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AS2870-2011 Design Bearing Capacity?

AS2870-2011 Design Bearing Capacity?

AS2870-2011 Design Bearing Capacity?

Looking at AS2870-2011 I see the Design action effects and Design bearing capacityare defined as follows

1.4.2 Design action effects
Design for serviceability and safety against structural failure or bearing failure shall be based on design actions due to-
(a) permanent action plus 0.5 imposed action; and
(b) foundation movement.
The permanent and imposed actions to be resisted shall be in accordance with AS/NZS 1170.1.

1.8.14 Design bearing capacity
The maximum bearing pressure that can be sustained by the foundation from the proposed footing system under service loads over the design range of soil moisture conditions.

The design load then is not even the maximum load for allowable strength design (permanent plus full imposed action) and consequently what is called Design bearing capacity is actually a bit less than Allowable Bearing Pressure and significantly less that Design Bearing Capacity for limit state design where design loads are multiplied by a load factor; e.g. Ed,dst = [1.2G, 1.5Q].

[edit] So my question is if I have been given a Design Bearing Capacity to AS2870 would I use this with serviceability load cases rather than Ultimate Limit State load cases?
The footing would need to be designed accordingly with an "elastic" distribution of soil pressure and I would design for no lift off for serviceability load cases.

RE: AS2870-2011 Design Bearing Capacity?

So what is the question?

RE: AS2870-2011 Design Bearing Capacity?

rowingengineer, question added.

RE: AS2870-2011 Design Bearing Capacity?

The bearing of a soil is normally a long term loading case for standard type situation in resi footings, so in as/nzs1170.2 would mean a long term action effect. The above 0.5 factor I believe is a long term factor for as2870 for want of a better explanation. So for service you would also apply the correct long term loading which I think without a standard near by would be 0.3.

RE: AS2870-2011 Design Bearing Capacity?

rowingengineer, yes the long term load case makes sense. The factor would normally be 0.4, so 0.5 is close.
Even so the combination specified does not correspond to any of the strength limit cases in AS1170.0 - 4.2.2; e.g Ed = [1.2G, 1.5ψlQ] It is closest to a serviceability limit state G + ψlQ

In my case I have been given a design bearing capacity to AS2870 for design of an industrial platform with pad footings, and I'm not going to get anything better. Fortunately it's in Region A so wind loads aren't dominating the design.

RE: AS2870-2011 Design Bearing Capacity?

Allowable bearing normally means working load type design, many engineers in geo don't like limit state.

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