The Materials Used in Timber Engineering
1.5 ENGINEERED WOOD PRODUCTS .1 General
The properties of these boards can be modified during manufacture in a similar manner to the other fibreboards.
There is further classification and marking:
• for conditions of use (see section 1.4.2): dry no symbol
humid H
exterior E
• for application: general no symbol
load bearing L
load bearing for all durations of load A load bearing for instantaneous or
short-term durations of load S
The load-bearing capacity is further classified as 1 for normal loading and 2 for heavy duty loading. The various symbols described are then combined to give a particular specification as shown below:
Fibreboard type: condition of use + application purpose + load duration + load-bearing category
(the last two are optional codes) so, for example, a heavy duty load-bearing hard- board for use in humid conditions for all types of loading is HB.HLA2.
1.4.7.1 Tempered hardboard
Sometimes known as ‘oil tempered hardboard’ the strength and durability charac- teristics are excellent. There are three drawbacks to its use. The movement with change in moisture content is very high so boards coming directly from manufac- ture at a low moisture content will pick up atmospheric moisture until the equilib- rium moisture content is reached with a consequent expansion. The magnitude of this movement can cause buckling of the hardboard in wall panels of about 50 mm when fastened to studs spaced at 600 mm. It is often a user recommendation that boards should be conditioned by applying water and allowing the boards to reach equilibrium. The boards then tend to shrink ‘drum tight’ onto the supporting frame.
The second problem is in hand nailing the boards. The board surfaces are so hard that starting a nail is difficult. Tempered hardboard has a smooth surface on one face and the other is ‘rough’ – technically described as the mesh face. It is somewhat easier to nail into the mesh face but mesh surface appearance is usually unaccept- able. The third limitation is in board thickness. The maximum thickness is around 8 mm, thus limiting its application to wall sheathing or the webs of built-up beams.
The BS 1142 grade THE has been replaced by the BS EN 316 grade HB.HLA2, the code that was described in the previous paragraph. For further information refer to the Wood Panel Industries Federation.
1.5 ENGINEERED WOOD PRODUCTS
manufacture of larger sections that can themselves be used as compression, tension and flexural members without any further manufacture or assembly such as required by a plywood box beam. The first of these products, Laminated Veneer Lumber (LVL), was simply the extension of plywood technology, whereas the two later prod- ucts, Parallel Strand Lumber (PSL) and Laminated Strand Lumber (LSL), required the development of new techniques and processes. The procedures of reconstituting wood fibre in these different ways gives products that are so uniform and consistent in their mechanical properties that the variations normally associated with sawn timber can be ignored. For example, there is a single Evalue equivalent to Emean, for the statistically derived Eminis practically no different from the mean value.
1.5.2 Laminated Veneer Lumber
The simplest description of Laminated Veneer Lumber (LVL) is a unidirectional plywood, i.e. the grain direction of all the veneers is the same. There are LVL products for specific applications where a small proportion of the veneers are set at right angles to the main body to give cross bonding for stability.
The standard dimensions available are widths up to 900 mm, thicknesses from 19 to 89 mm and lengths up to 20 m, although transport and handling becomes a problem over 12 m. The boards can be used on edge as a beam (similar to a verti- cally laminated beam with the laminates 3 to 4 mm thick) or flat as a plank particularly where concentrated wheel loads are applied. One of the problems with the beam application is that the potential lateral buckling instability for many appli- cations requires a section perhaps 75 mm wide ¥600 mm deep, i.e. a depth/breadth ratio of 8 to 1. The BS 5268-2 rule of thumb limit for this ratio is 7 to 1 so care has to be exercised in detailing the lateral restraints. As a plank, deflection can be a problem as the ratio of E/bending strength is typically 650 for LVL whereas softwood is 1100, so LVL used as a plank is more likely to be controlled by deflection than by bending strength.
The strength of LVL makes it particularly useful for I beams and built-up lattice trusses and girders. There is a single modulus of elasticity value due to its consis- tency (statistically the 5th percentile value is so close to the mean that differenti- ation would be unreasonable) and likewise the load-sharing factor, K8, has the value 1.04. The end connections can be a problem since, with split rings, nail plates and such items that do not have full penetration through the section, there is a poten- tial failure plane at each glueline. The cross-banded LVL finds particular applica- tion in these circumstances. Joints should be designed assuming the C27 strength class values of BS 5268-2 with restriction on the diameter of nail or screw when driven parallel to the glue line.
EN standards are being drafted for the production and use of LVL. All the present products in the market place, e.g. Kerto and Microllam, have Agrement certificates describing the product and giving design information.
1.5.3 Parallel Strand Lumber
Parallel Strand Lumber (PSL) is made by a patented process and marketed by Trus Joist MacMillan as Parallam. The process starts by peeling veneers in a
similar way to plywood from Douglas fir, Southern pine or Western hemlock, drying and then grading to remove excessive defects. The veneers are clipped into strands 13 mm or so in width ¥ 3 mm thickness up to 2.5 m long. These strands are then coated with a phenol formaldehyde adhesive, the strands are aligned and pressed and the adhesive cured at high temperature to form a billet.
The resulting product is not solid as there are voids between the compressed, bonded strands.
The standard size of billets are 68, 89, 133 and 178 mm in thickness and 241, 302, 356, 457 and 476 mm depth. The maximum size of billet that can be produced is 285 ¥488 mm ¥20 m length.
The moisture content on completion of the production of PSL is approximately 10% and as the dimensional changes arising from exposure to high atmospheric relative humidity are small the material may be considered dimensionally stable in most environments. On the other hand, prolonged exposure to liquid water – for example, from rain or a leaking pipe – can cause dimensional changes of up to 12% perpendicular to the axis of the strands and 5% in the direction of the strands due to water penetration into the voids in the material. Recovery on drying from this wet condition will leave a residual, permanent swelling of 50% of the maximum value. It follows that PSL used in an external environment must be protected from the direct action of the weather.
In all other ways PSL can be considered as a stronger more consistent form of the parent timber. Like LVL there is a single modulus of elasticity value and because of its consistency the load-sharing factor, K8, has the value 1.04. Other- wise the design of PSL follows the procedures of BS 5268-2. Joints should be designed assuming strength class C27.
Because PSL is the product of a single manufacturer located outside the European Union, it is unlikely that EN standards will be drafted for the product.
Reliance currently has to be placed on an Agrement certificate for the properties of the material.
1.5.4 Laminated Strand Lumber
Laminated Strand Lumber (LSL) is made by a patented process and marketed by Trus Joist MacMillan as TimberStrand. Logs are soaked, debarked and flaked into strands about 220 mm long and 1 mm thick. The strands are dried, coated with an isocyanate (polyurethane) adhesive, aligned and deposited as a mat 2.5 m wide ¥ 10.7 m long. This is heated under pressure to cure the adhesive and creates a billet which is then sanded and trimmed to the required size.
The billet sizes created are 2.4 m ¥ 10.7 m in thicknesses for structural use ranging from 30 to 140 mm. The timber species used is Aspen, which is a fast- growing timber of relatively poor structural quality. As solid timber, LSL has more consistent, higher properties than the parent material. Aspen is one of the least durable timbers, so exposure to environments that could promote insect or fungal attack should be avoided.
The material is more akin to OSB in that it does not have voids as in PSL. It is dimensionally stable and starts life at approximately 8% moisture content. The result of prolonged exposure in a high humidity environment is to take up mois- ture through the edges rather than the top and bottom surfaces. As a consequence, The Materials Used in Timber Engineering 33
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