Considerations and Suggestions about the Seismic Assessment Procedures of Existing R.C. Frame Buildings According to OPCM 3431 and Eurocode 8

In the European seismic countries, the majority of the building heritage was designed according to out-of-date or even non-seismic codes. The lack of considerations about the nonlinear response during the design stage leads to relevant uncertainties, regarding the potential development, location and extension of inelastic mechanisms, as well as their ductility capacity. Therefore, the reliability of the force-based assessment procedure obtained performing a linear analysis is in general questionable, due to the difficulties associated to the correct evaluation of the reduction factor of the internal forces. The direct consequence is the considerable gain in complexity of the assessment procedures proposed in the Italian seismic code and in Eurocode 8. Both codes promote the use of the nonlinear analyses and the displacement-based approach to assess the ductile response. In the light of these considerations, this work intends to furnish useful tools to perform a reliable assessment of existing r.c. frame buildings. In particular, four irregular structures, built according to out-of-date codes, were analyzed with the following aims: i) suggesting simplified approaches and improvements for the assessment procedures, concerning the evaluation of both seismic demand and capacity of the structural members; ii) checking the importance of defining properly the effective stiffness in linear analyses; iii) checking if the conditions to accept the results of linear methods proposed in the two codes are appropriate; v) giving a judgment about the reliability of the conventional 2-D pushover analysis.