Engineered Wood Products

In the early part of the twentieth century there were more abundant old-growth forests, which provided structural beams, timbers, joists, and other weight-bearing lumber. These old-growth forests have been reduced, driving up the cost of construction material and increasing conflict with forest conservation groups.

Lumber producers have developed laminating techniques that allow the use of younger trees to create a variety of building products. These products, called Engineered Wood Products, have a number of benefits over products fomr old-growth forests.

• They are made from more abundant young trees.
• The yield from a tree increases 30 to 50 percent.
• The engineered products are stronger than the natural products. A piece of engineered lumber the same size can bear more weight than its natural counterpart. Better yet, a smaller piece of engineered lumber can bear the same weight as a larger natural piece of lumber.
• Engineered lumber, with its greater strength, can span a greater distance than natural lumber.
• A length of engineered lumber is lighter than than the same length of natural lumber, which makes it easier to handle.
• Engineered lumber does not warp, crown, or twist, which helps it retain accurate dimensions.

There are five main categories of engineered wood products. These include laminated veneer lumber, parallel strand lumber, laminated strand lumber, wood I-beams, and glue-laminated lumber, also known as glue-lam.

Laminated Veneer Lumber (LVL)

LVL is similar to plywood because it is made from multiple layers of laminated thin wood veneer. Unlike plywood, the layers in LVL run in the same direction, parallel to the long direction, and do not have crossbands. The layers are glued with exterior grade adhesive, then pressed together and heated under pressure.

You can use LVL for permanent application in floor and roof beams, as well as door and window headers, or for temporary applications like scaffolding and concrete forms.

Parallel Strand Lumber (PSL)

PSL is made from long veneer strands. These strands are laid parallel to each other and bonded together with an adhesive in a specialized heating process.

You can use PSL as beams, posts, and columns. It is good for use in load-bearing applications.

Laminated Strand Lumber (LSL)

LSL is made from small logs that can’t be used for standard lumber. The bark is removed from the logs, and then the logs are cut into short strands. The strands are dried, coated with resin, and pressed into long blocks called billets by a process that includes steam injection.

You can use LSL for millwork such as doors and windows, or other applications that require high-grade lumber. LSL does not have the load bearing capacity of PSL.

Wood I-Beams

The wood I-beam shown in Figure 1 is a composite structural member made up of a web with flanges bonded to it with exterior type adhesives. The web is made of oriented strand board or plywood; the flanges are made of dimension lumber or laminated veneer lumber with grooves that fit over the web.

Like the steel I-beam that it resembles, the wood I-beam is exceptionally strong. You can use it for floor joists, rafters, and headers.

Glue-Laminated Lumber (Glulam)

The laminated lumber shown in Figure 2 is made of several pieces of kiln-dried lumber held together as a single unit, a process called lamination. The pieces are nailed, bolted, or glued together with the grain of all pieces running parallel.

Glulam is very versatile; it can be shaped into forms from straight beams to complex curved members. You can use it for headers, floor girders, ridge beams, stair treads and stringers, purlins, cantilever beam systems, and arches.

Laminating greatly increases the load carrying capacity and rigidity of the wood. When extra length is needed, the pieces are spliced with the splices staggered so that no two adjacent laminations are spliced at the same point. Built up beams and girders are examples. They are built as shown in Figure 3, usually nailed or bolted together and spliced.

Lamination can be used independently or with other materials in the construction of a structural unit. Trusses can be made with lamination for the chords and sawed lumber for the web members as shown in Figure 4.

Units such as plywood box beams and stressed skin panels can contain both plywood and lamination, as shown in Figure 5.

Probably the most common use of lamination is in the fabrication of large beams and arches. Beams with spans in excess of 100 feet and depths of 8 1/2 feet have been constructed using 2-inch boards. Laminations this large are factory produced. They are glued together under pressure. Most laminations are spliced using scarf joints as shown in Figure 6, and the entire piece is dressed to ensure uniform thickness and width.

The depth of the lamination is placed in a horizontal position and is usually the full width of the beam, as shown in Figure 7.