Arching followed by catenary response of steel shear connections in disproportionate collapse

Steel shear connections are mainly designed to sustain shear forces. There is limited research assessing the axial response of shear connections, which is important in the evaluation of robustness of steel structures. Structural collapse can be arrested following localized damage if an alternative load path with sufficient capacity is available. In this study, the formation of compressive arching action followed by tensile catenary action is investigated. It is shown that in a column loss scenario, connections may develop significant axial compressive force before catenary action begins; this phenomenon has often been neglected in assessments of connection robustness in the sense that axial force is reported as a tensile action only. The presence of arching action has been confirmed in two experimental programs, and one set is selected for further study using a numerical approach. A simplified analytical model is then presented and compared with the observed axial response of these connections. It is concluded that vertical eccentricity between the centres of rotation of the connections at the two ends of a beam is the principal factor causing the development of a compressive arching force. Another influential parameter that affects the formation of arching action is the stiffness of the surrounding structure.

Simplified Compression Arch Action Model for R/C Beam-column Assemblages

A set of analytical equations are developed for calculating the beam-column assemblage flexure action capacity and compression arching action capacity under a middle column removal scenario. The suggested equations covered most of the main parameters affecting the assemblage behavior including seismic detailing, longitudinal reinforcement ratios, concrete confinement, and the contribution of concrete flanged slabs. The proposed analytical model for predicting the flexural and compression arching action capacities is validated with a large number of experimental results. The model provides a good estimation for 79 beam-column assemblages with several geometrical, reinforcement configurations, and material parameters. The mean values of the experimental to the theoretical ratio for calculating flexure and compression arching capacities are 1.15 and 1.16, respectively. The predictions of previous compression arch action models are found to be more conservative. Finally, the proposed model is utilized in parametric studies including all key parameters that affected resistance of the beam-column assemblages against progressive collapse.

Bidirectional strut method: out-of-plane strength of confined masonry walls

An analytical method to determine the out-of-plane strength of confined masonry walls is developed. The method is called the “bidirectional strut method.” Walls with and without openings subjected to combined out-of-plane and axial loads are considered. The method is based on two-way arching action. Masonry compressive strut forces are transferred eccentrically to the concrete confining elements. Flexural and torsional effects, together with the variation of displacements along these elements, are considered. Analytical strengths of confined walls are determined using this method. These strengths are compared with experimental and other analytical strengths. A sensitivity analysis of the strength is carried out considering different variables. It is concluded that the bidirectional strut method accurately predicts the strength of the walls studied. The main variables that affect the strength are the wall aspect ratio, wall slenderness ratio, and the stiffness of the confining elements.