Photo © W.I. Bell. Photo courtesy WoodWorks
by Kenneth Bland, PE, and Paul Coats, PE
In building code history, wood structures have been highly regulated. Experience gained from past fires contributed to what is still the basis of today’s modern building codes, which are traditionally slow to change, and therefore, retain limits and restrictions established in response to what occurred centuries ago.
Today’s building codes may lack flexibility in materials and methods often justified on a case-by-case basis through code variances or establishing equivalencies. While wood has always been a material of choice for residential construction, options in the code for a certain class of commercial structures remain few—primarily steel and concrete. However, this is changing, and new developments in technology and updates to the International Building Code (IBC) are making it possible.
The 2012 IBC regulates allowable building height and floor area based on structural framing materials, either combustible or non-combustible, and fire-resistance ratings of walls, floors, and roof structures. With increasing recognition in IBC for fire sprinklers and other fire safety building features, the possible size and types of wood buildings has dramatically increased. The IBC permits one- and two-story business and mercantile buildings of wood construction to be unlimited if they have a reasonable separation distance from other buildings and a fire sprinkler system. Residential multi-family buildings can be taller than in the past—up to 30 m (85 ft) in some cases—and wood features in otherwise non-combustible buildings are more readily permitted. This is all timely for both ever-expanding environmental considerations and new developments in wood technology.
An alternative approach
National Fire Protection Association (NFPA), 5000, Building Construction and Safety Code, has a similar approach. However, it contains an alternative to regulating building height and area that emphasizes the number and frequency of specific fire-resistive elements, such as fire barriers rather than building size, in regard to fire risk. This idea has been codified in Appendix D of NFPA 5000—an approach that uses fire-resistance rated separations to compartmentalize buildings regardless of size. In other words, building size is not limited according to construction type, but rather a maximum area between fire-resistance rated elements in the building is established.
This idea has not yet been successful in the IBC, which still relies on traditional height and area limits by means of a table with standard ‘per story’ area limits. However, this could change in the future.
Basic areas and increases in the IBC
IBC recognizes two overarching types of construction: non-combustible and combustible. These are reflected in the construction type categories of the IBC. Types I and II are non-combustible, while Types III, IV, and V are combustible. IBC, Table 503 establishes building area and height limits in accordance with the construction type of the building. However, the tabular allowable areas are only a starting point for how the code regulates building area based on construction type. The presence of features such as fire sprinkler systems, separation from property lines, and fire-resistance ratings permit buildings much larger than the tabular areas.
Fire sprinklers and open frontage—the separation of the building from property lines and other structures—are the most important features for larger building areas. Sprinklering a building will yield a minimum 200 percent increase in tabular allowable area per story for most low-rise buildings, and 300 percent for single-story buildings. Fire separation of 9.1 m (30 ft) will yield an additional 75 percent. The increases are cumulative; with some additional separation, the code removes all area limits for many use groups in single and two-story buildings—they become what IBC calls “unlimited area buildings” (Section 507) .
The table in Figure 1 shows a comparison of allowable building size for common types of construction and commercial use groups in low-rise buildings. Construction Types III and V are primarily combustible (e.g. wood), and Types I and II are non-combustible (e.g. steel or concrete). The numbers in each cell (from top to bottom) indicate the basic tabular area per story, the maximum increase for open frontage (i.e. distance from property lines), and the combination of full-open frontage and sprinklers. The numbers for Type III-B (combustible, unrated except for exterior walls) and V-A (combustible, one-hour rated) are both roughly equivalent to those for II-B (non-combustible and unrated).
Looking at a similar table for single-story buildings (Figure 2), the unlimited category makes size distinctions even less.
Benefits of sprinkler trade-offs
Over the last 20 years, IBC has changed to recognize safety benefits provided by automatic fire sprinklers. Many buildings are required to be sprinklered because of a combination of floor area and use (i.e. occupancy group). Since sprinklers may be required regardless of construction type, designers should consider the most economical materials permitted by the code given the sprinkler mandate. Permitted area increases due to sprinklers should be considered when selecting the type of construction early in the design process, since sprinklers may be required by other provisions of the code.