Thanks for the several useful responses. I'm concerned that with such a
deep shelf increasing the leverage from the weight of the books, my
biggest problem is going to be mounting into the wall. I think I
probably have four studs along the run, but it seems like there will be
a LOT of force trying to bend or pull those fasteners out of the studs.
Unlike the situation with a kitchen wall cabinet or angled shelf
bracket, none of the load is being transferred the other way, into the
wall or wallboard.
I have a couple of the Lack floating shelves. They're ingenious in
design, but IKEA warns purchasers about a maximum load on them.
The protruding column is a concrete pier (this is in a highrise condo)
and I'm thinking that may provide some help to me. If I can anchor
solidly into that at the cutout points--seven inches in front of the
main wall--it seems like the weight of books on the back part of the
shelf will partially balance the weight on the front part of the shelf,
making these anchors pivot points that are largely supporting the
shelves vertically, and relieving stress on the wall mountings.
The torsion box seems a much more complicated project than sandwiching
U-channels in between plywood. Would that provide benefits for my
support problem, or is it more to avoid longitudinal sagging?