Fig 1 Timber bridge


Table 1 Design life

Design Life and Reliability

The design life required by clients for various structures or structural elements will vary considerably depending upon the nature, use and cost of replacement of the building or structure.

As an example, detached housing is usually (in an engineering sense), designed for a typical life of over 50 years. This does not mean that maintenance, repair or replacement of some parts of the house will not be required in less than 50 years (i.e., roofing, plumbing, painting, etc.) or that the house will not last well in excess of 50 years.

The intent is that the structural fabric of the house will last at least 50 years and will sustain all imposed loads for that period. Our society has come to expect this level of performance from dwellings. Other cultures may accept a lower design life, with the advantages of lower cost, portability, etc.

Similarly, for structures that are temporary, such as hoardings, temporary bridges etc., specific design lives of a few months to a few years may be appropriate.

The degree of reliability required for a particular structure or component is directly related to the level of safety required and the consequences or cost of failure.

Table 1 provides approximate categorisation for a number of applications. However, designers must use their professional judgement.

With respect to safety, Table 1 recognises the following:

• Failure of building envelopes/cladding etc. may not necessarily cause a high probability of death or injury.
• Redundant, loadsharing type structures (i.e. typical domestic house framing) have the ability to share load without total failure, even though one or a number of members or joints may have failed.
• For major non-loadsharing framing systems (i.e. large members, wide spacing, portal frames, trusses, cross-bracing, etc.), failure in a single member or joint may lead to collapse of the whole structure.

Reference: Timber Datafile P4 – Timber – Design for Durability of the NAFI Timber Manual.

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Timber Research Unit
Department of Architecture
University of Tasmania