Soil Suction Forces 1

[spats]

I ask this question at the risk of sounding stupid, but here goes. I'm designing metal building foundations for uplift. Unfortunately, I'm dealing with a contractor that insists on thickened slab/haunched footings, or he "will get his other engineer to do it". These types of foundations are always problematic because exterior columns are always well off-center from the haunch, and you wind up with significant overturning moments. I don't want to come back to the contractor showing huge chunks of concrete required, and am looking for other mechanisms to resist the uplift and overturning forces.

It seems to me that there has to be a significant amount of suction developed between the slab and the soil as the slab tries to uplift. I've tried to google the subject, but I'm not having much luck. In the same way as you can place a piece of newspaper over a ruler overhanging the edge of a table, and break the ruler with a downward blow, "soil suction" has to be able to resist a tremendous amount of uplift. A column of air exerts about 14.7 psi of pressure on the top of the slab. It seems that if you had less air pressure exerted below the slab due to the buried/confined soil, you could develop a significant amount of uplift resistance.

Any thoughts or technical information on the subject would be greatly appreciated.

 

[SteelPE]

I don't think I would ever attempt to do this.... and if my contractor come back and say "We'll get spats to do it" I will more than gladly give my projects to you and go flip burgers for a living.

 

[manstrom]

An odd topic, I wouldn't rely on any suction for resisting uplift.

There has to be a way to make his footings work, take a step back and see how you could resolve the forces to do what the footing does. Lateral load from the footing pushing against the soil? Friction?

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[Leftwow]

Can you send some drawings or pictures of something similar to a haunched footing?

 

[SlideRuleEra]

spats - IMHO, don't rely on suction. You are really depending on water (soil moisture) to create an airtight seal... have a drought and there goes the seal.

If you want an outside-the-box, but practical way to resist uplift look into using helix screw anchors with the top of the anchor embedded in the concrete. These anchors are used in the electric power industry to anchor guy wires on distribution power lines. I have used them as wind load and floatation tie-downs for small structures and as "dead men" for resisting overturning of a retaining wall - they have worked well for those applications.

Here are typical products:

http://www.bulls-industry.com/showpro.asp?id=27&new=&class_id=2&title=&class1_id=&class3_id=

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[SteelPE]

Interesting thought SlideRuleEra. So to calculate the uplift resistance on these you would calculate the cone of soil that would be ripped out of the ground at failure and then just apply a factor of safety?

 

[SlideRuleEra]

SteelPE - Yes, that is essentially it. These anchors are so common that a lot of research has gone into their capability. The manufactures usually publish safe load recommendations for several soil descriptions.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[spats]

One of the few things I found in my search for "soil suction" was an article in the April 2006 Building Safety Journal, which is published by the ICC. The article was about design of concrete floor slabs for storm shelters. In part it states "The soil was modeled with translation-type elastic compression springs. The springs acted in compression only - soil suction (spring in tension to counter the upward deflection of the slab) was not modeled."

 

[Lomarandil]

Agreed with SlideRuleEra -- it isn't modeled for good reason. That whole last clause is just academic filler speak.

 

[RM87]

As with the others, I wouldn't consider soil suction as a viable option. Do you happen to have a sketch of what's going on?

If the contractor is confident enough with his skills to construct his desired design, then I would design it for him - I would be a bit conservative with your design if you're feeling unsure. In your plans, I would suggest the use of a surveyor to stake out the applicable locations to mitigate that sort of liability, and let him know that any additional work as a result of his ineptitude, should be paid per your Schedule of Fees... etc.

What sort of metal foundation were you proposing in the first place?

 

[IDS]

Without a sketch it's not clear to me why you are getting this large overturning force. Suction will be zero for long term forces, and as close to zero as makes no difference for all loads for soils above the water table so clearly cannot be relied upon. If you have a continuous slab between columns then if you analyse that as a spring supported beam with point loads at column positions, and no tension from the soil, that should give a reasonable result.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Ron]

SteelPE...it's actually even easier than that. I've used the method SRE noted and you can get tested uplift values for helical anchors from the manufacturer. If you want to calculate the soil shear failure parameters, have at it, but the empirical values are likely already there.

spats....I would not consider soil suction as an uplift reaction...ever.

 

[Ron]

SRE....your opinion never needs to be humble....you know more than most of us will ever hope to!!

 

[rlflower]

As a structural engineer (read NOT a soils engineer), I have an opinion that I think may be worth your while.

Not knowing what type of soil we are dealing with - which is often the case, we have to assume the worst. With this in mind, I think se should explore some simple dynamics with regards to soils in general, but in particular a common type of soil that tends to give us the most trouble.

I am referring to what is commonly referred to as "expansive soils". These soils consist of various amounts of sand, silt and clay; however, they commonly have a significant amount of clay. This characteristic draws our attention in that the predominant presence of clay in the soil tends to cause the soil to swell when saturated and to shrink as it dries (picture the classic hexagonal shapes appearing after a saturated predominately clayey soil has dried).

So, what does this have to do with your inquiry? Soil suction may well be present when the soil is saturated, yet this characteristic "evaporates" (excuse the pun) as the water evaporates. As the soil dries, its molecular character requires it to pull away from itself and from an adjacent foundation structure. Thus, we cannot rely upon soil suction to be consistent.

There, that is my ten cents worth.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[spats]

For the benefit of those who asked, the attached is an example of a thickened/haunched slab footing.

Obviously the consensus is that soil suction should not be used, and I understand the objections. I would, however, like to point out one observation. I have added to existing metal buildings in many instances over the years, and commonly see tiny 4'x4'x1' thickened slab footings under main frame sidewall columns on the existing building drawings provided to me. I know there is no way they would even come close to working for uplift loads, yet they have been there for decades. Either they have never been close to being fully loaded, or something else is going on that we're not accounting for. By the way, I practice in Florida, so we're not talking about the dinky wind loads that I'm sure a lot of you normally deal with.

 

[manstrom]

If I had to sharpen my pencil and make that detail work in uplift, I would use the entire footing plus a good chunk of the slab. If that footing would start to lift, how much of the slab would work in bending / shear? 10'? 12? What about permanent dead load of the interior of the building?

Your design load combo should be 0.6D + 1.0W. You could probably back off on the 0.6 using engineering judgement since the dead loads can be calculated accurately.

Check the uplift loads, are they MWFRS or C&C?

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[rlflower]

Reviewing your detail, spats, I would suggest standard hooks for the top bars to add resistance to rotation due to the offset gravity load.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[IDS]

Are you talking about a net upward force from the column or just overturning?

Either way, the fact that existing buildings do not have problems indicates that the combined action of the footing and slab provides a stable system without requiring soil suction.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[RM87]

At the risk of being banal, have you considered connecting your column footing to another existing (or preferably new) foundation across the structure via grade beam, and just use that system as a couple with the soil resisting any OTM? It would help address differential settlement as well. It might be interesting to use this method with SRE's recommended helical anchors for some redundancy...?

 

[spats]

You guys can just throw out your views without any regard to numbers but you're not correct. Calculate a column with 20 kips of uplift (working) as opposed to less than five kips of dead load. Both loads are only 1 foot from the edge of the slab. Take 60% of the dead load and then figure out how it works. Then get back to me.