Suspended Slab

 
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Old 05-20-2011, 07:45 AM   #1
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Suspended Slab


I've always been curious about how to build a suspended slab, but I've never been able to find a good website explaining the steps taken to construct one. Some of the questions I have are:

How thick are they on average?
What is the spacing and thickness of the rebar?
What type of forms do you use for the underside and how often do you brace them?
What are the bracing methods?
What are your limitation?

I understand these are questions for an engineer, but I'm sure some guys out there can help give me a head start. Thanks!
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Old 05-20-2011, 08:30 AM   #2
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Re: Suspended Slab


When I've done a suspended garage floor we've used steel under the floor. The one three car garage we had 2 large steel beams and then steel flooring on top of that (I cannot remember what its called) but it looks like corrugated steel. Then the concrete just went on top of that. One thing I learned in doing it was to caulk the saw expansion joints we made in the garage floor...otherwise when a car pulls in with snow on it, it melts and eventually leaks in the room below.

Hoping to do another one, I'm talking with one HO about it since it will probably be just about as expensive as filling a LARGE garage with gravel.

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Old 05-20-2011, 08:48 AM   #3
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Re: Suspended Slab


There are a few ways to do this, but it depends on what is going below it.

Option #1: Use steel beams that are pocketed into the foundation and then spot weld metal decking on top of that. Then rebar. Then pour the slab. This option isn't very waterproof but if it is only interior, not an issue. If it is going to have cars parked on it and a theater room below it, not a good idea. Also, this way is tough to finish the ceiling below and condensation on the metal can become an issue if it is exposed to cold air above and warm air below.

Option #2: use a suspended pre-cast slab system. Hanson Eagle Pre-cast is who does it around here. There are a few others I'm sure. They bring in pre-cast slabs and crane them into place (you leave a keyway in the foundation to accept the slabs). Then, you waterproof this with a high quality waterproofer and caulk all the seams. Then you pour a slab on top of this for your finished slab. This costs about the same as option #1 in total price, maybe a tiny bit more, but is waterproof and will allow you to put that theater room or expensive equipment below it without nearly the worry of leaking.

Hopefully that helps.
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Old 05-20-2011, 11:26 AM   #4
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Re: Suspended Slab


You guys leave limited options.

Option 3 and usually the first option. Is to pour everything in place. Footings, columns, beams, and slab. Typically pouring the beams and slab at the same time.

As for forms you can go from all wood or use one of the exotic systems like Doka, Efco, Peri, Aluma-joist Or HV Titan.
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Old 05-23-2011, 07:03 PM   #5
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Re: Suspended Slab


at my townhouses they put 2x4 joists with plywood and braces and then put 6" square WWM poured it and ripped the floor out below. Same thing with condos sort of, they make a form with steel joists and columns and sheet it with MDO and pour and take the floor out
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Old 05-23-2011, 07:43 PM   #6
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Re: Suspended Slab


Quote:
Originally Posted by JustaFramer View Post
You guys leave limited options.

Option 3 and usually the first option. Is to pour everything in place. Footings, columns, beams, and slab. Typically pouring the beams and slab at the same time.

As for forms you can go from all wood or use one of the exotic systems like Doka, Efco, Peri, Aluma-joist Or HV Titan.
And you leave limited options as well.

I don't know that I agree with your option 3 being the first option. It is also the most difficult IMOP and involves the most form work. All the other options involve materials that stay in place, thus no need to buy a bunch of formwork that you may not already have laying around, and no need to wreck and remove same.

Option 4: Post Tension slab- similar to option 3 but cables or "tendons" are stategically placed and tensioned with a jack after curing of the slab and before forms are removed. Definitely involves a structural engineer, but allows for substantially longer spans and columns and beams are minimized.

Option 5: Composite slab: In this scenario, steel beams and metal deck are used with "Nelson Studs" welded to the beams thru the metal deck on 6", 12" or whatever centers the engineer (again needed) deems appropriate. This approach combines the strength of the steel beams with the concrete so that they work as an unit. Again, this increases allowable spans between beams and columns, in this case, steel.
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Old 05-23-2011, 07:47 PM   #7
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Re: Suspended Slab


Mudpad has it.

And everywhere I've been, a suspended slab can only be a PT deck, because the slab is actually suspended from the PT cable tendons
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Old 05-23-2011, 07:58 PM   #8
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Re: Suspended Slab


Quote:
Originally Posted by merrittbuilder View Post
I've always been curious about how to build a suspended slab, but I've never been able to find a good website explaining the steps taken to construct one. Some of the questions I have are:

How thick are they on average?

Depends on the loads and spans. I have poured elevated slabs as thin as 3" and as thick as 2'. (that one was supporting an elevated swimming pool)

What is the spacing and thickness of the rebar?

Again, depends on loads and spans. A recent composite slab on deck I did was # 4's on 10" centers EW, have done conventional reinforced slabs with two mats of #9's at 6" centers. Post tension cables reduce rebar substantially.Totally up to the designers, and I wouldn't second guess that one.


What type of forms do you use for the underside and how often do you brace them?

3/4 formply on 10k shoring

What are the bracing methods?

10K shoring is the industry standard, you need another engineer to design your shoring.

What are your limitation?

What I always tell my guys is there are two rules to forming concrete. #1, it has to hold concrete. #2 you have to be able to wreck it.


I understand these are questions for an engineer, but I'm sure some guys out there can help give me a head start. Thanks!
That's about all I know about elevated slabs, except to say that when they fail in the middle of a pour, it is not pretty.
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Old 05-23-2011, 09:29 PM   #9
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Re: Suspended Slab


You can check out Hambro systems, coreslab as Mudpad said, Insuldeck.

There are loads of options, I always try and see what is 'locally' available for it to be cost effective
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Old 05-23-2011, 09:44 PM   #10
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Re: Suspended Slab


i was on a commercial site on saturday and like they said above, there was corrugated steel decking then conc. I think the steel decking will stay though
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Old 05-24-2011, 01:43 AM   #11
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Re: Suspended Slab


when I was doing it we would pour all the columns first on a floor, then when all the columns were ready start with flytables or free forming with aluminum scaffold frames, braces, then aluma beams on top, 4x6 stringers on top of that then 3x4 form ply on that,

the "beams" would be about eight feet to the next one, which would be a difference in width from sixteen inches to 3 feet, each "beam" centered to a column, each flytable would be the width of one beam. Once the floor was poured the tables would be rolled out, or free form taken down, go up to the next floor. Lots of rebar, can't tell you specific spacing.
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Old 05-24-2011, 02:11 AM   #12
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Re: Suspended Slab


Quote:
Originally Posted by mudpad View Post
And you leave limited options as well.

I don't know that I agree with your option 3 being the first option. It is also the most difficult IMOP and involves the most form work. All the other options involve materials that stay in place, thus no need to buy a bunch of formwork that you may not already have laying around, and no need to wreck and remove same.

Option 4: Post Tension slab- similar to option 3 but cables or "tendons" are stategically placed and tensioned with a jack after curing of the slab and before forms are removed. Definitely involves a structural engineer, but allows for substantially longer spans and columns and beams are minimized.

Option 5: Composite slab: In this scenario, steel beams and metal deck are used with "Nelson Studs" welded to the beams thru the metal deck on 6", 12" or whatever centers the engineer (again needed) deems appropriate. This approach combines the strength of the steel beams with the concrete so that they work as an unit. Again, this increases allowable spans between beams and columns, in this case, steel.
Well your option 4 is what I was eluding to. And forming a deck with Titan HV is a dream to put together.

And option five requires a welder and specialty equipment to weld nelson studs. But I am pretty sure that is what Monticello was talking about.

Like I said it can all be done with wood or manufactured forms. And most of it can all be rented even the 3/4" mdo ply.


BTW Merrit I suggest the book Formwork For Concrete Structures by Peurifoy and Oberlender.
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Old 05-24-2011, 06:38 AM   #13
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Re: Suspended Slab


where i am you can get a 48pc skid of 13/16 formply for $36.75
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Old 05-24-2011, 08:04 AM   #14
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Re: Suspended Slab


Can only speak for the commercial side. Symons forms and Safway shoring. Both can be rented, and for us it's been cheaper to have their army of ants set up the shoring. They run circles around guys that don't do it every day.
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Old 07-03-2011, 07:39 AM   #15
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Re: Suspended Slab


I agree with aggie67 that experienced workforce doing this is something that will pay back over time.

Last edited by betongplatta; 07-03-2011 at 07:44 AM.
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Old 07-03-2011, 07:12 PM   #16
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Re: Suspended Slab


I'm thinking framingpro nailed it.

What was the span of the 2x4 joists? How thick was the concrete? Rebar?

Look at my avatar.
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Old 07-03-2011, 07:15 PM   #17
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Re: Suspended Slab


what is 6" square WWM?
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Old 07-03-2011, 10:14 PM   #18
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Re: Suspended Slab


cleveman -

That is probably a material that is/was also normally referred to as 6/6 10/10 mesh, which is welded wire mesh (6" o.c. each way and 10 gage wire). - Tough to unroll, flatten and keep in place, but very effective.
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Old 07-04-2011, 05:58 AM   #19
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Re: Suspended Slab


Quote:
Originally Posted by cleveman View Post
I'm thinking framingpro nailed it.

What was the span of the 2x4 joists? How thick was the concrete? Rebar?

Look at my avatar.

i think about a 5' span, 4 or 6" concrete and WWM not rebar
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Old 07-04-2011, 09:03 AM   #20
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Re: Suspended Slab


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cleveman -

Tough to unroll, flatten and keep in place, but very effective.
That's if you buy it in rolls. We buy it in 8' x 16' sheets. Slightly more expensive but a huge labor saver.

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