It's the tensile strength that keeps it sitting on the rebar. That pvc really wants to straighten out but can't because of the rebar obstacle. As long as the rebar is securely in the ground and there's also a good portion of it inside the pvc, it's very difficult for the frame to lift up. When you have one side in and you arch the pvc down and slip it onto the corresponding rebar, you'll feel how solid it is.

*Important note: the straight rebar should be almost 90-degrees to the ground. You can angle it in toward its opposing rebar piece JUST slightly but do it only by a couple of degrees. You want to have to force the pvc down on the rebar when arching. That's the strength of the hut.

One thing you may want to add is a piece of hooked or bent rebar between where the ribs hit the ground Tie this off to a grommet on your tarp. If the hut gets any lift from a strong wind, these tie-downs act as a back-up system.

GP, do you know where you'll be camping? I can come by and help you guys if you need it. I'll be there Sunday.