Influence of an In-Situ Inspection on the Reliability Analysis of an Ancient Timber Roof

Despite the durability of timber and its efficient performance seen in the built heritage, it has become a common practice, in Portugal, to replace ancient timber roof structures by concrete or steel roof ones. The main reason may be attributed to the difficulty in assessing the real condition of timber structures with respect to its actual level of conservation. In this work a reliability analysis of an ancient timber roof, from a Portuguese neoclassic building of the eighteenth century, is made to evidence the importance of different levels of information taking into account visual and geometric inspections. The impact of posterior knowledge obtained from non-destructive tests is evaluated by comparing the probability of failure and the reliability index on two distinct scenarios. The first scenario considers only prior information for the mechanical properties of timber elements and apparent cross-sections for the structural members. On the other hand, the second scenario considers the results of an in-situ inspection that provides the residual cross-section of the principal members, as well as the updating of the modulus of elasticity and density, based on a Bayesian Updating procedure that takes advantage of the results of a database of non-destructive tests. Latin Hypercube Sampling (LHS) was used in this study to generate a set of structural models, in which each model corresponds to a realization of the assumed random variables. Apart from the mechanical properties, the uncertainties related to permanent, snow and wind loads, are included according to the provisions of the Joint Committee on Structural Safety (JCSS). The presented results indicate that in-situ inspections have to be a priority on the assessment of ancient timber structures. The absence of a careful assessment of deteriorated elements can lead to incorrect conclusions about the structural safety. Additionally, the use of a probabilistic framework allows to a better definition of intervention plans by providing the reliability of distinct critical elements.