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Old 04-09-2013, 11:18 AM   #1
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Default Pouring a grade beam. Engineers and concrete guys unite!

So I am planning a rear deck build, and the design considerations are:

"Ground level" height
Designed to integrate into a future round patio, such that the distal edge of the deck has curves in it which resecces the distal edge of the deck upwards of two feet at one corner and again in the middle.
Installing french doors in place of an existing door and window with steps up from the deck level to a door landing
Composite decking material
40 Ft width (entire width of house, leaving approximately 5ft 6in to property line on both sides)
12 Ft depth
Possibly 2x6 joist boards
Ledger board attatched to CMU foundation wall with epoxy anchors
There is no appreciable elevation change from the foundation to the distal end of the deck.

My challenge is the foundation/framing of this deck. From my research, it seems that two suitable options are:

1: Install two rows of support piers to local code frost line depth (36"), run either treated beams or treated plates supporting untreated beams across the piers, and install the joists on a level system with hangars. One row of piers would be > 5ft away from house foundation, second row of piers would be at far edge of deck. A cantilevered design will not be possible with joists mounted at the same level as the beams, so all joists on curved edges will need to be supported from below at the edge with a separate pier.

2: Install a single full-length (40') Steel-Reinforced Concrete grade beam with near edge at approximately 9'4" from the house foundation, and a far edge approximately 10" from the foundation for a total thickness of 8" (or wider, but not more narrow) and total height of 12". Grade beam footings will be spaced at either 5' or 6' on center and extend to below the frost line, while the beam itself will only be about 12" below grade. The grade beam will be topped with a ground-contact rated 2x8 plate anchored to the concrete using J-Bolts. Because of this design, the distal edge of all joists will extend a maximum of 2' beyond the distal supporting edge of the treated plate which is the maximum allowed cantilever for deck design in my area, while the near side of the curves will extend to exactly the distal edge of the grade beam plate, significantly simplifying the framing of the deck. Additionally, the local code for single 2x6" deck joists specifies a span of no more than 9'5" for 16" O.C. construction. The near side of the beam is approximately 9'4" from the foundation, and adding in for the thickness of a 2"x6" ledger board further reduces that span to a little over 9'2"

Problems:

My engineering design experience is limited to my training as an Army Engineer officer - which is limited to say the least. Army design considerations include a 2-year life expectancy and expedient construction methods - which means if I was designing this deck for the Army, the foundation would probably consist of two pressure-treated 2x6s layed flat on the ground at each end. Obviously, I'd like my home deck to last longer than 2 years.

Documented instances of the use of grade beams to support deck construction is extremely limited on the interwebs...because nobody does it apparently. This means that it's hard to find good examples of what the generally accepted engineering practices are.

I don't know if the grade beam itself is supposed to extend below the frost line because it is buried in the ground, or if only the footings need to extend to this depth. I've read that the beam itself need not extend to the frost line, and I've read that it should. I've also read that installing 2" thick foam beneath the beam is a solution to the threat of frost heave.

Any insight on the options? Is there a third option that I'm missing (that isn't "why the **** don't you just install a patio?") Are my engineering figures correct?
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Old 04-09-2013, 11:42 AM   #2
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I'd do the pier method. sono tubes of the diameter and depth that match your local code with pressure treated 4x4 posts. Use those nifty simpson brackets I cant remember the part number anymore for you 2x6 (local code might require up to 2x10 for the supports which connect to the piers, 2x8 wouldnt be a bad idea) PT stringers. Remember with composite decking you want your stringers 12" on center to avoid it feeling spongy in the summer. Use the simpson brackets that are correct for whatever you're doing they're way ******* easier and stronger than trying to do carpentry. If you attach the deck the house, be sure to seal it properly (look it up) and bolt through the ledger board on the house, not lag bolts. Or give enough gap to the house at the ledger board that any water that gets down there will just run out (my preference, does require putting another run of posts next to the house).

Other notes, hidden fasteners are the bomb for decking. Use the correct ****** ******* screws, old galvanize + new PT = your two year army life span. Plastic trim boards are awesome, and so is composite railing. Spend the extra money to buy the better brand of composite railing so it doesnt feel like cheap plastic ****. And random, some brands to composite decking smell like toast when you cut them.
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Old 04-09-2013, 02:05 PM   #3
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Old 04-09-2013, 02:10 PM   #4
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I was recommending option 1.
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Old 04-09-2013, 03:17 PM   #5
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I will not be using posts for this construction. The purpose of posts is to get the deck off of the ground, the purpose of my deck is to be as low as possible. If I use piers, I will supporting the beams with, at most, a treated plate, which is a 2-by laid flat on top of the pier. The beams themselves would likely be a double 2x6 design, with piers spaced appropriately for the loading of the 2x6. You're right on the 12" spacing, I forgot to put in the original post that, although I was using max span numbers for 16" OC construction, my intention was to build with joists 12" OC. The deck is going to be attached to the house, but as in the original post, the ledger board for the deck will not be attached to the band board for the house - it will be connected via epoxy anchors to hollow CMU at ground level, approximately 2.5' below the elevation of the ledger board of the house. I am a fan of using hidden fasteners so long as they allow me to maintain 1/4" spacing between deck boards minimum for ventilation, as the low ground clearance will require a different primary ventilation system than raised decks with lots of room to breathe. Finally, I have no plans for perimeter railing as nearly the entire walking surface will be ~8" above grade with the exception of the landing and stairs down from the outswinging french doors - the stairway/landing design is going to be a semicircular or semi-octoganal landing with stairs cascading down the full perimeter to the deck surface.
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Old 04-09-2013, 03:32 PM   #6
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Sorry I guess the use of distal, which I always associate with flower reproductive system anatomy, was throwing my concentration. The thing I don't like about your option 1 is the thought to use un-treated beams. Thats not going to fly if you want it to last, though you can get a bit more life from the un treated beams if you put tar paper over the top of them before you put the decking down. I'm a little concerned with how the cantilever is going to work with the grade beam design, would they joists be continuous pieces (makes it seem like it would be taller than your goal)?

I did assume you'd be running a railing, is 8" below the minimum height for needing a railing? And I wonder how many drunken uncles will twist a knee stumbling off it at the family bbq.
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Old 04-09-2013, 03:50 PM   #7
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distal is usually referring to relationship to your body. so unless the deck is strapped to you...
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Old 04-09-2013, 05:27 PM   #8
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lol, yes, distal/proximal is relationship of body parts relative to the heart IIRC.

It seemed to make sense to me when I used it in the context of building a deck.

As far as railings go, City of Columbus requires railings for decks above 30". 8" shouldn't be a problem. I wouldn't ever place untreated beams directly on concrete or the ground, which is why, for option 1 - I would use either treated beams on the concrete piers to support untreated joists, or else I would use treated plates on the concrete piers to support untreated beams. The whole wood system should float an inch or two above grade with concrete piers either being flush with grade, or marginally raised above it. I figured I'd be pulling some topsoil out in favor of gravel over fill dirt anyhow, engineering the soil to be as high as I want it to be.

The cantilever design assumes that the 2x6s will be continuous 12-ft' lengths hung from the ledger board crossing the grade beam, and cantilevered from the grade beam to the band board. Span from the ledger board to the treated plate atop the grade beam would be about 9'4", and span from the grade beam to the band board would not exceed 2'. Currently, my planned height for the grade beam is in line with a planned height for simple piers - the top of the grade beam would be either flush with grade, or no more than about an inch above grade.

Result in height:
Grade: 0"
+ Flush grade beam: 0"
+ 2x6" Joists: 6"
+ 1 1/2" thick decking: 7-1/2"

And in the future, I plan on laying a round patio adjacent to the deck, with a diameter of 20' with a small fire pit in the middle. I'll probably gain an inch of grade height making the step down only 6-1/2" another stone walkway to the deck from another side will also make that step about 6-1/2". All other edges will eventually be landscaped so as to provide a walking suggestion to the mentally incapacitated.

For those wondering why I don't just build a standard height deck: If I threw a rock at my neighbors house, it would bounce off, come back, and hit my house. When I'm entertaining friends on a 3' high deck, that 6' privacy fence suddenly doesn't feel very tall or private anymore. My backyard (and deck) must be an aesthetic escape from the neighbors, otherwise it's pointless for me to build.
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Old 04-09-2013, 05:40 PM   #9
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I was talking against putting any untreated beams in the deck at all. At least with prices up here, the cost difference on this size deck would be a no brainer to never have to worry about the wood rotting from water splash or being eaten by insects. Almost all the deck orders I used to build for contractors only included treated wood or composites.

Also, what decking are you getting at 1 1/2 inch? Most standard decking is closer to 7/8". 1 1/2" is the final milled "2 inch" direction of a 2x4.
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Old 04-09-2013, 05:50 PM   #10
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Quote:
Originally Posted by Leafy View Post
Sorry I guess the use of distal, which I always associate with flower reproductive system anatomy
That would be a pistil.
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Old 04-09-2013, 06:06 PM   #11
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Quote:
Originally Posted by Leafy View Post
I was talking against putting any untreated beams in the deck at all. At least with prices up here, the cost difference on this size deck would be a no brainer to never have to worry about the wood rotting from water splash or being eaten by insects. Almost all the deck orders I used to build for contractors only included treated wood or composites.

Also, what decking are you getting at 1 1/2 inch? Most standard decking is closer to 7/8". 1 1/2" is the final milled "2 inch" direction of a 2x4.
Ahh, I understand this point now for no untreated beams. It makes sense and supports my plan for a maintenance free deck.

1 1/2 inch decking is a number I pulled out of my *** because I haven't put nearly as much research effort into the actual decking as I have the framing and foundation. 7/8" inch will indeed be the thickness of the decking.
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Old 04-09-2013, 09:28 PM   #12
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Paging Vashtheconcretepourer---
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Old 04-10-2013, 09:29 AM   #13
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Quote:
Originally Posted by sixshooter View Post
Paging Vashtheconcretepourer---
I forgot who our resident concrete guy was. Hope those security cameras are working out for ya, Vash.
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Old 04-10-2013, 10:04 AM   #14
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I used Azek decking and have been very happy with it. Also got the hidden fasteners - the kind that have little plugs to cover the holes. You don't even notice them.

Deck guy was very adamant not to use Trex products. They had problems with them in the past and the company wouldn't honor the warranty.
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