Influence of rigid floor assumption in seismic analysis of RC existing buildings

The study concerns with the seismic analysis of reinforced concrete (RC) building, with particular attention to the role played by floor system. The rigid-floor assumption is often used for RC existing buildings' vulnerability assessment, in order to simplify the study of their structural behavior. Seismic codes (EC8, NZS4203, FEMA-273) provide a set of quantitative criteria related to deformation of diaphragm and average drift of the associated storey and qualitative criteria related to the shape of diaphragm. Despite in many cases this hypothesis can be considered sufficiently accurate, it is not always valid because it is related to several features of structure as floor geometry and materials, relative stiffness between floor system and vertical structural elements, shape of building, etc. In fact, unlike new structures, the main problem of RC existing buildings is not having geometrical and mechanical regular features (for example, in a ribbed slab, the irregular depth of the top concrete slab) as well as to present irregular dynamic parameters. When the floor system cannot be considered rigid, it may be studied as flexible slab with the increment of uncertainty related to stress states of structural elements. In order to understand how the floor system influences the structural behavior, it has been investigated the seismic performance of several applications, extrapolated from real RC existing buildings, through numerical methods, with and without the rigid-floor assumption.