A simple but effective capacity model for check and design of beam-column joints in RC seismic buildings

Abstract

The design of beam-column joints is a crucial step of the design of seismic-resistant RC framed structures. Premature joint failure may prevent yielding of beams and the achievement of the target collapse mechanism. Hence, an effective capacity model is essential. In the past, Italian building code in force before NTC (2008) required that a minimum amount of ties, equal to that used in the column, be provided in the joint, but it did not stipulate any verification of the joint. Eurocode 8 (2005) and subsequently Italian NTC (2008 and 2018), recognize that shear in beam-column joint is higher than in columns and recommend the explicit verification of the joint. The suggested capacity model is based on stress analysis and is very demanding in terms of joint ties. The new draft of Eurocode 8 (2020) provides a different capacity model, based on research carried on by Fardis (2021), which better reflects the state of art on this topic under both theoretical and experimental point of view. This paper examines the capacity model for shear resistance of joints adopted in Eurocode 8 draft, underlining its advantages but also some difficulties in practical application. Based on a parametric analysis of a wide set of RC joints, it is showed that it is possible to assume that the joint rebars are yielded when the joint attains its ultimate limit state. This assumption does not determine any significant loss of precision in evaluating the shear resistance of the joint and leads to a capacity model consistent with other models adopted in Eurocode 8, e.g. those for shear resistance of RC beams. Furthermore, it allows a closed form solution of the problem that leads to a very simple procedure both for the verification of joint resistance and for the optimization of the design of horizontal and vertical rebars of the joint.