The use of engineered wood products is an essential component of nearly all wood-framed buildings. This article will focus on two specific types of engineered wood products, structural composite lumber (SCL) and glue-laminated (Glulam) timber framing. Understanding the intended uses and differences between various SCL products and glulam framing is essential for design professionals.
Structural composite lumber (SCL) is a term used to describe a family of engineered wood products created by layering wood veneers or strands and bonding them with moisture-resistant adhesives to form structural framing members such as beams, studs, and columns. SCL members provide numerous advantages over sawn conventional lumber, including higher strengths, dimensional stability, and resistance to moisture changes. SCL consists primarily of three products: Laminated Strand Lumber (LSL), Parallel Strand Lumber (PSL), and Laminated Veneer Lumber (LVL).
Laminated Strand Lumber (LSL) is manufactured from flaked wood strands and resembles oriented strand board (OSB) in appearance, though the strands are arranged parallel to the longitudinal axis of the member. Members are commonly fabricated in 1 ¼”, 1 ½”, 1 ¾” and 3 ½” widths, and in 9 ¼”-16” depths to match common i-joists. Stud options are available in equivalent 2×4, 2×6, and 2×8 sizes that are stronger, straighter, and (as needed) longer than sawn lumber. LSL is typically less expensive than other engineered wood beams.
Due to its high allowable shear strength, LSL beams have the capacity for larger penetrations than other engineered wood beam options. While not as strong as LVL or PSL beams, LSL is generally cheaper and is ideal for short spans. LSL is also ideal for use in rim conditions due to minimal shrinkage, cupping, and high fastener holding strength when used in highly loaded diaphragms or for shear transfer at plywood shear walls.
Parallel Strand Lumber (PSL) is manufactured from veneers laid into long, parallel strands and bonded together. PSL beams are primarily used in beam and header applications where high strength is required. Common PSL beam sizes are available in widths of 3 ½, 5 ¼”, and 7”, and depths matching I-joists from 9 1/2” – 24” deep. PSL columns are also available in sizes comparable with sawn wood members from 4×4 to 8×8 in size.
PSL beams are generally more expensive than glulam, LSL, or LVL beams. PSL beams can be stained or finished where an aesthetically pleasing exposed application is desired.
Laminated Veneer Lumber (LVL) is a commonly available engineered product that is manufactured similarly to PSL. Available sizes, strengths, and stiffnesses are similar to PSL but are generally cheaper, making it a commonly specified beam type. A benefit to LVL is that it can be fabricated in narrower beam widths (1 ½, 1 ¾”), and multiple plys can be nail-laminated together to form a larger beam. This is especially beneficial in retrofit options where lifting a wide, heavy beam into place is cumbersome or infeasible. LVL stud and columns are available as well from some manufacturers.
Glued Laminated Timber (Glulam) is manufactured by face-bonding layers of kiln-dried timber members, typically 2×4 or 2×6 in size, together with waterproof adhesives to form timber section. Glulams are popular due to their engineered strength, versatility, availability, and cost. Typical stock beam widths are available in 3 1/8”, 3 ½”, 5 1/8”, 5 ½”, 6 ¾” widths and depths exceeding SCL beams. However, custom glulams can be fabricated in almost limitless widths, depths, and profiles, giving glulam beams a distinct advantage over SCL beams in their versatility and architectural appeal. Glulams have a long history of being used beautifully in exposed, large open areas such as vaulted ceilings, churches, theatres, and a vast array of other public spaces. Manufacturing processes for glulams allow for members to be cambered, curved, and fabricated in unique shapes, such as arches or as bridge members. Different appearance grades for exposed conditions may also be specified to increase architectural appeal.
For exterior or weather-exposed conditions, glulam beams are generally preferred over SCL beams. Weyerhaeuser, one of the few manufacturers of PSL in the U.S., has a Wolmanized PSL product that is approved for weather-exposed framing beam applications, but it is relatively expensive. Few other SCL treatment options exist. Alternatively, pressure-treated or preservative-treated options exist for glulam members. Additionally, several naturally durable species of glulam beams are produced in the U.S., including Alaskan Yellow Cedar and Port Orford Cedar, which provide green alternatives to chemical treatments.
Both SCL and glulam beams may be used where a fire-rated exposed member is required, subject to meeting the provisions of Chapter 16 of AWC’s National Design Specification® (NDS®) for Wood Construction. Typically, only wider beam sections will meet the required fire rating due to the depth of charring of any exposed face. This often eliminates the use of LSL, and glulams are usually preferred over LVL and PSL due to cost, appearance, or available beam sizes.
Design professionals should be knowledgeable about specific product availability and costs in their areas during design as this can help drive which types of engineered wood beams are specified. Although SCL and glulam beams can be used interchangeably at times, they also have unique advantages and limitations to be aware of.