Earthquake source effect and impact of the applied methodology to assess the overstrength factors of RC bridges

Abstract

Seismic hazard assessment in several sites worldwide depends on two or more seismic sources. Many countries affected by subduction zones have strong motions earthquakes originated in at least two important seismic sources. In Mexico, interplate earthquakes (subduction process) have epicenters located in the Pacific coast with hypocenter depths less than 45 km and intraplate earthquakes (intermediate-depth earthquakes) normally have epicenters inside the continent with depth hypocenters greater than 45 km. Both seismic sources can potentially produce events with magnitudes Mw greater than 8.0 and, therefore, generate severe damage to the country's infrastructure. The design process of the bridges in Mexico is based on the chapter of Seismic Design of the Manual of Civil Structures of the Federal Electricity Commission. The standard allows to reduce the spectral ordinates of the design spectra by ductility and overstrength. It establishes an overstrength factor of 1.5 to reduce the design spectra, value not well supported by the Manual. This study evaluates the effect of the seismic source and methodology used to compute overstrength factors of common typologies of reinforced concrete bridges. The seismic capacity of the bridges was calculated with nonlinear static analysis and nonlinear time history analysis. The results showed that the importance of the seismic source on the overstrength factors depends on the seismic location of the bridges and, in general terms, nonlinear static analysis overestimates the bridges overstrength.