How to fix second story bathroom joists and calculate load 1

[Destruct2021]

Working on a 2nd floor bathroom remodel. We removed a fiberglass jacuzzi tub and a two sink vanity from a master bathroom. We want to install a 400# cast iron pedestal tub and a 400# double, marble-topped vanity. In doing the renovation we've removed the old tile and will reinstall new tile. We also had a water leak so we pulled off the drywall on the ceiling below so we have access.

We have 2"x10" joists on 16" center. The joists span 14' but the bathroom is 8'4" wide to the wall. (see attached drawing). The joists run perpendicular to the length of the tub and vanity.

We figure the tub with water with a person will weight 1000#. There is a joist right under the middle of the tub. This joist was cut for the drain out of the removed jacuzzi. The new drain tub will also be centered on this joist so we are going to cut the joist and put a double header 2"x10" plumber box. The long centerline of the tub will 21" from the wall. The drain for the tub goes underneath and then down right under the lip of the tub which will be about 6" from the wall. There will be three joists under the weight of the tub.

The tub will be supported on three joists.

The vanity sits on four legs with two of the legs being between two of the joists holding up the tub and the other two legs being between two joists not related to the tub weight. The long centerline of the vanity is about 7'4" from the wall by the tub and about 6'6" from the end of the joist supported by the downstairs wall.

QUESTIONS:
1. We did the joist math and we can't support the weight of the tub (probably can support the vanity). We want to sister, scab, and/or block the three joists under the tub to strengthen everything. We don't have access to either end of the three joists though otherwise we'd just sister the three joists and call it a day.

WHAT CAN WE DO?

2. How can we do the analysis to show have confidence we met building code as well have constructed a long-lasting, non-deflecting joist system?

Thank.

 

[phamENG]

If you want confidence in the analysis...you'll need to hire a local structural engineer to review the conditions and determine the adequacy of the existing system or necessary method of reinforcing.

 

[Enable]

Couple things

A) If you want my help you shall stop using # to indicate lbs. Quit it, and quit it now! You are the second person I have seen to use this ridiculous convention and I cannot sit idly by any longer!

B) Agree with phamENG, if you want confidence you'll need to hire a structural engineer. We can help (and you have provided enough info, so I will) but one presupposition for the usefulness of these forums are that the people asking questions are knowledgeable enough to be able to interpret the responses in such a way that they can reliably use the info. Not being an expert, you might take what we say and do all sorts of silliness. Not because you are not trying, but because you don't know!

Add to this the fact that we can only respond to what's being asked, and sometimes it's the questions not asked that are the most important!

C) Lots to discuss to help with your confidence level

1 - This is an existing structure, so you need to check all elements you are relying on for design. That means the joists and their supports, sheathing, and blocking. Essentially have to look at everything along the load path. Are the joists bearing properly on their supports (min 1.75" typically)? What connections exist between the joists and their supports (e.g. toe-nailed, or bracketed, or...)? What are the condition of the joists and their supports (any holes, notches, rot, etc)? Are the supports adequate (e.g. if they rest on walls do the walls have double header plates else do joists land directly overtop of studs)? These are the kinds of things you need to check first, before you go ahead with a new design.

2 - Your loads surely are light. If a person + tub weigh 1000lbs, what do you think the design load should be? I gaurentee you it is more than just the static load! Think about people slipping into and out of the tub. That's an impact load that needs to be accounted for. Moreover, water tends to rot out joists around tubs and if you want longevity, you will want to overkill this to make sure even if the system does degrade it'll be fine.

3 - When you say that the existing joist system cannot accommodate the loads, what do you mean by that? How they fail will tell you what / how you need to add additional elements. That is, do they fail in shear, bending, or in deflection? Can you tell us what the capacity vs factored loads are on these joists (provide a diagram + calcs so we can follow)?

4 - Can you describe / take pictures of what is below so we know what we have to work with in terms of adding additional structural elements?

 

[Destruct2021]

A. Used the # as lb in the Air Force forever (Royal Canadian).

What is the # symbol called?
1: The pound sign. This name came to be because the symbol comes from the abbreviation for weight, lb, or libra pondo, literally “pound by weight,” in Latin. When writing “lb,” scribes often crossed the letters with a line across the top, like a t.

B. Agree. Working on hiring a structural engineer but they are out 4-6 weeks with work locally. I'm a mech eng but this isn't my field. This has to be trivial problem for someone with experience in analysis and/or construction. (and that's not me)

C

1. I added pictures to the original pdf (attached). The joists along the exterior wall are sitting 2-3" over the wall. First time I pulled the fiberglass to see how they are attached. I can't tell how the joists are attached. Other than the big cut taken out for the jacuzzi tub drain the joists are in okay condition. Dry and no rot.

2. This is a good question. I figured the tub weight at 1000 lbs spread over 2'x 4.5' is 111 lbs/sq ft and 44 lbs/sq ft just sitting there. That weight will be over three joists. Other than then joists under the tub and vanity, I figure the designed dead load should be 10 lbs/sq ft and live load be 40 lbs/sq ft. Those values should be no issue with the construction as-is for the other joists under the bathroom. I think the designed live deflection has to be L/360 as it will be a tile floor. Then I have no idea how to calculate. I can see there are at least joist shear and moment capacities and inertias to calculate. I that just stepped me into the deep end.

Thanks

 

[Ron247]

I have used # informally in calcs, but not #lf or #psf.