Design of reinforced and unreinforced timber beams under uncertainties

Uncertainty in the strength of natural material, such as timber, is considerably higher than man-made construction materials, such as structural steel, from controlled and precise manufacturing process. Adopting a classical deterministic design approach, designer normally employs a high factor of safety to incorporate this material strength uncertainty in design; an approach that can lead to overdesigned structures with unpredictable performance. This paper presents the uncertainty in the strength of unreinforced timber followed by the evaluation of the performance of classical design method against nondeterministic design technique. The nondeterministic design is carried out with Monte Carlo technique in order to estimate failure rates corresponding to the adopted factors of safety utilised in the classical design. It is shown that increase in factor of safety does not necessarily yield a reliable design. Laboratory experiments are also performed on reinforced timbers. Different methods of reinforcement are adopted in this study: Fibre Reinforced Polymers (FRP) and Steel Reinforced Polymers (SRP). The uncertainty in the strength of reinforced timber is also quantified and modelled. It is shown that Near Surface Mounted (NSM) reinforcement of timber has a two-fold effect on its strength, namely, enhanced strength and reduced uncertainties, directly leading to reduction in the overall cost of the structure.