709 - Timber Sturctures
GENERAL
Structural timber and lumber consist of relatively strong and lightweight materials capable of supporting short-term overloads without sustaining permanent structural damage. Structural timber and lumber elements are used in formwork, temporary structures, sign supports, highway safety appurtenances, and bridges.
Wood is orthotropic, meaning its material properties vary in different directions. Strength is greatest in the direction parallel to the grain and is weakest perpendicular to the grain. The material properties of wood also vary between different species of wood. Design provisions and tabulated design values for different species and grades of lumber are given in the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Highway Bridges, and the National Design Specification for Wood Construction (NDS). Wood species can be divided into two broad classes, hardwood and softwood. Hardwoods are from broadleaf trees (oak, maple), and softwoods from conifers (needle trees such as pine). Species with similar mechanical properties are classified into groups. Timber and lumber are further separated into categories – dimension lumber, boards and timbers. Generally the preferred species for timber bridge construction are Douglas Fir/Larch and Southern or Mixed Pine.
Timber and lumber grading at most saw mills is done by visual inspection, and assigned by appearance and strength factors. See Figure 1 for some visual examples. Some factors are :
- Number, size, and position of knots and holes
- Bark on edges
- Decay
- Checks and splits
- Machining defects
- Twisting, bowing, and warp
MATERIALS
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CONSTRUCTION
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MEASUREMENT AND PAYMENT
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