elevated concrete deck

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Old 03-24-14, 04:08 PM
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elevated concrete deck

I'm in the planning stage of building a porch on the front of my house. I plan on doing most of the work myself - never built an elevated concrete deck so I need some good sound advice on how this thing should be constructed. I've attached a diagram so you can get an idea of what I have come up with so far.

There will be 6 coarses of 8 inch block on a steel reinforced footing. My local supplier has 22 ga. steel deck pans and suggested the pier location in the center for support. Are the piers next to the foundation walls OK or should I consider a beam of some kind attached to the foundation wall supported by piers? Also, for lateral stability, should the pans be attached to the blocks on top? My supplier said to lay the pans on top of the blocks. Seems to me that will just invite unwanted lateral movement.

Someone else suggested that I pour the deck in 2 pours to incorporate an expansion joint instead of cutting one green. That might be a good idea. And if the valleys in the pans are 3 inches deep how thick should I form the deck?
 
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Old 03-25-14, 01:46 AM
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1st thing i'd do is find a PE who's got insurance AND a current license,,, not sure why you'd want to use stay-in-place forms when plywood & supports are the traditional forming method,,, stay-in-place is for bridge decks & tall buildings we can't leap in a single bound,,, what did your 'friend' say about rebar ?

look at a section of bridge more closely - there's no joint either formed OR sawed - each piece of the deck is monolithic.

doing this isn't difficult but doing it correctly's VERY important,,, your elevated deck will stay in place by gravity & some little pins preventing lateral movement,,, unless you're building in cairo - then all bets are off down there - probably salem, too
 
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Old 03-25-14, 07:14 AM
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Hopefully BridgeMan will be along to answer you more professionally. I built my front porch with the commercial, stay in place corrugated decking and it worked quite well. I had a span of 8' and also did a rebar grid for extra strength. The corrugated panels were easy to work with without having to build, support and then strike a temporary wood deck.

My porch was engineered in from the beginning and had a steel beam at the house to support that end of the steel decking. Will you be supporting your patio from the house or your piers? If they recommended piers I assume the house will not be used for support in which case you probably do need a beam to provide continuous support for the decking. It's quite strong length wise but needs continuous support along it's shorter dimension.
 
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Old 03-25-14, 07:54 AM
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Here in West Virginia we are always concerned about ground movement due to underground mining. Water is a big concern also. If you notice, the drawing shows no access beneath the deck to remove forms as only 3 coarses maybe will be above ground. Backfilling is an option but it would act like a sponge and put pressure on the foundation wall. That span of wall has no lateral support inside the crawlspace. Stay in place forms seemed like a good solution. As far as finding a PE ... my money bags are just damp ... not full ... doing it myself is the only option.
 
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Old 03-25-14, 08:20 AM
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Steel beam

Thanks for the input Pilot Dane. Its much appreciated. Since this porch was an afterthought, it wasn't engineered in so piers next to the foundation wall seemed to be a better option to me rather than attaching it somehow to the foundation wall. My supplier suggested mid support in this 12 ' span. Do you think it would be neccessary to use a beam there, too?
 
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Old 03-25-14, 09:51 AM
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I think you need a beam at the house and across the center so the decking would be supported across the full width creating 6' spans the entire length. If you pick up a piece of the decking you'll know why. It's really strong lengthwise but can curl up very easy in the other direction.
 
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Old 03-25-14, 04:14 PM
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Not being that familiar with them, I suspect that metal form panels designed for concrete building floors would typically require mid-span support; especially since that's what your supplier, who's familiar with the product, also recommended. The much heavier bridge deck forms that I'm familiar with never require support at their mid-span, and are capable of spanning 14' or more with negligible deflection (while supporting the weight of 10" of concrete). I think I'd use just one steel or heavy timber beam at mid-span, supported by the concrete block piers as shown in the diagram. I'd look at resting the tops of forms on the new block porch and house foundation walls, if possible. Expansion joints shouldn't be required for such a short (less than 20') placement, while contraction joints sawcut or grooved at the one-third points would ensure any transverse cracking that happens will occur there, in a controlled fashion.

For simplicity and as a precaution, I'd nail preformed bituminous joint filler all around the perimeter, set at the desired thickness above the tops of the metal forms where you want finished-grade concrete to be. Doing so will make for easier puddling, striking off and finishing, as the top of joint filler will be the top of concrete elevation. For 3" deep valleys, I'd go with 3" above the flats, making the floor an average of 4-1/2" thick. That computes at less than 4 C.Y. for the dimensions shown (and suggested above), so one complete placement would be the way to go. You'll need a temporary working platform at the front and sides of the porch, so you're not standing in the mud trying to finish it. I wouldn't use rebar, unless the form metal is really flexible and flimsy.
 
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Old 03-27-14, 07:14 AM
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Thanks BridgeMan45 for your advice. As a welder, I know that 22 ga. steel is flimsy so it must have good support at each end as well at midpoint to reduce deflection. That being said, I'll most likely use a grid of rebar in the concrete also. Looks like I'll have to set the forms on 2 steel beams - 1 beam next to the foundation wall because it would be impossible to set the forms on the foundation now that the house is set. Thanks again.
 
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Old 03-28-14, 11:56 AM
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Don't make the common mistake of laying transverse (to the pan corrugations) rebar directly on the pans. Doing so compromises the rebar's ability to be completely "locked into" and integral with the concrete. Small, impervious spacers should be used, as I don't think bar chairs or dobies are made that short. Welding some short rebar stubs perpendicular to the transverse bars every 3' or so (at the flats locations) would work. You also may want to thicken the slab from the 3" above the pan flats dimension I suggested earlier, as you don't want rebar closer than 2" to the wearing surface of the finished concrete.
 
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