WOOD - Materials for Civil and Construction Engineers - page 427

Orientation of
support

(a) (b)

FIGURE 10.16 Strength axis of plywood.

of sawn lumber for many applications. Laminated veneer lumber (LVL) is
used for scaffold planks and in flanges of prefabricated I-joists. Both LVL and
parallel strand lumber (PSL) are used for headers and major load-carrying
elements. The strand products are used for band joists in floor construc-
tion and as substitutes for studs and rafters in wall and roof construction
(USDA-FS, 1999).

Laminated Veneer Lumber Laminated veneer lumber (LVL) was developed
in the 1940s to produce high-strength parts for aircraft. It is currently a wide-
ly accepted construction material. Veneer sheets, 2.5 mm to 3.2 mm (1/10 in.
to 1/8 in.) thick are glued with phenol-formaldehyde adhesive to form bil-
lets, which are 0.6 m to 1.2 m (2 ft to 4 ft) wide by 38 mm (1.5 inches) thick.
The veneer sheets are all aligned with the grain parallel to the length of the
member. The end joint between sheets is either staggered or overlapped to
minimize any strength-reducing effect. Continuous presses are used to form
sheets with lengths limited only by handling capability. Alternatively, the
billets may be manufactured in 2.4 m (8 ft) lengths and enjoined with either
scarf or finger joints to form longer pieces.
The veneer used to manufacture LVL must be carefully selected for uni-
formity. Visual grading may be used, but ultrasonic testing is commonly
used to identify and sort acceptable sheets. The ultrasonic testing can
rapidly detect flaws in the veneer, such as knots and splits that would limit
the strength of the finished product. Since the quality of each veneer layer
is controlled, the variation in product properties are less than for natural
products.
LVL is manufactured to meet specific stress grade requirements. Due to
the ply construction, the orientation of the member and load must be con-
sidered when selecting properties. Figure 10.18 identifies the common