Improved Bouc-Wen Model Implementation in OpenSees

The current implementation of the Bouc-Wen model in OpenSees exhibits unstable behavior for certain parameter values. In particular for values of the exponent n lower than one. Such values are used, for example, to describe the materials that exhibit a nonlinear behavior even at small strain values, for example soil. In this paper an implementation in OpenSees of a numerical algorithm for determining the ordinary differential equation internal to the Bouc-Wen model to account for this unstable behavior is proposed. In particular, a search interval reduction procedure, based on the bisection and false position method, is described. The Bouc-Wen constitutive law, modified by Gerolymos and Gazetas to account for soil degradation, was also implemented in OpenSees and the same solution scheme is used to avoid numerical instabilities. This model is used in a one-dimensional site-response analysis and in the dynamic seismic response evaluation for integral abutment bridges.