Scoping Review on Mid-Rise Wood Buildings
Executive Summary
The push to allow the building of five- or six-storey wood frame structures is coming mainly from the wood industry, developers and city planners who want to provide more affordable housing options and increase urban density. Planners have been reluctant to achieve increased densification with high-rise buildings, which may dramatically alter the urban landscape. However the economics of low- to mid-rise steel and concrete residential buildings are such that few are being built. The BC wood industry sees an opportunity to grow their domestic markets at a time when export markets for their products are depressed.
In a number of municipalities in the Lower Mainland, Southern Vancouver Island and the Okanagan, wood frame structures taller than the currently allowed four stories have been approved under 'special considerations' provisions. This emerging practice mirrors what has been happening in neighbouring jurisdictions in the western United States. Several local governments in Washington, Oregon and California have amended local laws to allow five- or six-storey wood frame structures with stipulations that include having the ground floor comprised of non-combustible materials or otherwise provide two hours of fire rated construction.
Other typical stipulations include:
- protecting exit enclosures with 2-hour fire resistive constructions,
- pressurized exit enclosures and elevator shafts,
- fire walls,
- automatic fire-sprinklers and
- stand-by power sources to ensure continuous operation of fire protection systems.
The objective of these measures is to try to ensure a building's structural stability and protect human life in the event of fire. The increased fire risk of wood buildings during construction is also a concern and consideration may be given to requiring the installation of sprinkler systems much earlier in the construction process.
Other challenges of mid-rise wood building construction include structural stability and building envelope integrity. Wood shrinkage causes changes to the physical properties of wood. According to Kevin Cheung, "the cumulative effects of multi-storey shrinkage can cause large expanses of interior and exterior drywall, panelling and siding to buckle." Newer engineered wood products may increase the performance of wood with respect to shrinkage and seismic events. Their use, however, may reduce the cost advantage of wood-frame construction and require the development of new knowledge and expertise on the part of designers, contractors, trades and building officials.
Seismic considerations are also a significant concern in BC. While an Italian-designed seven-storey wood framed building was successfully tested on the shaking table at Japan's National Institute for Earth Science and Disaster Prevention in 2007, it was built with engineered wood laminate materials that are not in use here. In the United States, the National Science Foundation is sponsoring a multi-university NEESWood project that "seeks to take on the challenge of developing a seismic design philosophy that will provide the necessary mechanisms to safely increase the height of wood-frame structures in active seismic zones." Testing of wood-frame designs more similar to BC construction methods is expected to take place next year.
Sound transmission can also be a concern with wood buildings, particularly for a multi-family residential building. Lightweight gypsum concrete and other sealers may help. Lightweight concrete poured on the floor after framing can be used to reduce sound transmission as well as providing a fire barrier between units.
Promoters of wood-frame construction cite a number of 'sustainability' advantages including lower energy usage and less carbon emissions. They assert that wood is easy to insulate to high standards and easily adaptable to changing energy codes. They also cite a 2006 European Commission finding that:
- Production of materials for wood-frame buildings uses less primary energy than concrete-frame buildings.
- The difference in life cycle emissions between wood and concrete framed buildings ranged from 30 to 139 KG of carbon per square meter of floor area.
- From a life cycle perspective, the net change in carbon stocks (tree biomass and wood building stocks) is insignificant when using wood-based building materials from sustainably managed forests.
Further analysis will be required to confirm the 'sustainability' comparison between different building frame materials.
Conclusion
The opportunity may exist to achieve benefits such as more affordable housing options and increased housing densities through changes to the BC Building Code to increase the allowable height of wood frame residential buildings from four to six stories. However, the technical challenges such as fire safety, structural and building envelope integrity and sound transmission must be carefully examined and appropriate mitigation requirements developed. Anticipated cost and sustainability benefits compared to concrete or steel frame buildings must also be substantiated through further research and analysis.
To view the complete scoping review, click here.
