Shear-Enhanced Gradient Inelastic Force-Based Frame Element Formulation for Analysis of Shear-Critical Reinforced Concrete Members.

A large number of structures in the United States and worldwide include nonductile reinforced concrete (RC) frames with columns and beams that are prone to shear failure. Due to the brittle nature of shear failures, accurate simulation of RC structures with shear-critical members is essential to predicting their overall capacity under severe loading scenarios (e.g., earthquakes) and designing effective retrofits and upgrades. In this paper, a previously developed gradient inelastic (GI) force-based (FB) beam-column element formulation capable of capturing axial-flexural interaction and predicting flexural failures is extended to account for axial-flexural-shear interactions in RC members in order to predict shear failures. The proposed shear-enhanced GI FB element formulation advances the original GI FB element formulation by developing higher-order cross section kinematics, i.e., beyond the plane sections assumption, and by developing a 3D concrete constitutive model. The higher-order cross section kinematics can simulate strain distribution of the cross section more accurately while using 3D concrete constitutive models at the element's cross sections permits simulation of axial-flexural-shear interactions. To incorporate the confinement effects of transverse steel reinforcement, through-the-depth stress equilibrium is strictly enforced in the transverse directions of the member's cross section. To eliminate strain localization phenomena, new gradient nonlocality relationships are introduced in addition to those of the original GI FB formulation. The proposed element formulation is implemented in the OpenSees structural analysis software and is shown to maintain continuous macroscopic section strain distributions over the element length during softening and discretization convergent responses, thereby eliminating the strain localization phenomena. In addition, the predictions of the shear-enhanced GI FB element formulation are compared with data available from experiments on RC beams and columns. [ABSTRACT FROM AUTHOR]