AS 1684 - Timber framing manual: Simplified or not? (Nat)

HIA receives many enquiries about the residential timber framing manuals or the AS 1684 suite of Australian Standards in relation to which is the most appropriate standard to adopt as the standard has several parts - the following information may assist members to choose.

AS 1684 or the Australian Standard for residential timber framed construction has four parts:

• Part 1 relates to design criteria that can be used as a basis for the preparation of span tables and design data for the other parts. This is the document commonly used by engineers.
• The other three parts provide building practices and procedures that assist in the correct specification and determination of timber members, bracing and connections. Part 2 and 4 are used in non-cyclonic areas and Part 3 is designed for the cyclonic areas of northern Australia . Part 4 is referred to as the ‘Simplified’ version.

As the name suggests the ‘simplified’ version was developed as a more user friendly document than Part 2 but there are differences between the two that users must be aware of that places limitations on the simplified version.

The criteria in both versions are specifically for conventional timber-framed buildings of one or two storeys.

Part 4 can only be used in the two lower wind classifications of N1 and N2 where Part 2 covers wind classifications up to N4.

The maximum width of a building designed under Part 4 is 12m where Part 2 can be used for building widths up to 16m and maximum external wall heights are 2.7m and 3m respectively.

There is a limit placed on the maximum roof pitch in both documents of 30 degrees for Part 4 and 35 degrees for Part 2.

To assess the amount of bracing a building requires under Part 4 only requires four steps, the bracing units are expressed as type A or type B and a simple table will indicate how many units are required.

Part 2 is a lot more complex in relation to bracing calculation and requires the raking forces to be determined and the calculation of bracing expressed in kilo Newtons per metre designed to counter that force.

The user friendly version is popular for its hard copy span tables and minimal information is required to determined spans where Part 2 has supplementary span tables on CD that requires more input to calculate timber sizes but delivers more choice.

The disadvantage of using Part 4 is that it generally over-engineers building components, the span tables are less comprehensive and can be less economical to use than Part 2 so this should be considered when deciding which part to adopt.

For further information HIA members can contact HIA’s Building Services staff on 1300 650 620 or hia_technical@hia.com.au.   

If you would like to become a HIA member, contact 1300 650 620 or enquiry@hia.com.au

Ref No. NAT BCA 08-17

The above is intended to provide general information in summary form. The contents do not constitute specific advice and should not be relied upon as such. Formal specific advice should be sought by members with respect to particular matters before taking action.  ABN 99 004 631 752