Construction Type and Cost

Under the IBC, most low-rise commercial buildings fall under one of the following occupancy groups:

• Assembly (Group A-2): Nightclubs, restaurants, taverns, bars
• Business (Group B): Banks, barber and beauty shops, dry cleaning and laundries, professional services
• Mercantile (Group M): Department stores, drug stores, markets, motor fuel-dispensing facilities, retail or wholesale stores, sales rooms

For these (and other) occupancy groups, structural wood framing is permitted in Construction Types IIIA, IIIB, IV, VA, and VB. The IBC specifies allowable height and area for each, and each has different requirements, largely related to fire protection. As shown in Figures 1–3, which highlight information from the International Code Council (ICC) Building Valuation Data, August 2015, the average cost for each construction type also varies widely.

Figure 2 highlights the difference in cost between two construction types commonly used for low-rise commercial buildings—Type IIA, which doesn’t allow structural wood framing, and Type IIIA, which is typically wood-frame. Both have similar allowable heights and building limitations, but the average Type IIIA building costs $16 to $22 per square foot less.

In Figure 3, Type IIB Construction is compared to Type VA—also commonly wood-frame—and shows an even larger savings of $24 or more per square foot for the wood building. Allowable heights and areas remain similar, with exceptions that include greater height for Type VA Assembly and Mercantile occupancies, and greater height for Type IIB Education and Business occupancies.

Source: ICC Building Valuation Data, August 2015

Heights and Areas

One of the changes in the 2015 IBC compared to the 2012 version is a restructuring of the section on heights and areas. The equations are simpler, but there are more of them, along with more tables that are also larger and more complex. Among the specific changes there are separate tables for allowable heights (IBC Table 504.3) and allowable stories (IBC Table 504.4), with results based on occupancy classification, construction type, and whether the project is sprinklered.

Although Types IV and V may be more appropriate depending on the objectives of a project, architects interested in choosing wood structures should be aware that almost any low-rise commercial building can be designed in wood using the Type III designation. For common low-rise commercial and mixed-use occupancies (Assembly, Mercantile, Business, and Residential), Type II and Type III Construction in IBC 2012 and 2015 have equal building height and story limits.

DIAMOND FOODS INNOVATION CENTER

Photos: Eckert & Eckert

Location: Salem, Oregon
Architect: ZGF Architects
Engineer: KPFF Structural Engineers

Designed to complement and expand an existing warehouse and shipping facility, this 7,350-square-foot office includes a partial second story and multiple double-height spaces. Walls are traditional wood-frame construction using 2-by-4 and 2-by-6 dimension lumber; the truss roof includes glulam and exposed structural tongue and groove decking; and the floors are comprised of wood I-joists with glulam in select areas. The facade is an innovative combination of Douglas-fir wood siding and heat-treated pine.

Opportunities for Unlimited Area

To determine allowable height and area, many designers look to the IBC tables covering occupancy versus construction type (IBC Table 506.2) and use the equations to determine the tabulated limits for their buildings. Sometimes overlooked is the fact that commercial projects of Type III, IV, and V Construction—including Assembly, Education, Business, Factory, Mercantile, and Storage Occupancies—may qualify for unlimited area in IBC Section 507. This is because of the open space that often surrounds commercial projects, such as parking lots and major roadways, which, in addition to sprinkler protection, increases safety by providing firefighting access to multiple sides of the building.

The typical baseline requirement is 60 feet or more of open space on all four sides. However, a project may still qualify for unlimited area if a portion of the building has as little as 40 feet of open space—if the exterior walls and openings on the side with the reduced frontage have a 3-hour fire-resistance rating.

Where a project includes multiple buildings that are less than 40 feet apart—such as a group of offices or retail stores—the IBC allows them to be considered as a single building for the purpose of determining whether unlimited area applies, providing they have 60 feet of open space around them, and provided they would all otherwise qualify for unlimited area.

Two-story buildings of Business, Factory, Mercantile, and Storage occupancies of any construction type can have unlimited area providing they have the required surrounding open space and are equipped with a National Fire Protection Association (NFPA) 13-compliant automatic sprinkler system (IBC Section 507.5). Additional allowances for Assembly occupancies related to unlimited area buildings can be found in IBC Sections 507.4 and 507.7.

BIG BOX RETAIL: WOOD SAVES NEARLY $1 MILLION

Cost and environmental studies compare wood to steel

To evaluate the opportunity for wood use in the commercial building subcategory known as big box retail, WoodWorks commissioned two studies—one cost comparison and one life-cycle assessment (LCA)—on the same big box project designed in steel versus wood.

Having received the drawings for a one-story, 54,800-square-foot steel-frame big box store in California, WoodWorks commissioned the design of a comparable building using wood materials. The two designs share the same geometry, structural layout, and column grid. They also have the same gross floor area, floor plan, and layout.

According to the cost comparison, the wood building design saved an estimated 22 percent, or $988,626, compared to the steel building design. The largest cost savings were associated with the structure and roof insulation. Structure cost savings associated with the wood design totaled approximately $425,000. In order of highest to lowest savings, they were concentrated in roof framing beams, roof decking, roof framing columns, primary roof framing such as trusses and joists, and wall framing. Savings associated with roof insulation represented the largest single element savings (more than $400,000) due to the cost difference between rigid insulation (steel design) and batt insulation (wood design).

The LCA study concluded that the impacts of the proposed wood building are lower than the steel building for all indicators except ozone depletion, where the proposed building results were 5 percent higher. Impacts were analyzed from raw materials through demolition/disposal and also with the additional stage of recovery/reuse/recycling. Highlights through demolition/disposal include:

• Global warming potential: Wood building saves 642 metric tonnes of carbon dioxide equivalent (CO₂e).
• Nonrenewable energy use: Wood building saves 9,116 gigajoules (GJ).
• Raw material supply and manufacturing: Wood building has an average of 30 percent less impact across all indicators.
• End-of-life transport: Wood building has 11 percent less impact across all indicators.

Adding the recovery/reuse/recycle stage had minor effects on the overall comparison between the wood and steel buildings. As with the analysis excluding this stage, the wood building outperforms the steel building overall and for all indicators except ozone depletion potential, where the proposed building results were slightly higher.

Detailed information on these studies is available in the WoodWorks report, which includes a section on opportunities for big box wood design.¹

BMR/ECO ATTITUDE STORE CONCEPT
Locations: Saint-André-Avelin, Sainte-Agathe-des-Monts, Saint-Jean-sur-Richelieu, Québec
Architect: Sophie Tétrault Architecte
Engineers: Rochon Experts-conseils S.I.M. Experts-Conseils AP Consultant

Founded in 1967 by a group of Quebecois building supply dealers, BMR seeks to incorporate regional materials into its member stores. Wood reflects the company’s heritage while its natural warmth enhances clients’ shopping experience. In addition to providing a more energy-efficient structure, an article in Les Affaires acknowledges sales increases attributable to the wood structure.