SEISMIC RELIABILITY OF CHEVRON BRACED FRAMES WITH INNOVATIVE CONCEPT OF BRACING MEMBERS

According to the most modern trend, performance-based seismic design is aimed at the evaluation of the seismic performance of structures in terms of mean annual frequency of exceeding a threshold level of damage, i.e. a limit state.A procedure for performance-based seismic assessment is herein briefly summarised and applied to concentrically "V" braced steel frames, designed according to different criteria.In particular, two design approaches are examined.The first one corresponds to the provisions suggested by Eurocode 8, while the second approach, proposed by the authors, is based on a rigorous application of the capacity design criteria aiming at the control of the failure mode.In addition in this work a new conception of bracing members is developed and applied with reference to V-braced frames designed according to the methodology proposed by the authors.It is well known that aiming at the safeguard of brace connections, Eurocode 8 provides a limitation to the brace slenderness.The drawback of this limitation is the oversizing of brace diagonals at the upper storeys, which prevents the development of a collapse mechanism of global type.This is the starting design issue for the conception of new bracing members.In fact, by introducing in the brace members a zone characterized by a reduction of the cross sectional area (Reduced Section Solution), it is possible to calibrate the yield strength leaving substantially unchanged the slenderness, so that the limits provided for the normalized slenderness can be still satisfied without brace oversizing.The aim of this work is to focus on the seismic performance of V-braced frames designed according to both Eurocode provisions and the proposed methodology.For the structures dimensioned according to the proposed criteria, the Reduction Section Solution is also applied with the aim to safeguard the connection without increasing the structural weight.Finally, a probabilistic approach based on the combination of probabilistic seismic hazard analysis (PSHA), probabilistic seismic demand analysis (PSDA) and probabilistic seismic capacity analysis (PSCA) is applied aiming to investigate the seismic performance of the designed structures.It is pointed out how the proposed design method leads to a very important improvement of the seismic performances with a negligible increase of the overall building cost.