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Building Code of Australia (BCA) Volume Two covers the design and construction requirements for houses (Class 1 buildings) and garages, carports and sheds (Class 10 buildings). The BCA Deemed-to-Satisfy Provisions for residential slabs and footings are covered in Part 3.2.
Under the Deemed-to-Satisfy Provisions there are two compliance paths for the design and construction of raft slabs:
Commonly, raft slabs are designed and constructed in accordance with AS 2870.
Accordingly, the information below focuses on the requirements in AS 2870. However, it should be noted a detailed description of all of the requirements prescribed in AS 2870 is not provided – rather, a general overview of some of the key components is given here.
AS 2870 covers the design and construction of residential slabs and footings. The current edition referenced in the BCA is the 2011 version.
The Standard includes substantial commentary at the back of the Standard to assist in its application. As with most ‘commentary’ or ‘explanatory information’ in codes and standards, it is included for information and advice only, and does not form part of the mandatory body of the Standard.
Here are some of the key aspects of applying the Standard and working out the requirements for a raft slab.
Site classification is based on expected ground surface movement and the depth to which this movement extends. Classifications range from A = little or no movement, S = slightly reactive clay sites, M = Moderately reactive clay and silt sites, H1 = Highly reactive clay sites, H2 = Highly reactive clay sites, E = Extremely reactive sites, P = Sites with unstable or soft foundations, fill etc.
The requirements for depth of footings and reinforcement for the slab differs based on the soil classification. A geotechnical engineer can provide you with a soil classification for the site and the local council may also have detail on the soil characteristics for the site.
Filling used for the support of a slab shall be controlled fill or rolled fill as follows:
A raft slab can be designed in accordance with either:
The approach of using the provisions of AS 2870 written into the standard is more of a prescriptive approach of following standard design and prescriptive tables. This approach dose contains certain limitations on its use depending on building geometry and being limited to only certain site classifications.
In terms of the prescriptive provisions under this approach Clause 3.2.2 prescribes the internal and external beam layout requirements for the slab design. Clause 3.2.3 prescribes the reinforcement requirements and 3.2.4 the construction requirements. Clause 5.3 contains further detailing requirement which may apply depending on the slab design itself.
Section 4 uses more of a ‘first principles’ approach to the design of the slab and this is generally done by a structural engineer, or someone with the relevant skills and experience.
The grade of concrete used shall be N20 with a slump of 100mm with 20mm maximum nominal aggregate size, or as specified by the designer. Thickness of the slab may also vary depending on engineering requirements, such as filled land, underfloor heating, polished concrete finish.
Reinforcement in rafts and slabs shall be in accordance with the following:
Where slab mesh is specified such as SL82 the full description is:
S = Square mesh, L = Low ductility, 8 = 8mm bars, 2 = 200mm square size
Edge rebates for slab on ground, stiffened raft or waffle raft with masonry cavity or veneer construction shall comply with the following:
Service penetrations are permitted through the middle third of the depth of edge and stiffening beams. The effect of other service penetrations shall be taken into account by the provision of extra concrete depth or reinforcement.
Termite management treatment, particularly of service penetrations, should be considered at this stage. Vapour barriers or damp proofing membranes may also be required for the slab and there may be specific requirements for these in your state or territory.
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