Combined Flexural and Cable-like Behavior of Ductile Steel Beams

Abstract

In addressing the propensity of steel building structures to experience progressive collapse due to extreme loading conditions (e.g., blast), current design guidelines propose the use of a threat-independent approach that is commonly referred to in the literature as “the missing column scenario”. Under this scenario, a column from a given story is assumed to be removed and the resulting structure is analyzed to determine if it could sustain the loads by activating one or more alternate load carrying mechanisms, with the idea of mitigating the potential for progressive structural collapse. This study specifically focuses on the ability of ductile steel beams to carry loads by transitioning from flexural behavior to cable-like behavior. Theoretical fundamentals of this behavior are described for rectangular and W-shaped steel beams with idealized boundary conditions and presumed fully ductile behavior. Two theoretical analysis approaches are used to model the beam behavior: rigid-plastic analysis and cable analysis. The main factors affecting the behavior, such as material and geometric properties as well as boundary conditions are described and corroborating nonlinear finite element (FE) analyses are presented and compared to the theoretical results. Open System for Earthquake Engineering Simulation, OpenSees was used in the FE analysis studies. Based upon theoretical and FE analysis results, a set of equations are proposed that can be used to predict the deflection at the onset of pure cable behavior. Additionally, the effect of elastic boundary restraints on the beam behavior was studied using FE analysis. An approach to evaluate the boundary restraints offered by the surrounding members in a given frame is also presented. It is shown that axial restraints have a much more significant effect on the behavior than rotational restraints. The theory presented in this thesis can serve as basis for designing ductile steel beams undergoing transition from flexural to cable-like behavior.