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Liquidbreaddiet
Advanced Member
Username: Liquidbreaddiet

Post Number: 573
Registered: 06-2005
Posted on Thursday, March 15, 2007 - 03:33 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

i need to beef up my deck structure for a new spa. THe deck is existing and drastically undersized. It is made of preasure treated 2x8s spanning 15 feet. This obviously will not hold a hot tub. Technically i would imagine if we had a couple feet of snow it would fail. That being said. Replacing the deck is not an option. I was wondering what size beam I would have to install at the midspan of the joist if the tub was to straddle the beam. Also if there are any other suggestions about ways to shore up the structure. Some information about the spa inclue that it is 92" by 92" with a dry weight of 1100lbs and a full weight of 4225.
"If I'm not supposed to eat animals, why are they made out of meat?"
 

Steve Funk
Intermediate Member
Username: Tundra45

Post Number: 318
Registered: 06-2004
Posted on Thursday, March 15, 2007 - 09:21 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

Wow, that's a big tub. I'm not an engineer here, but here's some info I gleaned from Wood Structural Design Data, Vol 1. Second Edition (1941). I know it's old, but they used to build with a lot of wood back then. It says not to use anything that will deflect more than 1/360th of its length. A 15' span divided by 360 is only half an inch! So, a nominal 4x6 beam 15' long can handle anywhere from 800 pounds to 2000 pounds depending on the wood species, grain structure, etc. Too small. The average 6x6-15' supports 1850 pounds before deflecting half an inch. So, three 6x6-15's or two 6x6's with posts in the middle should cover your application. Alternatively, you could place two 8' 6x6 beams with posts on each end just under the tub itself, say 4' apart. An average 6x6-8' is good for 3500 pounds. Were it me, I'd go for two 6x6-8's about 4' apart with 6x6 posts. As for the posts, you're going to have to consider what they'll be placed on too. Me, I'd pour a concrete piers 12" in diameter 30" below ground and 6" above ground for each of the four posts.
Disclaimer-
This is not advise, only my own rambling thoughts. Do as you want at your own risk.}
 

Dan Listermann
Senior Member
Username: Listermann

Post Number: 4149
Registered: 03-2004
Posted on Friday, March 16, 2007 - 02:42 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

If the tub is such that failure might cause injury ( high off the ground or something ), a professional who can visit the site should probably be consulted. Structural engineering without diagrams or pictures is very difficult.

--This space is STILL being left intentionally blank.-


 

Belly Buster Bob
Senior Member
Username: Canman

Post Number: 2733
Registered: 02-2003
Posted on Friday, March 16, 2007 - 04:06 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

here, you require a permit to install a tub that size. The permit requires an engineer's sign off for above ground installed vice "on the ground".
Check the manual for your tub, most of them need full coverage support on the underside and cannot just straddle beams. Most of the tubs these days get structural support from what they sit on.
Bellybuster Bob
www.bellybuster.netfirms.com
 

Belly Buster Bob
Senior Member
Username: Canman

Post Number: 2734
Registered: 02-2003
Posted on Friday, March 16, 2007 - 04:07 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

you sure on those weights??? That's just under 300 gallons, I would've figured quite a bit more
Bellybuster Bob
www.bellybuster.netfirms.com
 

Bill Pierce
Moderator
Username: Billpierce

Post Number: 6741
Registered: 01-2002
Posted on Friday, March 16, 2007 - 04:58 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

At 92 x 92 inches and let's say 36 in. deep, that's 176.33 cubic feet of liquid or 1319 gallons and 11,005 lbs. of water weight alone, plus that of the spa. I can understand your concern.
 

Steve Funk
Intermediate Member
Username: Tundra45

Post Number: 323
Registered: 06-2004
Posted on Friday, March 16, 2007 - 05:09 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

4225 - 1100 = 3125 lbs of water / 8.33 lb/gal = 375 gallons. Seems about right for a tub that size.
 

Bill Pierce
Moderator
Username: Billpierce

Post Number: 6744
Registered: 01-2002
Posted on Friday, March 16, 2007 - 06:09 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

I wonder if 92 x 92 inches are the inside or outside dimensions of the spa. At 231 cubic inches per gallon and at a depth of 36 inches, 375 gallons would have an area of 2406.25 square inches and be 49.05 inches on a side. Perhaps I'm a little generous in my estimate of the depth at 36 inches.

(Message edited by BillPierce on March 16, 2007)
 

Ric Heinz
Intermediate Member
Username: Rheinz

Post Number: 438
Registered: 01-2004
Posted on Friday, March 16, 2007 - 10:48 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

I'm with Bill. Your weights are way off.

A 7' X 7' X 2.5' deep tub would hold 122 cu. ft. of water at 62.4 lbs/cu.ft. Deducting say, 25% of this for benches and round corners gives me a water weight of 5733 lbs. Include your dry weight of 1100 lbs, plus four adults (another 800 lbs) and you'll have a total weight of 7633 lbs.

A normal engineer would design with at least a 2 to 1 safety factor. That means your structure will have to support at least 15,250 lbs.
Ric
Flatfender Brewing, NW Houston
 

Bill Pierce
Moderator
Username: Billpierce

Post Number: 6752
Registered: 01-2002
Posted on Saturday, March 17, 2007 - 03:26 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

Ric is right that I didn't consider the reduced volume for the benches and rounded corners. However, any way you look at it you can see how above-ground spas and pools need a lot of support and also why waterbed mattresses can be problematic.
 

Liquidbreaddiet
Advanced Member
Username: Liquidbreaddiet

Post Number: 574
Registered: 06-2005
Posted on Sunday, March 18, 2007 - 01:26 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

well I ran it through a beam calculator that first looks at loads and load patterns- i decided to locate the tub centered between 4 columns to transfer load to 4 points equally rather than 2. I am going to run 3-2x12s between each pair of columns underneath the existing joists and then double the existing joists. This configuration fits well within the allowable beam and joist deflection and takes into account live, dead, snow and tub loads for the area. My deflections end up being .117" where .317 is allowed by code. I believe that equates to more than a 2 to 1 safety factor right? Steve got it right when he calc'd out the weight of the water. Belly buster bob - I didnt mean to imply that only the beams would be holding up the tub- rather placement of the tub would center on the beam i proposed installing. FYI the weights I quoted came directly from the manual and if you follow the link provided you can confirm if you are still in doubt. Thanks for all the comments http://www.d1spas.com/spa_selector.asp?spa=7
"If I'm not supposed to eat animals, why are they made out of meat?"
 

Tom Burk
Member
Username: Tomburk

Post Number: 147
Registered: 03-2003
Posted on Monday, March 19, 2007 - 03:33 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

Decks often fail not so much from vertical loads, but from lateral movement of those loads. Fastener extraction or inadequate fasteners used to attach a deck to a structure or inadequate diagonal or lateral bracing could allow the deck to collapse.
Make sure that you use the properly coated fasteners or stainless for any of the newer pressure treated lumber, if thats the type of material your using.
 

Liquidbreaddiet
Advanced Member
Username: Liquidbreaddiet

Post Number: 576
Registered: 06-2005
Posted on Monday, March 19, 2007 - 06:56 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

Thanks tom - yes for the new beam I am following a proper bolting pattern using either hot dipped galv. or zinc coated through bolts as well as an additional nailing pattern. I will also be cross bracing in both directions at each of the columns with 2x material. I will also be using #1 pt lumber as well (it is special order and costs more but there is significant difference in load capability and where I can locate bolts. thanks again
"If I'm not supposed to eat animals, why are they made out of meat?"

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