Footing Design?
Footing Design?
(OP)
I have a situation where I have a footing with a large uplift, lateral load and overturning force (not an ideal situation). I have the weight of the footing and soil above the footing resisting these forces. The top of the footing is embedded 4'-0" into the ground due to frost concerns.
When it comes to the soil weight above the footing and the design of the footing for stability, would you use the theoretical "conical" soil above the footing or just the project soil above the footing (see sketch) to resist these forces?
When it comes to the soil weight above the footing and the design of the footing for stability, would you use the theoretical "conical" soil above the footing or just the project soil above the footing (see sketch) to resist these forces?






RE: Footing Design?
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RE: Footing Design?
A 4'-0" by 4'-0" footer is not that big. Step it off in your workspace. Plus the only ones who are going to see it are the contractors. Once it's buried, no one will know.
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I once had a monopole utility pole that had lines 90º to one another and was only about 60' tall and could not be guyed. It could not use a caisson foundation for site reasons and other utilities.
I had to use what amounted to a combination spread footing + tall pier foundation.
The spread footing was enormous.
I cant remember the exact dimensions, but I think it was something like 20' square and 4' thick.
When I watched them pour it, I felt like a dummy. But the numbers were run numerous times and it was what it was. When they back-filled ans set the pole, It looked great!
RE: Footing Design?
How thick is your 10'x10' footing? Looks like the height of your pier is making the condition much worse. Is the force in one direction only? If so a rectangular footing will be much more effective. With the values of your forces we could offer better input.
RE: Footing Design?
You are spot on with the height of the pier. Someone made the brilliant decision to keep the columns 3' above the slab which is only making my problems worse. Due to geographical location, I am also required to place the footing 4' below grade.
In the end, I ended up with a 9'x9'x2' footing not taking the soil cone above. I have a spreadsheet that I wrote that designs simple footings. I did not include the soil cone above the footing because in order to engage the cone I needed the footing to "slip" a little bit.
This problem is related to the same problem I posted on this forum with regards to metal building foundations a few weeks ago. I really don't like designing foundations for these types of buildings because the mfr does not care what the reactions are at the base of the column.
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This should help your OT moment on the footing.
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BA
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Good point, however, we are getting a written variance from the building inspector to avoid the part of the design.
Toad,
I thought about tie rods but the loads of concern are due to wind loads, not the "kick" loads associated with the dead and live loads placed on the structure.
RE: Footing Design?
A written variance will not alter the fact that, in the rare event of a flood, your building will be structurally unsound. I would not accept such a variance as you will likely be embroiled in litigation in the event of a failure during high water table.
Why not use drilled caissons with belled bottoms to resist uplift?
BA
RE: Footing Design?
RE: Footing Design?
Pure uplift? I have plenty to resist pure uplift. It's uplift in combination with the overturning that is killed the footing design.
With the 9x9 footing listed above I only included the soil directly above the footing. I guess this will give me a little added FOS.
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That shear causes overturning at the base, no?
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Yes, the wind shear at the base of the column is producing the overturning I am/was having problems with.
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Is the overturning force in both directions? The friction on the sides of the footing can be significant. Again you would need to have movement to get the friction but to me it seems reasonable. You are providing a safety factor against failure not against movement.
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Concrete is cheap and fixing foundation problems is probably the msot difficult thing on a building to fix.
Like my college prof said "why do we use concrete? Why? This S%#@ is cheap!!"
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So in summary, concrete is cheap but if you get really desperate take the soil wedge.
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I do not use lateral bearing pressure to resist uplift/overturning unless I am designing some sort of embedded pole. It this instance, I would not feel comfortable relying on lateral bearing pressure.
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In order for the footing to fail in OT or shear it would have to push the slab out of the way.
The slab will need to be designed to transfer the lateral load into other piers, walls, dead load friction, etc...
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We are beginning the circle again.
This is something that I had tough about but ultimately decided to avoid for reasons listed in your first post (making the design more simple). With the selected GC, I think simplification will be a good thing.
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You can offset some of the cost with reduced concrete volume and reduced excavation.
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There is an English idiom: "Do not spoil the ship for a ha'porth of tar".
I had a problem with the buoyancy of a tunnel as the grade was below design flood stage. I designed for some helical anchors but the client, in a design/build job, objected. He called a meeting of the top structural engineers in the the city to get their view on whether I was being too conservative or not. The question of whether a FOS of 1.5 was appropriate and whether the "conical" soil should be used using the friction coefficient of the concrete wall. They all agreed that using 1.5 and the vertical wall was very conservative but none knew of any authoritative cover for being less conservative. My boss then asked them what they would use. They all said they would use 1.5 and the vertical face.
Michael.
Timing has a lot to do with the outcome of a rain dance.
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Step 1 would be to have enough weight to counteract the uplift with a FS of 1.5 (ie) 0.6D > W (uplift)
How do you then account for a simultaneously acting moment? Under uplift, is it assumed that there is no contact pressure? The moment has to get resisted by soil bearing.
Sorry about a seemingly basic question.
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BA
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Yes, I have a spreadsheet that I wrote that accounts for the stability, bearing and strength of the footing under a few load combinations.
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A possible answer to the 1.5 factor query is 'yes definately, for all sections except maybe 1 or 2, we just have no way of telling which ones they are!' A quick lesson in the reasons why we have safety factors would then be appropriate.
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BA..... You obviously don't understand the code system and building official authority down here in the states these days. BO's do have the authority and the power to negate or ignore the laws of physics and nature, you know things like gravity and buoyancy and the like in a flood plane. But, then they do insist that you get flood insurance. They just sign the variance and wave their magic wand, and it can be so, either at .6 or .7. That is until you get to court, and then they are no where to be found, and you can't include them in the suit anyway, and the owner forgets he insisted you save him a few bucks. You keep trying to apply some common sense and conservative engineering judgement to the problem, and we can't do that here any longer, the code must say so, it must be there someplace, in all it's perfection and machinations. If they wanted a cheap building, their PEMB, then they may have to pay a little more for foundations to support it. And, it really shouldn't cost SteelPE more, in given away engineering time, than they saved on the supper structure, to now save them a few more bucks on those foundations too. And, he shouldn't stick his neck out a mile to save them those few bucks. Get on with it, an extra truck load of concrete per frame is less than we've spent on this thread!
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Would you mind posting a pdf of your spreadsheet or the excel file, if you are very generous.
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I'm not sure about that.... I will have to think about it. What I can tell you is that the spreadsheet was based off the combined footing spreadsheet created by Dr. Daniel T. Li. I took a sample of his spreadsheet and completely rewrote it the way I wanted the numbers run.
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You are correct when you say I don't understand the code system and building official authority in the USA but I know what a flood is as there are parts of Canada which get them annually. And as far as flood insurance is concerned, my guess is that flood insurers would not cover damages caused by engineering error or omission. Failing to provide for the effects of a flood in a flood plain constitutes engineering error, and should have been readily foreseen during design, a written variance from the BO notwithstanding.
Personally, I would not assume the risk of ignoring flood potential.
BA
RE: Footing Design?
They probably have pressure relief for the floor slab.