New answers tagged shelving
If the entire shelf is solid 2x4's (1.5x3.5") (not a 2x4 frame) and the support structure is adequate, about 3000 pounds, making a few somewhat pessimistic assumptions (the safer direction to assume.)
If you can avoid floating, I suggest you do. It's inherently weak. (And sooner or later, someone's probably going to try chinups or an entire Encyclopedia Britannica on your work.) If you gotta, Rockler sells these blind shelf supports: http://www.rockler.com/blind-shelf-supports. Hope you're in the US, or shipping/customs might be tough. (My review on ...
Thete are several kinds of supports sold specifically for this purpose, plus the option of using fairly standard triangular brackets upside down and arranging the shelf's contents to hide them.
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