AS 2870 residential and footing construction

[280275]

AS 2870 is a standard code for residential strip footing design in Australia. I take a look some structural drawings, I see that it is likely that the structural engineer doesn't follow what the code said and he has got his own standard and design.

For example, the section of a largest internal strip footing can be 600x600mm with 6L12TM (top and bottom) for a 2-storey residential building at site class M (bearing capacity 200KPa). The external edge strip footing is 450x600mm (600mm in depth) with 4L12TM (top and bottom). These dimension and trench mesh is not shown in the code.

I read an Australian book for strip footing design which is similar to pad footing design. But I cannot get the practical results ( e.g. the width is much less than 600mm).

I maybe have a wrong calculation or haven't undertood the code yet ???. Any idea ? Thanks.

Happy New Year.

 

[dbuzz]

AS 2870 does contain some standard designs that are “deemed to comply” with the code requirements but you are not necessarily limited to those designs. The Engineer (or Designer) can also design to engineering principles.

The commentary to AS 2870 states that “to allow for uneven loading and eccentricity, the minimum allowable bearing pressure is set at 100kPa” for strip footings. Typical values for applied pressure under the “standard” strip footings are 50kPa, 65kPa and 80kPa for double storey clad frame, masonry veneer and solid masonry construction respectively. This is because the footings are not just designed for allowable bearing pressure but also for providing sufficient strength and stiffness to accommodate some movement.

With Class M soils being moderately reactive, the Engineer may be basing the design on previous good performance or to suit current practise in that locality. Why don’t you call him/her to discuss?

For more information try reading the commentary to AS 2860 and the references nominated within.