Foundations for a spread footing

[ztengguy]

I have been asked to design some foundations for a metal building. I asked for a soils report, was told it was not going to be provided, and assume its clay.

So, I was going to use 2000psf as a bearing capacity, and notes all over the drawings to confirm in field, and 6" of stone under all footings.

What do you guys think?

 

[msquared48]

Here it is 1500 psf without a soils report.

Check with your local jurisdiction to see what they require. Not trying to be rude, but rt doesn't really matter what we think as we are not there. You are.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[jeffhed]

Depending on the size of the steel building, soil bearing capacity may not be much help. The net uplift on the frame columns governs most of the time in my experience, and I am in a higher seismic area, 90 mph wind loads. If the soil is clay, make sure it is not expansive, we have a lot of expansive soils in my area. Spread footings will not work in those situations.

 

[ztengguy]

Thanks, I am not sure how to check if expansive if they are not doing a soils report.

I know its up to me to decide what to do, but just wanted some 'second opinion' type stuff.

 

[Eldorado]

ZtengguyI agree with msquared48 on the 1500 psf, but you have a better idea of the clays in your area. In my area I can probably get 2000psf consistently and I still use 1500psf without a geotech report. Around the coastal areas of Texas 1500psf is not always conservative. Again, you know your area better. The real concern I have for your situation in not having a geotech report is not the bearing capacity, but the building pad preparation. What site improvements are you going to specify and what can you base it on.

I do disagree with jeffhed regarding spread footings not working on expansive soils. Performance expectations definitely need to be examined, but in my area, we build over expansive clays all the time. The building pad needs to be improved with the proper fill material with an adequate thickness, but the footings might still bear on expansive clays. A typical metal building design might incorporate a spread footing at the base of the rigid frames (typically at the exterior edge of the foundation) that has a continuous perimeter grade beam and probably a tie-beam from one leg of the rigid frame to the other. I generally use a 36" minimum grade beam depth (generally 30" minimum below surrounding grade) to help reduce soil desiccation. Owners in my area are typically aware that some movement (and maintenace) can be expected, but are willing to accept this level of performance as opposed to the added cost of a suspended foundation.

 

[MiketheEngineer]

IBC says 1500 psf w/o soils report. If this is a building of any size - DEMAND a soils report or tell them to take their business elsewhere!!!

 

[BAretired]

I agree with MiketheEngineer. A soil report is the proper way to go. Footings may be the wrong solution. How would you know without a soil report?

BA

 

[JedClampett]

You can reference Chapter 18 of the IBC. Any times the soils are questionable you need a Geotechnical Report. And how do you know if they're questionable or not? A Geotechnical Report. And there are a host of other reasons that might apply to your case.
It's truly amazing how cheap a Geotechnical Report is. I've seen quotes of about $4000 with borings for a decent one. I'm sure a shoddy one is cheaper. If the project isn't worth enough to justify that, it isn't worth your engineering it.
You can be sure there will be plenty of money available to study the building if it sinks into the swamp.

 

[dhengr]

I agree with Mike and BA.... And, I’m really tired of this kind of nickle and dime crap, and am happy I’m not doing that kind of work any longer. What percentage of the total construction cost, cheap building included, is some soils investigation, a report, and fair pay for a good and thorough Structural Engineering job? Those costs are likely saved in better/easier/cleaner foundations and building performance in short order. You know damn well who’s door they will come knocking on if anything goes wrong, so be sure you pay your insurance premium with that fee. They are counting on you as the insurer of last resort so they can save a few thousand bucks up front. We aren’t really doing good engineering any longer, we are just another layer of insurance, someone to blame it on, so they can do things on the cheap, and we do a little number crunching in the bargain.

 

[ztengguy]

Thanks, I saw the 1500 psf. Perhaps when the footing sizes are big, they will reconsider. I will request the excavations be inspected by a geotech rep before footings are poured.

Its a 6000 sf metal building. I know that its still a building, but small at that. I think the biggest reaction is about 15K

 

[JAE]

Some areas you DO NOT want to put gravel below footings if the gravel is somewhat free-draining (i.e. uniformly graded).

In our areas, we have loess soils which are silty clays - if you put gravel anywhere near footings you could get underground erosion - transfer of materials from one area to the other and result in sinkholes or settlements.

 

[paddingtongreen]

@ztengguy, you say the biggest reaction is 15kips but what is the uplift?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ztengguy]

looks like I have 11K uplift on the footings.

 

[msquared48]

Minimum 4X4X4 foot footing taking into consideration oa portion of the slab for uplift resistence too...

Rather substantial I'd say 2.5 tp 3 yards per footing? $1000 in place per footing?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[FixedEarth]

Why take this risk for excessive settlement by assuming soils values? Let us say you have a soils firm check footing bottom and they verify 3,000 psf is adequate. And let us say, you have only used 1,500 psf for allowable bearing pressure. So you now go on and get the structure built just to find diagonal cracks and differential settlement greater than 1.5 inch next year. This can be caused by having a 7 ft of Clay crust over 35 ft of compressible clay. Do you think those soils are rare & unreasonable- it is a job I did this year. Luckily, my client got a soils report before the foundation was designed.

Jeffhed has a valid point. I looked at a 5 story building with shallow mat foundation on expansive soils. The building was designed by national firm and the soils report was done by another national firm. The mat cracked and the room seperators has had buckled. The building owner decided not to hire the soils firm to do observations- an outside less experienced firm was brought in and they did not catch it.

About 15% of our revenues are from structures like these mentioned above. If your client insists on no geotechnical investigation report, use drilled piers & grade beams with structural slab and assume mud (100 psf skin friction).

 

[RFreund]

You could assume somthing reasonalbe as suggested 1,500psf to 2,000psf but I would state your soil bearing capacity that is required and that it must be verified by owner / owners representative. Also agree that uplift can be a controlling factor.

EIT

http://www.HowToEngineer.com

 

[kylesito]

I also throw my hat in for the 1,500 psf figure and agree with the other disucssions happening about uplift.

And I agree with the nickle and dime stuff on the geotech report is ridiculous. They end up paying for it anyway at some point. Either we assume ridiculous design parameters and give them an overly conservative design that costs more than what they need or we require the contractor to confirm any of our other assumptions during construction (which means they pay for the borings and charge a markup on it). I just don't get it.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi