رفتار ساختاری تیرهای چوب چند لایه سریشم شده پیش استرس شده از چوب فشرده

In this study, glulam beams were strengthened by inserting compressed wood (CW) blocks into the pre-cut 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.

Glued laminated timber or glulam have been used in Europe since the middle of the 19th century [1]. Glulam is made of wood laminations glued together to form a specific piece of wood for a specific load. The interest to use this technology is to decrease product variability and make it less affected by natural growth characteristics like knots. Besides, the glulam technology offers almost unlimited possibilities of shape and design for construction, and is widely used for load bearing structures in houses, warehouses, pedestrian bridges, etc. Reinforcement of structural wood products using bonded reinforcing materials and pre- or post-stressing techniques has been studied for many decades. In the earlier stages of the research, the focus was mainly on using metallic reinforcement, including steel bars, pre-stressed stranded cables and bonded steel and aluminium plates [2], [3] and [4]. Recently, research on glulam beams reinforced with fibre and fibre-reinforced polymers (FRPs), such as carbon, aramid and glass fibres, has been increased significantly, due to the high specific strength and stiffness of the FRP materials. Plevris and Triantafillou [5] studied the effect of reinforcing fir wood with carbon/epoxy fibre-reinforced plastics (CFRPs). Plevris and Triantafillou [6] investigated the creep behaviour of FRP-reinforced wood and developed an analytical approach to predict time-dependent deflections of timber beams reinforced with CFRP laminates with different thickness. Triantafillou and Descovic [7] also studied the effect of pre-stressed CFRP reinforcement bonded to European beech lumber. A lot of research work has been undertaken to investigate structural behaviour of glulam beams and solid timber beams reinforced by FRP sheets or bars [8], [9], [10], [11], [12] and [13]. Issa and Kmeid [14] undertook research on glulam beams reinforced with two types of reinforcement: steel plate and carbon fibre reinforced polymer. The reinforcement has changed the mode of failure from brittle to ductile and has increased the load-carrying capacity of the beams. Borri et al. [15] discussed the use of FRP materials to strengthen the existing wood elements under bending loads. Guan et al. [16] studied glulam beams pre-stressed by pultruded GRP tendons. Finite element models were developed and validated, which is capable of simulating the pre-camber introduced into the beam due to transfer of the pre-stressing force. Corradi and Borri [17] also studied reinforcement of timber beams reinforced with pultruded GFRP elements. The results indicated significant improvement in flexural stiffness and capacity compared with unreinforced timber beams. Fiorelli and Dias [18] developed a theoretical model of fibre glass reinforced glulam beams with necessary validation. Johnsson et al. [19] studied reinforced glulam using pultruded rectangular carbon fibre rods and established the anchorage length for this system. The proposed reinforcement method increased the short-term flexural load-carrying capacity by 49–63% on average. However, reinforcements currently used to strengthen glulam beams cover almost whole bottom surface of the beam. Although they are effective, however due to various limitations, applications of strengthening the beam using CFRP, GFRP and other metallic materials are limited up to date. Possible problems of the existing strengthening techniques facing are likely compatibility, de-bonding, stress relaxation and complex procedures. The above problems may be overcome by the newly developed strengthening techniques using compressed wood, which is to make use of moisture-dependent swelling nature of compressed wood. In the reinforcing practice, glulam beams were strengthened by inserting compressed wood blocks with the lower moisture content than the ambient one into the pre-cut rectangular holes on the top part of the glulam beams. Once the CW blocks were inserted, they would be gradually swelling due to absorbing moisture from air until they reached the equilibrium state, i.e. the balance between the moisture-dependent swelling and the constrained expansion by surrounding glulam. The expansion on the CW blocks on the top part of the beam would generate bending moments with respect to the neutral axis that would create a pre-camber of the beam. As a result, the up-lift deflection would also produce initial tensile and compressive stresses at the top and bottom extreme fibres of the beam before applying a service loading. When such beam is used in construction, the pre-camber will cancel out some downward working deflection and the initial extreme fibre stresses will cancel out some working stresses. Therefore, the beam size can be reduced (so material saved) but still having the desired load carrying capacity, or more load can be carried by the same beam, also with the bending stiffness increased. There are some major advantages of the above reinforcing technique, i.e. (1) it is very cost effective, (2) it uses timber to strengthen timber beams so that it is purely green, (3) very small amount of compressed wood is needed, (4) it is an easy and simple practice, (5) it will have significant impact on timber construction (domestic houses and large span commercial structures), material savings, sustainability and environmental protection by reducing CO2 and volatile organic compounds (VOC) emissions. This paper aims to investigate how the moisture dependent swelling of the compressed wood, which was inserted in a pre-cut hole on the top part of a glulam beam, could generate pre-camber and further enhance the structural performance of the beam reinforced. In addition, the ultimate failure modes of both the pre-stressed short and long glulam beams were presented.

A new approach to strengthen glulam beams has been established in this research. The use of compressed wood made of a lower grade wood through densification as a reinforcing material has been approved to be effective. As only a small amount of compressed wood is required and no bonding between the CW and the beam is necessary, the techniques developed are economical and environmental friendly. This research shows that the controlled moisture-dependent swelling of compressed wood is crucial to pre-stress glulam beams and to produce pre-camber as well as the initial stresses to cancel out working counterparts partially. Destructive bending tests for all beams pre-stressed by the compress wood have also indicated that there are significant enhancements on the bending capacity and the initial stiffness. For the short glulam beams reinforced by three 30 and 45 mm thick CW blocks corresponding to the reinforcement volume of only 1.2% and 1.8% of the total beam volume, there are increases of 19% and 22% in bending stiffness and 14% and 19% in load carrying capacity respectively. For the long beams the enhancements of the bending stiffness were 37.1% and 45.8% for the beam reinforced by five and seven CW blocks and a CW lamina respectively. In terms of load carrying capacity, a beam reinforced by five CW blocks and a CW lamina carries the maximum load of 64.3 kN, which is an 11% increase in comparison to the control beam. The beam reinforced by seven 45 mm thick CW blocks should show a higher improvement on load carrying capacity following its initial stiffness if there was no premature failure. Amount of CW used to strengthen the long beam can be increased to enhance the pre-camber and the corresponding initial extreme fibre stresses. Cyclic and visco-elastic creep behaviour of the pre-stressed beam systems are currently under investigation in the numerical modelling. In addition, stress relaxation is being investigated. This work will enhance further understanding of structural behaviour of glued laminated beams reinforced by compressed wood.