Boy Bob, you have asked a tough one. Hope someone else has the expertise to answer better than I have...
As an Architect I deal with quite a few structural engineers, but most are not trained in the structural capabilities of glazing. With an almost infinate list of glazing materials: sheet, float, plate, rough plate, tempered, annealed, heat strengthened, spandrel, laminated, wired, etc. as well as various grades of quality of each, most engineers stay away from structural responsibilites of glass. Many engineers would therefore in any special circumstance just specify required loading conditions in the construction specs for the glazing manufacturer to meet and take responsibility for. ie: "I'm not going to be responsible for that...that's your job".
Since this question is not a major construction project you will not receive any accurate calculations from a manufacturer or "generic" loading tables that are easily interpreted (unless your NASA or I.M. Pei, etc). I have never personally seen any standardized loading charts for horizontal plate glass. The quality of the glass and specific manufacturer has a large impact on an accurate calculation. If you talk to most glazing companies, they will probably just give you some general common sense approaches to your concerns, which would apply to the curios.
SUPPORTS:
In many curio loading conditions, the end supports will more than likely be the critical structural aspect of the shelf. Are they plastic? metal? 2 per end? 3 per end? a continous support (wood ribbon strip)? or just the small "pegs"?
The optimum support would be a continuous ribbon support around 3 sides. Most curios have a mirror in the rear, so rear supports are not possible. If only supported at ends, replace the clear plastic L-shaped "peg" type supports with metal ones. The plastic could shear off with a substantial load (depending on the design of them), especially if loosely fit. Make sure the support pegs are snug and fairly deep into the side of the cabinet also. A continuous rear support will *greatly* enhance the amount of loading allowed to the shelf and is probably the single best "easy" solution when available.
GLASS TYPE AND THICKNESS
Plate glass is the most common glass for curios. Tempered or heat strengthened glass is approx. 2 to 6 times stronger than plate glass, but only in a vertical condition. Replacing the plate glass with the others is really not going to help you much in a horizontal loading condition. Of course if the shelf fails, tempered glass won't break into large chards which possibly could save a sculpture or 2...but doubtful since the lightest bump can break the sculptures.
The single most important "self assurance" you can have in your curio is to increase the plate glass thickness. If your existing shelves are 1/4" plate, I would suggest increasing to 3/8" (or even 1/2") if the unsupported span is much more than 3 feet and unsupported at the rear (but only if your trying to cram them in and your willing to spend the money to be "extra safe"). The cost of upgrading the glass is probably a smart investment if you have a CH1, Coquille, Portland Head, etc. Clear spans 30" or less seem to hold up fairly well with 1/4" plate if you at least spread the sculptures a little and keep "Navesinks" off them. A 16" x 30" end supported 1/4" plate shelf seems to hold 10-12 average size sculptures fairly safely, but I would not cram them much tighter.
My personal suggestion would be to modify the shelf supports as best as your curio permits, and increase the glass thickness a little. If you are a very creative person, buy an extra piece of glass of same exact size, material, and support locations as your curio. Take it out to the shed and carefully start loading it with some weights until it shatters. Then never load your curio more than 60% of the critical load. It's fun being destructive occasionally!
-RodW
[This message has been edited by Rod Watson (edited 12-22-98).]
