"Joe Barta" wrote in message
.. .
Murray Peterson wrote:
"todd" wrote in
:
For the sheet goods, on average, plywood would be the best, then
particle board, then MDF for initial sag.
I'm a bit unclear on how you're going to build the shelf as a
hollow box. If you're going to go to that much trouble, and then
include a steel angle, I'd suggest just doubling up on the wood
for the shelf. Two of the 3/4" glued-up pine boards would deflect
only about 3/32 at the middle under a distributed 70 pound load
(assuming a shelf depth of 12").
You can get an estimate of sag using the Sagulator at
http://www.woodworkersweb.com/sagulator.htm.
Are the sagulator numbers good for long-term loading, or just
initial deflection? My experience with purchased bookshelves has
been poor -- shelves that progressively sag more as the years go
by.
BTW, the loading is not distributed -- almost all of the weight is
pretty close to centered.
I'm no engineer... more of a shoot from the hip from experience type.
I think a torsion box with 1/4" glued on ply top and bottom with a
total thickness of 1-3/4" should be strong enough for even the
heaviest stereo equipment.
I think the "sagulator" is just a guide and it has no use in
calculating the strength of engineered assemblies... which is what a
torsion box would be.
Joe Barta
We're in luck. I'm a mechanical engineer who specialized in materials
engineering. I wasn't attempting to calculate the strength of a torsion
box. I'm sure it would work. But for the original poster, I'm just
imagining that a couple of 3/4" thick boards that he can buy off the shelf
and laminate together would be easier to deal with and would also be very
effective.
Of course the sagulator is just a guide. For one, it doesn't take into
account end conditions, which can have an effect on deflection. It also
makes assumptions regarding the bulk properties of the material that may or
may not be true. And it can't do stuff like figure the deflection if a
thicker piece is applied to the front of the shelf to add rigidity.
todd