Stress Laminated Timber (SLT) is a construction system created by compressing multiple layers of timber together with steel tendons. The stress part in the name implies that stress or pressure is used to hold the timber boards together. So, the steel tendons, which run through the timber boards in the direction perpendicular to the timber layers, are pulled in tension while simultaneously pushing on anchor plates that squeeze the timber boards together. The system is fixed in place with a permanent hold at that initial compressed state. SLT has the advantage of being adhesive free, constructable on-site, and deconstructable after use, which is important for recycling. Wave layered timber (WLT) is an altered version SLT which aims to utilize timber boards with a wave-shape cut into them to enhance the performance.
Although there are already some construction projects using WLT, optimized design parameters for such systems are lacking and the performance under different service conditions are not very well reported. Indeed, while information on the performance of STL is present in the literature, WLT is a rather new system with very little scientific background detailing it’s physical and mechanical behaviour. Since the wave shape offers a different type of connection between the adjacent layers, compared to the flat planed timber used in SLT, the system behaves differently than conventional SLT.
Important to this work is the fact that timber is inherently susceptible to moisture, which leads to swelling and shrinkage, and that over time wood relaxes from its stressed state to one of lower stress when fixed in such a squeezed position. I am personally interested to understand how a change in moisture content influences the assembly and what happens to over time due to relaxation. Therefore, in the present study we investigate just that and look at how WLT behaves under varying relative humidity conditions and how the pressure holding the boards together changes in wet and dry conditions. To explore the influence of time alone, we measure the change in pressure as wood relaxes, while in a state of constant moisture content. We also looked at how the wave-shaped boards fit together when compressed.
From the results, we see that the relationship between force and moisture content is proportional, such that stress decreases linearly during shrinking and increases linearly when swelling. However, the additional effect of wood relaxation is observed, and the rate of pressure loss during shrinkage if found to be higher compared to the rate of pressure gain during swelling. I found it particularly interesting to see how much the pressure decreased when the wood was shrinking. This makes an interesting design scenario for construction, where deciding on the initial pressure is a balance between maintaining enough pressure to keep the assembly tightly together and not too much pressure that the wood gets crushed.
Keywords: stress laminated timber, moisture content, relaxation, compression
Authors
Steven Collins
Aalto University, Finland
Lauri Rautkari
Aalto University, Finland
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