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2.10, 2.40 or 2.70 3.00, 3.30, 3.60 or 3.90 4.20, 4.50 or 4.80 5.10, 5.40 or 5.70 6.00, 6.30, 6.60 or 6.90 7.20

Lengths of 5.70 m and over may not be readily available without finger jointing. * In accordance with Table NA.1, BS EN 336:2003.

materials to make a range of value-added systems such as structural insulated panels (SIPs).

EWPs may be selected over solid sawn timber in many applications due to certain comparative advantages:

• They can be manufactured to meet application-specific performance requirements.

• Large sections or panels in long lengths can be manufactured from small logs with defects being removed or dispersed.

• They are often stronger and less prone to humidity-induced warping than equivalent solid timbers, although most particle- and fibre-based boards readily soak up water unless they are treated with sealant or painted.

EWPs are more expensive to produce than solid timber, but offer advantages, including economic ones, when manufactured in large sizes due to the rarity of trees suitable for cutting large sections.

1.7.1 Glued-laminated timber (glulam)

Glued-laminated timber, glulam, is fabricated from small sections of timber boards (called laminates) bonded together with adhesives and laid up so that the grain of all laminates is essentially parallel to the longitudinal axis. Individual laminates are typically 19-50 mm in thickness, 1.5-5 m in length, end-jointed by the process of finger jointing as shown in Figure 1.8a and then placed randomly throughout the glulam component. Normally, the laminates are dried to around 12-18% moisture content before being machined and assembled. Edge-gluing permits beams wider and larger than the commercially available sections to be manufactured after finger jointing. Assembly is commonly carried out by applying a carefully controlled adhesive mix to the faces of the laminates. They are then placed in mechanical or hydraulic jigs of the appropriate shape and size, and pressurised at right angles to the glue lines and held until curing of the adhesive is complete. Glulam is then cut, shaped, and any specified preservative and finishing treatments are applied.

Timber sections with a thickness of around 33 mm to a maximum of 50 mm are used to laminate straight or slightly curved members, whereas much thinner sections (12 or 19 mm, up to about 33 mm) are used to laminate curved members. Glued-laminated members can also be constructed with variable sections to produce tapering beams, columns, arches and portals (Figure 1.8).

The laminated lay-up of glulam makes it possible to match the lamination quality to the level of design stresses. Beams can be manufactured with the higher grade laminates at the outer highly stressed regions and the lower grade of laminates in the inner parts. Such combined concepts permit the timber resource to be used more efficiently.

Design of glued-laminated timber members is covered in Chapter 6 where the strength, stiffness and density properties of homogeneous (single grade) and combined (having outer laminations of higher grade) glued-laminated members are detailed.

(c) Curved portal (London Chiswick (d) Truss system (Scottish Parliament)

Strawbery Lodge)

Fig. 1.8. Glued-laminated structures. (Part b: photo courtesy of APA, The Engineered Wood Association. Timber Limited, a member of the Glued Laminated Timber Association, UK.)

(c) Curved portal (London Chiswick (d) Truss system (Scottish Parliament)

Strawbery Lodge)

Fig. 1.8. Glued-laminated structures. (Part b: photo courtesy of APA, The Engineered Wood Association. Timber Limited, a member of the Glued Laminated Timber Association, UK.)

(c) photo courtesy of Axis

Face ply

Face ply

Grain directions

Cross ply (core) Back ply

(a) The structure of a three-ply plywood Fig. 1.9. Examples of plywood and wood core plywood.

(b) Five-ply plywood

(c) Three-ply blockboard

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