Seismic Retrofit of Existing Steel Concentrically Braced Frames with Intentional Eccentricity Braces

Concentrically Braced Frames (CBFs) built before the introduction of the capacity design concept do not usually meet modern seismic design requirements. They are vulnerable to seismic events and they could develop unpredictable failure mechanisms. An existing building on the EPFL campus which has these characteristics is selected as case study for this work. Retrofit measures are then deemed necessary for this building. Its most critical CBF is pinpointedandsubsequentlyretrofitted. Threeretrofitsolutionsareproposed. Acommonlyretrofit proposition using Conventional Buckling Braces (CBBs) is compared to two retrofit solutions applyingrecentlydevelopedhigh-performancebraceswithintentionalinitialeccentricitycalled Braces with Intentional Eccentricity (BIE) and Naturally Buckling Braces (NBBs). Compared to CBBs, these innovative bracing systems provide larger energy dissipation at small drift levels and offer larger post-yielding stiffness. In this work, these high-performance braces, that were only investigated in single brace configurations in previous studies, are examined in more realistic conditions. Initially, numerical models and equations defining the physical behavior of BIE and NBBs are validated according to existing experimental test results. Moreover, a new design procedure able to define cross-section properties and initial eccentricity values is established. Subsequently, with the purpose of evaluating the three different retrofit solutions, a nonlinear model of the selected critical CBF is developed and studied through nonlinear response history analysis. The model consists of a 3-story V-braced CBF. The obtained results demonstrate that the benefits resulting from both innovative bracing systems are successfully experienced by the structure when designed according to the established design procedure. Both global and local behaviors are positively affected, significantly improving the seismicresisting performance of the analyzed structure. Those findings suggest that BIE and NBBs appear to represent a viable alternative to CBBs for the retrofitting of existing bracing systems