Adaptive Modal Pushover Procedure for Seismic Assessment of Bridge Structures

During the last decade, application of pushover analysis in seismic assessment of structures is increased. Pushover analysis provides crucial information on responses of structures that cannot be obtained through conventional linear static methods. However this method suffers from some drawbacks. The major drawback of the conventional pushover is that it is basically restricted to a single-mode response, and cannot account for the contributions of the higher modes and also the progressive change in the modes shapes because of structural yielding. To remedy this shortcoming, recent years have witnessed the development of some adaptive modal pushover methods. These methods obviously consider the effects of the higher modes and progressive variation in the dynamic attribute of the structure in the inelastic phase. However, these advanced methods are generally proposed for building structures and their application to bridges is questionable. In order to extend the application of these methods to bridge structures, the differences in the structural system of buildings and bridges should be considered. In the common pushover methods proposed for buildings, the top storey displacement is used as displacement character in pushover curve while, in the bridge structure selection of each support displacement as displacement character are disputable. In this paper an advanced adaptive modal pushover method is proposed for seismic assessment of bridge structures. The proposed method takes into account the higher modes effects and progressive change in modal properties. This new method incorporates the adaptive pushover method in multi modal pushover analysis. In fact in the proposed method, three main concepts: 1) modal combination, 2) adaptive load pattern and 3) energy based pushover curve are used and their advantages are combined together. In the proposed method the pushover curve is provided individually for each mode. The pushover curve of the multi degree of freedom system (MDOF) is converted to the single degree of freedom system (SDOF) curve with unit mass. In each load step based on the simultaneous mode shape and energy concept, the MDOF system curve is converted to a SDOF system. Since the energy concept is used to define the capacity curve, the values and signs of the displacements in all support are considered. Therefore the ambiguity in selection of the support as control point (displacement character) is removed. In each step the increment of the displacement in the SDOF system is defined based on sum of the work done at each support level through each incremental force. At each step, the total work done in all supports is assumed to be equal to the work done by the total base shear. The proposed method is applied to an irregular four span viaduct under Tabas earthquake records. It illustrates a good estimation of the peak displacement response in the inelastic phase.