Laminated Veneer Lumber

Laminated veneer lumber (LVL) (Fig. 4.24), also known as microlam, is more economical than laminated timber as there is little waste in the production process. It is manufactured to three grades by laminating timber strands with polyurethane resin under heat and pressure. In one process, logs are cut into flat timber strands 300 mm long; these are then treated with resin, aligned and hot-pressed into billets of

Grades of laminated veneer lumber:

Purpose/ Environmental Type

Loading Conditions

Load-bearing dry (hazard class 1) LVL/1

Load-bearing humid (hazard class 2) LVL/2

Load-bearing- exterior (hazard class 3) LVL/3

exterior conditions

(subject to testing or appropriate finish)

Fig. 4.24 Laminated veneer lumber (LVL)

Fig. 4.25 Laminated veneer lumber (LVL) construction - Finnforest Office, Boston, Lincolnshire. Architects: Arosuo and Vapaavuori Oy. Photograph: Courtesy of Finnforest

Monocoque structures

Interesting and innovative built forms can be created using LVL (and other timber products) to create flat or curved form monocoque structures. These work on the well-established principles from the motor industry, in which the hard body skin acts in concert with any stiffeners to form the structure. Using this technology, structurally efficient and elegant forms, which may be slender, tapered, flat or curved may be produced. LVL is quickly becoming a significant material to complement the more established products such as plywood, OSB and glulam, particularly because of its availability in very large sections.

PLYWOOD Manufacture

Plywood is manufactured by laminating a series of thin timber layers, or plies, to the required thickness. The timber log is softened by water or steam treatment and rotated against a full-length knife to peel off a veneer or ply of constant thickness (Fig. 4.26). The ply is then cut to size, dried and coated with adhesive prior to laying up to the required number of layers. Not all the plies are of the same thickness; often, thicker plies of lower-grade material are used in the core. However, the sheets must be balanced about the centre to prevent distortions caused by differential movement. Plies are normally built up with adjacent grain directions at right angles to each other to give uniform strength and reduce overall moisture movement, although with even plywoods, the central pair of plies has parallel grains. The laminate of plies and glue is cured in a hot press, sanded and trimmed to standard dimensions for packaging. Decorative veneers of hardwood or plastic laminate may be applied to one or both faces. Most plywood imported into the UK is made from softwood (largely pine and spruce), from North America and Scandinavia. Smaller quantities of plywood produced from temperate hardwoods are imported from Finland (birch) and Germany (beech), while tropical hardwood products are imported from Indonesia, Malaysia, South America and Africa.

The standard sheet size is 2440 X 1220 mm, with some manufactures producing sheet sizes of up to 3050 X 1525 mm or slightly larger. Sheet thicknesses range from 4 mm to 25 mm for normal construction use, although thinner sheets down to 1.5 mm are available for specialist purposes.

Under the European fire classification of construction materials (BS EN 13501-1: 2002), an untreated plywood panel would normally achieve a class D-s2, d0 rating, excluding its use as flooring when the rating is class DFL-s1 (depending on a minimum thickness of 9 mm, a minimum density of 400 kg/m3 and fixing to a non-combustible substrate [class A1 or A2] without an air gap. The secondary classifications 's' and 'd' relate to smoke production and flaming droplets).

Grades

Plywood is classified according to its general appearance and physical properties (BS EN 313-1: 1996). The key characteristics are the form of construction, durability, and the nature of the surface. The durability of plywood is largely determined by the bonding class of the adhesive used. This ranges from class 1, to the most durable class 3 (BS EN 314-2: 1993), which can be used externally without delamination, providing that the timber itself is durable or suitably protected against deterioration.

Bonding classes for plywood:

Class 1 Dry conditions (suitable for interior use).

Class 2 Humid conditions (protected external applications, e.g. behind cladding or under roof coverings).

Fig. 4.26 Manufacture and standard types of plywood

Class 3 Exterior conditions (exposed to weather over sustained periods).

Phenol formaldehyde resins are the most frequently used for the most durable plywoods. Marine plywood (BS 1088-1: 2003) is a combination of a moderately durable timber bonded with phenolic or melamine-formaldehyde resin. The standard class of marine plywood is suitable for regular wetting or permanent exposure to salt or fresh water. The lower grades of plywood are bonded with melamine-urea-formaldehyde or urea-formaldehyde resins. In addition to the grade of adhesive and the durability of the timber itself, the quality of plywood is affected by the number of plies for a particular thickness and the surface condition of the outer plies which range from near perfect, through showing repaired blemishes, to imperfect. Factory-applied treatments to improve timber durability and fire resistance are normally available.

The standard BS EN 635: 1994 describes five classes of allowable defects (E, and I to IV) according to decreasing quality of surface appearance; class E is practically without surface defects. These are related to hardwood and softwood surfaced plywoods in BS EN 635: 1995 Parts 2 and 3 respectively.

The performance specifications for plywood to be used in dry humid or exerior conditions against the criteria of bonding strengthe and durability with respect to biological decay are described in the standard BS EN 636: 2003.

Biological hazard class conditions for the use of plywood:

Class 1 Dry conditions (average moisture content <10%).

Class 2 Humid conditions (average moisture content < 18%).

Class 3 Exterior conditions (average moisture content >18%).

These biological hazard classes correspond to the service classes in BS EN 1995-1-1: 2004.

The standard BS EN 636: 2003 also has a classification system based on the strength and stiffness of plywood based on bending tests. Plywood is assigned to a four-part code specifying bending strength and modulus in both the length and width directions. Plywood sheets should be identified according to their intended application with 'S' for structural and 'G' for general use.

Uses

Considerable quantities of plywood are used by the construction industry because of its strength, versatility and visual properties. The strength of plywood in shear is used in the manufacture of plywood box and I-section beams in which the plywood forms the web. Increased stiffness can be generated by forming the plywood into a sinusoidal web. Plywood box beams can be manufactured to create pitched and arched roof forms as illustrated in Figure 4.21. Stiffened and stressed skin panels, in which plywood and softwood timbers are continuously bonded to act as T- or I-beams, will span greater distances as floor structures than the same depths of traditional softwood joists with nailed boarding. Such structural units can also be used to form pitched roofs or to form folded plate roof structures or barrel vaulting (Fig. 4.27). Plywood of 8-10 mm thickness is frequently used as the sheeting material in timber-frame construction and for complex roof forms such as domes. The lower-grade material is extensively used as formwork for in-situ concrete.

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