(Civil/Environmental)
(OP)
5 Feb 14 10:10Greetings,
A local manufacturer is working on a rigid foam tank pad that would go under petroleum storage tanks. They have a data sheet on the material that indicates a compressive strength of 25 psi and actual test data that indicates a flexural strength of 60 psi and a modulus of elasticity of 1220 psi.
They asked me if I could tell them what the load capacity of a 14" thick piece by 12' diameter piece of this material is.
It's been a long time since mechanics of materials for me, and I've already told them that this wasn't something I felt comfortable doing, but I am still curious about how one would go about this process given the above information. Can anyone explain it to me?
Thanks,
Kurt
(Structural)
5 Feb 14 10:28Is the foam bearing on a foundation or on soil?
(Structural)
5 Feb 14 11:13Dow manufactures an extruded polystyrene that has various compression capabilities as high as 100 psi. You should be able to get polyurethane foam in this rating. Check the manufacturer's product data sheet for applications and/or contact the manufacturer.
Dik
(Civil/Environmental)
(OP)
5 Feb 14 11:27The foam will be bearing on soil.
Again, I've pretty much washed my hands of this, but I'm still curious about the process one would go through in analyzing this situation.
Thanks.
(Structural)
5 Feb 14 13:35I have used rigid foam under many concrete structures as a filler. It works great as a filler material since it weighs practically nothing and has a relatively high compressive strength. I have used it once or twice under footings.
From reading the info from Dow and similar, it appears that the compressive strength is rather high, but I would be concerned with punching (small bearing areas).
I have gotten some flak from county officials - "This isn't a structural material". but I haven't had any issues in using it for a uniform bearing condition.
(Civil/Environmental)
(OP)
5 Feb 14 13:58Personally, I don't think that the bearing capacity of the material will be an issue. My question is how to demonstrate that through calculations. If the material has a compressive strength of say 25 psi (or 100 psi), how does one go about calculating the overall bearing capacity of a uniformly loaded piece of it with an area of 100 square feet and a thickness of 14 inches? Obviously, I know how to multiply the area time the pressure and come up with a weight (force). But that doesn't account for the thickness. Is the thickness only necessary to resist the punching? And if so, what is needed to calculate the minimum thickness necessary to resist punching?
Thanks.
(Structural)
5 Feb 14 15:15Tr4driver:
I would be sure to protect that insulation material from UV and any other environmental degradation. Otherwise, you problem description leaves too much to the imagination for a very explicit answer. At the moment your only answer can be 25psi, as a uniform load, with some factor of safety. Are they trying to keep the heat of the fluid out of the soil or visa-versa? Otherwise, a compacted 10-12" sand cushion will probably be cheaper. 14" thick insul. and 12' dia. must be based on something that they’ve cooked-up, what are those constraints? What are the soil conditions and the tank construction? You should have no problem imagining that a steel floor plate in the tank should cause something less than 3.6kips/s.f. Some subgrade cushion and the rigid insul. should not be phased by a few protruding 1" round rocks; a 12" round conc. pile, in a swamp would be another matter. But, what do you do at the tank wall where the loads are in kips/lin.ft. and concentrated immediately below the shell? The insul. probably can’t take this and it shouldn’t be exposed out there anyway. You have to design your foundation (&/or tank) to distribute any concentrated loads to the insul. at less than 25psi.
(Structural)
5 Feb 14 15:17PL/AE. Check out this other current thread.
(Civil/Environmental)
(OP)
5 Feb 14 15:50I appreciate all of the information thus far. Here's some additional information:
As I stated earlier, the tanks will hold a petroleum product. They will be located within a secondary containment system that is lined. The polyurethane pads will be placed on the top of the liner (which will be on the top of the soil). The idea of the pads is to provide a uniform surface to place the tanks on.
I'm not sure where the 14" depth came from. Actually, that's what I'm trying to reconcile in my mind. Based on the tank volume and footprint, 25 psi will be more than adequate (with a factor of safety). I'm just trying to understand what role the thickness of the material plays. For instance, would a pad made out of the same material only 1" thick have the same load capacity? The area would be the same.
Another factor that I know must be considered is the wind loading on the tank. I would expect it to produce a non-uniform load distribution on the pad (which I can easily calculate).
I'm really just curious how one goes about calculating the thickness of a material (whether it be foam or concrete or whatever) required to generate it's rated compressive strength. That's the bottom line.
Thanks.
(Mechanical)
5 Feb 14 15:56depth would contribute to the total compaction or defomation
(Structural)
5 Feb 14 16:07One thing to keep in mind is how much stronger this material is than most soil (at least in Florida), with 25psi= 3600psf. But then of course you have to make sure you have good soil preparation, because unlike concrete the foam has no ability to span over a small weak or poorly compacted zone. Agree with dhengr- make sure the tank applies no more than 25psi to the foam, and make sure you have good soil preparation.
(Structural)
5 Feb 14 17:08Since the polyurethane is to be on top of the liner, within the containment, a tank leak into the containment will melt the polyurethane.
(Structural)
5 Feb 14 17:43the thickness will impact its deformation. The strength listed is typically put in context with a % deformation, ie 3.6psi @ 15 deformation. 14 14" thick piece will compress 14x more that a 1" piece. As may as three deformation numbers are sometimes listed, 1% 5% and 10%, with the 1% being the limit typically used for structural fill. That being said, your 25 psi is a high allowable load and likely associated with a 5% or 10% deformation as opposed to 1%.
Among other things I would be concerned about lateral loads, any time I use foam fill I typically have side walls that stabilize the slab that I am putting over the fill.
(Structural)
5 Feb 14 23:43Please ignore my last comment. Like some others, I was thinking polystyrene, which is soluble by petroleum products. Apparently, this is not the case with polyurethane foams, but I would want to make sure of that for the particular foam which is to be used.
(Mechanical)
6 Feb 14 00:01Polyurehane foam is soluble in air.Here in SoFla, the stuff decomposes to sticky crumbs, in unstressed, indoor, dark, air conditioned storage, in a year or two.I wouldn't use it for a structural anything.
Mike Halloran
Pembroke Pines, FL, USA
(Structural)
6 Feb 14 01:07I would agree that it is not a structural material when the conditions of use could lead to degradation and instability. Exposed edges will lead to loss of support at the periphery and subsequent failure. Is seems that the tank either needs only a very small layer to protect the liner, or it is an LNG tank, which I doubt would need lined secondary containment (from a practical viewpoint, not a regulatory one.) If the pad will have a perimeter stem wall around the foundation, it might be acceptable. If it is not a refrigerated liquid storage tank, I'd be inclined to use controlled low strength concrete or just run of the mill, cheap concrete as fill to build up the tank height. I guess it depends on the purpose of the foam.
(Structural)
6 Feb 14 09:22hokie66 beat me to the punch yet again. It will melt when exposed to petroleum products.As for analysis, I would look at it as an elastic layer in a multilayer system....similar to pavement analysis. You have the mechanical properties, plug and chug as a layered elastic system. Check Witczak or other pavement analysis/design texts for examples.hokie66...stop that! I read a post, I jump to potential failure modes (since that's mostly what I do), settle on something and then read the posts only to find you nailed it before me...happens too often!.....can I at least claim great minds think alike?
(Structural)
6 Feb 14 16:25Sorry, Ron. I'm usually the one who is subject to being beaten by time zones. But there seems to be some different opinions around about polyurethane foam. Maybe there is polyurethane foam and then there is polyurethane foam...I'm certainly not a plastics expert. Blocks or foam 14" thick are probably a lot different than the stuff that comes out of a pressure can??
(Mechanical)
6 Feb 14 17:05There are two basic kinds of polurethane foam; poly-ether and poly-ester.One is more durable.I forget which.
Mike Halloran
Pembroke Pines, FL, USA
(Structural)
7 Feb 14 14:28 http://msdssearch.dow.com/PublishedLiteratureDOWCO...Quick primer on some different products out there. There are specific foam panels designed for load bearing applications as the OP indicated. Besides in nearly every flat commercial roof, the other use structurally I have seen is to build up a structural floor slab to a different height without adding significant dead load. Then you top it with concrete.Also in Florida, injectable polyurethane (similar to Great Stuff that comes in a can) has become popular in the last few years as a soil improvement and foundation stabilization method.
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