This thesis deals with the shear design of Cross Laminated Timber (CLT) elements stressed by concentrated loads which are locally reinforced by means of self-tapping screws with continuous threads. A simplified model is presented using an effective width for the calculation of the shear stresses in the vicinity of point supports or concentrated loads. Laboratory tests supply material-mechanical principles to determine the interaction of rolling shear stresses and compression perpendicular to the grain. In addition to experimental tests theoretical models are developed to examine the load bearing behaviour of CLT-elements reinforced by self-tapping screws. Preliminary tests with plate elements provide initial experience with these reinforcements under biaxial load transfer. Finally a design concept validated by means of the test results is proposed.
In this study, glulam beams were strengthened by inserting compressed wood (CW) blocks into the precut rectangular holes with one-thirds of the beam depth from the top of the beams. This practice was to make use of moisture-dependent swelling nature of the compressed wood which was conditioned with the moisture content significantly lower than the ambient one. The test results showed that a pre-camber was produced in the mid-span of the beam reinforced due to expansion of the compressed wood blocks on the top part of the beam. As a result, significant initial tensile and compressive stresses were generated on both the top and the bottom extreme fibres of the beam, respectively. Subsequent bending tests revealed that the initial stiffness and load carrying capacity of the pre-stressed beams were increased significantly in comparison to the beam without pre-stressing.
Journal of the Society of Materials Science, Japan
Glued laminated timber (glulam) composed of mechanical-graded lamina shows higher strength reliability than lumber. However the glulam still has wider strength distribution than steel or concrete for structural applications because of the large difference between the mean and the lower limit value. This study aimed at reduction of the coefficient of variation of the glulam in bending strength by partial reinforcement technique using wooden sheets. The wooden sheets made from bamboo or white oak were covered to several defects on the surface of glulam such as large knots and finger joints. The strength performance and the reinforcement effect for the bending test of the glulam with reinforcement materials were evaluated.
As a result, it was clarified that a bamboo sheet (two layers) or a white oak sliced veneer sheet could reinforce the defects, and the average of bending strength was improved due to the improvement in the lower value in the strength distribution.
Fibre-reinforced polymers (FRPs) are effective in the flexural stiffening and strengthening of structural members. Such systems can be optimised if accurate numerical models are developed. At present, limited information is available in the literature on numerical models that can predict with good accuracy the nonlinear behaviour of FRP reinforced low-grade glued laminated timber beams. This paper discusses the development of a finite element model, which incorporates nonlinear material modelling and nonlinear geometry to predict the load–deflection behaviour, stiffness, ultimate moment capacity and strain distribution of FRP plate reinforced glued laminated timber beams manufactured from mechanically stress graded spruce. Beams with and without sacrificial laminations are modelled and their performance is compared to unreinforced glued laminated timber beams. The model employed anisotropic plasticity theory for the timber in compression. The failure model used was the maximum stress criterion. Strong agreement was obtained between the predicted behaviour and the associated experimental findings. It was deduced from comparing the results from the numerical model with experimental findings that the FRP plate succeeds in increasing the performance of the adjacent timber significantly. The model is a useful tool for examination of the effect of reinforcement percentage and will be used for optimisation of the hybrid beam.
Openings are usually required to allow services like plumbing, sewage pipes and electrical wiring to run through beams. This prevents an extra depth of the floor/ceiling, while preserving architectural considerations. The introduction of large opening causes additional tension perpendicular to grain in timber beams. The low tensile strength perpendicular to grain of wood allows crack formation. Crack propagation around the hole considerably decreases the load-carrying capacity of the beam. However, in most cases, crack formation and propagation around the hole can be prevented by the use of an appropriate reinforcement. Screw, glued-in rods, and plywood are alternative options for the reinforcement. Design of the reinforcement requires that the working mechanism of the reinforcement is fully understood and properly addressed. In addition, reinforcement should be designed for actions produced in the section of the beam weakened by the hole. The current paper uses a simple truss model around the opening to calculate the tensile force in the reinforcement. Two simple formulations for design of the reinforcement are derived and compared with numerical and experimental results, showing an overall good correspondence. The proposed truss model can be considered for incorporation in future codes of practice.
The paper reports on the activities of the RILEM technical committee “Reinforcement of Timber Elements in Existing Structures”. The main objective of the committee is to coordinate the efforts to improve the reinforcement practice of timber structural elements. Recent developments related to structural reinforcements can be grouped into three categories: (i) addition of new structural systems to support the existing structure; (ii) configuration of a composite system; and (iii) incorporation of elements to increase strength and stiffness. The paper specifically deals with research carried out at the Bern University of Applied Sciences Switzerland (BFH), the University of Minho Portugal (UniMinho), and the University of Trento Italy (UNITN). Research at BFH was devoted to improve the structural performance of rounded dovetail joints by means of different reinforcement methods: i) self-tapping screws, ii) adhesive layer, and iii) a combination of selftapping screws and adhesive layer. Research at UNITN targeted the use of “dry” connections for timber-to-timber composites, specifically reversible reinforcement techniques aimed at increasing the load-bearing capacity and the bending stiffness of existing timber floors. At UniMinho, double span continuous glulam slabs were strengthened with fibre-reinforced-polymers. All three examples demonstrate the improved structural performance of timber elements after reinforcing them.
A research study was undertaken to investigate the mechanical performance of glulam beams reinforced by CFRP or bamboo. Local reinforcement is proposed in order to improve the flexural strength of glulam beams. The glulam beam is strengthened in tension and along its sides with the carbon fiber-reinforced polymer CFRP or bamboo. A series of CFRP reinforced glulam beams and bamboo reinforced glulam beams were tested to determine their load-deformation characteristics. Experimental work for evaluating the reinforcing technique is reported here. According to experiment results, the CFRP and bamboo reinforcements led to a higher glulam beam performance. By using CFRP and bamboo reinforcements several improvements in strength may be obtained.
The use of glulam beams with changing depth offers the possibility to adapt the section modulus to the bending moment. In the case of single-span beams under uniformly distributed load, however, a change in beam depth will lead to a contrary effect for the shear stresses. Curved and pitched cambered beams feature not only high utilization rates in bending but also areas of high tension stresses perpendicular to the grain and shear parallel to the grain stresses, two stress components for which timber features only small capacities as well as brittle failure modes. Out of 245 cases of damaged or failed large-span timber structures, several failures document the possibility of a shear fracture (full separation) developing in grain direction from the curved part towards the supports, partly followed by a failure of the beam in flexural tension due to a change in stress distribution resulting from the change in section modulus. Reinforcements against tension stresses perpendicular to the grain in form of fully threaded screws or threaded rods can be considered state of the art. With respect to their application as shear reinforcement, not many research results are yet available, resulting in a lack of experimentally validated design approaches.
Within this paper, approaches to design shear reinforcement for glulam beams in the unfractured and the fractured state are presented, validated and discussed. The moment of failure, i.e. the transition from the unfractured to the fractured state is characterized by dynamic effects. This situation is not covered in this paper. The same applies to the subject of moisture induced stresses, resulting from the reinforcement restricting the free shrinkage or swelling of the glulam beam.
This paper describes a series of four-point bending tests that were conducted, under service loads and to failure, on unreinforced, reinforced and post-tensioned glulam timber beams, where the reinforcing tendon used was 12mm diameter toughened steel bar. The research was designed to evaluate the benefits offered by including an active reinforcement in contrast to the passive reinforcement typically used within timber strengthening works, in addition to establishing the effect that bonding the reinforcing tendon has on the materials performance.
The laboratory investigations established that the flexural strength and stiffness increased for both the reinforced and post-tensioned timbers compared to the unreinforced beams. The flexural strength of the reinforced timber increased by 29.4%, while the stiffness increased by 28.1%. Timber that was post-tensioned with an unbonded steel tendon showed a flexural strength increase of 17.6% and an increase in stiffness of 8.1%. Post-tensioned beams with a bonded steel tendon showed increases in flexural strength and stiffness of 40.1% and 30% respectively.
A new approach to reinforce glulam timber beams has been developed by using compressed wood (CW) which is made of a lower grade wood through densification processes. In the reinforcing practice, compressed wood blocks are inserted into pre-cut holes on the top of glulam beams to produce pre-camber and to generate initial tensile and compressive stresses on the top and the bottom extreme fibre of the glulam beam. In order to optimize the size, the number and the location of CW blocks, 3-D finite element models have been developed. 3D non-linear finite element models have been developed to simulate the pre-camber of Glulam beams locally reinforced by compressed wood blocks. The models developed have also produced the initial tensile and compressive stresses at the top and bottom extreme fibres with building-up moisture-dependent swelling on the CW blocks. With the pre-camber and the initial stress state that cancel out proportions of working deflection and stresses.