Effect of abutment-backfill limit state definition on the assessment of seismic performance

Over the last decades, several performance-based design and assessment procedures for bridges under earthquake loading have been put forward. For assessing seismic performance, an appropriate definition of limit states is necessary. For bridges with seat-type abutments, the importance of limit states related to damage in the abutment-backfill system is often overlooked, partly due to the use of joint gaps that substantially exceed the expected design seismic displacement of the deck; nevertheless, gap closure may still occur for earthquakes stronger than the design one, with beneficial or detrimental impact on the bridge behaviour. Here, the important limit states of ‘operationality’ and ‘collapse prevention’ are defined using different criteria for the various bridge components, including the abutment-backfill system; for the latter, displacement-based criteria that express damage in the backfill and the shear keys and their effect on the entire bridge were used for the longitudinal and the transverse direction, respectively. The effect of the selected criteria which are subject to uncertainty was studied for a typical concrete overpass. Nonlinear dynamic analyses were conducted for different levels of seismic action and the results were used to evaluate the performance of the bridge in either direction using a range of values for the criteria related to the abutment-backfill system. It was found that the abutment-backfill based limit states could be critical for certain levels of ground motion with regard to both the ‘operationality’ and the ‘collapse prevention’ of the bridge.