Valutazione numerica e sperimentale della performance sismica di edifici in c.a. a bassa duttilita. Edifici a nucleo e a soletta piena

This work reports seismic evaluation of two particular types of R/C (Reinforced concrete) buildings with low ductility capacity: U-core buildings, or torsionally deformable buildings, and flat-slab buildings. In both cases, although for different reasons, these structures are extremely vulnerable to seismic action. Special studies with suitable approaches are therefore demanded from seismic standard codes when treating them. Examples of U-core buildings are the typical multi-storey apartment blocks built in Italy between 1960s and 1980s with a rigid central staircase, i.e. the R/C U-core, in most cases designed only to withstand static actions, in accordance with the design code of those year. The main structural problem of these buildings is due to their in-plan irregularity, which involves torsional behavior when subjected to seismic events. This produces large displacements and deformations of the flexible edge and strong forces on the R/C U-core which represents the stiff edge. Flat-slab buildings do not generally have problems of low torsional stiffness since, if well designed, they are equipped with perimeter frames or walls. One of the major problems lies in the connections between the slabs and their supporting columns. Large bending moments and shear forces are concentrated at the connections, which are thus susceptible to non-ductile, sudden, brittle punching failure. Connections failing in this mode have very little post-failure rotational capacity or ductility. A common issue for these two types of buildings is therefore the low energy dissipation capacity due to limited ductility, which is translated by standard seismic codes in quite low values of the behavior factor 'q'. A good estimation of this factor, by non-linear numerical analysis or experimental evidences, is mandatory for a proper seismic evaluation of the cited buildings. Based on the foregoing, the present work is therefore divided into four parts. All of them are first preceded by a short Introduction where the major features of these buildings and an overview of the current developments in research and seismic codes are presented. Part 1 studies non-linear static methods (pushover methods) for seismic assessment of both regular and irregular buildings. The applicability and validity of such methods, in particular of the advanced pushover methods specifically developed to be applied to irregular structures, to the case study of U-core buildings is evaluated. As most innovative result, Part 1 presents and discusses the validation of the N1 method, a non-linear static procedure recently proposed by Bosco et al. and its improvement, in order to make it completely conform to the widely accepted N2 method. In the present work, the N1 corrected method is presented and validated with reference to a set of 22 steel and R/C frames, with regular and irregular mass distribution in height. This analytical correction was facilitated by introducing coefficient , calculated in closed form from the results of modal analysis. N1 thus becomes equivalent to N2 and can be used for practical applications in the same conditions, with the advantage that it can easily be adapted to the modern Displacement-Based Seismic Design of structures. Part 2, which is essential for the numerical study conducted on U-core and flat-slab buildings, reports the validation of the adopted non-linear numerical model through the reproduction of three experimental tests on complex RC walls found in literature. Part 3 represents the main part of this thesis. It covers seismic analysis of U-core buildings by means of non-linear dynamic analyses aimed at numerical evaluation of behavior factor 'q', to be used in less expensive but still reliable spectral response analyses. Numerical evaluation was conducted on suitably designed case studies representing typical U-core buildings designed only for gravity loads with various core eccentricities and torsional stiffness. Part 4 reports study of flat-slab buildings for which provisions in seismic design codes are still largely deficient. Results from experimental tests on four specimens of full-scale flat-plate slab-edge column connections made with normal concrete and steel fiber-reinforced lightweight concrete are reported. Quasi-static reversed cyclic loading was applied to the specimens to investigate their ductile behavior. Analytical interpretation of the experimental results in order to obtain correspondent values of ductility capacity and behavior factor is also reported. Numerical simulation and extension of the results have been also conduced. This part of the research is still far from being exhaustive and is suitable for further future investigatio