On Sun, 17 Apr 2005 09:58:42 -0500, j walker wrote:
What I am attempting to understand is how much sag should occur in a
2X8 floor joist if the span is 8 feet. The application is for a water
tank stand which will measure 8X8 and will hold an 1100 gallon water
tank. The rim joists are 2X12 and the floor joist ends rest on a
ledger nailed to the side on the 2X12. There are 9 joists in between
the 2X12.
I would recommend that you have an engineer do your load calculation
and framing plan for this.
Keep in mind a couple of things:
The liquid volume alone will weigh about 9160 Lbs. before you add
anything in for what will hold the water, temporary point loading
during servicing, base level allowance for live load, etc.
Let's say that your tank and water come in at around 10,000 Lbs.
(we'll leave out the other stuff for now).
You are proposing to carry this on eight joists which will rest on a
ledger. Assuming the ledger to be 2X stock (1-1/2" net) you will have
36" to rest your 10,000 Lb. load on (1-1/2 x 1-1/2 x 16 = 36).
That is about 278 Lbs. per Sq. In.
Let's think about the fact that you are using a 2 x 12 rim joist and
butting 2 x8 joists to it on a ledger.
The 2 x 12 is 11-1/4" and the 2 x 8 is 7-1/4", leaving 4" for the
ledger. Unless you specify that the ledger must be ripped from a 2 x
6 to the full 4" height, you may wind up with a 2 x 4 ledger sitting
1/2" above the top plate. I wouldn't want to see the fasteners handle
that load all by their lonesome. Neither would I want to see the
space shimmed with typical shim stock.
I would want to make sure that the plate was doubled and that the
joists rested directly over the vertical framing members below the
plate, and that the load was transmitted all the way to the foundation
in the same fashion.
It seems to me that the amount of sag is not the real problem. That
is not to say that it is not a problem at all. I would glue and screw
1/2" min. ply (not CDX) to the underside of the joists and glue and
screw the subfloor on the top, creating a sort of torsion box on the
cheap.
The other thing that I would consider is that you are attempting to
hold a lot of water on a framing system that will decline in its
ability to hold the weight if it is subjected to water damage.
Anything that holds water will eventually leak.
These sorts of considerations are what make people like me pick up the
phone and transfer the liability to my friendly local engineering
firm.
Tom Watson - WoodDorker
tjwatson1ATcomcastDOTnet (email)
http://home.comcast.net/~tjwatson1/ (website)