Structural reliability analysis of the flexural strength of timber beams

Abstract

Structural timber has a considerably higher variability of the strength properties both within and between members. Especially when it is compared to other structural materials, such as steel and concrete. The variability within a timber member is not considered in engineering design, where each member is treated under the assumption that it is homogeneous where the strength is assumed to have a constant value along the member. The code which covers the design of timber buildings and civil engineering works, Eurocode 5 is based on the limit state concept used in conjunction with a partial factor method. The codes give the characteristic design values for timber beams. Although from studies and tests that have been carried out in the past a discrepancy arises from the prediction of the flexural strength and what had previously been found in the laboratory. This may arise from a number of factors which the code does not consider; the drying process and defects in timber such as thin cracks, pitches and knots which have a great effect on the final strength. Past experimental data for flexural strength on bare timber beams will be supplemented with further tests on timber beams. In this way, it would be possible to assess the extent to which the design parameters effect flexural strength of timber beams, so that more research could be focused on those particular parameters. Probabilistic methods are the basis of all modern design codes. The purpose of reliability-based design is the systematic consideration of all the uncertainties involved in the design process. The uncertainties could, for example, be found in the expected loads and the strength estimation. Probabilistic analyses of structures can also be carried out at different levels. The Level 2 approach utilizes the First-Order Reliability Method (FORM) or the more accurate Second-Order Reliability Method (SORM) in order to estimate the probability of failure (and, hence the reliability) of a structure or structural component (such as a beam in flexure) from the joint probability density of all the basic random variables that influence structural behaviour. After finding the probability of failure, the reliability indexes and the sensitivity factors, it would then be possible to obtain a statistical model for the prediction of the structural behaviour of timber beams in flexure.