Keywords: reinforced glulam, prestressed, GRP, timber beam, finite element, precamber, bonding.

Many attempts have been made to improve the load carrying capacity of glulam beams by the addition of reinforcement. The earliest utilised steel with either thin plates glued onto the outer laminates of beams, or steel bars bonded into pre-cut slots between laminates. More recently, with the availability of materials such as pultruded glass fibre reinforced plastic (GRP) and carbon fibre reinforced plastic (CFRP), similar ideas have again been tried. Indeed, there are companies that manufacture and supply stock ranges of reinforced beams. The principle aim of the reinforcement is to further enhance the strength of beams made from good quality timber rather than as a means of enhancing beams made from lower quality material. However, the idea of reinforcing beams made from low grade timber was the subject of a recent investigation carried out at the Queen's University Belfast, the results of which are encouraging [1].

Attempts have also been made at prestressing glulam by the use of high tensile steel tendons. Both post tensioning, with the tendons passing through pre-cut longitudinal grooves in the glulam and anchored off at end plates, and pretensioning with bonded tendons have been tried [2,3]. With the high modulus of elasticity of steel compared to that of timber, prestress losses due to elastic shortening of the timber at the transfer stage and to creep in the timber over the longer term were perceived as major problems. It is not surprising, therefore, that no commercially successful method of prestressing via the use of steel tendons is in existence.

The concept of prestressing glulam with GRP tendons is new and has been recommended as a realistic method of upgrading low grade glulam [4].

With GRP tendons it is considered that the problems of losses and of potential tendon debonding in pretensioned systems are less daunting than with steel tendons. This is because GRP has a modulus of elasticity of only about 25% that of steel. This means that it has to be strained to a much higher level than steel in order to reach its limiting strength. Therefore, losses due to elastic and time dependent shortening of the timber after release are relatively smaller than with steel tendons. Also, the flat strip form of the GRP means that there is a relatively larger surface area than with the traditional round or square cross section of steel tendons and, therefore, that bond stresses are lower.

From a technical point of view, there are two fundamental ways in which prestressing would enhance the performance of a glulam beam as follows:

1. By introducing an initial compressive stress into those fibres that in service would go into tension and vice the versa, the grade of timber in flexure would effectively be increased.
2. The introduction of prestress via an eccentric tendon would cause the beam to curve along its length, thereby providing a precamber.

In this paper an investigation of bond stress between a tendon and the adjacent timber was carried out using 3-D finite element analysis. Analysis helps understanding of the mechanics and the limitations of this bond, which are considered as key factors in the development of a successful prestressing technique. In the modelling, both the glulam and the GRP tendon were treated as orthotropic elastic materials. Interaction between the glulam and the GRP tendon was simulated by properly defined contact surfaces. The load-deflection relationship of the pre- stressed beam was predicted and reasonably good correlation between the experimental results and the numerical simulations was obtained. The model that was developed is capable of giving an indication of bond stresses at the ends of beams, at the transfer stage, the anticipated full load condition and at ultimate load. With the validated model parametric studies can be undertaken to determine the sensitivity of the stress distributions to variations in the physical and geometrical properties of the materials. The equivalent of the transmission length, which is well known in prestressed concrete parlance, is identified and discussed for timber beams.

1

J.R. Gilfillan, S.G. Gilbert and D.P. Russell, "Development of a structural composite using home grown timber and fibre reinforcement", DOE report, Reference: UNQU9701, 2000.

2

B. Bohannan, "Prestressed wood members", Forest Products Journal, 12(12), 596-602, 1962.

3

J. Peterson, "Wood beams prestressed with bonded tension elements", Journal of the Structural Division of ASCE, 91(1), 103-119, 1965.

4

P.D. Rodd and D.J. Pope, "Prestressing as a means of better utilising low quality wood in glued laminated beams", In "Proceedings of International Conference on Forest Products", Daejeon, Korea, April 21-24, 2003.

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