A retrofit method for extended end-plate connections in portal frames using prefabricated cover plate components under loading

Many industrial buildings need to modify or add functional uses, or incorporate energy devices such as photovoltaics to meet low-carbon requirements. The beam-column connections with bolted extended end-plates (BCCBEEPs) of portal frame, designed based on internal forces, fail to meet the required load-bearing capacity and need to be retrofitted, but no effective retrofit measures are available. An innovative prefabricated cover plate component (CPC) method was proposed for retrofitting BCCBEEPs, which does not require dismantling the original connections and allows for quick on-site installation of pre-manufactured components. Finite element models of 28 specimens with two typical failure modes (end-plate yielding and bolt fracture) were created. The CPC retrofit's effectiveness in improving bearing capacity and seismic performance was evaluated by comparing key mechanical performance indicators. The impact of five preload levels on retrofitted BCCBEEPs was also explored. The results show that: without retrofit, the failure modes of the connection are end-plate yielding and bolt fracture. After applying the CPC retrofit measure, the failure location shifts from the connection to the beam component, and no failure occurs at the connection. The maximum reduction in the end-plate gap and bolt tension at the connection are 62 % and 20 %, respectively. With the strengthening of the connection, the yield and peak bearing capacities increase by over 13 % and 14 %, respectively. In terms of seismic performance, the degradation of strength and stiffness of the connection does not significantly weaken, while the initial stiffness increases by up to 55 %, and the energy dissipation capacity increases by up to 2.9 times. After retrofitting the connection under load, the ultimate bearing capacity remains unchanged, but the bolt tension is prone to exceed the limit, leading to fracture, especially when the preload level is 0.8. Each component of the CPC retrofit measure requires further optimization to ensure better collaboration with the end-plate and connecting bolts, achieving optimal load-sharing and maximizing efficiency.