TECHNICAL INSIGHTS
TECHNICAL INSIGHTS The articles featured in Technical Insights are to prompt thought and discussion to assist our members' question and evaluate their understanding of the technical requirements of Australian Standards and other national/international source material. Technical Insights is to intended to provide background information, a different viewpoint, a perspective from an individual with lived experience of disability or to prompt further discussion and/or research by you as an access professional.
Dimensioning and Tolerances by Howard Moutrie
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n June I posted this topic on the ACAA Forum and I feel it is worthwhile expanding and I apologise in advance that in the end this article becomes a forum for a particular issue I have rather than a purely technical discussion. There are two separate issues, the first is drafting techniques and standards and the second is building tolerances. Although separate, they are inter-twined. With respect to tolerances, I tried to get this acknowledged in the latest edition of AS 1428.1 (yet to be published) and although I had support from the building sector it was rejected by the committee. The result is that the standard asks for a level of construction which is outside the standard practice and difficult to achieve in some instances. Firstly, let us look at dimensioning. Architects, draftsmen and builders work, in accordance with AS 1100.301, on the premise that they dimension to the structural element, thus, the dimensions must allow for the application of any linings and finishes. So, for a toilet with a clear width of 1900mm the room should be dimensioned at least 1925mm. This causes no issues because the room is meant to be a minimum of 1900mm. Confusion can occur, however, when maximum dimensions are required. An ambulant toilet is required to be 900-920mm wide, however, this could be dimensioned at 930mm, which on face value seems incorrect but is actually allowing for the thickness of the wall tiles so that the finished 44
dimension is compliant. Confusion can also be created if the architects indicate a corridor dimension at 1540mm, meaning it to be clear, but the builder will build the structure at this dimension based on the recognised drafting standards. There is an exception to this. The above discussion relates generally to the setout plans or general arrangement plans. When detail plans are produced for the bathroom areas, these are typically produced for the fitout trades and assume the structure is built, thus, they may show the clear dimensions. A complication to this relates to the selection of fittings and finishes. For example, a wall tile may be 6mm thick or 10mm thick. If the tile hasn’t been selected care must be taken by the architect to make the correct allowance. A common problem I see on site is that the accessible toilet is designed with a standard basin, with all the correct clearances, but then a larger basin is selected and the result is non-compliance. The access consultant, the architect, the interior designer and the builder need to be aware of the implications of these selections. When reviewing the drawings it is important to understand the dimensioning methodology. This all ties into building tolerance too, but before I discuss tolerances it is worth digressing to a method of measurement which was developed
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