My latest geotechnical report recommends the use of a raft/waffle foundation due to expansive soil.
The building will be a 75′ x 100′ metal building on concrete slab.
A raft foundation, as I understand it, is basically a reinforced concrete slab, sometimes with an edge turndown to prevent skidding.
A waffle foundation, I believe, consists of a grid of strip footers that are reinforced and interlock each other.
Therefore, I assume that a raft/waffle foundation is a combination of these two. Am I on the right track?
Anyone here have experience with foundations like this?
Heck If I know….
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Another consideration on this project is the desire to install radiant heat in the floor slab. I am wondering how you would create the thermal breaks to keep the heat from going to ground.
Heck If I know....
this is interesting - a term I have not heard before, but the description is like some I have worked on. I did not5 know that there was a dynamicv that helped it deal with expansive soils. But for the Q on the radiant included if this thing looks on the bottom like a big waffle or say a coffered cieling. I would suppose thqt you would trench the middle beams and then only form the perimiter, chairing your rebar in both. I would lay in 2" XPS where the larger flat sections are, and just use astrofoil ( or whatever they call it down your way - foil covered double bubble wrap) fit into the shape of the beams. It is flexable and while nowhere near as efficient at resisting heat loss, it would be far better than nothing.
the other thing that would help is having sand gravel mix compacted under the slab and us excellent drainage around the perimeter, because the dryer the soil is, the better your thermal package will perform. Running water flowing slowly under the slab will suck a lot of heaat away from it, so if your soil carrys ground water, keep that in mind.
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You can read about one version of a "waffle" slab by going here http://www.michaelholigan.com/projecthouse2001/week2.asp
What they are discussing at that site, is what is called, in industrial and heavy commercial work, "pinned" slabs. The pins can be drilled caissons at diameters in the 4 to 6 foot range, belled out on their bottoms, and drilled way down to get to something substantial enough. Therma-Tru had an Arkansas GC pop down hundreds of these at depths up to 70 feet, when building their new 400,000 sf Oklahoma door plant.
What is apparently being discussed here in this thread, is what the structural guys call a "floater." You may recall a good article in a past FH (when they actually did articles like this) in which a builder did one of these in a marshy area.
Whoever started this thread with the q about floaters on expansive soils, should go into the FH archives and see that article.
Thanks. The first time I saw something like this,it was in San Antonio and I was a whole lot less old than I am now, so I didn't know much, but I was on the crew of laborers setting steel in trenches for a warehouse type building of a couple acres. There were what could be called piers about 20' OC and three feet diameter by six feet deep. These were interconnected by trenched beams about two feet wide and three feet deep. I had moved on by the tiome they poured it.
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Thanks for the information. Do you know how to access the FH archives?
There is another commercial project underway less than a mile from my report's site, they have a foundation design similar to what you call a pinned slab. They are drilling 'piers', or caissons, 25' deep to support grade beams and a slab. These piers are not belled at the bottom, however.
They started drilling yesterday, and were placing their rebar cages. They hit water at about 6'. The engineer won't allow them to displace with concrete. Last I heard they were trying to figure out what to do next.
Another problem for them is the fact that the only local concrete supplier has only 15' of pump hose (before the knuckle, which is too big to fit inside the rebar cage), and the engineer won't let them drop the concrete that far, even if they had a dry hole.
No one in the area has a pump capable of pumping out the water from that depth.
Life in the sticks, it's wonderful. :)Heck If I know....
There is a way to research the mag article archives, but I don't know it.
Are you the engineer-of-record for this project? If not, is there an engineer-of-record?
No, I am not the engineer-of-record, and there is not one yet. My engineering career ended after one year in college.
A local church has aked me to compile costs for an assembly hall; selecting an 'engineer-of-record' would be one of my tasks, if selected as the GC.Heck If I know....
Piffin, the engineer is calling for a capillary break of either 6" gravel, or 10 mil polyethleyne.Your suggestions as how to create the thermal break are good, here we call it 'foil bubble wrap'. I would be warming up an awful lot of concrete, wouldn't I?
I was thinking that the foil bubble wrap was as efficient as 2" XPS , considering real world applications. The XPS tends not to get placed without numerous gaps , whereas the mats of various types are normally taped tightly together. Your thoughts?
Thanks.Heck If I know....
On smaller structures here, for a floating frost protected slab, we use 18" of inch minus gravel and plastic and the foam. His requirements sound pretty minimalist to me.RE comparing the thermal break of one vs the other - the foam board is dense enough to support the loads - possibluy even more load bearing ability than the bubbles.But the bubble wrap bases a lot of its claims for R-value on the foil reflectivity of the surface in designing their own tests for heat loss. For a radiant barrier to have any appreciable impact, there must be an air void or vacumn in front of it. When material is directly in contact with the foil, it reverses effect and becomes a bit of a conductor rather than radiant barrier. You could prove this to yourself by heating a bricj so hot that it would scar and blister you to hold it. Put some AL foil between it and your hand and you can comfortably get to within a few inches of the brick having at least an inch on each side of the foil. But move the foil to be in contact with either the brick or the palm, and see how much changes.So the main strength of the bubble wrap in this situation is the trapped air in the bubbles, and the moisture barrier provided by the plastic material. I don't belive that this will compare favorably with the resistance to heat loss of the foam. It is the flexability of the product that makes me recommend it. I have done several slabs heated with pex and insulated with Foam from 1-1/2" to 2". I haveonly done one wioth bubble wrap under it. The owner was convionced that he had found a "better way" and it sounded fine to me back then so we used it. His reports since then of heating efficiency have been disappouinting compared to the compliments I hear from other owners/residents of homes done the other way.
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I am convinced that foam, if applied correctly, is the better insulator. I have seen some horrendous installations, with broken foam, gaps between sheets, etc. , all of which is no fault of the foam. Just being a realist as it pertains to the 'average' jobsite. At this time, most builders in the area are using the bubble wrap.
I am more impressed by the reports from your owners. That evidence alone will have me insisting on the foam. The devil is in the details, though, it has to be done right.
As for the frost/capillary barrier spec: Frost depth at this site is 18", so I suppose the design needs aren't as stringent as yours.Heck If I know....