A new method to assess the stiffness and rotation capacity of composite joints

Composite beam-to-column joints in buildings are mostly modelled as pinned joints in order to facilitate the design of the structure. In reality, due to the required reinforcement in the concrete slab, a certain joint rigidity and bending resistance is always available. The real joint behaviour corresponds therefore more to that of a semi-continuous joint. This is not only beneficial for the serviceability limit state but can also be advantageous at ultimate limit state. However, due to the lack of analytical design rules in EN 1994 to verify the rotation capacity of semi-continuous joints, these are commonly modelled as pinned joints, which impedes an efficient design of composite structures. In this context, a research program on the behaviour of composite joints, focusing on the ultimate rotation capacity, was initiated at the University of Luxembourg [1]. The aim was to identify the influence of two major joint components – the reinforced concrete slab and the steelwork connection – on the moment-rotation curves of composite joints under hogging bending moment. An experimental campaign comprising 8 tests on beam-to-column joints was conducted to determine the response of composite joints with variable reinforcement ratio and diameter of reinforcing bars. In addition to the experimental part, an FE model was developed with the software ABAQUS aiming to simulate the behaviour of internal beam-to-column composite joints. In this paper, the 3D finite element model and results of analyses are presented. The FE model has been defined by 3D solid elements with realistic contact definitions and non-linear material laws. The results of the numerical simulations presented a good agreement with the experimental data. Based on the experimental and numerical investigations, the influence of reinforcement and steelwork connection on the structural properties of composite joints is derived. A new analytical method to determine the stiffness and rotation capacity of composite joints is proposed. The accuracy of this new method is confirmed by existing experimental and numerical results.