I want to build a platform for a 400 gallon [about 3500lbs]. The platform will be 9x9ft. This will be 6 ft above grade. It will not be attached to the house. The hot tub is 90″ square. The extra platform is to allow for maintenance access around the perimeter. I used a programme at Lowes to design the platform. It suggests twelve [12] 6×6 posts. This is to cluttered for my liking. How to design a support with Four posts? Or what is the minimum number of posts? And how does this affect the dimensions of the other support members? Thanks Will |
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Seems ####bit cluttered to me too. add a couple of people to that load of water and you have two tonms spread over about 81 sq ft which firts a 50# load rating. Snow loads on some decks are required to be 70# so it is not all that hard.
First question to know though is what your local AHJ requires for a deck like this when you go to get the permit.
after that, it will help to know the elevation - how high up off the ground this will be
Then - do you need to have the posts at the perimiter or more directly under the pool? the more directly under the load the posts sit, the less dimension the joists need.
The only way I can imagine that this program came up with that result ( other than it being a piece of crap maybe) is that you made a mistake in setting the parameters, like calling for 2x4 joists set at 24" oc instead of 2x10@ 16 or @ 12" oc or telling it that you wanted no beams
I'll bet that's it. The program is probaly so cheap it does not have a tool for sizing beams and relies only on the rim joist so it caled out6 all those extra posts.
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Thanks for your response.
1.The platform will be 6ft above grade.
2.I would prefer that the posts be placed at or close to the corners. That means no direct post support under the tub.
3. Here in the country we do not need building permits or seek them.
4. Yes, the programme used to design the deck did not allow many options. No choice permitted for the joists or beams but 2x6s
Thanks
Will
2x6 is totally inadequate for ANY porch deck, IMO.
No Damn wonder that it called for that forest of trees to support the thing.my pref would be four posts at corners and two supporting an accessory beam directly under the load. That would make it fine with 2x10sSince you are trying to free span and maintain space for something, you should hirre a qualified professional to run the calcs for you.
But UI don't see what kind of storage you can have with this six feet off the ground. Counting the joists and neams under, you will only get as little over three feet of height to drive a wheelbarrow or mower into. That will be pretty expensive space per cubic foot.
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This is not something I would list under my area of expertise but I see a problem with 4 posts. Time takes its toll on exterior framing, rot, insects, frost, etc., can affect the performance of the structure. The trouble I see with 4 posts is, if 1 post fails the entire structure fails. I agree with Piffin's snow load analogy, yet snow loads come and go, and your deck will have to perform 24/7.
Though you are building your deck for today, you are in reality building it for 8 or 10 years from today.
A while ago there was a similar question, but the deck wasn't as high and someone suggested putting the tub on the ground and building the deck around it, creating a "sunken" hot tub. You could do something similar, but with the tub resting on a smaller platform on the ground with the deck around it.
They were probably sizing 12 6x6 posts without footings, to provide necessary bearing area. You can install footings and reduce the number of posts. But precisely how many of what size requires some engineering.
The important thing, at 6' high, would be cross-bracing to eliminate absolutely all wobble. Get a little wobble under a two-ton load and you're in trouble.
Here's what I did for a customer several years ago. Built the deck around part of the hot tub and set the tub on a poured pad. Nice and stable, no worries about any weight on the deck, easier to enter and exit the tub.
Thanks for the response. This is not an option. We have a deck off our bedroom at about 8' elevation. We plan to add platform support alongside the deck so that the top of platform is 2ft below the deck level .This will place the top of the hot tub 18" above the deck for ease of eggress and ingress.
Thanks
Will
You'll just have to add a lot of lateral and diagonal bracing then
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Will,
Check out this site - this tool is MUCH more sophisticated that the POC you used at Lowes. You need Excel, but it will give you all the load calcs for posts and beams - good way to play with options such as 4x4 vs. 4x6 vs. 6x6 posts, different beams & joists.
http://www.wwpa.org/_techguide/subscribe.asp
Wayne
Thanks Wayne:
I will take some time and work with the source you provided.
Your input is appreciated.
Will
Here's an important point that might not have been considered yet.
When I did my hot tub, the manufacturer was very clear concerning the support beneath the tub. It needed to be either a 4" concrete slab, offering continuous support, or a suitable substitute. Their warranty against leaks would be voided if the floor system flexed AT ALL.
In addition to that need for stiffness, there is the wieght concern that others have mentioned.
So, I could have either built a deck that was literally as stiff as 4" of concrete, with supports for two or three tons of dead load, or I could change my design.
The attached pic is the result, (since I was having too much trouble fitting all the necessary posts into such a small space.) Do yourself a favor. Get creative and figure out a way to put that wieght on the ground, or on a concrete foundation that transfers the load to the ground.
Unless you're the lead dog, the view just never changes.
Will:
You're spending quite a bit of time trying to work this out for yourself, and clearly you're not an engineer/architect. In this thread, some of the best advice given so far (IMHO), which I now wish to reiterate and bring back to your attention, came from Piffin. Specifically, he wrote:
"Since you are trying to free span and maintain space for something, you should hire a qualified professional to run the calcs for you."
That qualified pro would likely get all you need done in about an hour, or less. Well worth your while to get a design that's suitable to your aesthetic desires of 4 posts, and simultaneously appropriate for obtaining a structurally sound design.
Good luck with your project! Hope you'll post pics when you're done.
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