Slabs and Footings and the Australian Standards
As a home builder you are faced with many choices, from colours, to finishes, to how many stories, and many more – too many to mention.
The most important part of your building is the foundations. The foundations are made up of two components:
- The soils below the foundations which dictate:
- The type of footings/slab supported the soils.
The AS2870-2011 Residential Slabs and Footings Standard sets out a list of standard slab designs correlating to foundation soil types. This code provides designers with the most economical slabs/footing standards based on a number of criteria including both cost to the owner, and cost to the community.
The designs have been adopted based on performance criteria including:
- Width of cracking in both the slabs and the walls they support
- Amount of rotation (out of plumb) a wall may exhibit due to inevitable movement of the footings/slab.
The problem is, in my view, most home owners are unaware of the standard, and are under the false belief that when the standard is adopted they are getting a high quality result. This is simply not true and I believe the Australian Standards should be re-named the Australian Minimum Standard, as by definition, the rules set out in the Australian standards are the minimum allowable under the law, and not necessarily acceptable to all home owners.
I have visited many homes over the years and inspected cracked and moving walls which, in nearly all cases, comply with the tolerances of the Australian Standards, but are intolerable to the home owner.
- Cracks up to 2mm wide are within standard
- movement in walls up to 15mm out of plumb, are acceptable within the Standard
- Timber floors that feel bouncy may still be within the Australian Standard.
An engineered slab for a home can be designed and detailed to whatever tolerance you want. It is our job as structural engineers to listen to your requirements, and make sure you are properly informed so that a clear decision can be made on your greatest investment; one that balances economy with strength.