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Using AS 1684 for timber framing
AS 1684 – Four parts
AS 1684 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, Part 4 being the ‘simplified’ version was developed as a more user-friendly document than Part 2. But there are differences between the two Standards 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 simplified version is popular for its hard copy span tables and minimal information is required to determined spans. 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.
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