Probabilistic assessment of the flexural and shear strength of limestone roofing slabs (xorok) using structural reliability analysis

Abstract

Maltese stone roofing slabs (xorok) were an integral structural component of local architecture for centuries and are still found in many local buildings today. Together with an added architectural value, however, these elements are associated with a degree of uncertainty regarding their structural reliability and, therefore, their safety for use within the structure. Current documentation regarding these elements tends to mainly focus on their architectural value, without giving too much importance to the implications of their use on the structural safety of unreinforced masonry buildings. This research study aims to address this gap in knowledge by achieving a better understanding of the degree of structural reliability that is achieved by these stone slabs within the residential buildings when subject to loading as recommended by Eurocode 1, and how this reliability varies with a number of design parameters, namely: stone slab span, bedding plane orientation of the stone, failure mode, and nature and magnitude of the imposed loading. This study investigates how the strength varies with these design parameters in comparison to the applied vertical loading (point and uniformly distributed load). In order to answer this research question, a Level 2 First Order Reliability Method (FORM) Analysis was utilised. Laboratory experimental tests were carried out on the xorok, and existing databases of stone properties are used to obtain the mean values and standard deviations for the model uncertainty parameter so as to improve the mathematical strength prediction models for the different failure modes. Finally, systems reliability concepts are applied to obtain the overall safety of the stone slabs, and these results are then weighed against a design threshold level of safety in order to determine the suitability of the xorok as structural elements. The results showed that, for the samples tested, the systems reliability of most stone slabs did not meet the threshold reliability index, under some circumstances being significantly lower. Observations are made as to the significance of good quality workmanship and the implications that the different parameters may have upon the structural reliability and safety of the stone slabs.