Wood and Evolving Codes: The 2018 IBC and Emerging Wood Technologies

Building codes are evolving to support new technological developments for one of our oldest building materials
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Sponsored by Think Wood
By Andrew A. Hunt

Fire Protection

To understand the building codes’ fire protection provisions, it’s important to remember that codes divide construction into five types:

  • Type I and II: All building elements must be noncombustible.
  • Type III: Exterior walls must be of noncombustible materials.
  • Type IV (Heavy Timber): Exterior walls must be of noncombustible materials, and interior building elements are of solid or laminated wood without any concealed spaces.
  • Type V: Exterior walls, interior walls, and structural members may be of any material permitted by code.

The IBC and ICC require all building components within a particular type of construction to provide the same level of fire protection regardless of materials used. As a starting point, the IBC specifies a basic allowable area based on a single story, the type of construction, and occupancy classification. It then permits increases to allowable areas based on features of the building, including the addition of an automatic sprinkler system, side yard open space, fire walls, augmented exiting, and additional stories.

For example, the code allows low-rise, two-story business and mercantile buildings of wood construction to be of unlimited area when they are equipped with an automatic sprinkler system throughout and have 60 feet of fire-separation distance between the building and all property lines. Residential wood buildings with sprinklers and exterior walls made from fire-retardant-treated wood (FRTW) can be up to five stories in height and have additional “levels” when mezzanines are included. Under the 2018 IBC, mezzanines are permitted to have a floor area up to one-third of the floor area below and considered part of that story, and under certain conditions in dwelling units can be up to one-half of the floor area of the room below. The code also permits the use of wood for many features in buildings required to be of a noncombustible construction type, often even whole roof structures, based on other safety features.

Under the 2018 IBC, designers can use fire walls to create separate building portions that do not exceed the height and area limits set by code. This option can be exercised when sprinklers either aren’t an option or don’t afford the necessary increases for the project’s use and site characteristics. In Type V Construction, fire walls are permitted to be of wood-frame construction, allowing designers to divide the structure into separate buildings for purposes of size, each subject to its own height and area limits.3 Therefore, the size of a building can theoretically be doubled while maintaining the same construction type.

In addition to sprinkler and open frontage increases, a designer’s options also include increasing to a higher type of construction, which might include the use of fire-resistive construction throughout the building, fire-retardant-treated lumber for exterior walls, or heavy timber construction.

Rated Assemblies

There are several types of fire-resistive assemblies and components within a building. These include vertical assemblies (walls), horizontal assemblies (floors and roofs), and structural frame members (columns and beams). In most cases, these components and assemblies are required to have either a 1- or 2-hour fire-resistive rating. Fire-resistive construction is typically designated as the number of hours a representative test assembly or component will resist a standardized fire exposure when tested in a laboratory. One of the standards used for measuring fire resistance of building assemblies is ASTM E 119.

IBC Section 703.3 provides several methods for determining fire resistance of building elements, including but not limited to the following:

  1. Fire-resistance designs documented in approved sources.
  2. Prescriptive designs of fire-resistance-rated building elements, components, or assemblies as prescribed in Section 721.
  3. Calculations in accordance with Section 722.

Approved sources include documents such as AWC’s Design for Code Acceptance (DCA) series. DCA 3: Fire-Resistance-Rated Wood Floor and Wall Assemblies describes how interior and exterior wood-frame walls and wood I-joist floors can be used to meet building code requirements for fire-resistance-rated assemblies (see Figure 4).

IBC Section 721 provides prescriptive fire-resistance-rated wall and roof/ceiling assemblies for both traditional and engineered wood-frame assemblies.

The fire resistance of wood assemblies may also be calculated using the provisions of Section 722.6 of the IBC, which is based on the known fire resistance of many tested assemblies and assembly components. The calculation approach in this section is limited to 1 hour and is helpful in retrofit situations. The IBC also references Chapter 16 of the NDS, which has a broader application for calculating fire resistance of exposed wood members up to two hours.

By designing a building to meet the provisions of Type III Construction rather than Type V, the designer is able to take advantage of greater allowable heights and areas. For example, fire-retardant-treated wood (referenced in IBC Section 2303.2) is permitted in different locations in different types of construction, as noted in Sections 602.3 and 602.4. In Type III and Type IV Construction, this includes exterior walls and interior walls and partitions. In Type I and Type II Construction, fire-retardant-treated wood is allowed in nonbearing partitions, nonbearing exterior walls where a fire-resistive rating is not required, and portions of the roof construction. In Type I Construction, heavy timber roofs are permitted without fire-retardant treatment.

Heavy timber construction combines the beauty of exposed wood with the strength and fire resistance of heavy timbers. Modern versions include sawn stress-grade lumber, tongue-and-groove decking, CLT, NLT, and glulam. Under the code, fire resistance is achieved by using wood structural members of specified minimum size and wood floors and roofs of specified minimum thickness and composition; by providing the required degree of fire resistance in exterior and interior walls; by avoiding concealed spaces; and by using approved fastenings, construction details, and adhesives for structural members. Type IV Construction utilizes heavy timber elements as the structural members. This type of construction recognizes the inherent fire resistance of large timber and its ability to retain structural integrity in fire situations. The fire resistance in heavy timber construction typically comes from surface char, which insulates the wood member and leaves a significant portion of the member to continue supporting the structure during a fire.

The 2018 IBC allows CLT of a certain thickness (at least 4 inches for floors, 3 inches for roofs, and the minimum thickness specified in the manufacturing standard for walls) within Type IV construction. Fire-resistance testing has confirmed that CLT, like heavy timber, chars at a rate that is slow and predictable, maintaining its strength while serving its intended function for structural safety. In May 2018, the APA published the 2018 edition of ANSI/APA PRG 320: Standard for Performance-Rated Cross-Laminated Timber, an American National Standard that provides requirements and test methods for qualification and quality assurance of CLT. One of the changes in PRG 320-2018 addresses differences in fire performance for certain adhesives, which can lead to the early exposure of uncharred wood at lamination lines during the late stages of a fire; consequently, the updated standard mandates a compartment fire test protocol and an additional small-scale delamination fire test.4 CLT products manufactured to the standard have been recognized as code compliant in the 2018 IBC and will not delaminate during a fire.

Fire tests show that CLT

Photo courtesy of FPInnovations

Fire tests show that CLT chars slowly at predictable rates.


fire-resistance-rated wood-frame wall

Image courtesy of the AWC DCA3: Fire-Resistance-Rated Wood Floor and Wall Assemblies

The American Wood Council provides details for fire-resistance-rated wood-frame wall and floor/ceiling assemblies, such as the 2-hour assembly pictured here.

Fire Safety during Construction

The construction phase of a project presents unique risk scenarios that make the building more vulnerable than it is once complete, when features such as fire doors, gypsum wall board, smoke alarms, and sprinklers are in place.

Minimum safety precautions for fire during construction and the protection of adjacent public and private properties are provided in IBC Chapter 33. This section includes, among other things, provisions for fire extinguishers, standpipes and means of egress. The International Fire Code also includes detailed requirements.

In buildings under construction, arson and hot work are the most common causes of fire. For this reason, site security, rigorous procedures for workers, and access to fire hydrants are essential. Educating workers so they understand the vulnerabilities and how to avoid dangerous situations is also a must. To that end, the Construction Fire Safety Coalition (CFSC) provides resources through an online database on best practices than can help reduce the incidence and severity of fires during construction.

Chapter 33 of the IBC

Image courtesy of American Wood Council

Chapter 33 of the IBC provides minimum safety precautions for reducing the risk of fires during the construction process.

 

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Originally published in July 2018

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