As both a renewable resource and a material offering unique aesthetic qualities, wood is a very appealing option for a number of applications, both residential and commercial. With the ongoing development of manufactured wood products, engineers are able to use wood as a solution even where long spans and high load capacities are required. By making use of a mixture of products for different applications within a building, the engineer is able to optimize the design for both cost and material efficiency. This article offers a brief summary of popular wood products available on the market.

Glulam beams and posts in a new boathouse will remain exposed for visual appeal. Simpson Gumpertz & Heger Inc.

Dimensional lumber
Sawn lumber is the most common structural wood product. This term refers to any wood member cut directly from a log without additional manufacturing processes. “Dimension lumber” refers to anything that has a nominal thickness of 2 to 4 inches, while the term “timber” is applied to anything larger. Since it is not manufactured, sawn lumber is relatively cheap and readily available. Because of short spans and light loads, residential structures make use of sawn lumber for the majority of structural framing. This typically includes studs, floor joists, and rafters. Commercial buildings can also make good use of sawn lumber for the floor joists and studs where spans and loads remain relatively small. Sawn lumber is very durable when used in dry or protected environments. However, if used in exterior or wet applications, it can be pressure treated to add preservative qualities.

The National Design Specification (NDS) and its Supplement are used by the engineer for design properties and design requirements. When using sawn lumber, the engineer must not only size the members, but also specify the type of wood used. Typically, the engineer specifies a species or a species group, which is comprised of two or more individual species as listed in the NDS Supplement. A member will be stamped with one or more species from a species group or the species group name. As such, without closer inspection, there is no way to distinguish the individual species of a particular member within a stamped species group. All design properties are presented in the NDS Supplement per species group or per individual species. For dimension lumber, when a species group is specified, the design properties are based on tests of lumber from the species combined within that group. For timber, the design properties are based on the weakest species in the group.

One must note, however, that species groups with similar names do not always have the same individual species, and thus they do not have the same design properties. In addition, there are different design properties for the same species group names from different geographic areas where the timber is grown. Most lumber distributers are unaware of this distinction, and there is a risk they will supply something that has part of a name in it. Unfortunately, in this situation the design properties of a wood member provided may be less than the properties of the wood specified. The engineer should check with his/her distributer before starting a design to see what wood type and material is readily available in the area.

The nature of wood products also leads to a unique submittal/shop drawing process. Many distributers are unclear about what the engineer needs to see. If they provide design values copied from the NDS, or a similar reference, it is not a guarantee that this is the wood that will be delivered. The engineer should request to see the stamp on the wood, including the species/species group, grade, grading agency, mill number, and moisture content. Despite how sound a wood member may appear, engineers must rely on what has been certified and stamped on the wood without additional inspection.

Sawn lumber is a cost-effective and relatively simple solution where spans are small. However, when larger spans and higher loads are required, multiple sawn lumber members sistered together can be more expensive and cumbersome than manufactured wood products.

Engineered wood products
Structural Composite Lumber (SCL) is an efficient engineered wood solution that provides a variety of products composed of various types and sections of wood. The manufacturing process allows for a more consistent and uniform product overall. Typically, SCL randomizes natural strength-reducing wood growth characteristics, such as knots and slope-of-grain within individual members, resulting in increased design properties compared with sawn lumber. SCL generally includes laminated veneer lumber (LVL) and parallel strand lumber (PSL). In general, SCL is more dimensionally consistent and stable than sawn lumber, providing straighter and more consistent members.

LVLs are fabricated from thin, dried wood veneers similar to those used for some types of plywood. A log is cut with a rotary lathe to produce the thin veneers, ranging from 1/10 inch to 3/16 inch thick. These veneers are then flattened, cut, and dried. Then, they are glued together with a waterproof adhesive into an LVL billet, which is subsequently cut to size. The direction of the wood grain in all veneers is parallel with the longitudinal axis of the member. The parallel orientation of grain combined with veneers having randomized defects results in improved design properties when compared with sawn lumber. It is very important to note that manufactured lumber is proprietary, so design values may vary between manufacturers. The engineer should specify both the preferred manufacturer and the minimum design properties that were used for design. It is advisable to check with local distributors to see which manufacturer’s products and what sizes are readily available. As with sawn lumber, LVLs are intended for interior use unless specifically treated with preservative. LVLs can be used for roof rafters where dimension lumber is no longer a viable option because of longer spans, supported HVAC units, and substantial unbalanced snow loads.

Parallel Strand Lumber (PSL) is another commonly used structural composite lumber. It is a proprietary product manufactured by iLevel by Weyerhaeuser (formerly Trus Joist Macmillan), under the name Parallam. Long strips of veneer strands are laid parallel to the longitudinal axis of the member and bonded together with a waterproof adhesive under pressure and heat. PSLs are useful for beam and header applications where high strength is needed, and can also be used as columns where sawn lumber studs are not strong enough. As with LVLs, they are meant for interior use unless preservative treatment is applied. PSL is better treated with preservatives than LVL. It should be noted, however, that both have reduced design properties when treated because the water used in the treatment causes swelling and damages the adhesive bonds within the composite. PSL design properties can vary with size and intended application, so the distinction between beam or column usage should be specified as well.

Glued Laminated Lumber (Glulam) members are made with comparable properties and sizes as PSLs. Glulams are formed by bonding together individual pieces of dimension lumber called laminations, nominally 1 to 2 inches thick. To achieve increased length, laminations are end-joined with finger joints, which provide a large surface area and flat slopes to transfer axial and bending forces, and are face bonded to the desired depth. High strengths are achieved in a similar way as SCL by selecting high-quality material and dispersing defects throughout the members. Additional strength is achieved by placing the highest-quality laminations in specific zones within the members. Glulams can be manufactured as unbalanced bending members with designated compression and tension zones to achieve higher efficiency. The members are also manufactured with a designated top and bottom for simple span beam applications. Glulams are most often selected for their visual appeal where the structural wood framing will remain exposed.

Prefabricated wood I-Joists are efficiently shaped members — similar to W shapes in structural steel. They are formed by gluing either sawn lumber or LVL flanges to a plywood or oriented strandboard web. The composite nature takes advantage of the high strength of quality sawn lumber or LVLs in conjunction with the high shear strength of plywood. The “I” shape also results in a lightweight member for ease in constructability. Most wood I-beams are made with “knockouts” or precut hole locations to simplify the installation of mechanical and electrical system pipe/conduit. These members are useful for beam and girder applications where the higher strength of SCL is not needed, but sawn lumber is not efficient. Since wood I-beams are slender, more care has to be taken during erection to maintain stability.

When designing with SCL it is important to consider environmental conditions that may be encountered both during construction and in service. When SCL gets wet during construction and then dries under load, the long-term dead load deflection can be more than anticipated from standard calculations. We have encountered several instances where SCL members, even when continuously dry, have more than twice the calculated long-term dead load deflection. In addition, wood I-joist floor systems can experience undesirable vibration characteristics — even when designed for code-stipulated deflection limits. Some manufacturers recommend increased stiffness and deflection limits to reduce floor vibrations.

Comparison
Natural and engineered wood products are available in a variety of grades, sizes, and geometries that offer the designer many options for framing. Figure 1 (click to view Figure 1 in pdf) summarizes strength and size limitations of wood products, as well as uses for each type of product.

Figure 2 (click to view Figure 2 in pdf) includes a comparison of engineered wood systems in typical floor framing applications. The design assumes a spacing of 16-inches on-center, repetitive members, floor dead loads of 15 pounds per square foot (psf), floor live loads of 40 psf, and deflection criteria of L/360 for total load and L/480 for live load. Relative costs (based in the Northeastern United States) are for comparison. It is clear that for shorter spans, dimension lumber will be the most efficient system. Wood I-joists may be a viable option if issues with dimension lumber submittals, as previously mentioned, are an anticipated problem or if greater consistency between members is desired. For moderate spans, wood I-beams are both cost effective and comparable in depth to the dimension lumber systems. At long spans, wood-I joists are still cost effective, but other SCL options become reasonable.

Due to the uniqueness of natural building materials, the architect and engineer need to be aware of the benefits and limitations of each option. Consideration must be made not only to the final constructed building, but also to the methods of construction that may affect both the strength and stiffness of various wood products.

Matthew Johnson, P.E., is an associate principal, and Ariane Fund is a staff II engineer. Both are with Simpson Gumpertz & Heger Inc. (www.sgh.com), in Waltham, Mass., and can be reached at mhjohnson@sgh.com and aifund@sgh.com, respectively.