Objective Phenomenological Constitutive Law for Collapse Analyses in Distributed Plasticity Steel-Frame Models

Nonlinear static and dynamic analyses are increasingly being used to evaluate the seismic performance of structures. The level of detail that mathematical models should include depends on the required analysis. In a collapse performance evaluation of special steel moment frames, models must include material and element deterioration due to local geometrical instabilities that will allow triggering structural instability. However, capturing these effects in distributed plasticity models is challenging for two reasons: (1) the selection of a constitutive law that incorporates the geometrical phenomenon, and (2) the localization issues related to the material softening. This paper proposes a simple approach to implement phenomenological calibrated constitutive law that includes compression softening applied on a distributed plasticity model. Additionally, a regularization method is proposed to reduce mesh-sensitivity, ensuring an objective response. The constitutive law and the regularization method are tested under different modeling levels: Material, element, and structure. Furthermore, the model is used to perform collapse analyses of 2-, 4-, 8-, 12-, and 20-story special steel moment frame buildings. The proposed model can incorporate critical features, such as flexure-axial force interaction, postpeak deterioration, and an objective global response regardless of the element discretization.