My wife has an art studio downtown and she wanted some shelves. She wanted them to appear just to hang off the wall, to be very, very long, and to support about 40 pounds per lineal foot. Of course, the first thing I had to do was to buy a new table saw, so I asked my friend Philip DesAutels (who is studying at the North Bennet Street School in Boston) what table saw to get and he pointed me at the left-tilting, 240V, 10” Grizzly table saw which I bought and which I love). So I built the shelves -- about 50 lineal feet of what are basically boxes meant to be mounted to the wall using 2x2 furring strips.
Originally, I planned to mount the furring strips to the wall using 3-inch screws. I built a test shelf and mounted it to a set of studs with four screws. When I applied 40 lbs to the shelf, it bent heavily. Eight screws, and it bent somewhat. To actually mount it to the wall, I think I'm going to have to use 6” lag bolts into the studs.