The seismic response of irregular bridges could be complex if some irregular conditions were presented in the substructure. To evaluate as a simple form the influence of some irregular conditions, various irregular indices were proposed, considering elastic or inelastic parameters. With these indices, only one number, between 0 and 1, could be used to represent the bridge irregular seismic behavior for design and analyses proposes. However, applying various indices to the same structure could produces different results and it is difficult to define its application in the design procedure. In this paper, an elastic irregular index is applied to bridges with different irregular substructures, based on the variation of central and extreme piers length. As a regular model, a simple bridge with four spans and three piers of hollow rectangular section was considered, with unicellular box girder. The pier high variations include +25%, +50%, +75%, -25%, -50% and -75% of the original length, so seven irregular models were considered for each regular bridge. Monolithic connection between piers and girder are considered. Regular and irregular models were subjected to a 53 accelerograms database, registered in the most earthquake hazardous zone of Mexico. Through elastic analyses the maximum displacements of piers were defined for each accelerogram and bridge model. A new index is defined as the normalized difference between the maximum displacements of the irregular and regular models. Tendencies of the propose normalized difference and an irregular index available in literature were compared to define the best representation of the irregular condition of bridges.

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