Load sharing factors in timber bridge design

Abstract

It is well recognized that the structural capabilities of timber that are relevant to structural design vary not only because of natural variations in the material properties of the timber but also, randomly, as a consequence of several aspects of the structure itself and the loading. These factors are accounted for in the design process through the modification factors applied to the calculated resistance of the various components. One of these factors is the load sharing factor, which is intended to account for the enhanced strength of an assembly of components which results from the reduced probability of simultaneous occurrence of low strength in more than one component of the grouping. All the modification factors, including the load sharing factor, are usually derived by ignoring the fact that the two sides of the design equation, representing load effects and resistances respectively, may be interrelated. Despite its significance, the load sharing factor has been specified in design codes as a result of only cursory investigations, which have not rigorously considered the mechanics of behaviour of the assembly of timber components. The results of an analytical study are presented in this paper; the load sharing factor is developed by considering the interaction between the two sides of the design equation and also by explicitly taking account of the mechanics of behaviour of groups of timber components. It is shown that the load sharing factor can be conservatively presented as a relatively simple function of the number of timber components that can be assumed to be subjected to the same deformation as one another. Key words: factored resistance, design equation, load sharing, modification factor, modulus of rupture, strength variability, timber bridges.