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Five existing unreinforced masonry buildings, built in different Italian cities in a relatively recent time frame, were selected to study the seismic behaviour of modern existing structures, not designed to withstand seismic actions. The buildings were modelled using an equivalent-frame approach, adopting, and comparing two nonlinear constitutive laws and introducing specific modelling strategies to represent the distinctive features of modern existing URM structures. Nonlinear static analyses were performed to determine capacity and multi-stripe time history analyses to evaluate displacement demand at increasing return periods. Although non-exhaustive, the results show a trend of generally decreasing seismic vulnerability for more recent buildings. [ABSTRACT FROM AUTHOR]

Several building codes propose methodologies to account for epistemic uncertainties in the seismic assessment of masonry buildings by selecting a knowledge level and reducing material strengths by means of the associated value of the confidence factor. Previous works showed that, in the case of masonry structures, this approach has various limitations, such as the lack of proper consideration of experimental tests performed. This article focuses on the issue of imperfect knowledge on material properties of existing masonry buildings and proposes a probabilistic methodology for the assessment, based on Bayesian updating of mechanical properties. The use of a Bayesian approach allows to update the values of the material properties assumed a priori as knowledge on the building increases, by taking into account all the experimental information gathered during the assessment process. A large number of simulated assessments is carried out and the values of the confidence factors on material properties are defined through the comparison between the obtained results and those of the reference structure, assumed to be perfectly known. These factors are useful in a more general framework for the assessment of masonry buildings accounting for different sources of uncertainty. [ABSTRACT FROM AUTHOR]

The choice of modelling strategy and analysis options has a significant influence on the results of the seismic assessment of existing buildings and therefore it is very important to have an idea of the dispersion in the results due to different hypotheses regarding the structural model. This paper concentrates on pushover analysis, considered as the reference method currently adopted by engineers for the seismic assessment of existing masonry buildings, and on the equivalent-frame macro-element approach, assumed to be a satisfactory compromise between computational effort and accuracy in the results. A logic tree approach is used to treat the different considered options, including the definition of the geometry of the equivalent frame, the distribution of loads among the masonry piers and on the horizontal diaphragms, the degree of coupling between orthogonal walls, the definition of the cracked stiffness of structural elements and the modelling of masonry spandrels. By assigning a value of probability to each end branch of the tree, the distribution of the peak ground acceleration corresponding to the selected limit states can be obtained and, from this distribution, a quantitative estimate (in probabilistic terms) of the effect of modelling uncertainties on the seismic response of masonry structures is derived. [ABSTRACT FROM AUTHOR]