1、1 EVALUATION OF LATERAL LOAD PATTERN IN PUSHOVER ANALYSIS Armagan KORKMAZ1, Ali SARI2 1Visitor Researcher,Department of Civil Engineering, University ofTexas at Austin, Austin,TX78712, PH: 512-232-9216； armaganmail.utexas.edu 2Ph. D. Student, Department of Civil Engineering, University of Texas at A
2、ustin, Austin, TX 78712, PH: 512-232-9216； ali_sarimail.utexas.edu ABSTRACT Theobjectiveofthisstudyistoevaluatetheperformanceoftheframestructuresorvariousloadpatternsandvariety of naturalperiodsbyperformingpushoverandnonlinear dynamic time history analyses. The load distributions for pushover analys
3、es are chosen as triangular,IBC(k=2)andrectangular.Fourdifferentframedstructuresareused,whicharetypicalreinforcedconcrete(RC)framesystemsandhavefourdifferentnatural periods.Eventhoughthenonlineardynamictimehistoryanalysisisthebestwayto computeseismicdemandsFEMA-273andATC-40proposestouseofnonlinearst
4、aticprocedureorpushoveranalysis.Thispaperisalsointendedto compare the resultsof pushoverandnonlineardynamictimehistoryanalyses.Toevaluatetheresultsfromthe pushoveranalysesforthreeloadpatternsandalsofournaturalperiods,nonlineardynamictimehistoryanalysesareperformed.Earthquakegroundmotionsrecordedat50
5、 stationsduringvariousearthquakesoveralltheworldareusedintheanalyses.Pushoverandnonlineartimehistoryanalysesresultsarecomparedtochoosethebestload distribution for specific natural period for this type of frame structure. Keywords: Pushoveranalysis, nonlinear time history, load patterns, moment-resis
6、ting frame 2 INTRODUCTION Onlythelifesafetyandcollapsepreventioningeneralearthquakeresistantdesignphenomenaareexplicitlypreventedinseismicdesigncodes.Thedesignisgenerallybasedonevaluatingtheseismicperformanceofstructures.Itisrequiredtoconsider inelasticbehaviorwhileevaluatingtheseismicdemandsatlowpe
7、rformancelevels. FEMA-273 and ATC-40 use pushover analysis as nonlinear static analysis but nonlinear time history analysis has more accurate results on computing seismic demands (ATC-40,1996, FEMA-273, 1997). The purposes in earthquake-resistance design are: (a) to prevent non-structural damage in
8、minor earthquakes, which may occur frequentlyinlifetime, (b)topreventstructuraldamageandminimizenon-structuraldamageinmoderate earthquakes which may occur occasionally, (c) to prevent collapsing or serious damage in major earthquakes which may occur rarely. Designs are explicitly done only under the
9、 third condition. Theobjectiveofthisstudyistoevaluatetheperformanceoftheframe structuresfor various load patterns and variety of natural periods by performing pushover and nonlinear dynamic time history analyses. 3, 5, 8 and 15 story RC frame structures are used in the analyses and the load distribu
10、tions for pushover analyses are chosen as triangular (IBC, k=1), IBC(k=2)andrectangular,wherekistheanexponentrelatedtothe structure period to define vertical distribution factor (IBC, 2000).The four frame structures have been analyzed using nonlinear program DRAIN-2D (Prakash, V., Powell, G., Campbe
11、ll,S., 1993) and the results have been compared by recorded response data. Both nonlinear static pushover analysis and nonlinear dynamic time history analysis are performed. The correlations between these nonlinear analyses are studied. Theperformanceof the buildings subjected to various representat
12、ive earthquake ground motions isexamined.Finally,pushoverandnonlineartimehistoryanalysesresultsare comparedtochoosethebestloaddistribution(pattern)forspecificnaturalperiodfor these types of reinforced concrete frame structures. GROUND MOTION DATA For this study, it is considered as 50 different data
13、 used in the nonlinear dynamic time history analyses, given in the Table 1. All the data are from different site classes as A, B,C and D. The shear velocities for the site classes A, B, C and D are Vs 750 m/s, 360m/s to 750 m/s, 180 m/s to 360 m/s, and180 m/s, respectively. The ground motion dataare
14、 chosenfromdifferentdestructiveearthquakesaroundtheworldearthquakename, date ofearthquake,datasource,recordname,peakgroundaccelerations (pga) 3 forthe components, effective durations and fault types for each data are presented in the Table1., respectively. The peak ground accelerations are in the ra
15、nge 0.046 to 0.395g, where g is acceleration due to gravity. All ground motion data are recorded in near-field region as in maximum20 km distance. DESCRIPTION OF THE FRAME STRUCTURES 3,5,8and15-storyRCframestructureswithtypicalcross-sectionsandsteelreinforcementsareshown inFigure1.Thereinforcedconcr
16、eteframestructureshavebeendesignedaccordingtotherulesoftheTurkishCode. The structures have been considered as an important class 1 with subsoil type of Z1 and in seismic region 1.The dead, live and seismic loads have been taken account during design. All reinforced concrete frame structures consist
17、three-bay frame, spaced at 800 cm. The story height is 300 cm. The columns are assumed as fixed on the ground.Yieldstrengthofthesteelreinforcementsis22kN/cm2andcompressivestrengthofconcreteis1.6kN/cm2. Thefirstnaturalperiodofthe3-storyframestructureiscomputed0.54s. Thecross-sectionofallbeamsinthisfr
18、ameisrectangular-shapeswith25cmwidthand50cm height. The cross-section of all columns is 30cmx30cm. The first natural period of 5-storyframe structure is 0.72 s and the cross-section of beams is 25cm width and50cm height similar to 3-story frame. Cross-section of columns at the first three stories is
19、 40cmx40cm and at the last two stories, it is 30cmx30cm. The eight-story and 15-story frame structures have natural period of 0.90 sand 1.20 s.The cross section of beams for both framestructuresis 25cmx55cm. The 8-story frame structure has 50cmx50cm columns for the first five stories and 40cmx40cm f
20、or the last three stories. The cross section of columnsforfirst eightstoriesinthe15-storyframestructuresis80cmx80cmandatthelastseven stories, it is 60cmx60cm. NONLINEAR STATIC PUSHOVER ANALYSIS OF FRAME STRUCTURES For low performance levels, to estimate the demands, it is required to consider inelastic behaviorof the structure.Pushoveranalysisisusedtoidentifytheseismichazards, selectionoftheperformancelevelsanddesignperformanceobjectives.InPushoveranalysis,applyinglateralloadsinpatternsthatrepresentapproximatelytherelative inertial forces generated at
