WOOD - Materials for Civil and Construction Engineers - page 424

panels fall into a general class of material called flakeboard. Originally, the
strands had a random orientation, producing waferboard. In the 1970s,
recognition of the advantages of orientating the strands prior to gluing, then
forming panels by using plies with alternating orientation of strands, led to
the development of oriented strandboard (OSB). This is the dominant
strand product used in the market today. It's structural characteristics com-
pare favorably with plywood. Strandboard can also be made into structural
shapes, such as beams.
Dried lumber products can be glued together (laminated) to form struc-
tural shapes to meet a wide variety of applications. Typically, either south-
ern pine or western species of softwoods are used. The individual boards are
referred to as lams, which are glued together to form the glulam product.
Lams of southern pine are typically 35 mm (1 3/8 inch); Western species
lams are typically 38 mm (1 1/2 inches). The widths of glulam products are
typically 63 to 275 mm (2 1/2 to 10 3/4 inches), although virtually any size
member can be produced.
Engineered wood products can be broadly classified as either structural
panels/sheets or structural shapes as shown in Table 10.6. Panels are primar-
ily used for sheathing, whereas structural shapes are used as beams, columns,
etc. Sheet products and structural shape products can be combined to produce
a variety of composite structural members, such as I-joists and T-sections.

■ 10.13.1 Structural Panels/Sheets

Structural panel products include plywood, oriented strand board (OSB),
and COM-PLY. Plywood is manufactured as a composite of veneer plies,
with an alternating orientation of the grain. OSB is manufactured as a com-
posite of strands; the strands are oriented and pressed into sheets. The OSB
panel consists of three to five sheets glued together, with alternating orien-
tation of the strands. COM-PLY is a proprietary product manufactured with
a wood fiber core and veneer exterior. Panels may be used directly as sheath-
ing, roofs, sides, and floors. Sheet panels can be cut and bonded into struc-
tural components, such as glued or nailed box beams, stressed skin panels,
and structural insulated panels.
Conventional wood-based composite products are typically made with a
thermosetting or heat-curing resin or adhesive that binds the wood fibers to-
gether. Adhesives are chosen based on their suitability for the particular prod-
uct. Factors considered are the materials to be bonded, moisture content at
time of bonding, mechanical property, durability, and cost (USDA-FS, 1999).
Commonly used resin-binder systems for panels include the following:

Phenol-formaldehyde (PF) Used for materials designed for exterior ex-
posure, including plywood, OSB, and siding.
Urea-formaldehyde (UF) typically used for manufactured products
where dimensional uniformity and surface smoothness are of pri-
mary concern, such as particle board and medium density fiber-
board designed for interior applications.