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RugbyLock (Civil/Environmental) (OP)
12 Aug 08 19:41
I have a client who wants to have a hot tub on their deck and am unsure about what design weight to use.  I have been told that a 5x5 spa with water and people would be about 5000lb giving it a 200psf load.  Does this seem right to you?  Not enough?  Too much?
Helpful Member!  cvg (Civil/Environmental)
12 Aug 08 19:52
5' x 5' x 3' deep gives 75 cubic feet of water which weighs nearly 5,000 lbs.  Now add the weight of the hot tub, pump, piping, and at least 4 people but probably a whole lot more during a party and you are approaching more like 7,000 lbs.  You should probably assume a pretty hefty safety factor to handle impact loading as well.  This will give you more like 300 psf.  This is why most hot tubs are mounted on the ground, not on decks.
RugbyLock (Civil/Environmental) (OP)
12 Aug 08 20:00
wow..... no doubt...  Thanks for the reply, much appreciated!
RugbyLock (Civil/Environmental) (OP)
12 Aug 08 20:27
A second question with this.  Am I correct in assuming that the weight of the hot tub would be a live load since it's not permanently attached?  Would it be better to just count it as a DL and include it in seismic calcs for the deck?
csd72 (Structural)
12 Aug 08 20:56
Yes, I would definately treat it as a live load.

Ask the client if they know what models they are using, then you can get the weight of the unit and the water volume to calculate the total load.

Make sure that the client is not planning on putting a brick surround or something heavy like that.

Last of all, you should put the designed weight and area on the drawings to cover yourself in case they upsize it without telling you (it does happen).
JLNJ (Structural)
12 Aug 08 23:09
I would think the people and the water would weigh the same. If it were full of water and some people jumped in, the water would overflow.

200 psf is about 3' of water plus some addl DL. I'd say you must be in the ballpark.
RugbyLock (Civil/Environmental) (OP)
12 Aug 08 23:28
How much additional DL would you add?  Perhaps out of 300psf using 200 as LL and 100 as DL(or vice versa?) to account for some of the water still being in the tub during and event in the same manner as having to consider some of the Snow load during a seismic event?  I'm also considering not attaching the deck to the house because of the weight but I haven't done that before so I'm not sure what sort of seperation that I might need.
SLTA (Structural)
13 Aug 08 11:00
The manufacturer of the hot tub should be able to provide loading information, similar to a cut sheet for mechanical units.  That would hopefully answer most of your questions (and any unexpected ones, like a surprisingly heavy motor or the like).
msquared48 (Structural)
13 Aug 08 14:30
An 8X8 only holds 450 gallons of water, full - 3400 pounds of water.  Forget the people - they will just displace any excess water.  The tub is about 2000 pounds with electric equipment.  

Still say 3500 pounds including the people, plus 2000 gives 5500# over 64 sq. Ft. = 90 psf.

The loading from the smaller tub will not be much different in psf.  Use 100 psf minimum live load and you will be OK.   

Mike McCann
MMC Engineering

Dinosaur (Structural)
13 Aug 08 15:48
I'm more conservative than many folks, but I would ask how deep the hot tub is from the deck to the lip.  Then I would use that depth full of water as a design load.  If in a seismically active area, I would consider it as a dead load during seismic activity checks.  Otherwise, I would treat it as a live load.  The comment about the displacement of water is very close to my own thoughts.

Most Hot tubs I recall are around three to four feet tall.  So I guess the load would be 190 - 250 psf.

Good Luck.
cvg (Civil/Environmental)
13 Aug 08 17:02
just as an example:

Enjoy the sweet, sweet sounds of the Aquatic Melodies® surround sound system in this spacious lounge hot tub model. The Elation hot tub is a stellar performer with features that are sure to please even the most discriminating spa enthusiasts.

Seats 6 adults
500 gallons / 1893 liters
7'7" x 7'7" x 38" (231cm x 231cm x 96cm)
930 lbs / 6135 lbs filled (424 kg / 2690 kg filled)

http://www.calderaspas.com/Spa_Showroom_Hot_Tubs/model_hot_tub_elation.html

loading is approximately 185 psf.  Loading is not distributed evenly either, it is along the edges. The filled weight is to the normal fill line.  The tub should not overflow when people get in unless it is overfilled.  Of course, you should design for either filled to the top or with maximum number of people in the tub which during parties is always as many as can fit.
MiketheEngineer (Structural)
13 Aug 08 17:22
I have often built a special sub-deck just to handle these types of loads.  Those estimated above sound reasonable.   Find the model and capacity and go from there.  People will add weight since the sit around it (not in it) and most tubs are never completely filled but the water capacity assumes something like a 6'' level below top.
RareBugRA (Structural)
13 Aug 08 20:33
I would assume that your client wants a wooden deck.I am also conservative, for a larger than 4'x4' tub, in a seismic area, I would do a steel frame to carry axial, dead and live, possibly some threaded rod bracing for both deck and columns. See, the thing about wood (assuming you are designing with wood) is that even if it is wolmanized, after a few fears there you will see cracks and splits appearing, not to mention that it will rot. So let's just be safe and rely on steel for the weight of the tub and associated weights.

Best regards,

Rarebug, Chicago
dougantholz (Structural)
13 Aug 08 21:49
I would use wood, I would not use steel, and cvg has it about right.  I've done small pools on roofs of existing buildlings and while they are heavy, that over a large area the load is reasonable.  Steel is a problem because it probably doesn't look right for a residential application, not to mention the water.
The water displacement question comes up quite a bit.
Consider this - at capacity a hot tub typically has some free board.  This means that as people climb in, they won't make a mess and spill water over the side.  So we typically do one of two things: calc the depth of water if water is at the top of the pool and design that for the live load or two - calc the water at "capacity" and add 50 psf live or so.  That's just my judgment and that of a few other SE's in my office - others add 100 psf to a fully loaded pool.  Can be silly when you get the numbers together but hey it won't fail.  
msquared48 (Structural)
14 Aug 08 1:37
I designed my hot tub deck to 100 psf live load 10 years ago and have had no structural problems whatsoever.  

I feel that 250 psf is an overkill, even for an 8X8.  However, it will work...

Mike McCann
MMC Engineering

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