If the bath partition wall is parallel to the direction of the floor joists, it would be common for there to be double joists (or more, depending on how the load bearing from above the bath occurs). Do you know or can you see how the floor joists are installed beneath the partition wall?

There is nothing extra under the partition wall. The wall is perpendicular to the floor joists.

Are you saying that the floor has a sag without any load on it? Maximum deflection would be fully loaded, not just sitting there. If it has a sag now, something is wrong. We have TJI's, and the drywall crew stacked a huge stack where it was only supported by two joists. It was in the area of 4400 lbs.! Alarmed, I went down to the basement to measure the deflection, but it was only 1/8".

You should not be able to measure that much deformation in a newly framed floor. Are you assuming the floor below is level? Did you use a string or a laser device?

As Mushcreek mentioned the L/360 maximum code deflection limit assumes the joists are at the full design live load (30 psf) plus dead load (10 to 20 psf) and you have already exceeded it with only the unfinished framing dead load so this is a measurement issue, a material issue or an installation issue.

L/360 is the code maximum but at long spans that is never stiff enough to limit floor shake/vibration to a comfortable level (the code does not address this critical issue because it does not involve life safety) so L/480 or better should be used as well as a 20 sf dead load. The tub is too close to the support wall to be an issue for the joists but it might be an issue for the support beam.

The quick and easy way to test for adequate stiffness is for a 200 lb person to stand at the mid span on their toes and drop their heels to the deck. Care should be taken to avoid back injury (i.e. this should be done by a young person). If the floor shakes it is not stiff enough for normal use.

For a 24 ft span, I would have used L/480, 40 psf live load and 20 psf dead load. The TJI span table for that criteria would limit the span to 23'-6" and the bold type would alert me that the initial dead load deflection would exceed 0.33". I would have then selected the next stiffer joist which is 14" deep rather than decreasing the spacing to 12" o.c. Owner comfort should the design goal rather than cost efficiency.

This thread caught my eye, because our sub-floor is also bouncy. I will read the advice you have been given carefully.

JDS: What do you mean by the 200 pound person should drop their heels to the deck? This is not an expression I am familiar with. Are you saying the person stands on their toys and drops down flat on their back? If so, yes young people and mats please.

Carol

I said drop their heels to the deck not fall on their back.

Engineers call it a heel-drop test and it does compress your spine so its not for everyone.

Another method is to drop a sandbag but you can never find one when you need it.

Here is a full explanation of floor shake testing but you have to imagine that the guy was on his toes before the photo was taken.

Here is the only heel-drop video I could find from a medical exercise but its the same thing.

Thanks JDS. That is much easier. Interesting that the study was done by the University that is very close to my house. We will investigate the problem more.

Carol