| Popis: |
Switzerland is a country of low and moderate seismicity. Nonetheless, because unreinforced masonry (URM) residential buildings are a major portion of the built inventory, the seismic risk is not negligible. To meet a growing need for seismic evaluation of buildings, SIA 269/8, a Swiss code for seismic assessment of structures, was recently adopted. SIA 269/8 prescribes a risk-based seismic evaluation using the compliance factor concept, a factor that relates the seismic capacity of an existing building to the seismic capacity corresponding to the collapse safety requirement of a hypothetical (essentially) identical new structure. The goal is to achieve an acceptable low risk of casualties (between 10-5 and 10-6 individual annual casualty risk linked to compliance factor values between 25% and 100%). SIA 269/8 also defines a process on how to decide if a seismic retrofit is mandatory or not and how to select the retrofit measures that are commensurable to the actuarial value of the potentially saved lives. This paper presents a case-study investigation of a typical existing Swiss URM building with flexible floor diaphragms that features an in-depth analysis of both local failures and global building behavior. Prior to undertaking a global nonlinear static analysis, local failure modes such as the out-of-plane wall failure mechanism, support and resistance of the floor diaphragms, and the load transfer from the floor diaphragm to walls were examined. The main emphasis was on the interaction between the out-of-plane responding walls and the flexible floor diaphragms. The force-based method based on rigid-body motions and the Paulay & Priestley approach were considered. Being known for its conservatism, the latter is still the most common approach among the Swiss engineering community. Equivalent frame approach was then used to model the building globally, followed by the global seismic performance assessment using the N2 method that compares the displacement demand to the displacement capacity of the entire structure. The outcome of the SIA 269/8 evaluation procedure is that the governing compliance factor of 75% is dictated by the out-of-plane responding wall of the north façade, resulting in a corresponding cost limit of 7’500 USD. In other words, seismic retrofit is mandatory if the upgrade related cost does not exceed 7’500 USD. Following SIA 269/8, a seismic upgrade may be waived if the expected cost is higher than this threshold value as the achieved risk reduction is not reasonably justified. The findings of this case study indicate that an implicit assumption of global structural integrity may give a false sense of safety for URM buildings with flexible diaphragms as local failure mechanisms often govern the structural performance. Another distinct finding to emerge from this study is the importance of updating of geometric and material parameters of the existing structure for the seismic capacity assessment using in-site investigations and laboratory tests to improve the often limited knowledge of the current state of the building and reduce uncertainties. |
