Pilot experimental and numerical studies on a novel retrofit scheme for steel joints against progressive collapse

Current design standards require that beam-column joints should withstand a minimum tie force in the case of an extreme loading event resulting in the loss of one or more columns to prevent progressive collapse of a building. However, research has shown that nominally-pinned steel beam-column joints, which are commonly used in non-seismic areas and in the gravity frames of seismically-designed frames, are not capable of providing the required tie force while undergoing significant rotations imposed by a loss of column event. This paper proposes a set of novel structural details that can be added to industry-standard nominally pinned joints to increase both their tensile resistance and rotation capacity. The proposed structural details exploit the exceptional strength and ductility of duplex stainless steel pins (SSPs) under bending. SSPs are strategically placed in a way that they do not interfere with the behaviour of the joint under gravity loads. The monotonic fracture capacity of SSPs is first experimentally evaluated and the calibration of numerical models that can capture the fracture behaviour of the SSPs follows. Using the calibrated numerical models, a parametric study was carried out to identify optimal geometries of SSPs to reliably achieve the required levels of tie force and rotation in a joint. Finally, two previously tested vulnerable fin plate joints are used as design cases to demonstrate the effectiveness of the proposed structural details to increase the resistance against progressive collapse of nominally-pinned joints.