THE ROTATIONAL CAPACITY OF THE STEEL-CONCRETE COMPOSITE BEAMS UNDER HOGGING MOMENT AND THE SEISMIC PERFORMANCE OF THE COMPOSITE FRAMES

The use of steel-concrete composite framed buildings is particularly efficient for their high lateral stiffness, strength and ductility; therefore the structural performance achievable with these systems makes them particularly suitable for applications in seismic zone. However the current state of technical knowledge concerning the characterization of the structural behavior of steel-concrete composite systems subjected to seismic actions is not exhaustive and requires additional theoretical and experimental studies in order to better understand their behavior and improve the design procedures. Often these shortcomings mean that the choice of the framed structure as seismic resistant systems falls in reinforced concrete or steel buildings. In fact, in the case of the steel-concrete composite framed structures the complexity of the problem is increased by the identification of the role of the connection between the reinforced concrete component and the steel one. In light of this assumption, the research project undertaken in the three-years of PhD study has been addressed to the analysis of the seismic performance of the steel-concrete composite frames and the development of advanced computational models for the seismic design of this type of structure. In particular two aspects were selected and developed in this research field: the rotational capacity of steel-concrete composite beams under hogging moments and the rotational capacity of the connection between the composite columns and the foundation. All the decisions regarding the materials/connection models had the aim of characterizing as better as possible the three-dimensional FE models of the composite beams and the base column connection. The effectiveness of the models were investigated by the comparison between numerical and experimental results; the experimental tests on the composite beams subjected to hogging moments, had carried out by myself previously. After the calibration of the models, parametric analyses were performed. In the case of the beams an innovative approach for defining the moment-curvature relationships, considering the effect of the local buckling, was assessed together with an equivalent plastic hinge length, obtaining the procedure to calculate the rotational capacity. Also for the base column connection a plastic hinge length was defined, taking into account the effect of the fixed end rotation, in the case of a socket connection. Both the results, for beams and columns, represent the instruments for a reliable non linear analysis of the composite frames in which the dissipation of the seismic energy is addressed at the end of the beams and at the base connection, as typically aimed to the framed structures. The last step of this research was the implementation of the relationships assessed for the rotational capacity for the beams and the base of the columns, in the nonlinear model of a composite frame designed according to the requirements of the standards, both national and European. The results of the analyses, in terms of behavior factors and over-strength ratio, point out the good performances of the composite frames and their promising use to reduce the seismic risk of new buildings.