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engrjoseph (Civil/Environmental)
2 Sep 11 21:47
dear all,

i know the question might be funny, but i am quite confuse in designing a footing (e.g. isolated/spread footing). Some programs only consider the soil's sub-grade modulus(SAFE) and some the soil bearing capacity(STAAD).

Which is really more critical?
fattdad (Geotechnical)
6 Sep 11 20:42
consolidation characteristics or soil modulus.  Most shallow footing bearing pressures are based on settlement.

So, you may say, well, the subgrade modulus models this "spring-like" behavior.  Not really.  Subgrade modulus really considers the structure's response to the subgrade support. It really doesn't get into the details of the soils below the depth of a foot or so.

Bearing pressure (ultimate) is important, but settlement usually governs allowable bearing pressure.  Unless, that is, the engineer is just guessing. . .

f-d

¡papá gordo ain't no madre flaca!

trainguy (Structural)
6 Sep 11 21:20
engrjoseph,

With all due respect, this is a very basic question that should be asked to one of your (hopefully) senior colleagues.

You should not need a program to do this.  Usually, the ultimate or allowable bearing pressure should be provided by the soils engineer, and you can use this to design a spread footing.

tg
FixedEarth (Geotechnical)
7 Sep 11 1:52
If you are designing a combined footing or a mat and you want to use a beam on elastic foundation approach, you will need the vertical subgrade reaction.  This is prbably what SAFE software is asking you to input.  

If however, your geotechnical report states allowable bearing capacity, then you enter qallowable, which already considered settlement effects.  This is probably what STAAD is asking you to input.

If you have gravity loads and a uniaxial moment, the allowable bearing capacity from the geotechnical report will suffice.  If however, you had a biaxial moment, shear and vertical load and wanted the stresses induced on all corners to check for uplift, then the beam on elastic foundation method will be more accurate.  

www.FoundEng.com  

engrjoseph (Civil/Environmental)
7 Sep 11 3:34
Guys,

Thanks fattdad and fixedearth for the clarifications, it did really help.

Re: Trainguy:
   Yeah you are absolutely right, but almost all of my senior colleagues are way out of reach.


Thanks and Regards.

 

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