140 mm thick x 1220 mm deep x 14.63 m long. Design values for the strength and stiffness properties of TimberStrand are given in Table 1.16.

1.7.5 Parallel strand lumber (PSL), Parallam®

PSL, shown in Figure 1.13, is manufactured in the USA by Weyerhaeuser (iLevel TrusJoist) under the registered name Parallam®®. The manufacturing process involves peeling small-diameter logs into veneer sheets. The veneers are then dried to a moisture content of 2-3% and then cut into thin long strands oriented parallel to one another.

The process of stranding reduces many of the timber's natural growth and strength-reducing characteristics such as knots, pitch pockets and slope of grain. This results in a dimensionally stable material that is more uniform in strength and stiffness characteristics and also in density than its parent timbers. For bonding strands, waterproof structural adhesive, mixed with a waxed component, is used and redried under pressure in a microwave process to dimensions measuring 275 x 475 mm2 in section by up to 20 m in length.

1.7.6 Oriented strand board (OSB)

OSB is an engineered structural board manufactured from thin wood strands, flakes or wafers sliced from small-diameter round timber logs and bonded with an exterior-type adhesive (comprising 95% wood, 5% resin and wax) under heat and pressure; see Figure 1.14.

OSB panels comprise exterior or surface layers that are composed of strands oriented in the long panel direction, with inner layers comprising randomly oriented strands. Their strength is mainly due to their multi-layered make-up and the cross-orientation of the strands. The use of water and boil-proof resins/adhesives provide strength, stiffness and moisture resistance.

In the United Kingdom, OSB is often referred to as Sterling board or SterlingOSB. OSB has many applications and often is used in preference to plywood as a more cost-effective, environmentally friendly and dimensionally stable panel. It is available in various thicknesses of 8-25 mm with panel sizes of up to 2.4 m wide x 4.8 m long, which makes it an attractive product for floor decking, roof cladding, wall sheathing and for composite constructions such as SIPs, etc.

BS EN 12369-1:2001 [19] provides information on the characteristic values for the three grades of OSB complying with BS EN 300:1997 [20] for use in designing structures to EC5:

Table 1.9 Finnish birch plywood: Strength and stiffness properties
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