Module: Construction principles Unit: Principles of design with timber
Timber is, in many respects, the ideal construction material. It is strong in both tension and compression, has a high strength-to-weight ratio, it is light (compared with most alternative materials), can be easily worked and is easy to handle. Overall it presents few problems during construction.
In most design circumstances it is common for the design life to be specified as 50 years. However, a properly designed and maintained timber building can last for centuries.
For less permanent structures a 10-year design life may be assumed and it may also be considered that some elements of a structure have a shorter design life and will be replaced as part of the maintenance management.
The structural properties of timber are highly variable, dependent upon density, moisture content, temperature, grain deviation and knots.
The moisture content of timber varies with relative temperature and humidity. This variation results in radial and tangential movements, but it is relatively stable along the grain.
Changes in temperature can cause short-term differential movement in timber. The coefficient of thermal expansion of timber along the grain is smaller than that of steel or concrete and it has lower thermal expansion characteristics than those of masonry and steel.