The Integrity of Steel Gravity Framing System Connections Subjected to Column Removal Loading

Abstract

Large-scale system tests conducted on steel gravity framing systems (SGFSs), one of the most commonly used structural systems in the United States, have shown that enhancements may be required to achieve the desired levels of integrity to collapse when a column suffers damage that compromises its ability to carry gravity loads. One potential mechanism for SGFSs to arrest collapse is through the development of an alternate load path in a sustained tensile configuration resulting from large vertical deflections. The ability of the system to develop such an alternate load path is in part dependent on the ability of the gravity connections to remain intact after undergoing large local deformations. This study experimentally evaluates the performance of steel single plate shear and bolted angle connections when subjected to loading consistent with column removal scenarios. The characteristic connection behaviors are identified and key connection force and ductility capacities are reported for a number of different connection configurations. An approach to determine the deformations of the connection when discretized into fibers is proposed that enables fiber deformations to be estimated from system displacement. Here, the approach is used to determine the limiting displacements of the fibers at connection failure, an important parameter for analysis of floor systems in practice. An analytical rigid-body fiber displacement model is used to estimate the bare-steel framing capacity contribution to the resistance of a SGFS to column removal using the connection sub-assemblage results. For column removal scenarios in which the steel framing may be required to carry a large portion of the gravity load, the framing contribution was shown to be inadequate. Detailed FE models using ABAQUS were conducted to investigate alternate gravity connection details and retrofit strategies to enhance integrity. These new connection details are shown to improve the bare-steel framing contribution to the SGFS performance, relative to conventional connection details.