Regularization method to include material softening in fiber beam-column elements for seismic performance assessment of steel frames

Currently, nonlinear analyses are widely used to evaluate the performance of special steel moment frames (Special SMF). Most of the mathematical models used in these analyses used concentrated plasticity approaches whit lumped nonlinearities at the end of beams and columns. Even when these models are highly recommended due to their simplicity and computational efficiency, they present some setbacks related to the axial-flexural interaction and the load protocol calibration dependency. In order to overcome these problems, a distributed plasticity model can be used. However, these models are not intended to capture local geometrical effects such as buckling or necking into the material constitutive law. In addition, these models present localization issues when the constitutive law includes softening, leading to variability in the global response. This paper proposes a distributed plasticity beam-column model that overcome these limitations. The constitutive material law proposed herein captures softening due to local buckling under compression stress using a non-symmetric strain-stress curve. The parameters are calibrated through experimental data and validated for different axial loads and loading protocols. Moreover, to solve the softening-localization issue, a method of regularization is proposed. This regularization is verified through static and dynamic analyses of an 8-story Special SMF building. The results show that the proposed model can represent the behavior of W shape sections under different load protocols and axial load demands. In addition, the model can accurately incorporate critical features as maximum flexural capacity, flexure axial interaction, and post-peak softening. The regularization method yielded negligible variations under different plastic lengths of the column for all the analyses. The proposed model is simple to implement, and the results indicate that it can be used to evaluate the seismic performance of Special SMF.