Uncertainty analysis of FRP reinforced timber beams

Abstract

22Timber has been a popular building material for centuries and offers significant sustainable 23 credentials, high mechanical and durability properties. Availability, ease to use, convenience 24 2 and economy have made timber the most used construction material in history but, as it is a 25 natural material, uncertainty in its mechanical characteristics is considerably higher than 26 man-made structural materials. National codes and engineers usually employ high factor of 27 safety to incorporate timber strength uncertainty in design of new structures and 28 reinforcement of existing ones. This paper presents the results of 221 bending tests carried 29 out on unreinforced and reinforced soft-and hardwood beams (fir and oakwood) and 30 illustrates the reinforcement effect on timber capacity and strength uncertainty. 31Both firwood and oakwood beams have been tested in flexure before and after the application 32 of a composite reinforcement made of FRP (Fiber Reinforced Polymer) unidirectional sheet. 33The uncertainty in the strength of reinforced timber is also quantified and modelled. finding new uses for redundant historic constructions without affecting their significance. 56As a natural material, the strength of timber is appreciably reduced by the presence of defects 57 like knots, especially when located on the tension side, and distortion of the grain. For this 58 reason uncertainty in the strength of timber is considerably higher compared to an artificial 59 construction material (steel, concrete, bricks, etc.), which is produced through quality- UNREINFORCED TIMBER 111The bending strength of timber is governed by the modes of failure. Since the behavior of 112 timber in compression is different from that in tension, the failure modes could be highly 113 affected by this. Figure 1 show different characteristic failures of beams in bending. Simple 114 tension failure (Fig. 1a) due to a tensile stress parallel to the grain. This is common in 115 straight-grained beams made of high quality timber, particularly when the wood is well 116 seasoned and there is no diagonal cross grain. 117The most common failure mode is the cross-grained tension, in which the fracture is caused easy to see why a cross-grained timber would fail in this manner. 124As stated, an interesting effect of the analysis of the failure modes is that these usually occurs 125for different levels of bending loads. Failure mode in Figure 1b is usually activated for low 126 bending loads. This is also typical of low-grade timber where the high number of defects 127 facilitates the cross-grained tension failure. 128Failure on compression side is shown in Fig. 1c. This failure mode do not usually lead to the 129 collapse of the structure as the behavior of timber in compression is plastic (Fig. 3). Failure 130 modes in Figure 1a is usually activated for high bending loads as this occurs for straight- 131grained beams and tensile strength of timber is very high. In order to reduce the local crushing of the wood, the load was a...

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