Studying Higher Mode Effects on the Performance of Nonlinear Static Analysis Methods Considering Near-Fault Effects

다국어 초록 (Multilingual Abstract)

Daily development in civil engineering arena and the importance of economical aspects in the design of structures have motivated the engineers to shift their approaches from designing upon strength to designing upon performance. This is of more importance regarding the forces induced by earthquakes which have unpredicted and random nature. However, it is not possible to design or assess exactly the structural performance against strong ground motion using analytical methods which are applied for static loads and therefore precise dynamic analysis methods are needed. Despite significant progress in the analytical methods and engineering software, using nonlinear time history analysis is very difficult for engineers and designers due to unpredictable nature of future ground motions as well as complicated behaviour of structures. Consequently, the substituting methods are presented upon nonlinear static analysis. Coefficient method and capacity spectrum method, presented by FEMA356 and ATC40, respectively, are of this kind that have been used in the recent years and modified by newer documents such as FEMA440. Despite acceptable results, neglecting some important items, such as higher mode effects which can be decisive in tall and specific structures, made offering other procedures like Modal Pushover Analysis (MPA). In this study, the performance abilities of nonlinear static methods are studied in order to assess the dynamic response of structures under both far- and near-fault earthquake records. For this purpose, two reinforced concrete frames of 5 and 15 stories are analyzed by a series of nonlinear static and dynamic analyses. Analysis results show that conventional nonlinear static analysis methods cannot estimate accurately the deformation demands of tall building structures,particularly in the higher stories, due to the effects of higher modes. Although MPA method has almost improved the results, yet it has not high accuracy regarding near-fault earthquake records.

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