Modelling of the post-peak response of abutment-backfill systems: a new hysteretic model for OpenSees

The findings of numerous analytical/computational studies, tests, and in-situ observations have highlighted the importance of the deck-abutment-backfill interaction on the seismic response of bridges. These findings have allowed various researchers to develop models for the nonlinear behaviour of the abutment-backfill system. Such models have been extensively used in research, and the simpler among them have been adopted by modern seismic codes and guidelines and have also been incorporated in programs for the nonlinear analysis of structures. Although large-scale tests have captured the behaviour of the abutment-backfill system subsequent to the attainment of peak strength, this critical phase of the response has neither been studied to a sufficient extent from the analysis point of view, nor included in the constitutive models of available nonlinear analysis programs. The softening region of the response of the backfill has been addressed using a number of hardening/softening plasticity models that are rarely used in practical analysis wherein simpler models (like Winkler springs) are typically preferred. Capturing the post-peak range of the abutment-backfill response is very significant in seismic fragility analysis of bridges for the definition of limit states close to failure. In the above context, the main aim of the work presented herein is to introduce a new model capturing this range of the backfill response; it is an extended version of the ‘Hyperbolic Gap’ ‘material’ in OpenSees, named ‘Hyperbolic Gap Softening’ material, which allows the user to define a set of parameters that describe the softening branch of the backbone curve of an abutment-backfill system. In addition to tests of the new model using a simple structural configuration, results of response history analyses of a case-study bridge with different abutment-backfill properties are also reported, with a view to verifying the applicability of the proposed model and assessing the importance of capturing the post-peak abutment-backfill behaviour on the assessment of the seismic response of a bridge.