This paper describes an experimental test program and theoretical analysis which examines the reinforcing in flexure of glued laminated timber (glulam) beams using bonded-in carbon fiber reinforced polymer (CFRP) bars. A series of four-point bending tests were conducted till failure on unreinforced, passively reinforced and prestressed Douglas fir glulam beams in a simply-supported scheme. The focus of this research was to evaluate the reinforcing efficiency of both passively reinforced and prestressed beams. Test results showed that the flexural capacity of the reinforced, prestressed, prestressed & reinforced (bottom prestressed and top reinforced) beams greatly increased by 64.8%, 93.3% and 131%, respectively. While the maximum improvement of the bending stiffness reached 42.0%. Another important finding was that the extreme fiber tensile strain of timber beams at failure could be remarkably increased due to the presence of the tension reinforcement, which indicated it overcomes the effects of local defects and therefore the failure mode was changed from brittle tension failure to ductile compression failure. Based on the experimental results, a theoretical model was proposed to predict the flexural capacity of unreinforced, reinforced and prestressed timber beams, which was validated by the test data.
Timber beams can effectively be reinforced using externally bonded fibre reinforced polymer (FRP) composites. This paper describes a nonlinear 3-dimensional finite element model which was developed in order to accurately simulate the bending behaviour of unreinforced and carbon FRP plate reinforced glulam beams. The model incorporates suitable constitutive relationship for each material and utilises anisotropic plasticity theory for timber in compression. Failure of beams was modelled based on the maximum stress criterion. The results of the finite element analysis showed a good agreement with experimental findings for load-deflection behaviour, stiffness, ultimate load carrying capacity and strain profile distribution of unreinforced and reinforced beams. The proposed model can be used to examine the effect of different geometries or materials on the mechanical performance of reinforced system.
This paper describes an experimental test program and theoretical analysis which examines the reinforcing in flexure of glued laminated timber (glulam) beams using fiber reinforced polymer (FRP) and steel materials. A series of four-point bending tests were conducted till failure on both unreinforced and reinforced Douglas fir glulam beams in a simply-supported scheme. The focus of this research was to evaluate the effects of reinforcing materials, reinforcement ratio and arrangement on the flexural behavior. Test results showed that the flexural capacity, flexural global stiffness and timber tensile strain at failure were all improved considerably for reinforced timber beams when compared to the unreinforced control beams, in which the average improvement reached 56.3%, 27.5% and 49.4%, respectively. On the bases of the experimental results, a theoretical model was proposed to predict the flexural capacity and flexural stiffness of the reinforced timber beams. Most of the differences between theoretical and experimental results for both flexural capacity and flexural stiffness were within 10.0%, which showed a high accuracy of the proposed model. Subsequently a parametric analysis, which includes the axial stiffness ratio of reinforcement to timber, the relative location of tensile reinforcement, and the strength ratio of reinforced timber between flexural tension and compression, was undertaken to investigate the effects of the influential factors for both flexural capacity and flexural stiffness.
Reinforcement in glulam beams in form of screws or rods can restrict the free shrinkage or swelling of the wood material. The objective of the project presented was to evaluate the influence of such reinforcement on the magnitude of moisture induced stresses. For this purpose, experimental studies were carried out in combination with analytical considerations on the basis of the finite-element method. Taking into account the influence of relaxation processes, the results indicate that a reduction of timber moisture content of 3 - 4 % around threaded rods, positioned perpendicular to the grain, can lead to critical stresses with respect to moisture induced cracks. In addition, a substantial mutual influence of adjacent reinforcing elements has been identified. A reduction of the distance between the reinforcement thus results in a lower tolerable reduction of timber moisture content around the reinforcement.