Words by Tony Whitehead
Reducing variability of timber with CLT
New-generation timber-framed buildings have properties that make them ideal for surviving earthquakes, according to Dr Christian Málaga-Chuquitaype, a lecturer in structural engineering at Imperial College London. “Because of their relative lightness, they attract smaller forces during seismic activity,” he says, adding that the effect is particularly beneficial in taller buildings.
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“In developing countries, timber is considered as a structural option far more than it was. Engineered products such as CLT reduce the variability of timber, so it can be specified for structures with more confidence. I think we will see taller and taller timber buildings in these areas — especially where their performance in earthquakes can contribute to safe building.”
Timber already has a good reputation in earthquake-prone regions. These structures often survive and, being strong and light, tend not to collapse heavily and crush their inhabitants in the manner of poorly-built concrete homes. Much of the traditional housing in San Francisco is built from local redwood for precisely this reason.
Techniques to reduce earthquake damage
However, there are specific issues that should be taken into account: “Timber is very strong, but it is very brittle when it fails so we have to provide ductility or deformation capacity by other means. That is why modern timber structures are usually joined by steel rods and brackets. This works well, but to repair a building after an earthquake, you ideally do not want to have to replace hundreds of brackets and thousands of nails.”
The solution, he says, is to allow the walls to rock or slide: “They can then return to their original position by means of some level of pre-stress, for example. It is the same general system of damage avoidance that is deployed in concrete or steel structures — but the energy you need to dissipate is much less because the forces acting on the lighter structure are smaller.”
Though convinced of the potential of engineered timber in earthquake zones, Málaga-Chuquitaype adds a caveat: “When we use timber products in the developed world we can be sure that they are from a sustainable source and that they are engineered to a reliable standard. But, while it is helpful that timber is a local resource in many regions, we obviously don’t want to encourage the use of timber from illegal sources, or timber products that cannot perform as expected.”