I am building a 6 foot long shelf that is going to be supported at the ends
(no option of support in the middle). The shelf needs to support about 50-
70 pounds of weight (stereo equipment), so the "sag" (or deflection) in the
center is going to be a problem. I plan on building the shelf as a hollow
box, and using steel angle iron on the inside as a stiffener.
Just out of curiosity -- what commonly available wood product is best for
resisting deflection under a continuous load (i.e. shelving)? The easily
available ones at my local lumber store are: laminated pine, mdf, playwood
and particle board, I am assuming that the laminated pine would be best,
since all the grain is running longitudinally. Is my assumption correct?
IMHO, you need to revisit your design- there is always a way to add support
in the middle, even if it is only matte-black metal rods seated in shallow
holes so they don't float around. If the planned load for the shelf needs to
be centered, use 2 vertical rows of rods at the 1/3 and 2/3 points. Sounds
like you are basically building a bookcase. Can you put a back panel on it?
That will stiffen it up considerably. I'd use 3/4 pine plywood, or even 2x
lumber, for that long a span. A glued and nailed hardwood front strip
stiffens plywood, and dresses it up a bunch.
We could make a lot better suggestions if you could describe the proposed
shelf better, or even post a diagram of the design, or a picture of the hole
where it will go, somewhere.
If you like a heavier look, you could use 3/4"
material (plywood or solid wood) and simply put a
3-1/2 piece on the front and rear edges (and
upside down U and know you don't need to make a
complete box). That would hold a huge weight.
A 6' diagonal? That seems excessive, but if that's what you want..
Look at www.goedjn.com/sketch/shelf.gif
The top image is a top view of the layout of your supports.
You could get away with using 1x6s for these, but if you
use 2x4s, then you'll be able to sleep on the thing.
The ones that run along the walls should be fastened at
each stud with two 2.5" deck screws.
The two lower diagrams are details of the joint
where the outer crosspiece meets the the side-rails.
If the thing is going to fail, that's where it will
do it, so use tennons, a glue-block, and more deck-screws.
The inner diagonal is just a stiffener, so you
can skip the tennons, as long as you screw the plywood
upper deck to both the rails and the diagonals.
It actually is a bit short of enough -- the front edge of the shelf ends up
about 6 inches behind the front of the TV. That's what I get for having a
TV that size :-)
Thanks for the effort. I don't have a problem with the supports, since I
have a stud on each wall at the right point. I was worried about sagging,
but I'll be trying out the torsion box suggestion. Sounds like a good one.
I'll try to remember to post a link to some pictures of the completed job.
First, 50-70 pounds for a shelf is nothing, but it
the length is a problem. It is only a problem if
you are thinking about 3/4" material. So, use 2"
material, e.g., 2x8, 2x10, 2x12 solid wood (should
be available at any lumber store). Any kind of
plywood would be ok, but it needs to be at least
1-1/4" thick. Forget mdf and particleboard for
what you are talking about. Don't know anything
about laminated pine, never seen the product.
A hollow box is ok, but much more work than just
using 2" material. If you insist on a box, you
certainly don't need any metal stiffener.
BTW, you ought to visit a store, lay a 2x 8 down
on the floor supported by a couple of of 2 x4's
spaced 6 feet apart and then stand in the middle
of the 6 foot space. You will see how much the
2x8 deflects. Caution: don't do this if you weigh
more than 400 pounds.
How much is too much? If I = bd^3/12 = 7.25x1.5^3/12 = 2.04 in^4 for a 2x8
and the load W = 70 lb and Hem-Fir has a modulus E =1.4E6, the deflection
d = 5WL^3/(384EI) = 5x70(6x12)^3/(384x1.4E6x2.04) = 0.12 inches.
More porcupines :-)
Build the shelf like a floor structure, using 2x4 "joists" at 16" OC (on
edge, NOT flat). Assuming the shelf is less than 16" deep, you should only
need a 2x4 along the front and another along the back (I'd add cross
blocking every 2' or so to help stiffen things up). As long as you anchor
the ends well, this should hold plenty of weight. You could even stand on
it if you wish. Then cover this with the decorative covering of your choice
(plywood, paneling, sheetrock, or whatever).
If weight is an issue, you could probably build a torsion box out of 3/4
inch plywood. I'd make it 3-4 inches thick if I were doing it.
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