Structural design

Structural design in timber differs from design in other materials, due to the anisotropic nature of the material and also the great variation in material properties. The requirements of the Timber Structures Standard NZS 3603 reflects this.

The design method incorporates the current international practise of limit state design, with the Timber Structures Standard moving from working stress design to limit state design in 1993.

The large variation in material properties as well as a change in the available timber resource has meant that a significant effort has gone into grading methods (including the use of machine stress graders and kiln dried timber) and the derivation of accurate characteristic values in recent years.

This work cumulated in the release of a new standard Verification of Timber Properties NZS3622:2004 and Amendment No.4 to NZS3603 in 2005.

Key Points

• The anisotropic and variable nature of timber is reflected in the design method.

• Development of the standard has following introduction of new grading methods and a change in the timber resource.

• Limit state principles are used for designing in timber in New Zealand.

• Characteristic design stresses now originate from NZS3603 Ammendment 4.

• The design incorporates a series of modification factors to allow for service conditions.

Curved glulam columns

Source: Coutesy Timber Design Guide 2007

Structural design