Timber - Introduction to Design in Timber Wood is a very...

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Introduction to Design in Timber Wood is a very versatile raw material and is widely used in construction. Timber can be used in a range of structural applications including marine works: construction of wharves (e.g. fenders), piers, cofferdams; heavy civil works: bridges, piles, shoring, pylons; domestic housing: roofs, floors, partitions; shuttering for precast and in situ concrete; falsework for brick or stone construction. Timber is naturally occurring. This makes it a very difficult material to characterize and partly accounts for the wide variation in the strength of timber. However, this problem has now been largely overcome by specifying stress graded timber. The suitability of a particular timber type for any given purpose will depend upon various factors such as performance, cost, appearance and availability. The task of the structural engineer has been simplified, however, by grouping timber species into nine strength classes for which typical design parameters, e.g. grade stresses and moduli of elasticity, have been produced. Design of timber elements is normally carried out in accordance with BS 5268: Structural Use of Timber. The design principles which will be outlined here are based on the contents of Part 2 of the code (i.e. Part 2: Code of Practice for Permissible Stress Design, Materials and Workmanship). It should therefore be assumed that all future references to BS 5268 refer exclusively to Part 2. BS 5268 is based on permissible stress design rather than limit state design. This means in practice that a partial safety factor is applied only to material properties, i.e. the permissible stress and not the loading. Before discussing the design process in detail, the following sections will expand on the more general aspects mentioned above, namely: 1. stress grading 2. grade stress and strength class 3. permissible stress Stress grading The strength of timber is a function of several parameters including the moisture content, density, size of specimen and the presence of various strength-reducing characteristics such as knots, slope of grain, fissures and wane. The first step for assessing the strength of timber involves grading structural size timber. Grading may be carried out visually or mechanically. The latter approach offers the advantage of greater economy in the use of timber since it takes into account the density of timber which significantly influences its strength. Mechanical stress grading is based on the fact that there is a direct relationship between the modulus of elasticity measured over a relatively short span, i.e. stiffness, and bending strength. The stiffness is assessed non-destructively by feeding individual pieces of timber through a series of rollers on a machine which automatically applies small transverse loads over short successive lengths and measures the deflections.
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