Foundation Type for Storage Building

[JCWilson]

I am investigating the design of a 50' x 80' single story storage and/or work-shop type building that needs a foundation designed under it. Would a mat foundation thickened under the columns be the most appropriate? Or does the entire perimeter need to extend below the frost level?

Building type - moment frame with columns @ 20' on center on the long sides. Max loads at columns 15k vertical, 7.5k horizontal. Slab loading will probably be 200psf.

Soil Profile Type - S4

Location - Pennsylvania, USA

 

[ishvaaag]

Definitely to make the perimeter safe against frost is better, and there's no alternative way (except maybe for very impermeable soils) to ensure that frost won't cause a problem. Any restrain induced by the perimeter can be safely reduced to acceptable behaviour through reinforcement. At gates, it is of course safer to have elements of big stiffness.

This said, I have seen a number of buildings without going to frost depth and behaving well. I particularly remember one with structure in reinforced concrete, floor level plus 2 stories, where a 35 cm thick mat was placed upon 15 cm of stone and then quite malleable clay. I think to remember 45 cm square perimeter ties -and maybe between some columns- were used there. This 20 year old building has not given a problem, yet has not the required depth to be safe against frost, at least not the customary 90 cm or better 120 cm used here to such purpose (which may be an excessive requirement, anyway, since daily average temperatures almost never stay under zero ºC in this area, or just a few days).

Of course if you are thinking of a thin slab on grade you may end needing some thickening around columns, maybe even at your small loads. However, within the limits of putting in place what required (that is different in the mind of every designer) we have come to the conclusion that using single thickness is by far the cheapest and soundest way to proceed (this usually referred to foundations under 1 cellar) as soon as a mat or a piled mat starts to look what required, which has been in the tens of times in our case, given gypsum layers underneath being quite common in our area.

 

[SirAl]

Depends to a large extent upon the nature and condition of the subsurface soil profile. The 'S4' rating just doesn't provide enough information.

 

[BigH]

Yes - please provide some details on S4 rating. I might have to blush later, but I've never seen such ratings before. Is it from some structural code?

 

[JCWilson]

BigH, yes, I guess you could say it is a structural definition.

Soil type S4 is defined in the seismic section of BOCA 1996, Table 1610.3.1 as "A soil profile characterized by a shear wave velocity less than 500 feet per second, containing more than 40 feet of soft clays or silts." I believe it is similarly defined in the UBC(?)

I was hoping that it would mean a lot more to someone with a geotechnical background to the point that some foundation design information could be extrapolated from it. Unfortunately for me, that is all that was made available.

 

[Focht3]

Who determined / provided the "S4" classification of the site? Was a geophysical survey done of the site? If a geophysical survey was done, why was it needed?

It appears, based on the paltry information currently available, that the "S4" value was simply "plucked out of the air" (as opposed to being pulled from a dark, smelly place.) It's pretty clear that you need to have a geotechnical study performed in order to answer your original questions.

If someone thought that choosing an assumed poor site condition would allow the use of conservative design parameters, thereby saving the cost of a geotechnical study, they were clearly wrong. All the assumption has done is raise a lot of concern - concern that may not be justified.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[ishvaaag]

Just as an extension of the mat example I referred to, the clayey mud there was of such nature that you sank between 5 to 15 cm in one your feet if no careful on where treading.

Just adding the rolling stones (a solution already in place for surrounding spaces) was a blessing to walk, for even to extract your feet from the mud was a disgusting effort.

 

[JCWilson]

Focht3, you pegged that one.

Soil profile type S4 was selected as a worst-case seismic coefficient for structural analysis. That seemed easy to do in this part of the country where it doesn't govern anyway - for a simple building like this.

 

[Focht3]

You still need a geotechnical study - not a "soil boring test" to help you sort this out. I'll bet the site is no where near an S4 designation...if I'm wrong, meet me at the Brew Moon in San Antonio and the first six pack's on me! (I get to share in it, of course.)




Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[Horseman]

What if my soils are generally characterized by low chroma (10YR 2/0, 2/1, and 4/2), mottled, hydric, silt loams ?

 

[Focht3]

Those are agricultural descriptions - they are difficult to correlate with engineering properties in general. Rough correlations can be made locally, of course - by experienced geotechnical engineers.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.