An adjusted design approach for concentrically braced frames in low-to-moderate seismicity areas

Abstract

Steel concentrically braced frame (CBF) configuration is a common construction application in Europe. In the low-to-moderate seismicity context, European building codes provide two alternative design methods for CBFs; engineers have to choose between a "non-dissipative" method (DCL) neglecting all seismic provisions, and a "dissipative" one (DCM), applying its complex and expensive ductility requirements. Currently, the preferred method is the former one, because of its simplicity. Such a choice may lead on one side to oversized profiles that are unduly expensive, on the other side to possibly unsafe solutions due to the unpredictable nature of the regions characterized by low-to-moderate seismicity, where rare but strong earthquakes are foreseeable. On the other hand, enforcing engineers to apply strict "high-dissipative" rules seems too conservative for this case, which would result in over-safe, but uneconomic structures. This article proposes an adjusted design approach for the low-to-moderate seismicity design of CBF structures, aiming to satisfy both economy and safety criteria. The proposed approach is based on the exploitation of the three features of CBF systems, which have not been deeply investigated so far: "frame action provided by gusset plates", "contribution of compression diagonal and its post-buckling strength and stiffness", and "energy dissipation capacity of non-ductile bracing joint connections". The paper investigates these aspects by means of incremental dynamic analysis of case studies, based on the numerical models calibrated on full-scale experimental tests published elsewhere by the authors. As a result, it provides design recommendations and presents economic comparisons between the buildings designed with current Eurocode approach and the proposed one.