Laminated lumber is commonly used whenincreased wood load-carrying capacity and rigidity are required. Usually made of several pieces of 1 1/2-in. -thick lumber, called laminations, the pieces are nailed, bolted, or glued together with the grain of all pieces running parallel (fig. 6-1). 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 of laminated lumber.
Laminations may be used independently or withother materials in the construction of a structural unit. Trusses can be made with laminations for the chords and sawed lumber for the web members (fig. 6-2). Special beams (fig. 6-3) may be constructed with laminations for the flanges and sawed lumber for the webs.
Probably the greatest use of laminations is in thefabrication of large beams and arches. Beams with spans larger than 100 ft and depths of 8 1/2 ft have been constructed with 2-in. boards. Laminations this large are factory-produced. They are glued together under pressure. Most laminations are spliced using scarf joints (fig. 6-4), and the entire piece is dressed to ensure uniform thickness and width.
Plywood is a panel product made from thin sheetsof wood called veneers. An odd number of veneers, such as three, five, or seven, is generally used so the grains on the face and back of the panel run in the same direction. Cross-lamination (fig. 6-5) distributes the grain strength in both directions, creating a panel that resists splitting and, pound for pound, one of the strongest building materials available.
Figure 6-2.-Truss using laminated and sawed lumber.
Figure 6-3.-Laminated and sawed lumber or plywoodbeam.
Figure 6-4.-Scarf joints.
Figure 6-5.-Grain direction in a sheet of plywood.
Dry from the mill, plywood is never "green." From ovendry to complete moisture saturation, a plywood panel swells across or along the grain only about 0.2 of 1 percent and considerably less with normal exposures.
There is probably no building material as versatile as plywood. It is used for concrete forms, wall and roof sheathing, flooring, box beams, soffits, stressed-skin panels, paneling, partitions, doors, furniture, shelving, cabinets, crates, signs, and many other purposes.
Plywood is generally available in panel widthsof 36, 48, and 60 in. and in panel lengths ranging from 60 to 144 in. in 12-in. increments. Other sizes are also available on special order. Panels 48 in. wide by 96 in. long (4 by 8 ft), and 48 in. wide by 120 in. long (4 by 10 ft), are most commonly available. The 4 by 8 ft and larger sizes simplify construction, saving time and labor.
Nominal thicknesses of sanded panels rangefrom 1/4 to 1 1/4 in. or greater, generally in 1/8-in. increments. Unsanded panels are available in nominal thicknesses of 5/16 to 1 1/4 in. or greater, in increments of 1/8 in. for thicknesses over 3/8 in. Under 3/8 in., thicknesses are in 1/16-in, increments.