ABSTRACT
Timber is a non-ductile material, which means that it fails in a brittle manner when the failure stress is exceeded. Engineers have the choice to either specify a certain strength grade of timber or a species, although in practice strength grades are most popular. The codes of practice help by specifying design stresses, known as grade stresses for various grades and species. Grade stresses include shear, bending, tension and compression stresses. The engineer can modify these to account for moisture content, load duration, load sharing, section size and notching. The grade stresses are modified depending on the load duration. Long-term loading normally means dead loads, whereas short-term loads include wind and snow. Grades stresses and Young’s modulus can be increased if closely spaced members jointly support a load, i.e., floor joists or rafters. This is because imperfections are less critical when more than one member supports a load.
