Flat Topped Steel Shelters
Flat Topped Steel Shelters
Letter to a Client: By Paul Seyfried
I recently reviewed some shelter sites featuring flat-topped steel shelters. Flat-topped shelters derive no benefit from earth arching, so they can’t be buried very deep….a few feet or so. I’d get in one if a tornado was bearing down on me, and hope something heavy doesn’t fall on top of my shelter, like a utility pole, Peterbuilt, or John Deere tractor. The few feet of shielding (if not blown off during the initial blast winds) provide only minimal fallout protection and NO initial radiation protection.
Without exception, the pictures on flat-topped shelter sites suggest that the designers do not understand the concept of radiation shielding geometry on entrances. I see vertical exits merging directly into the ceiling of the shelters, and main entrances lacking 90-degree turns and without long enough horizontal runs.
Many shelter designers use the term “military grade” on their nomenclature, but this term is meaningless. Someone please tell me what “military grade”…. is. Is it a launch control center at a Minute Man III field in eastern Wyoming? That would mean a 3,000 psi overpressure rated facility, with two concrete containments…. one suspended on gimbals with 8 feet of travel, inside. These are buried 88 feet deep, and have very thick stainless steel blast doors weighing several tons.
Flat-topped shelters are seldom if ever built to NBC shelter standards (heavily reinforced concrete shelters excepted) and should be considered to be ‘storm shelters’, only. Blast doors must be carefully engineered to withstand both ‘over’ and ‘under pressures’. Many people have had very unpleasant experiences with their “storm shelter doors”, even during a tornado experience. During last year’s worst tornadoes in OK, over a dozen shelter owners reported that the sheet metal doors on their storm shelters had blown completely off. Most, thankfully, were not home or in their shelters when this occurred. Having inspected some of these sleazy doors at trade shows, I’m not at all surprised.
These designers don’t seem to understand that it’s not just rain and wind that we’re dealing with. Like Ron White, the comedian, said in his routine, “It’s not THAT the wind is blowing…it’s WHAT the wind is blowing.” How about a water heater hitting your door at 250 mph? Ed York (hardened facility construction advisor for NATO and good friend) made the timely comment; “A Ford station wagon, skimming the earth’s surface at 600 mph is a formidable weapon” comes to mind. Ed was educating me as to why one would want to place their air intake and exhaust goose necks below grade in rock cribs instead of having them waving in the breeze. I could tell by the way he would hesitate, and his gaze, that he was sizing up the IQ of the idiot across the table from him [that would be me] while choosing his words. I got his point loud and clear. A shock wave pulverizes everything in its path…. buildings, trees, vehicles, utility poles, and spreads this tidal wave of ground-up ‘jelly’ in front of it at very high speed. This mass also aids in pulverizing more stuff along the way, and adds to the mass of debris. BUT…. it also fills in low places like basements, ditches, voids, Swiss shelter entrances (remember the sloped ramps on their entrances) with this blended crap once the overpressure drops below about 7 psi. Above that, the wind is sucking out basements…. people, too. Anything sticking above grade is not going to be happy. And that’s why Ed York greatly favored a flat, horizontal steel hatch instead of a vertical door. It’s a low-drag target. A vertical door must be seven times stronger to resist a 100-psi overpressure wave due to reflected overpressure (not to mention the crap that arrives with it).
Flat topped steel shelters may have been properly engineered to carry the small, static, ‘dead load’ for a minimal dirt cover, but no consideration has been given to the dynamic, ‘live load’ from impact and movement during a blast, or for proper shielding loads. Some shelter designers (right on their web sites) claim their flat-topped shelters to be even stronger than arched shelters! I know of no ‘real engineer’ that would agree with that statement. The arch provides the strongest geometry for weight bearing and is a pure compression form. Arches appeared in architecture as early as the 2nd millennium BC and some have survived for many thousands of years, bearing witness to the strength of that concept. You’d think we’d catch on by now.