Australian Standards Combination Loadcase Factor For Pad Footings

[ooox]

Is there anywhere besides AS2870 that gives recommendations for load combinations to check against (soil) bearing failure.

I know the British Standards uses unfactored load case combinations but there doesn't seem to be any advise in the Aussie codes apart from the residential slabs and footing code.

 

[csd72]

Soil bearing capacity is usually given in allowable stress. When I have needed to give ultimate loads I have used the same load combinations as the column loading it (it is where the load comes from that counts not whether it is steel concrete, timber or soil).

By the way, there is an AS/NZS code forum under Engineering codes Standards and Certifications category.

 

[asixth]

Personally I don't like to combine ultimate limit state and geotechnical-structural design. The geotechnical engineer will give you an allowable bearing capacity and the structural engineer needs to ensure that the working loads do not exceed this bearing.

This can be any combination of Permanent Load (Dead Load), Imposed Load (Live Load) and Wind Load. For wind load, a working load must be used which is 1.5 times less than the ultimate wind load (refer AS1170.2:1989).

As for where the load combinations come from. I don't know because AS1170 (latest revision) is written in a limit state format.

AS4678 tried to incorporate a limit state approach to geotechnical parameters however there was some criticism because the material is not as well known as other building materials that structural engineers deal with so it is difficult to apply and material strength reduction factor (?).

 

[ishvaaag]

For mat-slab foundations it may be worth considering the reduction of live loads for cases with a number of stories, that can be substantial since the whole mat is an element receiving loads of many columns many stories high and unlikely to act at nominal live load value. Also, the failure would be less than critical in the sense that the mat ties together all columns. This reduction is permissible by spanish CTE code.

 

[asixth]

As ishvaaag has pointed out, area reduction factors for live loads can be found in AS1170.0 or .1. For footings supporting a large area the total live load run down can be decreased by 50%.