Code Methodology

This section contains information on limit state design and working stress design.

Terminology

Vertical actions on a structure from gravity and self-weight are called loads.

All other external actions on a structure, including those from wind and earthquake are called forces.

The resulting internal effects such as axial force, shear force and bending moment are called internal actions.
 

Limit State Design

Timber design in accordance with NZS 3603 is based on the “limit states” method of design and refers to the Structural Design Actions Standard, AS/NZS 1170. 

The designer should be familiar with the requirements of both NZS 3603 and AS/NZS 1170. 

When a structure, or part of it, fails to fulfill basic functions, it is said to have reached a “limit state”. 

Two limit states are identified, namely:

• the serviceability limit state and

• the ultimate limit state. 

The serviceability limit state (SLS) deals with deformation and deflection limits which affect the appearance or function of a building.

The ultimate limit state relates to the strength and stability of all or part of a building.

The ultimate limit state (ULS) is deemed to be satisfied if the structural timber members are proportioned so that their design strengths are not less than the actions produced by the factored design loads from AS/NZS 1170.

Limit State Design - Printable PDF

Working stress design

Working stress, allowable stress or permissable stress design is a method of design in which structures or members are proportioned for prescribed working loads at stresses which do not exceed their elastic limit.

This limit is usually determined by ensuring that stresses remain within the limits through the use of factors of safety.

Working stress design has been replaced internationally by limit state or ultimate state design.

Working stress was the design method used for timber design prior to the release of NZS3603:1993.

Because NZS3603:1993 developed from earlier working stress design standards, soft factors were used to convert the timber properties from working stress to characteristic strengths.

Working stress design is of historical interest only in New Zealand now.

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