From what I'm reading the Plywood is not sitting on top of the 1.5 square tube
beam

..the Weld tab is going to be carrying the load of the 18" x 7' 3/4" Plywood and also the sheite ya shove on the shelf.



I assume you are using Weld tabs so that the wood sits flush with the 1-1/4" steel, for Aesthetics