Highly loaded and large span timber beams are often used for halls, public buildings or bridges.
Reinforcement of beams may be required to extend the life of the structure, due to deterioration or damage to the material/product or change of use. The paper summarises methods to repair or enhance the structural performance of timber beams. The main materials/products cross sections and geometries used for timber beam are presented. Furthermore, their general failure modes are described and typical retrofitting and reinforcement techniques are given. The techniques include wood to wood replacements, use of mechanical fasteners and additional strengthening materials/products.
Cracks in timber members influence the stiffness and load-carrying behaviour but only rudimentary rules are given to evaluate cracked members. Therefore, an investigation to gather information about the most frequent characteristics of cracked timber structures has been carried out. This investigation provides the main characteristics of both the timber elements and the crack distributions encountered. These main characteristics have then been used to define a numerical model in order to investigate the impact of cracks on the stiffness and load-carrying capacity of timber beams. Based on these results, the existing rules considering cracks in timber beams can be evaluated and new rules can be developed.
The increasing number of wood structure amongst large and potentially public buildings gave a new impulse to the assessment of timber structures. For assessing the state of timber elements, cracks are a key indicator. Therefore, experimental and numerical investigations on not cracked and partly cracked timber members were carried out and analysed. The results show no influence on the stiffness and modulus of elasticity for partly cracked beams. The experimental results were used for the development of analytical and validation the numerical solutions for the assessment of the residual load carrying capacity of cracked timber members. Several models predicting the residual load carrying capacity depending on the crack situation are presented.
The use of relatively new constructions products like Cross laminated timber (CLT) is increasing significantly. It is planned to extend the production of CLT by producing them out of beech or of beech and spruce in combination as hybrid product. The objective is to provide high performing materials which compensate weak points in soft wood products. In order to use and implement the product, the mechanical behaviour of a CLT plate of beech were investigated. The potential of beech is shown in terms of known strength values. Experimental tests for the evaluation of the strength and stiffness values for beech CLT for different situations as well as delamination tests were performed. Failure cases of the mechanical tests are presented and discussed where the rolling shear failure was in major focus for the discussion.
Until today, all known timber building systems allow only slabs with a uniaxial load bearing action. Thereby, in comparison to normal reinforced concrete slabs, timber slabs are often thick, expensive and complicated to build. The reason for this is that there is no efficient connection technology to rigidly connect timber slab elements to each other. Alternative solutions are hybrid structural systems with concrete or steel, however, this combination of materials results in some disadvantages especially in terms of weight, ecology, construction time and costs. In the framework of a large research project a new timber slab system has been developed and already tested in first real applications. The developed slab system is designed for housing, commercial and industrial buildings. The slab system works as a flat slab carrying vertical loads biaxial and consists of timber slab elements like CLT glued together on site with a high performance butt-joint bonding technology. Research about the central slab element, the butt-joint bonding and fire tests have already been performed. The research showed the feasibility of this innovation. In 2015 a first prototype was built in Thun, Switzerland. A large three year research project started 2016 with the goal to reach market maturity.