Footing and sub-structure design
In most states and territories, a builder or designer can design the base construction system of a low-rise building themselves once they know the soil classification for the site. They can use the NCC provisions or either the detailed design tables in AS2870 (residential slabs and footings) or AS1684 (timber framing – when using a sub-floor system).
However, the design and construction of footings is best left to a specialist designer, generally a structural engineer, who will assess the soil conditions, nominate the appropriate soil classification for the site, as well as embedment depth, bearing capacity for footings and/or edge and internal beams.
A specialist can also assess the site conditions and decide on the most appropriate footing or slab type for the building design, as well prescribing the appropriate depth and width for the footings or slab beams. They can also determine the proper selection and type of steel reinforcement.
The design engineer will generally provide design details and recommendations on the cladding types and detailing based on expected movement. Included in their assessment will be details such as placement of articulation or expansion joints where using masonry elements to combat expected movement and the design may also include site drainage requirements to prevent ponding or directing water away from the building.
However, there are some restrictions on different base construction types, particularly slab types, set out in AS2870, which may limit, for example, an infill slab to only be used for certain sites due to limited expected movement of the site. Other restrictions may include only a minimal range of solutions for E and P class sites depending on the nature of the soil characteristics and expected movement of the site.
Generally, most of the different base construction systems can be used for various site classes but the depth and width of beams or pads will get more substantive – as will the reinforcement types to be used, particularly for class M–P sites consisting of reactive clay soils.
The dimensions of footings generally depends on the size and type of structure, and framing and cladding system to be used. Other factors influencing the design will include: the weight of the structure; the site classification and any unique site conditions or known site movement issues for that area; or characteristics unique to that area, such as salinity (salt level) of the soil which can influence reinforcement type or concrete selection.
This last point in particular reinforces the need to not just assume all designs and sites are equal and why using someone with local experience can have a number of benefits.