Analytical Implication of Structural Seismic Response Parameters Exposed to Pulse-Type Ground Motions

This study evaluates the performance of mid-rise steel bending frames subjected to near field earthquake records according to the fourth edition of the Iranian seismic code 2800. Since the location of the epicenter of the next earthquake tremors is accurately unpredictable, the horizontal component of ground motion records could be applied at any direction relative to the structure. This issue must be noted during analyze process. Incidence angle of the seismic input has a considerable influence on the structural demand. This research considers the seismic directionality effects on three-dimensional symmetric steel rigid frame structure. The mentioned structure is exposed to different free-field three-directional ground motion records oriented along four incidence angles, whose values are assigned between 0 and 45 degrees with an increment of 15 degrees. The ensemble of ground motions mentioned above, includes two strong earthquake records in Iran, i.e. the Tabas 1978 and the Bam 2003 main shocks. Furthermore, two powerful pulse type records of the Imperial Valley earthquake 1979 entitled, El-Centro Arrays 6 and 7 (E06 and E07) as well as the Holtville Post Office (HVT) and Mexicali (MEX) records were selected too. The HVT and MEX records display neutral and backward directivity effects respectively. Moreover, in this research, the main criterion for selecting the mentioned near field records for nonlinear time history analyses, has been the presence of a single coherent pulse with a long period and high amplitude characteristics which emerged simultaneously as well as multiple wave-like features in the time history. The analytical assessment of the results of this research has been done with a major focus on direction of imposed records relative to the structure main skeletal axes. Yet, the corresponding effects on the fluctuation of seismic response parameters, such as lateral displacement and story drift which are of notable importance in the design process, were also assessed remarkably. It is noticeable that the results of conducted nonlinear analyses which performed through applying the ground motion components along the building plan principal axes, may underestimate the structural seismic demands.The strong ground motions that would generate the highest seismic structural demands, when applied in the most flexible direction of the resistant skeleton. The results indicate that each record would have unique direction which is the most flexible one that would generate the highest structural demands. The strong ground motions in the mentioned situation were able to release a high kinetic energy content. This release of energy occurs generally during a relatively short time domain which significantly increase demands in its own direction.

Keywords:

seismic design , rigid frame , near-fault record , incidence angle