Wayne Whitney wrote:

In article , John Hines wrote:

Wayne Whitney wrote:

Does this floor system need reinforcing, and if so, how? Note that I
will be careful not to cut away any of the joists when installing the
plumbing.

You reinforce the area by "sistering" the joists, which is where you
attach new joist to the existing joist, and bolt the two together.

How often should they be bolted? The span is 12 feet. I assume the
bolts (1/2"?) should go through the center line.

No, like the other guy said, spread them out. The bolts don't have to
be that big, there job is holding the two joists together.

Since the bathtub is over just two joists, would it be useful to add
blocking between those two joists and the adjoining joists, to spread
the load over four joists? That would be easier.

No, blocking is to keep the joists from twisting, and to tie them into a
single structure, not to spread load.

As to whether the joists need reinforcing, here's a stab at an answer:
the 32" x 66" bathtub is 14.7 square feet. The extreme load of 1350
pounds I calculated comes out to 92 pounds per square foot. Looking
at a span table for 100 psf live load and 10 psf dead load for 2x10
southern yellow pine gives a span of 12-7 for SS, 12-1 for No. 1, and
10-10 for No. 2. Of course, my joists are not southern yellow pine, I
guess they may be douglas fir. But they are also a full 2" thick, not
1.5" thick.

I'm no engineer, I can barely spell it. G I'm just answering your
question on sistering. What you can look at, is the difference between
a 2x10 and a 4x10, since that is what sistering the joist is doing.

This computation makes it sound like the existing floor system is
borderline, depending on the grade of the lumber and effect of 2"
thick douglas fir versus 1.5" thick southern yellow pine. Did I do
this analysis correctly?

I dunno, I'm a software guy, so I'd load it to the hilt, and when it
crashed through the floor, build is stronger next time. G