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Footing Design?

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?
 

RE: Footing Design?

If we are talking temporary loads I'd be tempted to use the friction angle of the soil to add capacity.  However, concrete is cheap.  Don't hold back on concrete volume for footings if it simplifies your design.

RE: Footing Design?

(OP)
The loads causing the problem could be classified as temporary.  I am getting a little desperate because as it stands, I have a 50x50 building that is requiring a 48'x48' footing.  It just seems absurd.

RE: Footing Design?

(OP)
I'm sorry, it's suppose to read (I have a 48'x48' building that requires a 10'x10' footing).

RE: Footing Design?

I agree with Teguci as far as the footing.  Make it bigger, if you need to. If the Geotechnical Engineer will give you an angle that the soil acts, use it.  Otherwise just use the soil over the footer.
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.

RE: Footing Design?

10' x 10' footings were the small ones on a lot of projects I have been involved with.  

RE: Footing Design?

(OP)
How many 48'x48' buildings have you done that required footing this large?

RE: Footing Design?

None, I guess.
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?

The soil cone above for sure. You couldn't pull a plug out of the ground that is the same shape as the footing--at least that is my opinion. Also ask the geotech for allowable friction on sides of footing. While concrete in the ground is cheap it isn't free.

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?

(OP)
ron

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.  

RE: Footing Design?

have you taken the weight of the groundslab into account?

RE: Footing Design?

(OP)
I have taken the weight of everything into account (footing weight, soil directly above footing, frost wall, frost wall footing, soil above frost wall footing).  As far as I can tell, I have nothing left to take.

RE: Footing Design?

can you take some of the shear in the pier into the slab?
This should help your OT moment on the footing.  

RE: Footing Design?

(OP)
The slab is not really structural at the moment.  We are looking at using a 5"-6" lightly reinforced slab.  I don't think it would work really wall as a column.... plus it is in a potential flood plain.

RE: Footing Design?

If it is in a flood plain, you must use 60% of the buoyant weight of the foundation and soil.  Is that why the footing is so large?

BA

RE: Footing Design?

Footings on PEMB sometimes employ tie-rods connecting foundations across the entire width of the building, no ?

RE: Footing Design?

(OP)
BA,

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?

SteelPE,

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?

How much more dead load do you need to resist the uplift?   

RE: Footing Design?

(OP)
CTW

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.
 

RE: Footing Design?

The wind loads produce a shear at the top of the footing pier, no?
That shear causes overturning at the base, no?

RE: Footing Design?

(OP)
Toad

Yes, the wind shear at the base of the column is producing the overturning I am/was having problems with.

RE: Footing Design?

sounds like taking the shear out before it gets to the footing is the best approach, if you can achieve that through the slab or a grade beam under the slab ?

RE: Footing Design?

Is 4' to the top of the footing or the bottom? I agree that you would need movement to mobilize the cone. However if the design is governed by 0.6D, or a 1.67 safety factor, then I would say it is reasonable to include the cone. The sizes go crazy if you don't count everything.

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.

RE: Footing Design?

can you count on lateral soil bearing on the tall pier?  

RE: Footing Design?

I guess I have always been conservative with ftg's.
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!!"

RE: Footing Design?

And so we have come full circle.

RE: Footing Design?

(OP)
I guess we did come full circle.

So in summary, concrete is cheap but if you get really desperate take the soil wedge.

RE: Footing Design?

Toad the lateral pressure on the pier is a good point. Seems it would make a significant difference. Enough so that increasing the width could be cost effective.

RE: Footing Design?

(OP)
ron,

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.

RE: Footing Design?

If you tie the slab to the top of the pier (you'd have to make the slab structural/ meet min As) then the column shear can go into slab.

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...

RE: Footing Design?

(OP)
Teguci

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.

RE: Footing Design?

have you considered screw piles? Uplift capacity is about the same as downward.

You can offset some of the cost with reduced concrete volume and reduced excavation.

RE: Footing Design?

Interesting discussion.
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.

RE: Footing Design?

How does one design a footing subject to uplift and moment?

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.

RE: Footing Design?

Do you increase the foundation weight more than what's needed for uplift in order to get partial bearing to work?

RE: Footing Design?

Deep foundations (piles) seem to be the best option.

BA

RE: Footing Design?

(OP)
slick,

Yes, I have a spreadsheet that I wrote that accounts for the stability, bearing and strength of the footing under a few load combinations.  

RE: Footing Design?

paddingtongreen, love the story!

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.  

RE: Footing Design?

BA and Csd72 are talking about the same animal, by slightly different names, in my way of thinking.  Why not use helical (screw) piles immediately under the footing, and tied up into the footing for uplift, but also acting as added bearing cap'y. on the other side of the footing, depending on the direction of the moment.  Also, tie rods across the bldg. btwn. piers on the same frame work wonders at solving this problem.  You can't reject all the solutions offered, and then ask, what's the simplest solution, come on guys, what's the solution.

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!   

RE: Footing Design?

@SteelPE:
Would you mind posting a pdf of your spreadsheet or the excel file, if you are very generous.

RE: Footing Design?

(OP)
slick,

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.

RE: Footing Design?

dhengr,

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?

I am with BA. An owner can't relieve you of your professional responsibility. It is the EOR's alone and when you seal drawings it means that to the best of your knowledge that the design complies with the governing code.

They probably have pressure relief for the floor slab.

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