Cross-laminated timber (CLT) is changing the way urban builders scrape the sky.
From London to Tokyo, the race is on to build the tallest wood-framed skyscraper in the world. Prized for its workability, low cost and visual aesthetics, wood was widely used by urban builders until the early 20th century, when fires triggered by the 1906 San Francisco Earthquake leveled the largely stick-built city. Until recently, the other knock on wood was a vertical one, in that stick-framed buildings generally top out at five stories, owing to the accumulation of dead and live loads in excess of the allowable loads for lumber.
The emergence of cross-laminated timber has changed all of that. One of several engineered wood products referred to collectively as mass timber, CLT is normally an assembly of dimensional studs glued together at right angles to create massive wood slabs that can be used for load-bearing wall or floor elements, achieving a structural integrity comparable to those of steel or concrete but with the benefits of wood. This unique composition allows CLT to have strength in two directions, as opposed to glulam, which generally has one-directional strength. Heat-resistant glues aim to make CLT as fire safe as concrete or steel, and the comparatively light weight means CLT is easier to transport, resulting in lower freight costs and reduced carbon emissions.
How High Can CLT Go?
For high-rise builders with an eye on CLT, the only question left seems to be how high can it go? At 18 stories, the Brock Commons student dormitory at the University of British Columbia in Vancouver is currently the tallest wood-framed building in the world (see a time-lapse video of construction here). Architects in Chicago (River Beech Tower) and London (The Oakwood Tower) are racing to more than quadruple that height with plans for 80-story residential towers, but could be bested by Sumitomo Forestry, which expects to complete construction of a 1,148-foot tower in Tokyo by 2041.
Dubbed the W350, Sumitomo’s mixed-use plyscraper is designed so CLT components can be replaced across a rolling, multiyear schedule to extend the building’s lifespan and allow for additional recycling of the CLT into smaller dimensional pieces or biomass products. Engineered for both fire resistance and seismic resiliency, W350 will cost twice as much as a conventional steel tower, but Sumitomo expects CLT costs to ultimately fall in line with concrete and steel as the construction practice proliferates.
Simpson Strong-Tie and CLT
Easier material handling and installation of mass timber products like glulam (glued-laminated timber) and CLT are likely to provide immediate labor cost savings to builders, who can use low-torque, fast-driving fasteners such as Simpson Strong-Tie® Strong-Drive® SDWS Timber screws and Strong-Drive® SD Connector screws for easy CLT field assembly. Simpson Strong-Tie has also developed a range of connectors suitable for CLT applications, because we expect CLT construction to mushroom as builders and designers continue to explore durable, cost-effective alternatives to steel and concrete for specialized construction projects.
Economically and environmentally, the use of CLT for high-rise construction offers an upside to timber builders and architects looking to raise their sights without losing their wallets. Sustainable, recyclable, and approaching the structural attributes of steel, wood appears poised to reach its next apogee as a building material.