On Jul 13, 12:28 am, "Art" wrote:
"BobK207" wrote in message
ups.com...
On Jul 12, 8:37 pm, "Art" wrote:
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.
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 1/64"
the 6ft span (again with all the 100 psf distributed load concentrated
at midspan..assumed worst case) again looks ok stress ~660 psi &
deflection 1/16"
looks ok as is, no mods needed
sistering the rim joist IMO won't really do much,
but if you WANT to stiffen the floor, sister the 8 ft joists, use
glue & small dia (.113 to .131) nails..forget the bolts, you want
good shear transfer & its hard to beat glue & lots of small nails (~4"
oc)
Your joist to sill bearing loads (~100psi) look ok so the sisters
don't really have to bear on the sill. Non-bearing sisters will be
easier to install & they'll be there just to stiffen the spans
I spent more time typing than doing the design or calcs...so hopefully
someone will check my concepts.
Its all about checking the load path so if I'm not understanding the
existing condition...oops! 
cheers
Bob
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.
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