September 1, 2012
By Jim Taggart, Dip. Arch., MA, MRAIC
Something old is new again on the B.C. construction landscape. Currently, more than 100 mid-rise or six-storey wood-frame residential structures are in development or have been completed. Since the B.C. Building Code (BCBC) was revised in 2009—increasing the permissible height of light-frame wood construction for residential buildings to six storeys—there has been a lot of interest in the building and design community to pursue new projects. It is easy to understand this emerging trend in high-density residential construction when considering the environmental, social, and economical benefits.
As environmental awareness and concerns about climate change escalate, wood is emerging as a suitable material for mid-rise residential and commercial construction. The environmental benefits of wood include:
Peter Busby, CM, AIA, FRAIC, MAIBC, MAAA, MOAA, BCID, LEED AP, an architect at Perkins+Will, recognizes the environmental benefits and versatility of using wood.
“Wood has become an increasingly important part of our practice. It is the only structural building material made by the sun, benefitting nature throughout its life. We can extend those benefits by using wood—two-thirds carbon by weight—as an enduring carbon sink,” he says. “Design professionals, policy-makers, contractors, and developers need to work to ensure wood’s viability in all building types and sizes in the future.”
The ability to build residential structures greater than four storeys in wood can fulfil numerous community objectives. Since these mid-rise structures have lower construction costs, they provide an opportunity for affordable housing. The modest increase in height also allows local government to slightly increase density without permitting high-rise towers, creating more pedestrian-friendly communities and addressing urban sprawl.
Modern six-storey light-frame wood construction in British Columbia incorporates highly detailed, scientifically researched, and safe processes. These mid-rise building solutions currently being developed and refined in the province could lead to more sustainable communities and affordable housing that will positively change the face of North American cities.
A building code revision to permit mid-rise wood construction nationally is expected to be considered for adoption as a future target of the National Building Code of Canada (NBC) 2015 code cycle, which would bring Canada closer in line with other international jurisdictions. For example, the states of California, Washington, and Oregon have permitted similar opportunities for developers for more than a decade.
Reviving an old idea
With concrete and steel structures predominately represented in urban centres for most of the 20th century, it is easy to forget wood was once the material of choice for mid-rise construction. In many Canadian and U.S. cities, 100-year-old heavy timber post-and-beam offices and warehouses still stand in testimony to wood’s durability and strength.
In the new generation of wood buildings, diversity is the key, with different technologies proving successful according to the scale, type, and location of specific projects. Around the world, several methods of wood construction have been successfully employed, including:
Collectively, these structures (and others like them) have transformed the understanding of what is possible using only standard dimension lumber and the engineered wood products readily available from local retail lumber yards.
Advancing light-frame wood construction
Light-frame wood construction has been a mainstay in the North American forestry, construction, and design industries for small commercial and residential buildings. Perhaps because of familiarity, many architects and engineers have taken the technology for granted, and may have overlooked its potential for larger-scale buildings.
This situation has been perpetuated in part by NBC, which has historically applied strict limitations on the size and height of combustible light-frame wood construction. In 1990, however, the maximum allowable height for residential (Group C) buildings of wood-frame construction was increased from three to four storeys. In 1995, this was extended to Group D and E occupancies—the former including business and service buildings and the latter including mercantile occupancies.
Light-frame wood construction has always been known for its economy, versatility, and speed of erection, but not necessarily for its strength, accuracy, and precision. While that perception may have been legitimate several decades ago, new materials, innovative engineering solutions, and offsite prefabrication have substantially increased the quality and sophistication of light-frame wood construction.
For the most part, these individual changes have been unobtrusive, hidden within wall, floor, and roof assemblies, but collectively their impact has been profound. As a result of these modifications, light-frame wood construction is now considered a viable, affordable, and environmentally responsible alternative to concrete and steel.
Mid-rise design and construction
When constructing and designing mid-rise structures with wood, there are various factors to consider.
Key aspects of design that become critical in this new generation of mid-rise structures include increased dead, live, wind, and seismic loads that are a consequence of building to a greater height. The responses to such an imperative within more complex mid-rises involves the adaptation of structural and architectural design details that address key construction-related issues.
Earthquake engineering researchers examined the seismic performance of a full-size, six-storey wood building on the world’s largest shake table at a facility in Miki City, Japan, as part of the BCBC review process. (To read more on this study, see “Seismic Testing” by Steven E. Pryor, PE, SE, John W. van de Lindt, PhD, and Shiling Pei, PhD, in the June 2010 issue of Structure Magazine. Visit www.structuremag.org/article.aspx?articleID=1078#ArticlePDF). The structure was built from B.C. forest products and with commonly used Canadian construction techniques. The strongest test simulated an earthquake expected to occur once every 2500 years. Results confirmed seismic risk can be kept at an acceptable level for six-storey, light-frame wood buildings when they are constructed and designed well.
Recent testing has confirmed noise and vibration (i.e. flanking) transmission can be further reduced by breaking the continuity of the subfloor and ceiling system at the acoustic separation. Additionally, blocking members can be added in the floor cavity to decrease joist vibration, and concrete topping is now universally used on all floors to minimize vertical transmission of airborne noise. Research has shown the addition of an extra layer of floor sheathing is also an effective method for lowering the transfer of noise and vibration.
For construction up to six storeys, fire separations (other than firewalls) are required to have a fire resistance rating of one hour. BCBC does not prescribe any particular solution, leaving designers and code consultants the flexibility to determine the best option for their project. Solid, nail-laminated 2×6 dimension lumber construction is one way to achieve the required one-hour fire resistance rating for wood elevator shaftwalls.
With mid-rise construction, the exterior cladding must be non-combustible or constructed of materials expected to limit vertical fire spread. These include stucco, brick, and/or siding material (i.e. metal or vinyl). This is intended to minimize the risk of fire spreading to upper units if windows in an apartment below break because of heat from a fire.
Architects and code consultants have demonstrated the use of mineral wool (rather than fibreglass insulation) in exterior walls, combined with heavy-timber-compliant members as window headers and jambs, offer a level of protection equivalent to a layer of exterior-grade gypsum sheathing behind the exterior cladding. Mineral wool insulation can also be used in the common walls between different dwelling units, providing enhanced firestopping and flame-blocking attributes to these areas.
“When a wood building project is completed with all fire protection measures in place, the building should perform as well as any building constructed with other materials during a fire,” says Thomas Leung, P.Eng., Struct.Eng., MIStructE. “A wood building with fire-rated assemblies according to the building code requirements and fire suppression system installed is as safe as any building constructed with other materials—whether it’s a two-, four-, or six-storey building. In particular, wood-framed buildings, being much lighter than other material-type buildings, can have superior performance in an earthquake.”
Construction site methodology
The evolution and refinement of prefabrication for walls, and potentially other building envelope elements, have changed traditional material ordering and delivery systems. Computer-assisted material sorting and selection ensures use is optimized and waste minimized. A single prefabrication shop may be supplying various projects at one time; any needed waste collected can be reduced, reused, and recycled onsite, making the process both energy- and material-efficient.
When detailed and constructed accurately, prefabrication can speed up construction, minimize inefficiencies, and reduce potential for onsite errors.
Sustainability and affordability
Wood’s environmental advantages are considerable when compared to other major construction materials. At a time when ‘carbon neutrality’ is emerging as a key design goal in high-performance buildings, these advantages are becoming increasingly important.
The extraction and processing of wood products is less energy-intensive than other materials—such as steel and concrete—and often uses carbon-neutral wood waste as an energy source. Therefore, substituting wood for other materials in the construction of any building can help contribute to a reduced carbon footprint.
The conversion of wood into durable building products prolongs the benefit of carbon storage, and through reforestation, the sequestration cycle in the forest continues. Increased urban density, which can be the outcome of mid-rise construction, can help reduce the environmental impact of development in the built environment.
With mid-rise wood-frame construction as a new practice in British Columbia, it is not yet possible to make a statement about overall cost savings relative to other forms of construction. However, based on projects under construction in 2011, it appears the above-grade price of mid-rise construction in wood could prove to be less than steel or concrete building.
Once this new technology has been streamlined, and particularly if the prefabrication benefits are fully appreciated, construction times will be reduced. Increased speed and more effective use of materials will improve the affordability of housing as advanced wood-frame, mid-rise building technologies become mainstream.
Light-frame wood assemblies can now legitimately take their place alongside other sophisticated construction systems. While six-storey structures may still seem like a new challenge for this technology, Canadian building researchers believe in the possibility of safely extending light-frame wood construction to eight storeys. Indeed, the construction of taller, better performing, and lower-cost wood structures is possible with the evolution of a fully integrated design approach to mid-rise buildings, combined with the introduction of new building materials such as CLT, and the improvement of existing onsite practices.
Jim Taggart Dip. Arch., MA, MRAIC, teaches history and theory in the architectural science degree program at the British Columbia Institute of Technology (BCIT), and is the editor of Sustainable Architecture and Building magazine (SABMag). Taggart worked for more than a decade in the design and construction industry and has been focused on public and professional education since 1992. In 2001, he was inducted as a Fellow into the Royal Architectural Institute of Canada (RAIC). Taggart can be reached at email@example.com.
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