Mechanical behavior of the extended end-plate connections of a portal frame with various retrofitted constructions under loading

Abstract

With the mounting number of photovoltaic power generation panels on roofs, the proliferation of heavier cranes, the addition of interlayers, and extreme weather conditions, the loads imposed on portal frame structures have observably increased, resulting in a strong demand for retrofitting. However, there remains a dearth of quantitative assessment regarding the efficacy and superiority of retrofitting techniques for beam-column connections with a bolted extended end plates (BCCBEEPs). Given that the retrofitting process cannot be characterized by complete unloading, it is crucial to accurately comprehend the impact of preload on retrofitting. In this study, finite element models (FEMs) of BCCBEEPs with seven different retrofitted constructions were established and validated against experimental results, and their mechanical behaviors were thoroughly compared and analyzed. Moreover, the influence of the preload level on the retrofitted BCCBEEPs was explored. The analysis results reveal that: excessive reinforcement of the component was found to shift the weak link to the connection, making the bolts more prone to failure and reducing energy dissipation capacity and ductility. Compared to an un-retrofitted connection, the carrying capacity of the PSPF retrofit measure, for which steel plates were pasted on the inside of beam flange, was found to be increased by a maximum of 38%. However, the energy dissipation capacity was decreased by 39%, the bolt tension was increased by 61%, and the prying force was increased by 58%. Importantly, not only components, but also connections, must be strengthened. Excessive preloads can lead to increased vulnerability at connection areas of retrofitted connections, especially in cases of over-reinforcement. The higher the preload level, the greater the reduction in bearing capacity, which could potentially fall below that of an un-retrofitted connection. When the preload level reached 80% of the strength of un-retrofitted connection, compared to the no-preload connection, the PSPF measure experienced a decrease of 18% in carrying capacity, a reduction of 31.8% in ductility, an increase of 14% in bolt tension, and a rise of 27% in end plate prying force. Therefore, when the preload level exceeds 20%, it is advisable to accurately assess the impact of the preload on the retrofit before implementation.