moment frames designed according to threeseismic design codes, namely, the Iranian standard no. 2800(IRC2800), the Eurocode 8 and the Japanese (BCJ) codes ofpractice. The performance assessment of structures was exam-ined by way of non-linear static (pushover) procedure, asadopted in Fema 356 and ATC 40 documents and other cri-teria regulated in them. The performances of all deformation-and force-controlled elements were assessed based on theprovisions adopted in Fema 356. Extensive analyses showedthat, in general, structures designed with Eurocode 8 met all theacceptance criteria for the design-intended life safety perform-ance level currently adopted in the code for residential buildingsunder the seismic demands. For structures designed withEurocode 8, the beam-sway mechanism was observed at thetarget displacement, while they are the most economical struc-tures in terms of the total amount of required steel compared tothose designed with IRC2800 and BCJ. Such better responseis attributed to the way that the code distributes demands overthe structure and the way that the code considers the highermode effects. Structures designed with Eurocode 8 show moredesirable response over the yielding phase compared with theD

esigncode

BCJ

Eurocode

8IRC28

00

Num

berof

storeys

36

912

36

912

36

912

Totalw

eigh

t:kg

f68

6318

554

2913

351

718

6792

1606

723

858

3812

973

1917

170

3232

948

027

Redu

ctionor

increase

intotalb

eams

weigh

tcompa

redto

IRC28

00:%

+5·8

−6·9

−16

·7−3·1

+1·0

−13

·8−40

·4−35

·30·0

0·0

0·0

0·0

Redu

ctionor

increase

intotalcolum

nsweigh

tcompa

redto

IRC28

00:%

−12

·6+18

·8−4·9

+16

·6−11

·5−1·1

−17

·2−8·5

0·0

0·0

0·0

0·0

Redu

ctionor

increase

intotalstructure

weigh

tcompa

redto

IRC28

00:%

−6·2

+8·1

−9·9

+7·7

−7·2

−6·4

−27

·1−20

·60·0

0·0

0·0

0·0

Table

6.Com

parison

oftheam

ount

ofsteelu

sedin

thefin

al

design

ascompa

redwith

IRC28

00

19



other two codes. BCJ also showed desirable performance withthe exception that in some minor cases column-base hingingwas significant at the target displacement. In most cases,the lower parts of the structures were critical for structuresdesigned with this code. In contrast, in rare cases, IRC2800resulted in structures that did not satisfy the life safety require-ments. Similar to BCJ, violation of life safety requirementswas observed at the lower part of the structures and columnbases. It should be noted that the three-storey models designedwith IRC2800 satisfied the life safety level completely.Moreover, remarkable degradation in stiffness (in the post-peak branch of response) was observed for structures designedwith IRC2800 which directly relates to inappropriate stiffnessdistribution over the structure. In addition, IRC2800 providedstructures with the highest amount of required steel. Theresult suggest that the IRC2800 needs some improvementregarding the seismic design of steel moment resisting frameslocated in high-seismicity areas. Also, it seems that BCJ shouldassign more strength to the lower part of moment-resistingframes.

Yield displacements of medium-rise structures and peakstrength (lateral load capacity) of short- and medium-periodstructures are close to identical in all cases. However, notabledifferences exist in yield displacements of low-rise (three-storey) buildings as well as the peak strength of long-periodbuildings.

AcknowledgementsThis research is completed in part under the IIEES(International Institute of Earthquake Engineering andSeismology) project no. 8103-523. The authors gratefullyacknowledge Dr Abdolrahim Jalali, assistant professor ofearthquake engineering at Tabriz University, Iran, and formerPhD candidate of earthquake engineering at Tokyo Institute ofTechnology, for providing the Japanese building code of prac-tice (BCJ) and useful discussions on the code.

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CSI (2008) SAP2000: Structural Analysis Program. Computersand Structures, Inc., CA, USA.

Fema (Federal Emergency Management Agency) (2002)Fema-356: Pre-standard and commentary for the seismicrehabilitation of buildings. Federal EmergencyManagement Agency, Washington, DC, USA.

Fema (2005) Fema 440: Improvement of nonlinear staticseismic analysis procedures. Federal EmergencyManagement Agency, Washington, DC, USA.

Freeman SA (1998) The capacity spectrum method as a toolfor seismic design. Proceedings of the 11th EuropeanConference on Earthquake Engineering. Paris, France(CD-ROM).

ICBO (International Conference of Building Officials) (1997)Uniform Building Code (UBC) Volume 2: StructuralEngineering Provisions. International Conference ofBuilding Officials, Whittier, CA, USA.

Mansouri B, Kiani A and Amini-Hosseini K (2014) A platformfor earthquake risk assessment in Iran case studies:Tehran scenarios and Ahar-Varzeghan earthquake.Journal of Seismology and Earthquake Engineering 16(1):51–69.

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Marino EM, Nakashima M and Mosalam KM (2005)Comparison of European and Japanese seismic design ofsteel building structures. Journal of Engineering Structures27(6): 827–840.

Tremblay R, Bruneau M, Nakashima M et al. (1996) Seismicdesign of steel buildings: lessons from the 1995 Hyogo-kenNanbu earthquake Canadian Journal of Civil Engineering23(3): 727–756.

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