These aren't gorilla racks. I'd guess it's stamped 12ga steel. I have mine loaded up with 2-300lbs per shelf and I'm not concerned with the structural integrity. I do have a group 31 battery(60lbs in a 12\"x6\" footprint) and in the middle of the supports, it definitely causes some sag in the wire shelving. If you're pushing weight capacity, I'd cut some mdf or plywood to help distribute the weight. I might in the future just to aid sliding things around. Overall, I'm satisfied. If I needed more, I'd pay more for something more heavy duty, but I dont. Even with tools and various other heavy items, the only \"issue\" is the heavy battery too far from the supports which was easily fixed. I'll be buying another 2.