The 14 th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China A F.E.M. MODEL FOR THE EVALUATION OF THE SEISMIC BEHAVIOR OF INTERNAL JOINTS IN REINFORCED CONCRETE FRAMES. G. Manfredi 1 , G.M. Verderame 2 and G.P. Lignola 3 1 Professor, Dept. of Structural Engineering , University of Naples “Federico II”, Naples, Italy 2 Assistant Professor, Dept. of Structural Engineering , University of Naples “Federico II”, Naples, Italy 3 Assistant Professor, Dept. of Structural Engineering , University of Naples “Federico II”, Naples, Italy Email: gamanfre@unina.it, verderam@unina.it, glignola@unina.it ABSTRACT : The seismic performance of reinforced concrete buildings cannot be fully understood aside from the beam-column joints. The capacity design (and the subsequent strength hierarchy) or the displacement ductility design are modern design principles and they are strongly subordinates to the beam-column joint panels behavior which can reduce substantially the global ductility, if the joint is subjected to a premature failure. Both the cracking of the joint panel zone, the slipping of the passing through steel reinforcement rebars can generate additional deformability and they can alter the strength hierarchy between the elements converging into the beam-column joint. The present work focuses on these two aspects using a refined finite element non-linear modeling technique. A numerical-experimental comparison is discussed that validate the adopted numerical model. Numerical results were compared to experimental data and were found to be in good agreement with the test data, thus validating the methodological approach. KEYWORDS: Reinforced Concrete, Beam-Column Joint, Shear, Bond, F.E.M. Modelling 1. INTRODUCTION The behavior of the beam-column joints is a crucial aspect in the seismic design of Reinforced Concrete (RC) structures. The joints are geometrically circumscribed portions of the structures where the high stress transfer from the converging elements burden the concrete core and the reinforcing bars with very high gradients. This aspect can be particularly critical especially in the case of seismic resistant frames where the internal longitudinal steel reinforcement is subjected to tensile loads on one side and compressive loads on the opposite side of the internal joints at opposite column faces. Such conditions are particularly severe for the bond behavior of the passing bars through the joint, especially if it is referred to the high stress states and to the reduced dimensions of the joint core. These aspects, coupled to the mechanical non-linear behavior of the concrete, can cause a reduction of the flexural capacity of the converging elements and a significant variation of their deformability, due to the variation of the stress state in the reinforcing bars at both joint’s opposite lateral sections (Hakuto et al., 1999; Fabbrocino et al., 2004). Usually, according to the main international design codes, the beam-column joint failure is driven by the evaluation of the so-called “Joint Shear” assumed as the main parameter controlling the resultant of all the actions given by the converging elements. The joint shear, V j , evaluation is based on simple equilibrium equations involving beams and columns (see Figure 1), it is an internal force acting on the free body along the horizontal plane at the midheight of the beam-column connection. In the case of internal joints, the horizontal equilibrium related to the lateral cross sections yields to Eqn. 1.1: ' ' ' j c s c c s c V T C C V T C C V = + + − = + + − (1.1) Where column shear, V c , is the shear at the base of the upper column while C c , C s and T are, respectively, the force resultant in compression in concrete, and the compressive and tensile resultant in the reinforcing bars. These forces are evaluated on the right side of the joint. On the contrary, the forces evaluated on the left side of the joint are marked with an apex.