The Materials Used in Timber Engineering
1.6 MECHANICAL FASTENERS .1 General
dimensional change is more noticeable at the corners of a sheet where moisture take up is through two edges.
LSL, like PSL, has a single modulus of elasticity value and the consistency is reflected in a K8value of 1.04. Joint design is somewhat more complicated than with the other two engineered wood products as limitations are imposed by the Agrement certificate on the fastener diameter, type and orientation to the top and bottom surfaces of the board. For example, bolts and dowels up to 12 mm diameter should be used with C24 values but not into the edges of a board, and for diameters greater than 12 mm C27 values should be used but not into the edges of a board.
For the same reasons as PSL, the only source of technical information for designs in the UK and Europe are an Agrement certificate.
1.6 MECHANICAL FASTENERS
1.6.4 Staples
Staples can be used for lightly loaded connections between a sheet material and solid timber, and to give close contact during curing of a glued joint. It is impor- tant that they are not over-fired by a stapling gun. Their use in environments where corrosion of the steel can occur is not recommended as there is a high stress con- centration due to the forming of the staple at the shoulders. The combined effects of stress corrosion can lead to fracture at the shoulder resulting in the fastening becoming two slender pins.
1.6.5 Screws
Conventional wood screws manufactured to BS 1210 require preboring and are therefore very much slower to insert than nails or improved nails. Newer types of screw with a modified thread pattern may be described as ‘self-tapping’ and do not require preboring. The use of screws is therefore increasing in general applications as well as special situations, for example, where security against vibration is required or demountability is a feature. Screws are available in various finishes or materials to cover different environmental conditions.
1.6.6 Bolts and dowels
The bolts and dowels usually used in timber engineering are of ordinary mild steel, either ‘black’ or galvanized. Part of the strength of a bolted joint is achieved by the bearing of the bolt shank onto the timber, therefore there is no advantage in using friction-grip bolts except where a special joint can be designed as timber shrinkage could negate the clamping force completely. The washers used within bolted connections should have a diameter of three times the bolt and a thickness of 0.25 ¥bolt diameter (Form G washers to BS 4320) under the head and nut. If a bolt is galvanized the designer should ensure that the nut will still fit. It may be necessary to ‘tap’ out the nut or ‘run down’ the thread before galvanizing. Stain- less steel bolts may also be used in corrosive environments.
1.6.7 Toothed plate connector units
The relevant British standard is BS 1579. Toothed plate connectors (see Fig. 1.8) can be either single or double sided, square or round, and can be placed on one or both sides of the timber. This type of connector is always used in conjunction with a bolt, complete with washers or steel plate under head and nut. The flat part of the connector sits proud of the timber.
The hole in the connector is a tolerance hole, therefore with single-sided con- nectors there is always bolt slip. Using double-sided connectors it may be possi- ble with careful detailing to eliminate slip in the connection. The British standard The Materials Used in Timber Engineering 35
calls for an anti-corrosion treatment, but does not specify which one. This is left to the individual manufacturers.
BS 1579: 1960 lists connector reference numbers for each connector detailed in the standard, but this classification system is not used widely enough to enable a specifier to use it and rely on its being understood by all concerned. The standard has not been metricated but BS 5268-2 uses metric equivalents in referring to connectors.
1.6.8 Split ring connector units
The relevant British standard is BS 1579. A split ring may have bevelled sides (Fig.
1.9) or parallel sides. It takes shear and necessitates the timbers being held together by another connector – usually a bolt (Fig. 1.10). A small amount of joint slip is likely to take place. The British standard requires the connectors to have an anti- corrosion treatment. A special tool is required to groove out the timber for the shape of the split ring and a special ‘drawing’ tool is required to assemble the unit under pressure. Units can carry a relatively high lateral load.
1.6.9 Shear plate connector units
The relevant British standard is BS 1579. Shear plate connectors are of pressed steel (Fig. 1.11) or malleable cast iron (Fig. 1.12). A special cutter is required to cut out the timber to take each connector but no tool is required to draw the timber together. A connector surface is flush with the surface of the timber. The timbers must be held together with a bolt. A small amount of joint slip is likely to take Fig. 1.8 Toothed plate connectors.
The Materials Used in Timber Engineering 37
Fig. 1.9 A split ring connector.
Fig. 1.10 A bolt connector.
Fig. 1.11 Pressed steel shear plate 67 mm outside diameter.
Fig. 1.12 Malleable cast iron shear plate 102 mm outside diameter.
place. Shear plate connectors are located in place for transit with two locating nails.
The connectors must be given an anti-corrosion treatment.
1.6.10 Punched metal plate fasteners
Most applications of punched metal plate fasteners are for the manufacture of trussed rafters. The requirements and specifications given in the following are based on BS 5268-3 ‘Code of practice for trussed rafter roofs’. Punched metal plate
fasteners are usually fabricated from hot-dip zinc-coated steel sheet or coil to BS EN 10147 or austenitic stainless steel to a grade defined in BS EN 10088-2. The thickness of the plate should be not less than 0.9 mm and not more than 2.5 mm.
A zinc coating should be equivalent to or better than Z275 of BS EN 10147.
The majority of plates produced have integral teeth pressed out at right angles to the plate to give teeth at close centres which will subsequently be pressed into the timber by specialist machinery. The strength capacities of the various nail configurations are determined by test and published in the plate manufacturer’s Agrement certificate. As an alternative, plates with pre-punched holes through which treated nails are driven can be used. The principles of BS 5268-2 with regard to nailed joints and steel plates may be used to determine the strength of a joint.
Joint slip will take place as with any nailed joint. For joint slip characteristics of a particular plate the manufacturers of the plate should be consulted.
1.6.11 Other fasteners, gussets and hangers
Rag bolts, rawl bolts, ballistic nails, etc., all have their use with connections for timber. Steel gussets or shoe plates, etc., can be used with timber in a similar way to that in which they are used with structural steelwork, except that in this case nails and screws can be used as well as bolts.
In recent years the use of proprietary I beams has brought an increased range of joist hangers, eaves and ridge supports and hangers, indeed methods of jointing timber members for almost any imaginable situation (for example, seismic condi- tions). These products are the result of extensive testing, confirmed by use in practice. Obviously the finish of any fastener, hanger, etc., must be suitable for the service conditions.
1.7 ADHESIVES USED IN TIMBER ENGINEERING