Shelf strength - span length.
My son is building a project and is using oak ply for the carcass. It is intended to be a bookcase/shelf as far as I know. He is making it per the wishes of someone and I am a little concerned that the design, while cool looking, may not stand the test of time. The dimensions are approximately: 70" long, 28" high, 14" deep. There is one shelf lengthwise at about the mid point and one vertical piece joined to the underside of the top and the top of the horizontal shelf. There will be a back, so that will add strength.
I am concerned that the shelf, and perhaps even the top, will sag over time. He has proactively mechanically affixed a couple of slim profile metal plates to the bottom of the horizontal shelf to add stiffness/strength.
Are there any guidelines, etc, that can be referenced as to what span would be acceptable prior to sagging, etc? TIA.