Shelf strength & thickness for 4ft adjustable bookcase span

We built a library for an attorney who wanted his bookcases as large as possible with adjustable shelves. Our limit, because of sheet goods, was 4' wide with shelves basically 46 1/2". We told him that there was a possibility the shelves could sag but that we would try to make them as strong as we could. We ended up gluing 2 layers of 3/4" oak veneer core plywood together then put a 1 1/2" x 3/4" routed oak face on both the front and the back of the shelves. We told him that if the shelves did sag he could just turn them over. The bookcases are full of big ass law books, been there for ten years and he's never turned them over.
Mike O.
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