Footing design combinations for overturning & bearing stress to Australia Standard 1

[Ey_2]

I have a relative simple question in regards to footing design
Given the allowable bearing pressure. What combinations do you use to check the stability when lateral loads and moments are involved?
For gravity I can do G+Q but I don't know what load combinations do I use for wind & earthquake?

Or do you times a factor to get ultimate bearing pressure? If so, where can I find clauses that allow us to do so?

 

[rapt]

Human,

I said serviceability design factors were not appropriate! Then tsalked about doing a service stress check based on allowable bearing pressure which is normally a working (or in other terminology, service) stress allowed


Ey_2

Ws is defined in the codes and should be used if doing a working stress check. In the old days W was working stress, and ultimate wind was 1.5 * W. These days they are based on different return periods.

Es is not defined. All Earthquake loadings are Ultimate. A bit of a disconnect between 1170.1 which says to use Es and 1170.4 which only defines E which is actually Eu, so the earthquake loading code never defines an Es. There is one comment in 1170.4 for importance level 4 buildings must remain serviceable under a design event for importance level 2 structures, but that is still really a ULS strength check to make sure emergency buildings are still able to be used, but using a shorter return period to determine this.

I will try to find out.

Does you geotech report give both ultimate and allowable bearing capacities. If it does not you could ask for them.

 

[human909]

I said serviceability design factors were not appropriate! Then tsalked about doing a service stress check based on allowable bearing pressure which is normally a working (or in other terminology, service) stress allowed

Sorry if misunderstood or misrepresented what you were saying.

I was responding you use of unfactored and service wind loads as well as to this comment:
Not sure why people are defining Ultimate Load Combinations for a service stress design.

From both the title and text I see the problem as an ultimate limit state design and certainly not a "service stress design". Stability and 'overturning' are explicitly ultimate limit cases under the code.

My understanding has always been that the short term loading reductions are not applicable to footing design. So it is
G + Q
G + wcG + Ws
G + wcG + E
G + Ws
G + E

G+Ws for example can be quite unconservative. It isn't much good if your structure overturns when W is greater than Ws. Or G turns out to be slightly less than the assumed value.

 

[Ingeniator-Glenn]

My last work place (first engineering job) does all their own foundation engineering as well as structural, however the type of structures are generally simple (residential, commercial steel framed structures etc). I spent more time researching on these geotechnical points than structural, probably simply because there is no one specific guideline, defined set of rules across across the industry that I have seen.

My understanding as others have said above:
- ultimate bearing capacity calculations considers a FOS of 3 typically as this is termed allowable. However you could actually calculate your allowable based on limiting settlement for the intended structure.
- settlement limits are not defined anywhere, however there are some good papers or, limits as given by transport bodies.
- typically I see people working on just working stress method for limiting bearing stresses. Typically G + Q, FOS 1 as failure mechanism is deflection.
- ultimate for stability, uplift and sliding, FOS 1.5, 0.9G, Hydrostatic of 1, Destabilising Deadload 1.0-1.5 (as per 1170.1 i believe pending how certain your are), Stabilising of 0.9 Deadload, imposed load of 1.5

Try Appendix J of AS4678 - i feel this is the best available information across Australian Standards.

P.S long long time reader first time poster

 

[captain_slow]

Try Appendix J of AS4678 - i feel this is the best available information across Australian Standards.

I was going to mention AS4678 too; it is clearly a limit state code and appendix J is an extremely useful appendix (albeit with some outdated references). Having said that this code still assesses soil capacity for bearing as "allowable" (please do correct me if I am wrong).

 

[rapt]

Human,

But it is not logical to use a ULS load combination with a service stress limit like the allowable bearing pressure which is what the poster said that he had. Designers may be doing it, but it is not logical. A ULS stability loading would operate with and ultimate bearing capacity and appropriate factors.

If the factor between ultimate bearing * capacity factor to allowable bearing was the same as the factor between Ultimate Load and Service load, you get the same answer which ever one you used!

I will get back regarding earthquake loading next week, unless Agent666 wants to let us know his ideas now.

 

[human909]

But it is not logical to use a ULS load combination with a service stress limit like the allowable bearing pressure which is what the poster said that he had. Designers may be doing it, but it is not logical.

I agree. I think we were approaching the question from opposite ends.

And like I said earlier, from what I have gathered you a have vastly more experience than I do, so in general I would defer to your expertise but it won't stop me from questioning!

 

[blihpandgeorge]

References to ASD design in the US, or others outside of Australia are only confusing the question. The OP states this is in reference to Australian Standards, where ASD has not been in codes for a long time. The Aus Standards do not give Allowable loadings for wind or seismic. They give Ultimate or Serviceability but not Allowable. Using background from other codes may have some basis for gravity loads, but without a proper comparison of the derivation of wind and seismics loadings in Australia these comparisons are guesses at best.

For the OP - you will need to go back to the geotech and get Ultimate and Serviceability parameters from your geotech. The publishing of Allowable values by goetech is unfortunately common place, and for gravity only cases can suffice, but they are not useful for lateral loads. Both the piling (AS2159) and retaining wall (AS4678) are written in limit states codes, so there is no excuse to not get them. The appendix J of AS4678 is a useful guide, however it is for Ultimate and Servicablility, not Allowable.

 

[r13]

Below is a comparison of the three design cases. You be the judge on which one is preferred.

 

[Ey_2]

Thank you everyone.
Looks like the best way is to get the ultimate bearing pressure.
So, If ultimate bearing pressure is provided. For gravity can I simply compare the stress under 1.2G+1.5Q with ultimate bearing stress or do I still need to compare stress under G+Q with allowable bearing stress for the settlement issue that r13 mentioned?
Also, For lateral, if ultimate stress is used, do I still need to apply a FOS, which is about 2.0-3.0 for overturning? My view is no as FOS is more of a ASD way. For limited state we have destabiliting factor which is greater than 1 for working out the design overturning moment and stabiliting factor which is 0.9 for capacity, so as long as the capacity is greater than design overturning moment then it should be satisfied. Am I wrong?

 

[r13]

IMHO, the answer to both of your latest questions is "no", unless instructed by your design code, or your geotechnical engineer, otherwise.