As a natural product, wood has a number of properties which need to be taken into account when used for construction or exterior cladding. We have already considered anisotropy and the tendency to warp or distort. In addition, we must think of the finite sizes available, the propensity of solid wood with a larger cross-section to split, the presence of knots, the direction of the grain, and the resulting major variation in strength between different sections. Moreover; much wood is unsuitable for use due to aesthetic considerations (knots and grain).The process of sawing, edging and re-sawing the trunk into boards or lengths results in considerable waste. The remnants of wood which are produced when sawing tree trunks into lengths of solid wood, or entire trunks which for one reason or another are unsuitable for sawing, can be used in the manufacture of wood products.There are now many
8.8 Cross sawn wood. 8.9 Rift sawn wood.
such products on the market, manufactured by taking wood waste (sawdust, chippings or veneer) and bonding them together with resins or mineral-based adhesives. The exact choice of bonding agent will depend on the intended purpose and climatic conditions during use (Fig. 8.1 0).The resin-bonded products can be classed into the following main groups:
• laminated beams;
• products made up of chips or fragments (e.g. chipboard);
• products manufactured from wood fibre.
Brief descriptions of the products that can be used for cladding or in curtain walls are given below. (Source: Ambrozy and Giertlova) They appear in two categories, laminated beams and layered products.
Wood types Bonding agent Applications
Relatively short, knot-free boards with a rectangular cross-section, glued to each other under pressure.The grain of each section runs in the same direction.The boards can also be joined laterally with a butted joint or finger joint, the latter providing the most effective joint. Usually standard spruce or pine, although other woods can be used. Modified melamine resin, phenol/resorcinol-based resin, polyurethane resins. Supporting structures (beams, joists) and in door- and window frames. In facades it can be used for curtain walling.
It is possible to produce pieces with a large cross-section and considerable length which will not split or warp. Curved pieces are also possible.The anisotropic properties remain: swelling and shrinkage according to moisture content, and the differences in permissible stress load // grain and ±grain. Because the wood is dried before assembly, the swelling or shrinkage in interior conditions will be very small.
Lengths 13-40 m, breadth 60-320 mm, height 100-2300 mm.
(Source: Ambrozy Giertlova)
In this group we shall consider plywood, Laminated Veneer Lumber (LVL), and 3- and 5-ply solid wood sheets. Plywood is by far the most commonly used product in this group. The others are included here by virtue of their relatively recent introduction and their use as an exterior load-bearing element.
Structure Thin sheets (0.5 to 4 mm) are shaved from the tree trunk while rotated on a lathe. It is also possible to cut flat layers from the trunk, which serves to maintain the natural markings of the wood. The sheets, or veneers, are then laid on top of each other with the grain running in different directions, and glued under pressure.The sides are then trimmed and can be tongue-and-grooved. An odd number of layers is always used so that the grain of the outer two layers runs in the same direction. This helps to maintain stability. Many sheets will have a 'good' side and a 'not so good' side.Triplex consists of three layers and comes in thicknesses of 3 to 12 mm. Plywood consists of five or more layers.
Wood types Many different woods and qualities are used in the production of layered products. The outer, visible layers may be of a different wood than the interior layers.
Bonding agent Phenol, phenol/resorcinol or resorcin resin.
Dimensions Thicknesses of 6 to 80 mm, in sheets of 2500 x 1220 mm, 3100 x 1530 mm, and 2440 x 1220 mm. In most cases, the grain runs parallel to the longest side. Other dimensions exist, depending on the wood type.
Applications The number of possible applications is as great and varied as the number of different layered product types available. In exterior wall structures, plywood can be used within a frame of uprights and cross-pieces ('noggins') to form a load-bearing or reinforcing construction. In this case, five-layered moisture-resistant plywood is generally used. For exterior cladding which must meet certain aesthetic requirements, plywood of which (at least) the outer layer is of a high durability wood, in most cases a tropical hardwood, is used. When using plywood in an exterior situation, it is important to ensure that edges and fixing points are properly protected. The edges remain vulnerable because of the exposed end grain even when durable wood types and waterproof adhesives are used because of the risk of delamination.Vacuum and pressure preservation techniques can be applied on plywood.
Product Laminated Veneer Lumber (LVL), Furnierschichtholz (FSH), parallel-grain plywood.
Brand names Kerto-S, Kerto-Q, Microllam.
Structure Thin layers glued together with the grain in the same direction (Kerto-S), or with a few layers laid with the grain in the transverse direction (Kerto-Q).The butts of each layer are offset in relation to each othenThe advantage of this product compared to plywood is its far greater strength in one direction. In addition, much larger pieces can be produced: 1800 mm in breadth and up to 26 metres in length. Each layer is 3.2 mm thick (Kerto) or 2.5 to 4.7 mm in the case of Microllam.
Wood types Kerto: European spruce (Picea abies)
Microllam: Douglas-fir (Pseudotsuga menziesii); Southern Pine (Pinus)
Applications Kerto-S is mainly used as a linear construction element. Its structure gives it a far greater strength lengthways than in any other direction, and it is stronger than plywood. It can be used for timber curtain-walls.
Kerto-Q is used if load-bearing ability in two directions is required. In sheet form, it is used in roofs, floors and walls with a structural function, and is also used in joists and uprights with high lateral stress.The sheets are extremely level with high stability and good aesthetic properties.
Dimensions Max 1800 (b) x 26000 mm (l)
Thicknesses: Kerto-S ... 27-75 mm Kerto-Q ... 27-69 mm
In Kerto-Q, one of the five layers is laid with the grain perpendicular to that of the other layers.
8.10 Production methods of wood products.
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