Influence of geometrical and mechanical parameters on the seismic vulnerability assessment of confined masonry buildings by macro-element modeling

A new innovative structural system was developed in the reconstruction of Reggio Calabria city after the destructive 1908 earthquake, which was termed "confined masonry". Confined masonry structures are seismic resisting structures, where masonry walls are confined by reinforced concrete beams and pillars. During the construction of a confined masonry structure, masonry walls are used as formworks to build the reinforced concrete elements. The reinforced concrete frame plays the important role of confining masonry walls, and therefore helps in increasing the ductility of the whole structure; this implies better performances of the confined masonry with respect to the traditional masonry structures. In confined masonry structures, openings are confined by reinforced concrete frames, while wall intersections and floor slab-wall connections are realized by means of reinforced concrete elements. As observed after several severe earthquakes, confined masonry structures showed a reliable anti-seismic behavior due to several reasons. Among them, the confinement action of the reinforced concrete frames, the in-plane floor stiffness, the plan and elevation regularity. Nowadays many confined masonry buildings still exist in Reggio Calabria city and, in the frame of seismic risk reduction, the evaluation of their seismic vulnerability is of a great importance. However, when performing a large-scale seismic vulnerability assessment of a class of buildings, the mechanical properties of the structural elements cannot be precisely evaluated by means of in-situ experimental tests. These properties can only be estimated within reasonable intervals. In this paper a parametric study have been performed in order to investigate the influence of some mechanical parameters on the seismic response of confined masonry buildings. In particular, the effects of the quality of masonry in terms of shear and compressive strength, deformation capacity and Young and shear modulus have been considered, as well as the influence of the longitudinal bars and stirrups. Finally, the compressive strength of concrete is considered and its importance on the seismic response is estimated. The seismic performances in terms of push-over curves have been evaluated according to the Italian Seismic Code by means of 3DMacro structural analysis software. Masonry walls are modeled through an innovative "macro-element", which allows to take into account different collapse mechanism: bending failure (rocking), shear failure by diagonal cracking, shear failure due to sliding. Secondary element, such as pillars, beams, architraves, are modeled by using nonlinear frame element having concentrated plasticity at their ends.