Distributed inelasticity planar frame element with localized semi-rigid connections for nonlinear analysis of steel structures

A macro-element model with spread of inelasticity and localized nonlinear semi-rigid hinges is presented in this paper. The element is formulated through force-based approach. Spread of inelasticity along element length and section depth is captured by fiber discretization of monitoring sections, and introduction of localized connections enable representation of any type of linear or nonlinear semi-rigid (partially restrained) connection behavior along element length without any increase in number of degrees of freedom. The element is used in conjunction with corotational formulation for the capture of nonlinear geometric effects. The proposed element is used for the analysis of steel structures considering the presence of beam to column semi-rigid connections in this paper. For a true representation of the hysteretic energy dissipation characteristic of a beam to column connection a quadra-linear moment-rotation based model is implemented incorporating pinching, damage, the possibility of residual moment capacity and degradation of stiffness under various scenarios. The behavior is calibrated via experimental data available in the literature. Robustness and accuracy of the proposed element and hysteretic connection model are assessed through numerical and experimental data, and the importance of accurate modeling of cyclic behavior of semi-rigid connections is demonstrated through analysis of a steel moment resisting frame.