Effect of Plastic Hinge Properties in Nonlinear Analysis of Highway Bridges

Current design practice of highway bridges is moving towards an increased emphasis on nonlinear static analysis methods. Modelling for such analysis requires the determination of the nonlinear properties of the bridge elements that are expected to behave nonlinearly. Nonlinear static analysis is carried out for either user-defined nonlinear hinge properties or automated-hinge properties, as available in the software SAP2000. User defined hinge properties can be obtained using the recommendations of the Seismic Retrofit Manual by the Federal Highway Administration. Automated-hinge properties in SAP2000 are computed automatically from the element material and section properties according to Caltrans criteria. The bridge designer needs to be aware of that the majority of old bridges were built with little or no consideration to seismic forces and plastic hinge detailing requirements. Therefore, the use of automated-hinge properties for old bridges in nonlinear static analysis may lead to unrealistic displacement capacities. In this study, pushover analysis of two highway bridges built with little attention to seismic forces was performed in an effort to evaluate the difference in global response predicted by using the user-defined nonlinear hinge properties or automatedhinge properties in the software SAP2000. The results demonstrated that user-defined hinge model is capable of capturing the effect of local failure mechanisms, in the plastic hinge region, on the global response of the bridge; while the automated-hinge model can not capture this effect. Therefore, automated-hinge properties should be used with a lot of care, especially for old bridges that might include local failure mechanisms in the plastic hinge region.