A SIMPLE APPROACH FOR SEISMIC RETROFIT OF LOW-RISE CONCENTRIC X-BRACED STEEL FRAMES

Existing concentric steel braced frames (CBF) have been generally designed without considerations of principles based on plastic mechanisms. This leads to most of the inadequate responses shown by CBFs during recent earthquakes. On the contrary, the seismic performance of CBF has been demonstrated to substantially improve when the design is specifically addressed to obtain resistant ultimate mechanisms characterized by the yielding of diagonals. The design approach suggested by numerous modern seismic codes, such as the Eurocode 8 and the recent Italian guidelines NTC08, focuses on detailed design provisions aimed to assure new CBFs a ductile ultimate mechanism, characterized by the yielding (reasonably distributed along the building height) of diagonals, while the buckling of beams and columns is prevented. This approach could also represent a useful support in the definition of efficient retrofit strategies devoted to improve seismic performance of existing CBFs. In the past the authors proposed a simplified approach for the assessment of existing CBFs carried out considering a procedure developed in the light of the direct Displacement Based Design (DDBD) method and where the role of diagonals assumes particular relevance for defining the seismic safety level of CBFs. In the paper a retrofit procedure for CBFs is developed and applied to some cases of study. Non-linear time-history finite element analyses are presented in the paper with reference to the examined cases considering a set of spectrum-compatible accelerograms. The derived results are then presented by examining the ability of the proposed procedure to carry out suitable retrofitting interventions.