I didn't buy these to mount shelves for their intended purpose. Instead, I bought them to give extra support to a concrete countertop I made. Most of the weight of the tabletop rests on the bar itself, so they're just adding a little extra support to the bar. For that purpose, they seem to get the job done. They had some flex when mounted vertically, but not much. I'm sure they're great for attaching shelves.
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