More revolutionary is a renewed interest in timber as a structural material, and particularly in engineered products such as cross-laminated timber. CLT was developed around 15 years ago, and although there have been small improvements in the machinery and glues used in the laminating process, the basic product remains the same.
“The difference now,” says Viktor Rönnblom, structural engineer and timber specialist in WSP’s Skellefteå office in northern Sweden, “is that we know much more about how it can be used and how it performs.” This also applies to other engineered timber products, he says, mainly glulam (glued laminated timber) and laminated veneer lumber, he adds.
CLT has been described as “plywood on steroids” and it is formed in a similar way, from alternately oriented sheets of timber. Glulam, used more for beams and columns, is again similar but the grain in the strips is aligned. LVL uses thinner, veneer cuts with occasional cross layers.
“Engineering timber in this way removes natural variations and makes its properties much more consistent and predictable,” explains Rönnblom. “And because we have had substantial buildings to monitor for over a decade now, we know how engineered timber structures perform in wind, in fire, and we know about their stiffness and their dynamic properties.” This is enabling the design of taller timber structures, although the tallest complete pure timber structure is still only 14 storeys.
Further increases in height will inevitably be incremental, says Rönnblom. “The way to go is to build, evaluate, and then maybe add five storeys to the next design, and so on. I don’t think you should jump from 15 to 40 storeys. The effects of any miscalculation could grow exponentially.”
In any case, it is likely that building regulations will stymie any sudden leap in the height of timber structures. Worldwide, these tend to forbid buildings higher than six or so storeys because of fears over how timber structures will perform in fire. Tallwood House in Vancouver, which has a 17-storey hybrid CLT structure, had to win special dispensation from British Columbia’s Building and Safety Standards Branch — and that was only granted after the designers agreed to enhanced fire and seismic standards that exceeded those for a concrete or steel building. This involved complete encapsulation of most of the CLT and glulam components with three or four layers of fire-rated Type X gypsum board.
In fact, says Rönnblom, fears over fire safety are more to do with perception than fact. “Put glulam on a fire and it is quite difficult to get it to burn out completely,” he says. “It is also very predictable, charring at 1mm per minute, with the strength of the timber behind largely unaffected. It makes it predictable, easy to calculate and in many ways it performs better than concrete or steel.”
Height is not everything. Rönnblom points out that engineered timber can play different roles in different buildings: “For example, you can use composite CLT-concrete structures. In that case, the CLT is both formwork and part of the load-bearing structure.”
One of timber’s most important qualities, Rönnblom says, is that it is light, so foundations can be smaller and therefore cheaper and quicker to lay. “You can also put timber buildings in places where concrete would be unsuitable — in soft ground, for example. Timber lends itself to prefabrication, and it is easy to handle on site, so you can build quickly and efficiently.”
There is one market in particular for which engineered timber would seem well placed, as WSP‘s Robert Kilgour points out: “The trend in big cities is towards densification — squeezing more and more accommodation onto land that is already built.” The lightweight nature of CLT makes it ideal for building extensions because it reduces the modifications you have to make to the existing structure.”
WSP is currently advising an Australian client on adding a ten-storey extension to a six-storey block, he adds. “We are looking at how the timber will perform structurally and what would have to be done to connect it effectively to the existing structure. In this sort of situation, the lightness of engineered timber, along with the potential for rapid construction, makes it an attractive option as cities continue to densify.”
Last but not least, Rönnblom says, people enjoy inhabiting timber buildings: “They feel nice, and have pleasant acoustics.” This makes them a desirable proposition for housing, and also for commercial projects where a growing body of research is linking the productivity of employees to their comfort and contentment with the working environment.
His last point is an important one, for no matter how technically advanced a new material is, it will struggle to catch on if clients do not like it, or if it does not sit well with contemporary architectural tastes. Engineered timber is attracting attention as much due to its perceived ecological credentials as its structural capabilities — even though the carbon used in the manufacturing process shouldn’t be overlooked. But no matter. Progress in materials science is never wasted, and an designer can never have too many options.