Mine hasn't been twister tested yet, but I feel pretty safe with it. Above ground obviously. Door is a boxed frame heavy steel unit with massive hinges (3) and large through frame crossbolts (2). The door frame is fully boxed and the door swings inward, hopefully to aid in egress should debris be blocking the door. I keep a 2# sledge and wrecking bar inside, just in case (along with water, blankets and other essentials to ride it out until rescue). It's vented and heavily bolted to the concrete pad. I think any storm that could suck the door off would probably take the entire unit out. If it's that bad, I'm not sure I'd want to be in an underground shelter with the possibility of drowning or suffocating. The water table at my house would require a pretty heavy weight to keep from floating.
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