As interest in structural timber and its sustainability characteristics increases, the question arises: Does building tall with timber make sense? Paul Fast looks at factors that inform the answer.
The tallest mass timber building in the world is the 18-storey Tallwood House at Brock Commons
at the University of British Columbia (UBC) in Canada, soon to be eclipsed by the 24-storey HoHo Tower in Vienna. Sumitomo Forestry Co. of Tokyo has announced plans to build a 70-storey timber tower by 2041, and a recently published research paper explores the possibility of an 80-storey tower in Chicago.
As engineers, we see a unique allure in the prospect of building something that spans longer or rises higher. Although stretching our limits is a noble objective, doing so just to secure bragging rights misses the mark. When our firm was tasked with designing the structure for Brock Commons, the question was not ‘Can it be done?’ (no brainer), or ‘Should we do it because it would be the tallest timber tower in the world?’ (vain), but rather ‘Can we do it economically? Can we build a timber structure, with all its inherent sustainability advantages, for the same price as a more conventional steel or concrete building?’
Above: Brock Commons, image by Seagate Structures.
So will building tall with timber become mainstream? Here a few factors that will inform the answer:
1. Wood is the weakest material of the big three. It makes sense for heavier lifting to be done by the stronger materials and lighter lifting by the weaker material. Larger columns and walls are required when building with timber, which increases manufacturing costs and reduces useable floor space. The taller the building, the more these realities point to hybrid structure for the foreseeable future, at least until we can quickly grow stronger wood species (which I think will come… after all, concrete strength can now exceed 140 MPa!).
2. Harvesting the two-way spanning capability of cross-laminated timber (CLT) will enhance the viability of mass timber in tall buildings. If CLT slabs are point supported to create a structural system akin to a concrete flat plate, beams are eliminated, erection time is reduced, building height diminishes, and building services are more easily installed. Thus it makes sense to build taller when constructing micro-residential units where tighter column spacing is feasible. In our experience, large clear span requirements (for instance in standard 9m x 9m commercial space) forces timber into deep beam and slab solutions that are not only more costly but also problematic for systems installation.
3. Tall buildings in dense urban centres with high construction costs lend themselves to timber construction. Off-site prefabrication of large wood floor and wall panels results in faster construction and quieter job sites.
4. Acoustic considerations have a significant impact on the economics of timber construction. Dry systems (versus conventional concrete topping solutions) are increasingly being researched and developed; however, too little mass can become counterproductive from a wind loading perspective when building taller. Successful development of acoustic solutions will drive down the cost of timber alternatives.
5. Differential settlement issues due to elastic deformation, shrinkage, and creep are exacerbated when building taller, particularly in the case of hybrid construction where core elements are steel or concrete and columns are timber. These designs require rigorous fine tuning.
6. Weather factors impact tall timber construction in many regions. The development of inexpensive, temporary rain-enclosure systems will enhance the viability of taller timber towers that are often subject to lengthy periods of rain exposure during construction.
7. ‘Soft’ factors are also impacting tall timber construction. For office environments, funky is in and T-bar ceiling is out. Many tenants are calling for exposed timber and ample daylight, in essence a warmer ambience. The demand for more sustainable construction systems such as carbon-sequestering and rapidly-growing timber is also on the rise. These preferences open the door for timber in taller buildings, even at a small price premium. Overcoming public fear about fire still has a ways to go, notwithstanding extensive research results that should allay such concerns.
In light of these factors, does it make sense to build taller with wood? In the right location for the right building type, it absolutely does. However, let’s not force a square peg into a round hole… when your timber structure starts groaning, remember there are other materials that will gladly help out.
Paul Fast is the Founder and Principal of Fast+Epp, a structural engineering firm with headquarters in Vancouver. The firm worked on the Tallwood House at UBC, the proposed 12-storey Arbour project at George Brown College in Toronto, and is currently involved in several other tall wood buildings.