Effects of changes in seismic design criteria in the transverse and vertical response of Chilean highway bridges

Recent earthquakes in Chile have produced extensive damage in highway bridges, such as large transverse residual displacements, yield of shear keys, and unseat of the main girders, showing that bridges are highly vulnerable structures. Many of these damages are attributed to problems in the design codes. After the 2010 Maule earthquake, new structural design criteria were incorporated for the seismic design of bridges in Chile, among which the increase of the strength of the exterior and interior shear keys and the seismic bars, and the mandatory use of diaphragms at the ends of the girders stand out. The present study assesses the effects of the changes in seismic design criteria in the transverse and vertical response of Chilean highway bridges. This assessment is accomplished by comparing fragility curves of a typical bridge designed using the Chilean standards before and after the 2010 Maule earthquake, taking into account the seismic soil types and the different seismic hazard zones. Firstly, the evolution of bridge seismic design codes in Chile is described. Secondly, the performance of bridges and their main failure modes during the 2010 Maule earthquake are summarized. After this, four structural configurations representative of typical bridge typologies were designed using the different design criteria and then they were modeled for the evaluation of their seismic behavior. Finally, fragility curves were generated using Incremental Dynamic Analysis (IDA), nonlinear analytical models and ground motion records from previous earthquakes. The calculated fragility curves show that bridges designed with the current design codes have a seismic performance that depends, to a large extent, on the type of soil in which the bridge is located. The changes in seismic design criteria significantly decrease the probability of collapse of bridges, while the vertical response is unaffected by those changes. Despite those changes in design criteria, the results of the models show that the shear keys behave as sacrificial elements, avoiding damage to the columns and cap beams. Design recommendations that improve the seismic performance of non-skewed bridges are provided.