Mid-rise wood frame buildings are becoming more and more common in Canada. More than 250 buildings in British Columbia already completed since 2009. The first five- and six-storey buildings are scheduled to start construction in Alberta, Ontario, and Quebec, where the provincial building codes have also been amended to allow mid-rise wood frame construction. Therefore, the performance of the building materials and construction practices used to design and erect these buildings are being closely monitored by code officials, designers, and building component manufacturers.
Several key design aspects that become more critical in this new mid-rise construction include:
Increased potential for cumulative shrinkage and differential movement between different types of materials, as a result of the increased building height.
Increased dead, live, wind and seismic loads that are a consequence of building to a greater height.
Increased fire-resistance ratings for fire separations as required for buildings of greater height and area.
Ratings for sound transmission as required for building of multi-family residential occupancy, as well as other uses.
The use of structural wood framing in five- and six-storey (and greater) structures has been allowed by various provincial building codes in part by the development of engineered wood products (EWP) that are dimensionally stable with predictable physically properties. Unlike EWP with its composite makeup, solid wood is a heterogeneous material that undergoes varying amounts of shrinkage in different directions depending on the species, grade, and environmental conditions in which it's used.
Although EWP has given designers and engineers the tools they need to be confident in their structural design of mid-rise wood frame buildings, the professionals still have not addressed the problem of water degradation and fire susceptibility of the building or wood frame components during the construction phase. EWP is still intended for dry use only and is designed to only withstand the elements during normal, modest construction delays.
Weather and Moisture
It stands to reason that a six-storey wood building will take approximately 50% longer to frame that a four-storey equivalent built using the same techniques. As such, the risk of materials getting wet is considerably greater. This situation is exacerbated by the increased number of laminated solid sawn members and composite EWP that are required to carry the increased loads on the lower floors. If exposed to weather, these have a tendency to retain water longer and swell more than non-laminated members.
While there have been more than 250 five and six-storey wood-frame buildings completed in B.C. since the 2009 code changes, many municipal building officials and fire safety officers have not yet had firsthand experience with the application.
The Expansion of Mid-Rise Wood-Frame Residential Construction in BC by Canadian Wood Council. (click to download)
PinkWood Ltd. has recently developed a new series of Engineered Wood I-Joist called PKI-23 Series I-Joists. We designed this new series specifically to address some of the issues faced by builders in humid climates where weathering during construction delays can cause significant problems to EWP - OSB swelling and mildew growth. An additional benefit to the PKI-23 series joists, is that they come standard with a flame spread rating (FSR) of less than 75. This, compared to bare wood with a FSR of 100-185, helps reduce the risk associated with construction site fires and provides added protection to the finished floor structure once the building is complete. Like all PinkWood I-Joists, the PKI-23's are manufactured with solid sawn SPF flanges which also limit their vulnerability to swelling and delaminating due to water penetration. This issue has been seen with some laminated veneer lumber flanged I-Joists in the market.
For more information on the availability of our PKI-23 series I-Joists please contact us.