SEISMIC FRAGILITY ANALYSIS TO ASSESS THE IMPACT OF TIMBER VERTICAL ADDITIONS ON EXISTING RC BUILDINGS: A CASE STUDY

Construction of vertical expansions on top of existing structures can be considered as an economical and practical solution to contain urban sprawl. The use of timber, due to its lightness, suitability for modularity and prefabrication, and due to environmental benefits, emerges as an interesting option for the realisation of vertical additions in existing buildings. Unfortunately, in seismic-prone areas, the construction of vertical expansions clashes with seismic safety. Current standards require that the safety level of the existing structure after the expansion be that prescribed for new buildings. This requirement represents a major obstacle to vertical expansion because vintage buildings rarely comply with the provisions of the seismic standards even without any addition, making costly retrofit intervention necessary. In this paper, a case study is selected and analysed to try and understand how the vertical timber addition of existing buildings can impact their seismic behaviour. The selected case study is a 1980s four-story RC frame building with masonry infill walls. Following current design practices, a one-storey timber vertical expansion was designed by adopting a cross-laminated timber shear-wall system. The seismic response of the building with and without the timber expansion was then simulated via finite element modelling. The outcomes of the numerical simulations were then used to perform a seismic fragility analysis and assess the effect of vertical expansion on the seismic performance of the case study building. The results of the analysis showed how, in some cases, the vertical timber addition not only does not worsen the safety level of the building but can actually improve it. This outcome opens new paths for future research to further investigate the effect of vertical expansions and maximize their beneficial effect on the seismic response of existing buildings.