WOOD BUILDING SYSTEMS

Building designers interested in using wood for commercial projects have a variety of options, from exposed mass timber systems using innovative products such as cross-laminated timber (CLT), nail-laminated timber (NLT), and glued-laminated timber (glulam), to traditional dimension lumber framing that can be used to economically and effectively meet the objectives of almost any low-rise commercial project.

Boudin SF

Russell Abraham Photography

Location: Santa Rose, California
Architect: Hedgpeth Architects
Engineer: MKM & Associates

This 7,233-square-foot restaurant and coffee shop features a wide range of wood products. The roof is dimension lumber and Alaskan yellow cedar glulam; the walls are LVL, LSL, and PSL; and the exterior siding was milled from redwood members reclaimed from the site’s previous building. Acoustical ceilings are made from Douglas fir planks, spaced to allow sound to be absorbed by cavity insulation between the ceiling and roof.

Wall Framing

Options for wall framing include solid-sawn and finger-jointed dimension lumber, glulam framing, and structural composite lumber (SCL) products. Ceiling height is the primary driver of wall height, and it is common for commercial spaces to have ceiling heights of 12 feet or higher. Solid sawn dimension lumber, ranging in size from 2-by-4 to 2-by-12, is commonly used for interior and exterior walls. For posts under concentrated load locations, 4-by and 6-by solid-sawn lumber may be available depending on the region. Designers can also use multiple 2-by members nailed together to create larger composite members.

Again depending on the region, structural framing lumber is available in four main species groups: southern pine, Douglas fir-larch, hem-fir, and spruce-pine-fir. Lumber in each of these groups can be made from a range of species as defined by the grading rules. Visually graded lumber is the most commonly used, with grades that include select structural, #1, #2, stud grade, etc. In some areas, machine-graded lumber may also be available, including machine stress rated (MSR) and mechanical evaluated lumber (MEL). Because product availability varies widely, designers are advised to consult with local distributors and industry experts to determine which products are most appropriate for specific applications.

Related to dimension stud lumber is finger-jointed lumber (referred to as "end-jointed" lumber in the IBC), which is comprised of shorter sections of wood glued together into longer members. Per IBC Section 2303.1.1, if approved for the building application, properly graded and stamped finger-jointed lumber can be used interchangeably with the same grade and species of solid-sawn lumber. Some finger-jointed lumber is labeled as “vertical use only” or “stud use only.” This lumber is not appropriate for non-vertical uses, such as headers, beams, joists, or anywhere sustained bending or tension loads are expected. For fire-rated assemblies, finger-jointed lumber must be stamped “HRA” to indicate that a heat-resistant adhesive was used in its manufacture.

Glulam framing is manufactured using small pieces of lumber that are glued together to make significantly larger beam and column elements. Glulam beams are manufactured in a variety of sizes, with custom products greater than 4 feet deep and 100 feet long available.

SSCL is made from a variety of wood fiber products smaller than dimension lumber, such as veneers, and strands. This category of products includes laminated veneer lumber (LVL), laminated strand lumber (LSL), parallel strand lumber (PSL), and oriented strand lumber (OSL). These products are often used for floor beams under heavy loads and as studs in tall walls (see section below).

Manufacturers offer some glulam and SCL products specifically sized for wall applications. They can also be used for headers, heavily loaded trimmers and jacks, and sometimes posts (though solid-sawn is more frequently used for this application). While detailed structural design is beyond the scope of this course, it is valuable to know the high-level approaches to structural code compliance in framing design—i.e., prescriptive versus engineered.

With prescriptive framing design, applications suitable for each framing approach are defined and the calculation of loads onto framing members is not utilized. Prescriptive approaches applicable to some commercial buildings can be found in IBC Chapter 23 (conventional construction), and portions of the Wood Frame Construction Manual (WFCM), a referenced standard published by AWC. In both cases, the scope of the prescriptive approach is limited to 40 pounds per square foot (psf) live load on wood floors, which limits their use to commercial occupancies on the ground floor.

With an engineered approach, the wood framing system is designed to handle code-required minimum loads as defined by the IBC and referenced standards, with loading often calculated using ASCE 7-10: Minimum Design Loads for Buildings and Other Structures, and wood framing commonly designed in accordance with AWC’s National Design Specification (NDS) for Wood Construction, and Special Design Provisions for Wind and Seismic (SDPWS). The WFCM also provides an engineering approach to wood framing components.

Examples of Tall Wall Construction

Photos courtesy of WoodWorks

This restaurant in California includes 24-foot walls made from 2-by-8 Douglas fir.

Photos courtesy of WoodWorks

This jewelry store in Tennessee includes 22-foot walls made from 2-by-8 Douglas fir. Prefabricated off site, the walls were delivered with insulation and brick veneer already installed.

Tall Wall Design

Although defined as "low-rise," many commercial structures require "tall walls"—20 feet and taller—to achieve desired interior heights. Wood is both appropriate and effective in these applications.

Wood-frame tall walls offer the same benefits as other wood stud walls:

• They’re able to resist snow loads on the roof and wind loads on the wall, without requiring an additional load-bearing frame.
• When wood sheathing is added to studs, the wall is effective at resisting the lateral racking loads caused by high wind and seismic events.
• They can be easily insulated to provide excellent thermal resistance.
• They can be finished with a wide range of finishing materials.
• And they can be easily modified to adapt to changing tenant/owner needs.

For commercial structures, larger lumber sizes and engineered wood products can be used to obtain the same strength for walls that are taller and longer. Shear walls and connections can be easily designed to provide the required lateral resistance. Thermal requirements can be achieved with insulation. And, by paying attention to details and selecting appropriate finishing materials, tall stud walls can meet or exceed the more stringent fire separation requirements associated with most commercial structures.

Although the design of tall walls tends to be the purview of structural engineers, it is helpful for architects to understand two nuances that can have a significant impact on cost. For example:

• While a designer may wonder what is required to reduce the weak axis unbraced length of wall studs, the NDS Commentary says that any code-accepted wall sheathing, which includes wood structural panels and gypsum board, is valid. At the same time, it is important to have mid-height wall stud blocking to brace the stud during construction, as there will be loading before the sheathing or gypsum is installed.
• If ASCE 7 is used to calculate wind loads for components and cladding, structural components and connections that will be subject to wind from a larger area may have reduced loading requirements. This is similar to a live load reduction factor.

For a fast review of wall stud checks, the WFCM includes tables of allowable spans for different stud sizes and grades under various wind and gravity loading conditions and locations in the building.

New Heights Physical Therapy

Photo: Eckert and Eckert Photography

Location: Portland, Oregon
Architect: Constructive Form Architecture and Design LLC
Engineer: James G Pierson Inc.

This 7,000-square-foot office recently underwent a major adaptation from a metal shop to an Assembly/Business occupancy. The roof is a mix of dimension lumber, glulam, LVL, LSL, and PSL. Flooring is comprised of I-joists, glulam, LVL, LSL, and PSL. Walls are dimension lumber studs.