Hot Tub on Exterior Deck at Height & Self Supported 3

[m_struct]

A client would like to extend his exterior deck to support a 2000kg hot tub (4.4 kips). The new deck steps down from the existing and approximately 4m x 5m in plan, 2.5 m (9 ft) above ground and on sloping site. Is there any guidance for loading, corrosion, and safety for hot tubs on exterior decks?

Loading - The client says the hot tub is 2000kg (tub + water + 6 people). If 6 people are in the tub, how many more would be standing around the tub? What do you recommend loading wise? Dynamic factor for live load (say 2x LL)? Thinking of drunk people and jumping around - high consequence for failure.

Bracing & Layout – Deck is self-supported and about 1.5m below the ground floor of the house. Looking at post and brace on each side of rectangular deck.

Corrosion - Chlorine is present but it is open-air, well ventilated area (open-air). My first thought was with (4) steel posts and beams, with timber decking and joist running over the steel beams. For corrosion purposes, would be recommended to go with full timber? Timber option = 9 posts (3 x 3 grid) with one post at each corner of the hot tube.

Safety – Client wants tub on outside face of the deck. The handrail is about the same height was the edge of the tub, so would that mean the handrail should be double height at that point? Another option would be to recess the hot tub into the deck. This seems much safer and would require a subframe to support the tub. Any suggestions?

 

[phamENG]

I wouldn't worry about the weight of the people in the hot tub. I'd design for the distributed load required by your local code over the entire area(40psf for most parts of the US - I have no idea what you guys do in New Zealand). Then do an area load equivalent to the hot tub (usually light) and water (very heavy) in the location where it will be placed. Structure magazine has an article in their archives based on some research done on wood decks and they came back with a recommendation of between 9 and 12psf lateral live load applied to the surface of the deck. So if your deck is 4m x 5m (13.2ft x 16.5ft if I remember my conversions), you'd have a total lateral live load of about 2600lbs. That's based on the maximum load at which a group of people spaced to make a 40psf vertical live load can make synchronized movements before the dynamic deflections create enough discomfort for them to become self limiting. So once you figure out your code required gravity live load, apply at least 0.25 in the lateral direction at the same time and you'll capture the upper bound.

Bracing - This is always tough. Even if you don't transfer gravity loads, get lateral loads back to the primary structure if you can. Those connections are not easy to detail and, in my area of practice, are never installed correctly.

Corrosion - this is environment dependent. On the east coast of the US, if it's not inside a climate controlled building envelope I require galvanizing for steel (though can be talked into certain coating solutions under some circumstances). If you do steel, your lateral connections become easier as you can do moment frames or braces in steel that are more likely to be installed properly. Wood is a viable option, though. Careful with your stainless spec - certain types of stainless don't play nice with chlorine. You may need a duplex stainless alloy to get the kind of longevity you're after.

Set your guardrail height based on the highest walking surface. If it's reasonable that somebody will walk at a higher elevation to move around the hot tub, then use that for the datum for your guardrail measurement.

 

[Retrograde]

According to ASNZ1170.1 residential decks more that 1.0m from the ground should be designed for a live load of 2kPa and a point load of 1.8kN (not to be applied together).

Attached are some technical design guides for timber decks. They relate to Australian practice but may be of some use.

 

[NorthCivil]

if possible better to land the tub directly onto a foundation below. Then dress the deck around the tub like trim around a window.

Thats likely what they are after right - where the tub is flush with the floor of the deck?

Decks are already dodgy enough as it is, without trying to hoist a couple tons of hot tub onto a timber deck fully exposed to the weather and elements, that will start to lose its strength after the first stretch of bad weather (even if it is all treated timber and stainless connections, yes, no structure is immune to weather)

 

[Ingenuity]

A new ‘dick’ for a Kiwi.

Make sure you use lots of ‘caulk’ for any gaps.

Sorry, could not resist the schoolboy Aussie humour (humor for the North Americans) to a Kiwi.

Carry on - nothing to see here

 

[Settingsun]

Phameng, thanks for the lateral load reference. They're pretty scarce. The Aust code for walking tracks gives 0.25kPa (5psf) for a more sedate crowd (AS 2156).

I wouldn't replace steel with timber for durability in this environment. That's just trading rust for rot.

Ingenuity, no comment on the specified number of tub occupants? Hang your head.

 

[m_struct]

Yes, it is a big deck mate . . .

As the deck is 9ft above ground, and is that height to take in the views below, setting it on a slab would not be an option.

Steel sounds better (rust instead of rot). The idea was to fully support the tub with steel - steel girders (2) and (2) steel channels in the same plane as the timber joist. Looks like over-engineering, but given the variables - durability issues and bracing, seems about right.

 

[dik]

I would design the tub area for a full depth of water and the remaining area for about 40 psf.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[3DDave]

Any chance of a full tub of water and a crowd on a hot-tub cover that might be installed later?

 

[Canuck65]

We would design for the following loads:
LL of 1.9 KPa over the whole surface of the deck(or design snow load if it exceeds LL). Snow load sometimes exceeds live load in my area. Residential code does not require companion loads between LL/SL.
+ DL of hottub and water over the footprint of the hottub.

Commonly built with pressure treated wood and corrosion resistant fasteners in my area.

Do not forget that the DL is a sustained load, which affects the calculations for wood in my area. Reduced capacity of the wood joists under a sustained DL.

The guardrail height can be contentious. Homeowners want the regular guard height (42"), however, if the hottub is close to the guard then building departments in my area are clear that the guard must be 42" above the top of the hottub. Visualize people sitting on the upper lip of the hottub that is located 6" from the guard and then leaning back. It is a judgement call (i.e. no code mandated horizontal distance) as to how far the edge of the deck is away from the hottub before one can justify dropping the guards back down to 42" above the deck surface.

 

[m_struct]

The client has asked for the flexibility of the relocating the hot tub on the deck. As the vertical bracing layout was set based on the location of the hot tub, the deck would need to have a reliable diaphragm or more bracing lines to allow for this flexibility.

I considered a diagonal strut fixed crossing the underside of the deck/joist and fixing to the bottom face of each joist. The flat surface of the strut seems like it would be water trap and durability issues from the hot tub water.

Is there any reliable diaphragm action from the timber decking and joist? Maybe a steel strap, but again durability issues.

 

[Settingsun]

How were the unbraced posts stable before if there's no diaphragm?

What's the problem with more vertical bracing?

 

[phamENG]

Yes, you can get reliable (if not terribly strong) diaphragm action from timber decking. No idea about NZ codes, but in the States the NDS/SDPWS has a table for timber diaphragms (Table 4.2D on page 24 of the 2015 edition - should be available for free viewing online).

 

[XR250]

One problem I have about counting on timber diaphragms is that you have no control over what will be used as a future replacement for the floor decking. If they end up going with a composite and a hidden fastener system, then you can lose that benefit.

 

[JoelTXCive]

I second everything that others have said.

Like Canuck mentioned above...I would also worry about long term deflection of the wood under the sustained load.

 

[EdStainless]

You need to talk him into a recessed mounting of the hot tub, and then support it with a steel sub-structure separate from the deck. It could be shop fabricated in galv steel for reasonable life. And then that allows you to build a 'normal' deck.
Around here when people do this they have the top edge of the tub about 6-7" above the deck surface, keeps grills and chairs from sliding in.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[XR250]

We do hot-tubs on top of wood decks around here regularly. Never had an issue. Usually spec 2x10's @ 12" o.c. spanning 8 ft. so low risk of long-term deflection being an issue.

 

[STrctPono]

I like the idea that EdStainless proposed and have seen it several times but the only issues are whether or not this affects access to the equipment below and what happens in the future when they get a different size hot tub...

 

[EdStainless]

I mentioned that because a neighbor has a set up like that.
They got a large tub. We cut out a larger opening in the deck and built a new platform on top of the steel structure and set it in. What he likes in that it looks clean and motion of one part does not effect the other.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[m_struct]

I like the recessed steel option. However, I mentioned that a few weeks back and it did not get the nod. Next time.

@XR250 - What governs the 12" OC spacing? Is it the decking or the joist? What decking do you recommend?

The typical details for fixing vertical brace to timber post typical show bolts, like a M16 (0.75") bolt or so. However, engineered wood screws or higher Ga screws appear to be be easier to install while providing better performance. Looking at SS. Any suggestions?