Plywood

Plywood is a flat panel made by bonding together, and under pressure, a number of thin layers of veneer, often referred to as plies (or laminates). Plywood was the first type of EWP to be invented. Logs are debarked and steamed or heated in hot water for about 24 hours. They are then rotary-peeled into veneers of 2-4 mm in thickness and clipped into sheets of some 2 m wide. After kiln-drying and gluing, the veneers are laid up with the grain perpendicular to one another and bonded under pressure in an odd number of laminates (at least three), as shown in Figure 1.9a. The outside plies, always made of veneer, are referred to as faces (face ply or back ply) and the inner laminates, which could be made of either veneers or sliced/sawn wood, are called core. Examples of wood core plywood include blockboards and laminboards, as shown in Figures 1.9c-1.9e.

Plywood is produced in many countries from either softwood or hardwood or a combination of both. The structural grade plywoods that are commonly used in the United Kingdom are as follows:

• American construction and industrial plywood

• Canadian softwood plywood and Douglas fir plywood

• Finnish birch-faced (combi) plywood, Finnish birch plywood and Finnish conifer plywood

• Swedish softwood plywood.

The plywood sheet sizes available sizes are 1200 mm x 2400 mm or 1220 mm x 2440 mm. The face veneer is generally oriented with the longer side of the sheet except for Finnish made plywoods in which face veneers run parallel to the shorter side. Structural plywood and plywood for exterior use are generally made with waterproof adhesive that is suitable for severe exposure conditions.

The structural properties and strength of plywood depend mainly on the number and thickness of each ply, the species and grade and the arrangement of the individual

(a) Bending about either of the axes in the plane of the board x

(a) Bending about either of the axes in the plane of the board

(b) Bending about an axis perpendicular to the plane of the board Fig. 1.10. Plywood - axes of bending.

plies. As with timber, the structural properties of plywood are functions of the type of applied stresses, their direction with respect to grain direction of face ply and the duration of load.

Plywood may be subjected to bending in two different planes, depending on its intended use, and the direction of the applied stress and, therefore, it is important to differentiate between them:

(i) Bending about either of the axes (i.e. x-x or y-y) in the plane of the board, as shown in Figure 1.10a; for example, in situations where it is used as shelving or as floor board.

(ii) Bending about an axis perpendicular to the plane of the panel (i.e. z-z axis as shown in Figure 1.10b); for example, when it is acting as a web of a flexural member such as in ply-webbed beams.

BS EN 636:2003 [15] details the requirements for plywood for general purposes and for structural application in dry, humid or exterior conditions. It also gives a classification system based on the bending properties. An indication of how the characteristic values for plywood panels can be determined is given in BS EN 12369-2:2004 [16]. The information given in this standard is not suitable for structural design and for this purpose the characteristic strength and stiffness values of products with CE certification (marking) should be obtained from the manufacturers or suppliers. Often such information is available from manufacturers' websites.

The relevant grades, national standards and the quality control agencies relating to the structural grade plywoods that are commonly used in the United Kingdom are detailed in Table 1.6.

Indicative strength, stiffness and density values for the American plywood grade: C-D exposure 1 (CDX) and Swedish plywood grade P30 are given in Table 1.7.

In Tables 1.8-1.11 characteristic values for a range of Finnish plywoods that are used in the United Kingdom are given, based on the Handbook of Finnish Plywood [17].

In Tables 1.12 and 1.13 strength, stiffness and density values for unsanded CANPLY Canadian Douglas fir plywood and Canadian softwood plywood are given, respectively, based on data published by CANPLY Canadian Plywood Association [18].

1.7.3 Laminated veneer lumber (LVL)

LVL, shown in Figure 1.11, is an engineered timber composite manufactured by laminating wood veneers using exterior-type adhesives. In production, LVL is made with thin veneers similar to those in most plywoods. Veneers, 3-4 mm in thickness, are peeled off good quality logs and vertically laminated, but unlike plywood, successive veneers are generally oriented in a common grain direction, which gives orthotropic properties similar to those in sawn timber. Certain grades of LVL also include a few sheets of veneer in its lay-up in the direction perpendicular to the long direction of the member to enhance the strength properties. LVL was first produced some 40 years ago and currently it is being manufactured by a number of companies in the United States, Finland, Australia, New Zealand and Japan.

In the USA, LVL is manufactured from species such as southern yellow pine or Douglas fir by Weyerhaeuser (iLevel TrusJoist) under the name Microllam®; and in Finland LVL is manufactured from Spruce by Finnforest under the name Kerto-LVL. Kerto-LVL is produced as a standard product when all veneers are parallel (Kerto-S®) and also as Kerto-Q® in which approximately every fifth veneer is in the perpendicular direction.

Standard dimensions of cross-section for Kerto-LVLs are shown in Table 1.14 and the characteristic values for their strength and stiffness properties are given in Table 1.15.

1.7.4 Laminated strand lumber (LSL), TimberStrand®

LSL, shown in Figure 1.12, is manufactured in the USA by Weyerhaeuser (iLevel TrusJoist) under the registered name TimberStrand1®. LSL is produced from strands of wood species (often aspen), up to 300 mm in length and 30 mm in width, or species combinations blended with a polyurethane-based adhesive. The strands are oriented in a parallel direction and formed into mats 2.44 m wide by up to 14.63 m long, of various thicknesses of up to 140 mm. The mats are then pressed by steam injection to the required thickness. TimberStrands are available in dimensions of up to

Table 1.6 Details of the commonly used structural grade plywoods in the United Kingdom

American plywood grades

Canadian plywood grades

Finnish plywood grades

Swedish plywood grades

Grade

American standard

Quality control agency

Grade

Canadian standard

Quality control agency

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