Photo courtesy APA–The Engineered Wood Association
Roof structures are one of the primary exceptions noted in the IBC allowing significant use of wood in many commercial buildings. Projects where the roofing structure is a defining feature of the building—as seen here in the Skagit Multi-Modal Transportation Center in Mount Vernon, Washington—often take advantage of this permission. Photo courtesy APA–The Engineered Wood Association
Sprinklering a building, regardless if it is required by use group floor area, can also reduce costs elsewhere in the code. The 2012 IBC has at least 20 significant ‘trade-offs’ (or, ‘trade-ups’ for what may be considered better fire protection than passive systems). These include:
- reductions in both corridor wall ratings and opening protection;
- relief from areas providing areas of refuge in exits;
- increased exit travel distances;
- reductions or elimination of certain fire-resistance-rated separations;
- lowered requirements for fire and smoke dampers; and
- decreased restriction on interior finishes.
A sprinkler system can often pay for itself by eliminating features that may otherwise be required in the building, while also opening the door for wood construction at the same time by increasing the allowable building height and area.
Due to the fire protection afforded by active suppression systems, increases in building area and size are set forth for all construction types in IBC, Chapter 5, when sprinklers are installed.
The use of sprinklers does not eliminate all passive fire protection features required by IBC. The possibility of sprinkler failure due to human error (e.g. closed valves), or other reasons, necessitates an established level of passive protection that remains in the building code. A reasonable balance between active and passive fire protection is routinely a topic of debate in the code development process.
Type III construction and wood
By definition, Type III construction in the IBC requires non-combustible exterior walls (e.g. steel or concrete). However, there is an exception for the substitution of fire-retardant treated wood walls in exterior walls. Therefore, a Type III building can be comprised entirely of wood and still meet the construction type’s minimum requirements. This allows even greater areas and heights, as shown in Figure 1 and 2 (page 51 and 52).
Fire resistance-rate construction
The type of construction is categorized as non-combustible or combustible and further subdivided based on fire-resistance rating of building elements. The combustibility of a product is measured by ASTM E136, Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750 C (1382 F), which measures mass and volume loss when exposed to fire. While this test demonstrates whether the material will burn, it does not consider how the product reacts to a sustained fire when supporting loads.
To determine how a structural element or assembly will react under fire exposure, ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, or Underwriters Laboratories (UL) 263, Fire Tests of Building Construction and Materials, are used. Fire-resistance testing provides results based on the duration the element or assembly maintained its structural integrity. This performance-based approach begins to breakdown the traditional approach to regulating a product based on its combustibility and focuses on its ability to perform the necessary function.
Fire resistance of assemblies is typically achieved through the use of gypsum board or spray fire-proofing to protect beams and columns from fire exposure. Fire-rated wood assemblies also rely on gypsum wallboard, but solid or engineered wood elements can be sized to achieve a specified fire-resistance rating. As wood elements burn, the char becomes an insulator preserving the strength of inner fibers. Calculations require the members to be designed in accordance with approved methods for calculating the fire resistance of exposed wood members. The method called out in Chapter 7 of IBC uses Chapter 16 of the National Design Specification for Wood Construction (NDS) published by the American Wood Council (AWC). AWC’s Technical Report No. 10, “Calculating the Fire Resistance of Exposed Wood Members,” gives examples of calculations.
Primary exceptions
Apart from the increased height and area due to sprinkler systems and open frontage, two primary exceptions allowing significant use of wood in many commercial buildings are for roof structures and pedestal buildings. By virtue of a footnote in the construction type table, most roof structures may be wood even in non-combustible construction types. Structures where the roof itself is a defining feature of the building often take advantage of this permission.
