Nonlinear Seismic Response Analysis Of RC Frame Structure Accounting For The Influence Of Ground Motion Spectral Shape

With the development of Performance-Based Earthquake Engineering (PBEE),and the availability of advanced structural analytical software and faster computers,nonlinear dynamic time-history analysis has recently become more widely used forboth seismic design and evaluation of structures. Selection and modification ofground motion records is one of the most important issues for reliable seismicperformance evaluation of the structure. Generally speaking, intensity, frequencycharacteristics and duration are considered as the key indicators to select groundmotions. However, time-history analysis results, such as internal force anddisplacement, would be totally different even if input ground motions were takenbased on those indicators, due to complexity of ground motion characteristics.First, the methods of selecting ground motion records for dynamic analysis aresummarized and the concept of ground motion spectral shape is introduced in thispaper. Conditional Mean Spectrum (CMS) is constructed based on the supposedscenario and the attenuation model proposed by Boore and Atkinson. Then thespectral acceleration of18ground motion records from the Great WenchuanEarthquake and the standard response spectra based on Chinese code are comparedwith this CMS. Comparison results show that CMS strongly depend on the selectedattenuation model, which is not well-documented in China up to now. So it is still faraway to adapt the method based on CMS to select and modify ground motionrecords.Then,15ground motion records are chosen from the Great WenchuanEarthquake and PEER database. In order to eliminate the influence of the site soilcondition, average shear wave velocity of selected ground motion records is withinthe scope which is corresponding to medium-stiff soil site according to Chinese code.And a3-storey and8-storey reinforced concrete planar frame is taken as case studiesto represent the short-period and medium-period structures respectively. Maximumroof displacement and maximum inter-storey drift ratio are taken as engineeringdamand parameters. In order to investigate the efficiency of spectral acceleration atfirst-mode period,S a (T1), two scaling procedures are used to modify the originalrecords. One is based on peak ground acceleration (PGA) and PGA value for all15records is scaled to400gal. Another is based onS a (T1). Nonlinear dynamic analysis results under15scaled ground motion inputs show that for both short-period andmedium-period strctures, variability degree of nonlinear responses of structuresbased onS a (T1)-based scaling procedure is less than based on PGA-based scalingprocedure. This result indicates scaling ground motion records based on reasonableintensity index is beneficial to reduce the record-to-record variation in seismicresponses.Finally, in order to investigate the influence of ground motion spectral shape onnonlinear seismic response of RC structures, correlation between engineeringdemand parameters of structures and spectral acceleration at the second-mode andthe third-mode period, that isS a (T2)andS a (T3)is investigated. The results indicatethat engineering demand parameters of the structures has the good correlation withbothS a (T2)andS a (T3). That means engineering demand parameters are alsoinfluenced by the spectral acceleration values at the higher mode periods of thestructure. Furthermore, incremental dynamic analysis (IDA) is conducted on eachframe. IDA results show that when the structures are in the nonlinear stage, spectralacceleration at period less thanT1has no obvious influence on nonlinear seismicresponses and spectral acceleration at period larger thanT1has significant influenceon nonlinear seismic responses, for both short-period and medium-period strctures.This result indicates that the effect of period elongation during larger nonlinearitymust be considered.