Effective April 6, 2009, the British Columbia Building Code allowed an increase to the maximum building height for mid-rise, wood-frame, residential construction from four to six storeys.* With these changes, the provincial government wishes to encourage a new market for local wood products, to lower building costs, and to stimulate “green” building practices.
In response, design professionals and insurers have expressed concerns about the technical requirements and insurance risks. The engineering concerns primarily focus on fire safety, structural integrity and wood shrinkage. Because these design changes are untested, the insurance industry has voiced concerns regarding the distribution of risk.
The new Code changes address earthquake load and effects and, in particular, place restrictions on the configuration of timber shear-wall systems. Structural engineers will receive further directions for designing and locating shear walls. The goal here is to encourage more uniform responses from lateral load-resisting systems. These changes are expected to provide a more conservative approach until further research is completed.
In addition, the government introduced new Code provisions that established a lower limit on earthquake design force for five- and six-storey wood-frame buildings. The rationale behind these provisions is to reduce the risk of soft-storey earthquake behaviour which may result in building collapse. Soft-storey behaviour tends to occur in the lower storeys of taller buildings where there is lower resistance to lateral forces. For example, soft-storey collapse may result where the structural system is reduced to allow enlarged commercial space or a large ground-floor atrium.
Under the Code changes, mid-rise wood-frame buildings of more than four storeys in height will be sprinklered to the American National Fire Protection Association 13 Standard for the Installation of Sprinkler Systems. The changes include retention of the current ratio of building height to cumulative floor area and an overall 18-metre height limit to the uppermost floor level. By limiting the combustibility of cladding, the Code changes aim to reduce the risk of fire spreading to adjacent buildings.
Insurers have expressed some apprehension about issues of fire safety and water ingress. It remains to be seen how the changes will affect insurance coverage and premiums since there are questions whether fire sprinkler systems can adequately protect taller wooden buildings. Additionally, insurers and others have questioned if some fire departments have the capacity to adequately respond to fires in such buildings.
The Code changes have addressed some fire safety issues. Since the Code does not change the ratio of building height to cumulative floor area, fire risks, theoretically, are not expected to increase in relation to the following: ignition; interior fire spread beyond origin; failure of sprinkler system to control fire; occupants’ ability to recognize fire; occupants’ ability to evacuate the building. The requirement to have non-combustible cladding will reduce fire exposure to the building face and to adjacent buildings.
There are other considerations that will have to be taken into account. The increase in height for wood-frame buildings raises concerns about wood shrinkage and resulting building movement. Site conditions may affect original design assumptions and the performance of wood products depends on their specific grade and limits in moisture content. As a result, for taller wood-frame buildings, design professionals will need to pay close attention to the properties of wood materials.
The provincial government has provided funding to the Association of Professional Engineers and Geoscientists to establish a technical bulletin for engineering practice issues arising from these Code changes. This proposed bulletin will provide information and design techniques in relation to structural, fire-protection and building-envelope engineering issues. The overarching objectives of the proposed guidance are the safety, health and welfare of the public. This guidance may alleviate some of the concerns for design professionals and insurers.