Derivation of fragility curves for traditional timber-framed masonry buildings using nonlinear static analysis

Recent earthquakes and two experimental campaigns on timber-framed masonry walls have shown that timber-framed masonry buildings possess a good displacement capacity and hence can withstand severe earthquakes without collapse. In the present paper, timber-framed masonry panels with diagonal braces are studied. Using a simplified model based on non-linear (NL) lumped plasticity strut elements, NL analyses are carried out of typical traditional buildings in Lefkas (Greece) with diagonally braced timber-framed masonry walls in their lateral load resisting system. Furthermore, an investigation is carried out regarding the foundation of the buildings. The key feature of the Lefkas buildings is their dual structural system. The primary system consists of a stone masonry ground floor and all upper floors are made of timber-framed masonry walls. Timber posts in the ground floor, a few centimetres apart from the stone masonry, constitute the secondary structural system which is connected to the upper floors. This latter system is activated once the ground floor stone masonry piers fail. Two different structural models are developed to simulate each system. Pushover curves are derived from the NL analyses of the buildings and are then converted into capacity curves assuming the fundamental mode dominates. On these curves four damage states (slight damage, moderate damage, heavy damage and collapse) are defined on the basis of criteria related to the actual response of the building. The first three damage states are defined on the capacity curve of the primary system, whilst the ultimate damage state is related to the response of the secondary system. Then, fragility curves in terms of spectral displacement are generated, adopting a log-normal statistical distribution of the probability of damage.