July 4, 2013
By Jack Keays, MSc., P.Eng.
As architects, engineers, and builders push toward using sustainable, ‘green,’ and cost-effective building materials, National Building Code of Canada’s (NBC’s) tight framework with regard to combustible construction is coming into the spotlight.
Combustible and non-combustible construction, and combustible elements in buildings required to be of non-combustible construction, are terms with which most industry professionals are all too familiar. Materials permitted for use in construction depend on the occupancy, height, and area of the proposed structure.
Historically, constructing buildings of non-combustible materials was one of the only methods available to prevent large-scale conflagration. Densely populated areas, such as towns and cities, moved away from wood construction and built structures from brick, stone, and steel—with the intent of limiting fire spread between buildings and reducing the risk of major losses..
Building regulations have existed for centuries in various formats; changes to such regulations have been driven by issues prevalent in the industry at the time. Examples of this include the move toward non-combustible materials following major fires in towns and cities, such as the Great Fire of London in 1666 and, more recently, amendments to the Ontario Building Code (OBC) to address the issue of falling balcony glass in Toronto. (For more on laminated glass, see “Expanding Laminated Glass Performance” by Valerie L. Block, CDT, LEED AP, in the September 2012 issue of Construction Canada. )
The Canadian code
NBC is the model building code adopted or adapted, in part or fully, in provinces and territories across Canada. The first edition was published in 1941, and has been continuously updated with the intent of embracing new technologies, materials, and methodologies. While changes and revisions have been made, many of the early principles on which NBC requirements are based—such as area and height limitations and construction requirements—have remained largely unchanged.
The use of non-combustible or limited combustibility materials in construction has been an NBC requirement since its first edition. The definition of non-combustible construction was introduced in the 1960 version of the code; before this, non-combustible construction was not clearly defined.
The intent of non-combustible construction was to obtain a degree of safety from fire hazards by using only specific materials for structural members or assemblies, and by limiting the amount of combustible material incorporated into a building’s construction. The definition of non-combustible construction in the 1960 NBC is:
Non-combustible, as applied to a building construction material, means a material that falls in one of the following groups (a) through (c):
(a) Materials that are classed as non-combustible when tested in accordance with [Canadian Standard Association] CSA specification B54.1-1960, Determination of Non-combustibility of Building Materials.
(b) Materials having a structural base of non-combustible material, as defined in (a), with a surface not over 3.2 mm (1/8-in.) thick, which has a flame-spread rating not higher than 50.
(c) Materials, other than as described in (a) or (b), having a surface flame spread of not higher than 25 without evidence of continued progressive combustion and of such composition that surfaces that would be exposed by cutting through the material in any way would have a flame-spread rating higher than 25 without evidence of continued progressive combustion.
This definition allowed for certain materials with limited combustibility—such as treated wood and plywood—to be considered non-combustible. Classifying such materials in this manner was not the code’s intent, and the definition was amended in the 1965 NBC to return to the intended meaning.
Non-combustible, as applied to an elementary building material, means such material has been tested and found to comply with the relevant provisions of CSA B54.1-1960. The current standard for testing non-combustible construction is Underwriters Laboratories of Canada (ULC) CAN 4-S114, Standard Method of Test for Determination of Non-combustibility in Building Materials. While the test standard has changed, the definition and intent in the current code is primarily the same as the 1965 definition.
Building height restriction can be traced back to the height where a fire-hose stream could reach when fighting a fire from the structure’s exterior. The height of combustible construction was also limited due to any increased fuel load in the building, or anticipated issues associated with additional fuel load and potential fire spread within the facility.
Up until 1990, a residential building of combustible construction was not permitted to exceed three storeys in height. That year, however, the code recognized the benefits of sprinkler protection in controlling fires, as well as limiting the resultant smoke and heat. This paved the way for residential building height to increase to four storeys, provided a sprinkler system was installed throughout.
At a provincial level, the British Columbia Building Code (BCBC) was amended in 2009 to allow residential buildings of combustible construction to increase from four to six storeys. This change came after a review of potential risks and mitigating features, and after exploring how mid-rise wood construction has been regulated in other jurisdictions. (For more on wood buildings, see “Mid-rise Makeovers: B.C. Code Changes Encourage Building with Wood,” by Jim Taggart, Dip. Arch., MA, MRAIC, and “The Sky’s the Limit: Designing Airports with Wood,” by Eric Karsh, M.Eng., P.Eng., Struct.Eng., MIStructE, Ing., in the September 2012 issue of Construction Canada. )
Building area restriction was originally intended to control a fire’s size by limiting the amount of combustible contents in a floor area. This constraint was due to concerns with how the facilities performed in fire and increased fuel load. Before the 1941 NBC, the area of buildings of combustible construction was regulated by municipal building departments and local bylaws. As NBC evolved, the maximum area permitted for a structure of combustible construction has increased, and the current code permits a residential building of four storeys high to have an area up to 1800 m2 (19,375 sf) when there are sprinklers throughout.
Increases in building area have been associated with the introduction of sprinkler protection. However, this author feels the rationale behind permitting expanded areas has largely been arbitrary and opinion-based as opposed to statistics or analysis.
From an esthetic or constructability standpoint, it is impractical to construct a building entirely of non-combustible material. Taking this into consideration, in structures of non-combustible construction, NBC allows combustible elements such as:
While allowing such combustible elements provides some leeway to specifiers and designers, strict definitions preclude use of other combustible elements.
By moving to an objective-based code in 2005, designers and owners were no longer restricted to the ‘one-size-fits-all’ approach of the prescriptive-based code. This allowed for different building arrangements, more cost-effective solutions, and specific site issues to be addressed by means of an ‘alternative solution’ to NBC (Division B, “Acceptable Solutions”).
To include combustible elements in a building of non-combustible construction, which are not specifically permitted by NBC, the Authority Having Jurisdiction (AHJ) often requires an alternative solution application be submitted. This must provide rationale and technical justification in support of the deviation from the acceptable solution.
Non-combustible construction is described in NBC Sentence 188.8.131.52.(1). The objective and functional statements associated with this are:
To limit the probability that fire or explosion will impact areas beyond its point of origin by limiting the severity and effects of fire and explosions and retarding failure of collapse due to the effects of the fire.
To limit the probability of collapse of physical elements due to fire or explosion by retarding failure or collapse due to the effects of fire.
In support of the alternative solution, the applicant must demonstrate the objective and functional statement are satisfied and the proposed alternative solution performs at least “as well as” the acceptable solution.
Alternative solutions are often supported by material testing, in comparison with the acceptable solution, trade literature (i.e. product properties), or other assessments. To justify the inclusion of combustible elements, compliance with the intent of NBC can be demonstrated by a consideration of the following factors:
Essentially, the alternative provides evidence the proposed combustible elements do not increase the risk level acceptable under provisions within NBC.
Current movements within legislation
At national level, a task group of industry experts are currently reviewing potential changes to the 2015 NBC to allow an increase in height and area of buildings of combustible construction.
Provinces are also considering this change. In Ontario, Bill 52, Ontario Forestry Industry Revitalization Act, 2012—currently under consideration by the Legislative Assembly of Ontario—is seeking to make modifications to the 1992 Building Code Act to allow the height of wood-frame structures to increase from four to six storeys. The proposed change reads as follows:
Building code restriction, wood frame buildings:
(1) The building code shall not prohibit a building that is six storeys or less in building height from being of wood-frame construction.
(2) For greater certainty, subsection (1) does not prevent the building code from:
(a) imposing requirements on buildings of wood-frame construction; and
(b) prohibiting specified classes of buildings from being of wood-frame construction.
The proposed bill does not state the building types that will, if passed, be permitted to be wood-frame construction. Additionally, while the bill may be passed, it may take some time for changes to be made to OBC.
Historically, constructing buildings of non-combustible materials was the only method to prevent large-scale conflagration. To this day, a major concern with combustible construction is how such structures will perform during a fire. Fire safety features of modern facilities include sprinkler protection, compartmentation, and the use of limited flame-spread materials. These features limit a fire’s ability to grow or spread beyond its area of origin. Structural integrity and fire resistance can be achieved by using:
To move building regulations forward, and to successfully prepare alternative solutions, one must understand where the individual code requirements originated. Many started as a ‘quick-fix’ measure to address specific concerns within the industry at the time.
With advances in the understanding of fire science and behaviour, fire suppression systems, and fire service capabilities, certain requirements in the code may be outdated and no longer mandated to serve their original purpose. Moving forward with a performance approach allows for the suitability of a construction type to be determined by its quantitative performance in fire—performance based on statistics, testing, and hard data.
Jack Keays, MSc., P.Eng., is a project engineer at Sereca Larden Muniak Consulting Inc. Based in Toronto, he holds a master’s degree in fire safety engineering from the University of Ulster in Northern Ireland. Keays has more than five years of both national and international experience and is a licensed professional engineer with the Association of Professional Engineers and Geoscientists of British Columbia (APEGBC). He can be contacted via e-mail at email@example.com.
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