I recently purchased one of those small (8x10) steel storage shed kits from the local home improvement warehouse type place here. I'm not sure how much anchoring I need for it. The instructions simply say "this shed must be anchored", and they provide instructions for anchoring it to bare ground using cable anchors, anchoring it to wood posts, or to a concrete foundation, using their concrete anchors.
I have constructed a foundation/floor for it by building an 8x12 frame out of 2x4 treated lumber (I made the foundation 2 feet longer so I could stack my firewood on it as well), covered with 1/2" treated plywood. The shed is securely attached to this foundation by a 2 1/2" screw at every mounting hole, however the foundation itself is not anchored. The shed is also nestled in to a wooded area, between trees and our house, so it is somewhat shielded from wind.
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Yes, I realize I could just go ahead and add cable anchors as well, "extra protection won't hurt", but I'd prefer not to spend the money if they really aren't necessary. Yeah, they're only about 30 bucks, but times are tight, and $30 is $30. I've already spent way too much on this project as it is, but I needed to get all this junk out of my garage. :-)