Optimum Selecting and Scaling of Accelerograms for Nonlinear Time History Analysis of Structures

Seismic design and performance assessment of structures require employing earthquake loadings and determining the responses of structures using nonlinear analysis. Among the structural analysis methods, nonlinear time history analysis (NTHA) is the best option for this purpose because it demonstrates realistic behavior of structures against earthquake loadings. However, nonlinear time history analysis of structures involves still some problems related with assembling the suitable set of accelerograms that can properly represent the desired seismic level. Then, in order to perform a nonlinear time history analysis, appropriate record selecting and scaling strategy is required. Because of probabilistic specifications of ground motions, the identification of records to be applied in the evaluation of structural responses is a critical task. Furthermore, record selection directly influences both the median estimation and the dispersion about the median. The literature for selecting a pre-defined number of recordings within a ground-motion data set is developing consistently due to the recent intention of earthquake or structural engineering practice. Therefore, the characteristics of the selected ground-motion records and scaling strategies are more important for estimation of seismic responses and affecting the accuracy of structural responses are depended to the records selection [1-4]. In this research, a probabilistic based method has been utilized for optimum selecting and scaling of acceleration ground-motion records, which could be led to the reduction of dispersion in nonlinear structural responses. The utilized methodology in this research constrains the scaling to the differences between each individual record and corresponding estimation from the ground-motion prediction equation (GMPE) model. The record selecting and scaling are such that in addition to minimizing the dispersion of nonlinear responses, the median linear responses will be matched to the target spectrum. Furthermore, the applied method will preserve the seismic essence of the selected ground-motion records [4]. In addition, the method emphasizes the significance of preserving the basic seismological features of the ground-motions after being scaled. The final selection of the recording set is accomplished by estimating the standard deviation of all combinations resulting from an available accelerogram data set. In this study, by investigation of responses of MDOF steel moment resistant frames, the accuracy of the implemented method is verified. Because of the lack of accelerograms data for our country (Iran), the mean spectrum of relatively severe earthquakes has been produced as the design spectrum. The mentioned spectrum has been provided using 11 records (each record with two horizontal components) for soil type-2 with shear wave velocity between 375-750 m/s, minimum PGA equal to 0.3 g and minimum Mw equal to 5.30. In this investigation, selections of ground motions are managed at two stages. At first stage, 20 records with considering the design earthquake are selected. For second stage, among 20 ground motions, 10 records are selected for performing the nonlinear time history analysis. For nonlinear time history analysis, 4-story and 12-story 2D steel frames have been investigated. Designing of the mentioned frames have been carried-out using ETABS commercial software, and evaluation of seismic behavior of the frames has been performed using SeismoStruct framework [5]. For nonlinear modeling of steel material, Menegotto-Pinto constitutive modeling has been utilized in the finite-element procedure. The obtained results show that the dispersion of responses using the present study is less than the responses obtained from 2800[6].