Code-Conforming Wood Design

September 2014 » Project + Technology Portfolio » Commercial/Industrial/Government
Structural design limitations of traditional dimension lumber are falling away.
Paul Coats, P.E., C.B.O.
Interior of the Crystal Mountain Restaurant at the Campbell Basin Ski area on Mt. Rainier, Wash. Photo: Western Wood Structures, Inc.

Whether the structure of a commercial building is comprised primarily of wood, steel, or concrete depends on a number of factors. Certainly, compliance with building codes is a primary consideration, but what role do they really play when it comes to choosing wood? Wood has grown significantly as a preferred material for building designers and construction professionals during the last few years due to economic, environmental, and energy efficiency advantages. Nevertheless, these decisions have to be backed by building codes.

To provide a concise resource for the allowable use of wood in commercial buildings according to the International Building Code (IBC), the American Wood Council has partnered with the International Code Council (ICC) to produce a document that summarizes the parameters for wood construction in the IBC. The resulting Code Conforming Wood Design (CCWD) series contains easily digestible information for building designers and engineers about building size according to occupancy and construction type, including thresholds for automatic sprinklers, and a series of topics from establishing fire resistance, to finishes, protection from decay, the use of wood in noncombustible construction types, and precautions during construction.

Eight occupancies are discussed in CCWD, including:

  • Group A — Assembly occupancies
  • Group B — Business occupancies
  • Group E — Educational occupancies
  • Group F — Factory/Industrial occupancies
  • Group I — Institutional occupancies
  • Group M — Mercantile occupancies
  • Group R — Residential occupancies

Increasing options for wood

Designers and engineers are sometimes surprised to find that single- or two-story business (use group B), factory (use group F), storage (use group S) and mercantile (use group M) commercial buildings of wood construction are not limited in area if they have an automatic fire suppression system and 60 feet of open perimeter on all sides. But Section 507 of the IBC permits it without further qualification. Of course, not all buildings qualify for unlimited area. But whereas the IBC certainly contains hard limits for height of wood construction, its area limits routinely fall well beyond the size of most commercial buildings.

To illustrate the point, the CCWD provides a series of pre-calculated building size tables based on occupancy, percent of open perimeter, number of stories, and whether the building is sprinklered or not. Upon entering the tables with these four parameters, in a quick glance one can tell the allowable area per story.

Table 1 of the CCWD indicates that an unsprinklered single story restaurant (use group A-2) of Type VB construction, with 30 feet of open perimeter on all sides, can be 10,500 square feet per floor. However, Chapter 9 of the IBC requires restaurants greater than 5,000 square feet to have a sprinkler system, as indicated by a footnote in the table. Note, it is often the case for several common occupancies that the basic height and area table in Chapter 5 appears to permit larger unsprinklered buildings than Chapter 9 actually allows unless fire barriers are introduced to compartmentalize the building.

Table 1: Group A Nonsprinklered Buildings — Maximum floor area per story. Courtesy of American Wood Council

Therefore, if the building is sprinklered, moving to Table 2 for sprinklered A-2s, the same building can be 28,500 square feet. So, unprotected wood frame (non-rated wood frame) is permitted for all restaurants below the sprinkler threshold of 5,000 square feet, and the allowable area for the same single-story restaurant is nearly tripled when the sprinklers are introduced. The allowable area is nearly doubled again to 54,620 square feet if the building is Type VA construction (1-hour rated throughout).

Table 2: Group A NFPA 13 — Compliant Sprinklered Buildings — Maximum floor area per story. Courtesy of American Wood Council

The pattern is the same for other common commercial use groups such as M, F, and S, except the Chapter 9 sprinkler thresholds for these occupancies is typically 12,000 square feet and their base allowable areas are greater than for assembly occupancies, such as restaurants. These buildings can be wood frame up to the size at which sprinklers are required (for any construction type), and then size increases allowed when sprinklers are used permits wood buildings of much greater size.

In this way, it is true that for most commercial buildings, the IBC size limits rarely prohibit the use of wood framing. Beyond Type V construction, heavy timber (Type IV) and Type III construction have considerably greater allowances than Type V, although the exterior walls in that case must be of noncombustible materials or fire retardant treated wood.

Special occupancies

Section 510 of the code contains alternatives for buildings sometimes referred to as “pedestal” or “podium” buildings. Through the use of a special horizontal fire resistance rated separation, two construction types can be contained in a single building. This is typically used to put wood frame construction over a Type I parking structure, with the required three-hour rated separation in between. There are several independent alternatives in Section 510, and each subsection stands on its own as a separate set of protection requirements. Code changes in recent cycles have added flexibility by, for instance, widening the number of use groups that can be in either the upper or lower building, and permitting multiple stories of Type I construction below a wood frame structure above. These special occupancy sections also contain special provisions for the protection of exits.

Construction types for commercial wood framing

Type VB offers the most flexibility for commercial wood frame structures since there are no fire ratings required for the structural frame, although there may be fire resistance ratings required for individual elements such as means of egress and exits. Often, Type VA (one-hour structure) is practical since a one-hour rating is relatively easy to achieve for floor and wall elements. There are no special restrictions on materials used in exterior walls as there are in Types III and IV construction.

Type IV (Heavy Timber) construction will evolve as building and fire officials consider the effect of recent mass timber technology and the benefits it brings for structural design. Cross laminated timber (CLT) has been introduced into the 2015 IBC as Type IV construction, with provisions for CLT exterior walls that are protected with gypsum. Mass timber technology is eliciting greater interest in tall wood buildings, even high rise, due to the various aesthetic and environmental benefits of wood. Heavy timber has inherent fire resistance due to its mass, and when protected with gypsum or other membranes, can achieve high ratings while also limiting the effects of the additional fire load created by the massive wood. Type IV construction already has greater allowable areas and heights than both Type V and Type III, but design details are less flexible, primarily due to a restriction on concealed spaces in the building structure, part of the requirements for Type IV. As code requirements evolve to accommodate mass timber technology, alternatives for the protection of concealed spaces are likely to be added to the code.

Type III construction is already popular for residential use group building such as apartment buildings. The code explicitly allows fire retardant-treated wood to be used in the exterior walls instead of noncombustible materials, making it possible to be all wood. Type III commercial construction of other use groups, particularly in urban areas where masonry exterior walls are common, is well established.

Construction type classifications are strictly for building size limitations as they function in the building code. Therefore, even if concrete or steel is chosen for portions of the building, it could be classified as Type V or any other combustible construction type as long as it does not exceed the building size limitations for that construction type. In that case, alterations and additions, either horizontally or vertically, can be accomplished with wood.

Evolving tools for establishing fire resistance

Testing building assemblies and elements in a test lab to ASTM E119 or the corresponding UL standard and using tested assemblies or elements in design is the common path for establishing fire resistance in building design. However, alternatives for establishing fire resistance that are listed in the code (Section 703.3) are used increasingly.

For instance, Chapter 16 of the AWC National Design Specification for Wood Construction (NDS) outlines methods for calculating fire resistance of exposed timber and engineered lumber products such as glued laminated (glulam) beams and columns. AWC also recently updated its Technical Report Number 10 (TR-10), which gives background and examples for this method. The Component Additive Method (CAM) for wood frame assemblies developed decades ago by the National Research Council of Canada had additionally become well established in the code and is widely used (see AWC Design for Code Acceptance No. 4, DCA-4).

The building code also permits fire protection engineering analyses that are based on the testing results for various assemblies and protected elements. This can be used to establish fire resistance without the need for new testing. However, new testing is continuing. AWC recently sponsored tests for heavy timber elements protected with gypsum membranes; the results will be published soon.

Finishes and non-structural features

The CCWD provides a full run-down of building features that can be wood and how the code addresses them, with references to code sections. Wood finishes are limited by flame spread classification and a single IBC table, truncated in the CCWD for wood construction types, provides guidance. Flame spread classifications for all common species of wood are known. AWC’s Design for Code Acceptance No. 1 (DCA-1) documents these for design professionals if information from the manufacturer is unavailable. Wood doors and windows, trim, and appendages such as balconies and decks, all have specific code provisions that are shown in the CCWD for reference as well.

Conclusion

Codes are becoming more complex, and it’s safe to say that fire safety and life safety for buildings are well provided for by the IBC. As fire and life safety systems have increased in efficiency and use, codes have accommodated a wide choice of materials. Tools such as the CCWD are allowing wood to be considered for its many advantages without compromising safety. Other tools will be forthcoming and in demand as well, as some long-standing structural design limitations of traditional dimension lumber are falling away, making wood a more popular alternative for a wide range of applications.

A full copy of the Code Conforming Wood Design Series and other technical resources mentioned in this article are available for free download from the American Wood Council at www.awc.org.

Paul Coats, P.E., C.B.O., is the Southeast regional manager for the American Wood Council (AWC). Prior to joining AWC, he was on the code development staff of BOCA International (the International Code Council). He can be reached at pcoats@awc.org.


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