More about strengthening a floor for a whirlpool bath

[Art]

Please excuse me for starting a fresh thread on the subject but I got a lot more info on how the floor is constructed.

We removed an old whirlpool made by Jacuzzi. Jacuzzi says the floor needs to be 55 lbs/ sq ft. For some reason the new Sanijet tub recommends 100 lbs/sq ft. The latter number seems to make more sense.

So I want to strengthen the floor and plan on using a sheet of 3/4" plywood on top of the old floor.

Here is the new information. The portion of the floor with the tub is cantilevered out 2 feet. There is basically a box made up of 10 inch joists that are 8 feet long with the last 2 feet of them sticking out the side of the house to make the cantilever. That means that some of the tub will be sitting on the sill plate. That is good, I would think. However the parallel joists that make up the cantilever portion terminate at a single perpendicular joist which is fastened to another long joist which is part of the main floor system. So imagine a house of parallel joists with a box attached to the middle of one joist consisting of one parallel joist for the attachment and perpendicular joists that stick out beyond the sill plate 2 feet.

The part on the sill plate should be plenty strong. I am worried about the part that attaches to the long joist in the main part of the floor system. I am thinking all of the weight is on that one joist (except for what is on the sill plate). I cannot put posts underneath. I am thinking of sistering that joist and bolting all 3 joists together (the joist that is the end of the box, the joist that the box is attached to, and the new sister joist.) I will probably have an engineer look at it but I know you guys have great ideas, and I rather verify your ideas with the engineer rather than asking him to come up with a fix.

Thanks again for your assistance.

[BobK207]

Art-

Quick answer....if I'm understanding the existing conditons

you've got 8 ft 2x10 joist that span ~6ft & cantilever ~2ft and a "rim" joist that collects the ends of the cantilevers

since deflection goes as span^3 the 2 ft cantilever section is actually stiffer than the 6 ft span section.

Of course the actual loading (edge of tub or a real distributed load) depends on how the tub bears on the floor

anyway based on my rough calcs (assuming worst case 100 psf distributed load all concentrated at the end) the 2 ft cantilever looks ok stress ~300 psi & deflection

[Art]

Thank you for your response. I guess what worries me is that the short rim joist attaches to the middle of a long parallel joist (lets call that long joist, joist "A") which is part of the regular floor system. The middle of the joist "A" is holding all of the weight of the tub except that portion which is being held by the sill plate. Seems like Joist "A" is being asked to do a lot. So are the nails between joist "A" and the rim joist.

[BobK207]

Art-

Its all about relative stiffness.......stiffer elements (stiffer load path) take more load than more flexible load path.

I have an idea about how your floor is framed but without a picture or a sketch I'm really not 100% sure that I'm clear on the framing scheme.

Based on my understanding of your description

The joists cantilever over the sill ~2', the ends of these joists frame into a "rim joist" which frames into another joist (a long joist that is parallel to the cantilevers)

if this is correct..... then the rim joist just stabilizes the ends of the cantilevers & not much load finds it way out the end of the cantilevers thru rim to the parallel joist. So the parallel joist really isn't being loaded through it's connection to the rim joist

my premise is that the cantilevers (cuz' they are so short & stiff) are bringing all their load back to the sill thus the rim joist along with joist A are doing virtually nothing to support the load on the cantilevers.

If the connection from the rim to joist A went away would the performance of the cantilevers suffer?

think of your cantilvers like a series of parallel diving boards, add a rim joist to the ends of the diving boards.

Now add another LONG diving board but support its far end AND frame the rim joist into this long diving board.

Put a 100 lbs at the end of each short diving.....what supports the majority 100's of pounds of load?

the short boards? or the long board?

My premise is that because of the stiffness of the cantilever sill support vs the rim connection most (nearly all) of the load exists the cantilver thru the sill support.

cheers Bob

[Wayne Whitney]

Is the tub centered or nearly centered over the wall below? If so, almost all the weight will be carried by the wall, and you need only concern yourself with whether that wall is strong enough. Your 2x10 joists will be strong/stiff enough to carry the tub load to the wall. [Imagine cutting through the joists at the edge of the tub over the 6' span; the joists just have to be able to carry the weight back to the wall as a cantilever on either side of the wall.] Sounds reasonable, Bob?

Hmm, that's not really true, is it? A unit point load at the end of the 2' cantilever will induce a moment at the support of 2 ft-lbs. A unit point load at the middle of the 6' span will induce a moment there of 1.5 ft-lbs.

Cheers, Wayne

[Art]

The position of the tub with respect to the sill plate certainly is key and I should have included that in my original post:

The portion of the bottom of the tub that holds water is 29 inches wide. Towards the top the tub widens to 44 inches wide plus figure a 5 inch surround. So the heavy part of the tub starts 12 inches from the inside wall which is 4 inches thick. So the heavy part of the tub starts 17 inches inside from the cantilever joists. So 7 inches of the tub is cantilevered,

6 inches are sitting on joists directly above the sill plate (which is on a concrete foundation wall), and 16 inches are on the other side of the sill plate putting its weight on the joists that lead to the rim joist that connects to a long joist which is part of the main floor system.

[Wayne Whitney]

I am not an engineer, but I would say that since your tub is less sitting on top of a strong support (the concrete wall), the bending moments induced in the 2x10 floor joists, even at 100 psf load, are comparitively small. So there is certainly no need to strengthen the floor system as far as bending of the joists goes; you should check the bearing area to be sure the joists won't crush over the wall. It would be a very different situation if you had your tub in the middle of a 16' span, say.

Cheers, Wayne

[BobK207]

Wayne-

yes, what you wrote is true but there's more to it since we have a "distributed load".....total load increases linearly with span length

thanks for asking, you made me double check my thinking...here's a little more

deflection at the END of a cantilever for a distributed load is P*L^3 / (8*E*I) deflection at the beam mid span for a distributed load is 5 * P*L^3 (384*E*I)

E - material modulus I - joist moment of inertia

working out the constants so we can compare cantilver .125 * P*L^3 / (E*I) beam .013 * P*L^3 / (E*I)

Yup, cantilevers look more flexible than beams

BUT the key is that L for our cantilever is 24" & L for the "beam" span is 72"

& P in each case is the TOTAL of the distributed load for each

so the 6' span P would be 16"/12"*6ft *100 lbf/ft >>>>> 800 lbf & the 2' cantilever p would be 16"/12"*2ft *100 lbf/ft >>>>> 267 lbf

Of course I simplified the situation since we really don't have a cantilver or a simple beam span ....

we really have an "overhung beam" but I'm too lazy to do the analysis & can only remember a few simple loading cases like simply supported beam & cantilever both with distributed load or point load

good enough to do rough stiffness comparisons & I'm pretty sure they'll give conservative results (ie calc more defection than the real situation)

so if you play the numbers..... the 6' span gets a double whammy 3x the load AND 3x the span

of course a cantilever is not a stiff as a simply supported beam .....IF they're same length AND support the same load

BUT in our case the "beam" is 3x as long ...... that distributed load AND the L^3 terms are killers

even with the .25 vs .013 factors in our formulae ( a ratio of about

20) the combined effect of increased load & length gives us an "inverse" ratio of 81

so the 6 ft beam span deflects about 4x as much as the 2 ft cantilever when exposed to the SAME distributed load,

even under the SAME point load, the 6 ft span deflection would be ~35% higher

your moment analysis is correct for the SAME point load BUT the beam span sees an effective point load 3x the cantilever (because of the distributed load) thus the ratio of 1.5 vs 2.0 is overwhelmed by the load increase.

cheers Bob

PS about the bearing stress....each joist sees 16/12 * 8 ft * 100psf ~1070 lbf

even if all of the load through the joist into the wall sill you've got bearing area 1.5 * 3.5 = ~5 in^2

about 200 psi bearing even a conservative 400 psi (not the code ~600 psi) gives a really good margin

Art-

The Sanijet requiement of 100 psf seems REALLY high......what's the total tub weight, how much water does it hold & what is the "foot print"

[Art]

Thanks for all the info. Here is the exact tub. Note that they say the base is structural. I am not sure whether that means it spreads out the weight of the water. I did question them about the 100 psf but they said that is their recommendation... they would not budge on it:

[BobK207]

Art-

750 lbs of water, 250 lbs of person, 150 lbs of machine total 1150 lbs

~3.5' x ~5' foot print

~65 psf ...............sounds like the 100 psf is a "cover their butt" requirement

that is their recommendation... they would not budge on it

[Wayne Whitney]

Well, since we were talking about a tub, I was considering constant total load. But you are right that the 6' beam deflection is still greater than the 2' cantilever deflection, the constants in the deflection equations are much less for the beam but not by a factor of

3^3 = 27.

Cheers, Wayne