Soil Suction Forces
Soil Suction Forces
(OP)
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.
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.






RE: Soil Suction Forces
RE: Soil Suction Forces
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
RE: Soil Suction Forces
RE: Soil Suction Forces
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&am...=
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Soil Suction Forces
RE: Soil Suction Forces
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Soil Suction Forces
RE: Soil Suction Forces
RE: Soil Suction Forces
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?
RE: Soil Suction Forces
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Soil Suction Forces
spats....I would not consider soil suction as an uplift reaction...ever.
RE: Soil Suction Forces
RE: Soil Suction Forces
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.
RE: Soil Suction Forces
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.
RE: Soil Suction Forces
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
RE: Soil Suction Forces
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.
RE: Soil Suction Forces
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/
RE: Soil Suction Forces
RE: Soil Suction Forces
RE: Soil Suction Forces
RE: Soil Suction Forces
This link is worth a read though: http://www.structuremag.org/?p=342
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Soil Suction Forces
Other things he does for shed designs is not assume that the maximum wind pressure acts upon all surfaces at the same time. He assumes the wind is very turbulent and, for large areas, does not hit everywhere with maximum force at exactly the same time therefore allowing him to further reduce total uplift (he did his masters project on this and actually tested wind speeds instantaneously across a perpendicular plane 50 metres long).
For cyclones/hurricanes he also reduces the air density designated in the code thus reducing wind pressure and uplift, since cyclones/hurricanes are caused by a low pressure centre and the wind velocity around the centre of the cyclone/hurricane is inversely proportional to the air density. This one can easily be checked by looking at the local meteorological website where they keep pressure and velocity data for cyclones/hurricanes.
Of course, his competitors say that, out of the thousands of sheds he's designed which have been through many cyclones and survived, all of the sheds just didn't undergo "design loading". It's always easier to justify introducing assumptions which make the structure heavier and more costly, than to take risks and introduce assumptions which increase economy and efficiency.
RE: Soil Suction Forces
RE: Soil Suction Forces
Nice story nonplussed! I wish there was some more technical info to go along with that. I like the idea of the lower air pressure in a hurricane, but it doesn't seem like it would make a lot of difference, and I'd have to have some serious technical backup.
Canwesteng - I too am using a chunk of slab around the haunch to increase resistance. I generally go all the way to the slab joints. An example slab area would be 14'-3" wide x 9'-6" into the building. I make the haunch rectangular, with the long direction parallel to the slab edge, to concentrate the foundation weight as close to the slab edge as possible. Regarding top and bottom slab reinforcing: I assume you're talking about just in the area of the haunch and the design slab area around it?
RE: Soil Suction Forces
RE: Soil Suction Forces
RE: Soil Suction Forces
Pressure in the eye of a major Category 4 hurricane could be, say 27.5 inches of mercury. It would be even lower for a Category 5 storm. Atmospheric pressure in the maximum wind field, close to the eye, may not be as low as the eye, but should be similar. That pressure is only 91.7% of typical 30.0 In Hg sea level atmospheric pressure.
As usual, there is no free lunch, you will get reduced wind pressure... but you will pay for it with increased storm surge. The differential 2.5 In Hg translate to about 3 feet of water. Therefore the storm surge will be about 3 feet higher than wind speed alone would predict.
MikeE55 - I agree with you, if the ground is wet, as in a hurricane, there could be a certain (indeterminate) amount of suction. Simultaneously there would be friction. The uplift load is only a vertical component force trying to lift the slab. At the same time, wind pressure is providing a horizontal component force trying to slide the slab. I would expect these two components to interact, making the slab more resistant to uplift than expected.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Soil Suction Forces
Sometimes, its best to pass on jobs, let someone else pay the attorney down the road.
RE: Soil Suction Forces
Using 0.6D + 0.6W and assuming 20 kips uplift and 5 kips dead load, I get a net uplift acting on the foundation equal to 9 kips. Using the reduced weight of concrete (0.6*150 PCF), I calculate 100 cubic feet of concrete is needed or 3.7 cubic yards. Using a hairpin to resist large thrusts has always been questionable in my mind, a grade beam between frame columns with continuous reinforcing across the building is typically how I will resist the thrust and overturning. When countering the uplift, consider portions of the thickened perimeter slabs, grade beam, and triangular area of slab connecting perimeter thickened slab to grade beam as contributing to the total volume of concrete resiting the uplift.
RE: Soil Suction Forces
What does the geotech. engineer have to say about soil uplift resistance or slab-soil suction resistance? Let me guess; the contractor said; "if you need a geotech. report, I'll find some other clueless "engineer" to design it."
I would think drilled piers would do the job, but you do need some geotech. data to design properly.
Regarding text books on Metal Building Foundations, the comparison is made with "conventional" buildings. Assuming you have the same required column spacing (column-free spans), what is the difference between a metal building and a "conventional building"?
Presumably a conventional building has hard walls, the weight of which may add to uplift resistance. Does that mean a metal building with CMU or tilt walls is "conventional"?
Presumably a conventional building has a "heavy" roof. Does that mean a metal building with deep joists and a heavy built-up or concrete roof is "conventional"?
In case you're wondering, I'm not a metal building engineer and have no relation to any metal building companies. I am just bothered by the twisted logic which, if taken seriously, can cause the client / owner an increased budget just to make the design easier.
Sure, you could use a braced frame design and roof diaphragm to transfer lateral loads to shear walls at the endwalls. Compare the cost and see if that works out?
By the way, that will eliminate some of the horizontal reactions, but it won't make the uplift forces go away.
RE: Soil Suction Forces
RE: Soil Suction Forces