Footing Design?

[SteelPE]

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?

 

[ToadJones]

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

 

[SteelPE]

Toad

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

 

[ToadJones]

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 ?

 

[ron9876]

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.

 

[ToadJones]

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

 

[ToadJones]

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!!"

 

[Teguci]

And so we have come full circle.

 

[SteelPE]

I guess we did come full circle.

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

 

[ron9876]

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.

 

[SteelPE]

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.

 

[Teguci]

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...

 

[SteelPE]

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.

 

[csd72]

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.

 

[paddingtongreen]

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.

 

[slickdeals]

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.

 

[slickdeals]

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

 

[BAretired]

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

BA

 

[SteelPE]

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.

 

[csd72]

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.

 

[dhengr]

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!