How do you calculate the loads a deck can hold. In the end a hot tub will be on it?
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For something that critical to safety, and considering the number of people hurt each year from deck collapse, it is worth it to have an engineer do the calculation for you. Every part is vital -- footings, fasteners, spans, corrosion resistance, lateral stability.
More people are injured or killed from decks than most people realize. Frank Woeste, professor emeritus of wood engineering at Virginia Tech is quoted in JLC as saying, "Except for hurricanes and tornadoes, more injuries and deaths may be conected to deck failures than to all other structural failures stemming from typical residential code loads, including seismic and snow loads."
The math is trivial. I) Determine the proper size of joists for 40#/sqft live load and 12" joist spacing. You can look up the sizes in a code book.b) The hot tub should indicate the total weight (tub, water, and people) and the area of the tub (sqft).3) Divide that weight by the sqft of the tub. This should give you a load in #/sqft.II) Your joist spacing in inches needs to be 40/(the result from "3)")*12. For example: a 3000 weight over 64sqft. step "3) gives 3000/64 = 3000/64 = 47#/sqft.And the joist spacing is 40/47*12 = 10".---If you are lucky your AHJ will accept this math as prescriptive.
Perhaps I'm opening myself up for criticism here, since I'm no Physical Engineer, but I think that your calculations might leave the deck weak.
If I assume that the deck would be built with southern pine, then the span charts for 40psf live load, with 12" joist spacing tell me that I can use a #2-graded 2X6 that spans up to 10' - 9".
Using your calcs, the spacing changes to 10", but I still have 2X6's spanning almost 11 feet, holding a hot tub that weighs 3000# -- probably sitting in the middle. And even with the 10" spacing, that sounds very bouncy to me.
I agree with the posters who said that this is a perfect case which demonstrates the need for an engineer's stamp.
Unless you're the lead dog, the view just never changes.
PE stands for Professional Engineer... :-)
When you looked at the span chart I think you assumed that the load would be spread evenly over the 11' span. I think probably it would not be. Maybe if you had an 11' hot tub that held 6" deep of water :-)
I wouldn't use 2x6 joists for 11' span joists on any deck, much less one that had a hot tub on it. Even 2x8s at that span can give border line performance - again - without a hot tub.
BTW - I live in Apex - actually "in the County".
Edited 11/25/2006 7:24 am ET by Matt
Don't feel hesitant about disagreeing with GHR here. He/she has consistantly shown lack of complete consideration of design standards in advising, proving how daangerous it can be to try getting free engineering advice online.The maximized poiint loading for something like this what correct to point out. You only error is using the word bouncy. I doubt that a deck with 2x6 joists would do any bouncing under the load of a hot tub. It will restrict its movement to sagging or falling only! LOLAnother serious issue for the OP to consider is that since most decks are built with PT framing, and since new ACQ treatment has been shown to cause advanced early deterioration of fasteners, this deck should be built using SS or double hot dipped galvanizzed hangers and nails. I would prefer SS and a design that places all that extra load in direct bearing over posts, for the extra insurance especially should the fasteners fail at some point in the future
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Good points Paul...
What Piffin said about GHR is 100% correct. Just ignore him.My biggest concern with a hot tub on a deck is longevity. If a deck only last a few years, it's not that big of a deal to replace the top boards or rebuild the whole thing.But put a hot tub on there and you complicate the process considerably. Now when you fix the deck you have plumbing, wiring, and the hot tub itself to deal with.As far as design, I'd figure the typical 40# live load and add the weight of the full tub. That may be overkill to some degree. But I think a situation like this calls for some conservative design.You don't wanna be in the tub drinking champagne with Bambi and Thumper and have the thing collapse on you.
I'd horsewhip you if I had a horse.
The math for what you describe is trivial, but your method for determining load capacity is wrong.
What you describe assumes the total load on the joists is from the hot tub. But the live load is the load from the hot tub plus a uniform square foot loading for the portion of the joists not covered by the tub.
Joist capacity is only one part of sizing and detailing a deck. Most deck failures result from ledger boards pulling away from the building or connections failing.
Thank you for proving my point.
I don't prove anyone rigth or wrong. I simply gave a method that the local AHJs accept as prescriptive. Like me they are engineers. Not want-to-be's.
To get around an engineered solution for a low to the ground deck, normal deck building procedures can be used and the area directly under the tub is filled with compacted washed gravel, which the tub actually sits on.
If you are up high or don't want the tub to penetrate the deck it's typically easier to install a number of concrete piers directly under the tub with beams of some sort to distribute the weight than to engineer a strong enough deck to hold the weights involved.
Having said that, if you need to have the deck support the hot tub it depends on the size of the tub and local building practices. Some towns are very specific about deck construction and publish required specs. Others simply require an engineer's stamp, or at least require a structural design that they will spec the sizes of beams and posts for.
Locally, the building department will size the supports for us.
Beer was created so carpenters wouldn't rule the world.
"How do you calculate the loads a deck can hold. In the end a hot tub will be on it?"
you don't.
you call your engineer - and sleep at night.
EDIT: and if you don't have one - you find one.
Edited 11/24/2006 10:08 pm ET by alrightythen
Just thought of guy who worked for me. he is neighbour wanted a deck that would support a hot tub. I think he spent something like 5 hours going thru numerous calcs and formulas figuring out how to build it to support all the weight.
for the time spent he could have got an egineer to do it and throw a stamp on it. taking a whole pile of possible liabilty off his shoulders.
Edited 11/24/2006 10:14 pm ET by alrightythen
for five hours, he must have read the whole book, not just done the engineering.Good Night, Martha!
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apprentice
As you see, the real answer is get an engineered design unless it is an "on ground" application. As a point of reference, our 7'x7' hot tub holds 385 gallons (~3200#s) of water and has an estimated weight of 3900#s.
I've done a number of hot tub decks and have always overbuilt just for the peace of mind. If the tub is to be set in the deck, build a four post sub deck below the tub area using oversized footings, 6" x 6" posts, notched for a double band and through bolted then double up 2" x 8"s on 12" centers. Be sure to put some diagonal bracing to prevent lateral movement of the sub deck. Also account for access areas on the tub itself so you don't build it in some fashion that prevents you from servicing the hot tub. Most tubs are around 7' x 7' so this structure would be ample to carry the cumulative weight involved.