An increasing range of “Engineered Timber Products” has been developed, tested and found its way into the Australian market.
The practicalities of sawn timber in the market place mean that only the smaller sizes (generally less than 200mm deep) are readily available. In the past, very large cross-section members were cut from large straight hardwood trees to achieve large spans in bridges or heavily laden buildings such as warehouses. Now, these cross-sections are impossible to obtain from sawn timber. However, large timber members can still be used to carry large loads, span larger distances or cover large areas. This is achieved by fabricating larger members from many smaller pieces.
All of the products can be made with timber selected for appearance on visible faces. This makes them suitable for architectural functions.
Manufactured wood products can therefore be used in structural applications, appearance applications, or both. Large sized sections can be used for engineering applications to span large distances or carry large loads. As well, large sized members can be used architecturally as panels or create specific visual effects. Breaking up large, flat surfaces can improve the acoustic performance of spaces, so exposed beams or portal frames may contribute. Acoustic panels can be made from plywood with patterns of holes.
Properties and Design
Fabrication of these members makes use of the high strength and stiffness of wood fibres parallel to grain, and minimises the impact of the lower strength and stiffness perpendicular to grain. Design properties of all manufactured wood products are available from the manufacturer. Properties of common structural grades for plywood and glulam are given in the timber structures code AS1720.1.
For all manufactured wood products the properties of the finished product are generally higher than those of the timber stock used in the production of the material.
Manufacturers of most engineered wood products give design procedures, design properties, design data, and design aids for their products. The design aids may include span tables and installation tips. Special care needs to be taken to ensure adequate restraint of slender members during construction. (They may require temporary bracing.) These details are generally detailed in the design guidelines.
There are quite a number of processes in commercial use around the world for producing large sized timber members. Manufactured wood products include:
• LVL (Laminated Veneer Lumber), a higher strength linear product that can be manufactured in large depths and long lengths.
• Glulam (Glued Laminated Timber) – prismatic members that can be fabricated in large depths, widths, lengths, and formed into a number of shapes, including curves.
• Plywood is often selected to act as a structural diaphragm in which shear stresses must be carried. It is also useful for the webs of built up sections in which shear forces will be transmitted (webs in Box beams or in I-shaped sections). It can be used as feature panelling or for floors in which it carries out-of-plane loads.
• ESL (Engineered Strand Lumber), a prismatic product made from clumps of wood fibre arranged in a mainly linear pattern and held together in a glue matrix.
• OSB (Oriented Strand Board), a panel product manufactured from thin wafers of wood held in a glue matrix. The wafers are oriented during manufacture so that the grain is mainly parallel to the length of the board.
• Other fabricated products include various fabricated beams or trusses. This includes I-shaped beams, box beams and a number of open web parallel trusses.