Grades of timber, their properties and construction applications are discussed. Trees may be classified as: softwood, giving timber which is relatively cheap, available in large sections, but only of modest strength; temperate hardwood, yielding stronger timber, but too expensive for structural use; and tropical hardwoods, which give the strongest and most durable timber, but limited by costs to external and particularly marine applications. Property values depend upon the direction of measurement relative to the grain. The limited fire resistance means that exposed timber is usually limited to roof applications. Kiln-dried timber has a maximum thickness of about 75 mm. Larger sections may be created by gluing planks or veneer, maximum sizes being limited only by transport considerations. Design must consider creep under sustained load, as well as the inherent strength of the material. Connections are a critical design issue, as there is no plastic flow to relieve stress concentrations. The available connections are: direct bearing, glued joints, dowels, and timber connectors. 8 refs.
Primary author(s): Lawrence A
81, No.17, 2003, p.14,16-18