Implementation of Nonlinear Elements for Seismic Response Analysis of Bridges

General purpose finite-element software tools have put nonlinear analysis within an engineer’s reach for the assessment of bridge response to seismic loading. Although these tools can capture strength, ductility, and nonlinear material and geometric effects more accurately than response spectrum or linear methods, the response is extremely sensitive to modeling techniques, even when the same input parameters for bridge geometry and material properties are used in different software packages. The resulting discrepancies in nonlinear response are due to mathematical formulations of the element response, for example, concentrated or distributed plasticity, and software-dependent implementation of the formulations and their constitutive models. To illustrate the effects of modeling choices and the ability of two widely used software packages, CSiBridge and OpenSees, to reproduce analytical solutions, concentrated and distributed plasticity models were applied to cantilever bridge columns with simplified steel and concrete constitutive models. Discrepancies in stiffness and strength owing to the location and length of plastic hinges can be resolved for these simple component models. The modeling strategies were extended to two ordinary standard bridges designed by Caltrans. Although modal analyses show the bridge models have approximately the same distribution of mass and stiffness in the two software packages, only nominally consistent results can be achieved when using more realistic constitutive models for nonlinear static analyses.