Retrofit of Soft Storey Buildings Using Gapped Inclined Brace Systems

Abstract

Although a soft storey mechanism is generally undesirable for the seismic response of building structures, it could provide potential benefits due to the isolating effect it produces. This thesis proposes a retrofit strategy for buildings that are expected to develop soft storey mechanisms, taking advantage of the positive aspects of the soft storey response while mitigating the negative ones. After a review of traditional considerations that are made for soft storey structures, the work starts by comparing the behaviour of an RC frame building with two infill configurations; in the first configuration, it is assumed that masonry infills are distributed over all storeys uniformly, while in the next step and in order to consider soft storey effects, it is assumed that masonry infills are not present at the ground storey. Results of incremental dynamic analyses indicate that structures with uniform infill are less likely to collapse. However, if the displacement demands at the first level of soft storeys could be sustained, their overall performance would be significantly improved. Following this initial study, a gapped inclined brace (GIB) system is proposed with the aim of significantly reducing the likelihood of collapse whilst ensuring that the seismic damage concentrates at this single level, protecting the rest of the structure located above. The GIB system achieves these aims by reducing P-Delta effects at the first floor of soft storey buildings without significantly increasing their lateral resistance. The mechanics of the proposed system are defined and a systematic design procedure is explained and illustrated. The theoretical relations that are derived for GIB systems are verified through numerical analyses. Results of cyclic static and incremental dynamic analyses demonstrate that the overall seismic performance of soft storey buildings retrofitted using a GIB system is greatly improved, indicating that the GIB system produces an efficient and intelligent soft storey mechanism at the first level of such buildings, which provides several advantages over conventional approaches. The last part of the thesis discusses various uncertainties that remain about the potential of GIB systems, including the best likely connection details for GIB systems, which should be investigated as part of future research.