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Discussion Starter · #1 ·
Hi all!

So I planned my bathroom renovation, ordered a new bigger bathtub and ripped everything out. Now that I have the new bathtub I just calculated that it contains about 16 ft³ of water when full.

Calculations: average inside dimensions: 2.75' wide x 4.5' long and the overflow is 1.5' from the bottom and that comes out to 18.5 ft³ Take out about 2 ft³ for rounded corners and a couple of bumps on the sides and I get about 16 ft³ of water.

16 ft³ is about 120 US gallons. At 8.33lbs/gal that comes to 1000lbs and the tub itself is about 120lbs. Add 2 standing people (I'm 220 lean pounds - I wish - and my wife is 120lbs) and a rubber ducky and I get near 1500lbs.

Trying to figure out if my floor can handle that load I started to educate myself and got to the American Wood Council Span Tables for Joists and Rafters. Since I seem to be looking at a fairly high live load, I'm looking at the highest figures on the table which are 60psi live load with 20psi dead load with L/360 deflection limit. The plan is to put tiles on the bathroom floor using an uncoupling membrane. The overall dimension of the bathroom is 8' by 10'. The center of the tub is about 2 1/2' from the steel beam supporting one end of the joists. The tub has a supporting base which spreads the load over a 3' x 6' area and there are 4 joists directly supporting the tub. I will be using 3/4" T&G plywood over the joists.

The floor structure is 2x10s (old style 2 5/8"+ thick) every 16" with a span of 16' and is in good condition. I think no. 1/no. 2 Spruce-Pine-Fir is what was commonly used (there are no stamps) 64 years ago when this house was built (Montreal, Canada). If I am right to use the 60psi/20psi table, then I get a required Modulus of Elasticity (E) of 2.3 million psi and a Required Bending Design value of 1,951 psi. The design values for no. 1/no. 2 Spruce-Pine-Fir 2x10s for normal duration are E of 1.4 million psi and 1,005 psi for design value in bending.

I am thinking of doubling the 4 floor joists directly underneath the tub. The basement is open to work in but I can't add columns or cross beams. Do my assumptions (60psi live/20psi dead), calculations and conclusions make sense?

Thanks.
 

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JUSTA MEMBER
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Hi all!

So I planned my bathroom renovation, ordered a new bigger bathtub and ripped everything out. Now that I have the new bathtub I just calculated that it contains about 16 ft³ of water when full.

Calculations: average inside dimensions: 2.75' wide x 4.5' long and the overflow is 1.5' from the bottom and that comes out to 18.5 ft³ Take out about 2 ft³ for rounded corners and a couple of bumps on the sides and I get about 16 ft³ of water.

16 ft³ is about 120 US gallons. At 8.33lbs/gal that comes to 1000lbs and the tub itself is about 120lbs. Add 2 standing people (I'm 220 lean pounds - I wish - and my wife is 120lbs) and a rubber ducky and I get near 1500lbs.

Trying to figure out if my floor can handle that load I started to educate myself and got to the American Wood Council Span Tables for Joists and Rafters. Since I seem to be looking at a fairly high live load, I'm looking at the highest figures on the table which are 60psi live load with 20psi dead load with L/360 deflection limit. The plan is to put tiles on the bathroom floor using an uncoupling membrane. The overall dimension of the bathroom is 8' by 10'. The center of the tub is about 2 1/2' from the steel beam supporting one end of the joists. The tub has a supporting base which spreads the load over a 3' x 6' area and there are 4 joists directly supporting the tub. I will be using 3/4" T&G plywood over the joists.

The floor structure is 2x10s (old style 2 5/8"+ thick) every 16" with a span of 16' and is in good condition. I think no. 1/no. 2 Spruce-Pine-Fir is what was commonly used (there are no stamps) 64 years ago when this house was built (Montreal, Canada). If I am right to use the 60psi/20psi table, then I get a required Modulus of Elasticity (E) of 2.3 million psi and a Required Bending Design value of 1,951 psi. The design values for no. 1/no. 2 Spruce-Pine-Fir 2x10s for normal duration are E of 1.4 million psi and 1,005 psi for design value in bending.

I am thinking of doubling the 4 floor joists directly underneath the tub. The basement is open to work in but I can't add columns or cross beams. Do my assumptions (60psi live/20psi dead), calculations and conclusions make sense?

Thanks.

You forgot to subtract the water displaced by a body ( bodies?) in the tub.

That will give you a lot of weight reduction.

And go ahead on double joist the structure, for security.


ED
 

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You forgot to subtract the water displaced by a body ( bodies?) in the tub.

That will give you a lot of weight reduction.

And go ahead on double joist the structure, for security.


ED
I'm 179 lbs and I displace almost half the tub volume.
 
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Discussion Starter · #4 ·
Thanks Ed and Mike;

I did think of that and that is why I mentioned occupants standing before sitting. I assume that the "standing" load is of short duration (unless you take a shower in a full bath :) and maybe should not be considered but I wasn't sure. Still leaves 1160 lbs after water displacement.
 

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Discussion Starter · #6 ·
When I first got the tub, I installed it temporarily connecting a drain and filling it up with a hose. My 4 year old went swimming in it with both my wife and I kneeling in front of the tub and playing with her. Back to 1,500lbs on those 4 joists.
 

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retired framer
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Spanning 16 ft I would double up all of them, if you want to go nuts, you could use LVLs for the 4 under the tub.
 

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Discussion Starter · #9 ·
Spanning 16 ft I would double up all of them, if you want to go nuts, you could use LVLs for the 4 under the tub.
I actually looked into LVLs for those 4 joists. From all the manufacturers I looked into, the limitations as far as perforations through LVLs are really restrictive. Max 1" holes (which is OK for re-running the electric cables) but maximum 3 holes in each span... I have a dozen or 15 cables to run through those spans! I-joists would probably be better, lighter but a bit harder to manoeuvre in view of their 2 1/2" (or 3 1/2") flange thickness.
 
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