Checking the seismic safety of existing structures is a complex task, mainly due to uncertainities connected with material properties, loads and structural geometry. Before O.P.C.M. 3274, with the exception of applications to masonry buildings (and after important seismic events), no italian code has defined methods specifically devoted to existing, structures considering also r.c. buildings. included. The real novelty of new code applications consists in the development of reliable and applicable produres for the analysis of existing structures, especially of the reinforced concrete ones. The more credited methods (also for practical purposes) may be divided into two classes: linear and non linear ones (among them pushover analysis). These methods may be applied either to static analysis and to dynamic analysis. Static analysis is carried out by assigning a standard distribution of lateral forces along the heigth of the considered building following the assumed shape of “first vibration mode” of regular structures. Dynamic analysis enables to analyze also geometrically articulated structures by considering modal shapes derived from eigenvectors and eigenvalues of a generalized system. The methods are shortly, with particular emphasis on the linear one, in order to estimate the ultimate PGA for the considered structure. This method represents an appropriate compromise between reliability and rapidity of calculations. By using design spectrum with behaviour factor q, it is possible to consider, for existing buildings, a minimum value of ductility.
