Moore, Oklahoma pretty much wiped off the map

[Sea Skimmer]

Duration. A bomb blast is intense but short lived. A tornado lasts for an order of magnitude or more longer.

And the bombs are orders of manitude more intensive, and have debris moving at supersonic speed, made of metal.

The mass of the air is much less, but the speed is much much greater so the kinetic energy available for scouring could actually be higher. I've found plenty of images where tornados have stripped the topsoil off, but none specifically looking at wind driven scour pit effects. It might be they're negligible, or possibly just filled in again with debris as the twister passes.

Yeah I find about nothing that is more then the sod got ripped off. Anchoring against that is very very feasible.

[Broomstick]

OK... the problem is not just "will this object survive being lifted several hundred meters into the air and dropped" but will the people inside survive that treatment? The reason for putting it underground is to keep it from becoming airborne. Nor is it just the wind - in an EF5 you have entire buildings lifted into the air and striking other buildings.

You think I am not aware of what a tornado can do? Sorry big difference between this will cost money, and claiming above ground is impossible as you did. I'm not talking about a bunker either.

If something costs $1 million and you only have $10 thousand then it might as well be impossible. You're saying physically impossible and I'm saying fiscally impossible. Is that clearer, now?

Probably. But I do assure you that if if you built a hill out of concrete you will withstanding absolutely fucking anything including railroad cars being thrown at it. If money were no issue like that then the USAF found you could build a structure which could in principle survive partly falling into a nuclear crater.

Yeah, but in the real civilian world money IS an object.

[The Duchess of Zeon]

The duration problem doesn't exist, Skimmer is right. The difference in magnitude between atomic blast and an EF5 is so great that any structure designed to survive a nuclear shockwave would not experience relevant fatigue loading from an EF5. Please remember, velocity SQUARED is the relevant factor when dealing with energy applied to an object by a blast wave.

[madd0ct0r]

we're not talking about fatigue loading Duchess, we're talking about scour.

An example I'm happier with is scour around structures due to high energy storm waves. Given enough time to act, the scour effects can dig down multiple meters, potentially to below the foundations. Each wave, or peak in the wind, or blast front can only take away so much in the time it passes over a given area. There is a equilbrium shape where scour stops, but it normally takes a LONG time to get down to it. The only time I've seen it come close was when a typhoon parked off shore and sent storm waves crashing into a beach area for a month. In that case the beach went down by about 6m.

[Sky Captain]

Tornado passes over in few minutes at most. Storm surge currents and waves can last for days or even longer, big difference there. Maybe if shelter was built in a hill of loose sand the wind could blow away enough sand to expose the foundations, but in normal dense soil it shouldn't be a problem. Also in pictures showing tornado damage there aren't significant depressions near existing building foundations that would indicate scouring by wind.

[The Duchess of Zeon]

we're not talking about fatigue loading Duchess, we're talking about scour.

An example I'm happier with is scour around structures due to high energy storm waves. Given enough time to act, the scour effects can dig down multiple meters, potentially to below the foundations. Each wave, or peak in the wind, or blast front can only take away so much in the time it passes over a given area. There is a equilbrium shape where scour stops, but it normally takes a LONG time to get down to it. The only time I've seen it come close was when a typhoon parked off shore and sent storm waves crashing into a beach area for a month. In that case the beach went down by about 6m.

You realize that scour is governed by a complicated set of factors including viscosity that make it much worse in water than in air unless the surface material of the solid being affected is incredibly loose like desert sand, right? Even a highly particulate tornado hovering over a town for a month (!!) like in your example would probably only drive surface scour to something like 6mm, not meters, because of the differences in density and viscosity more than cancelling out a rough doubling of wind speed--by a few orders of magnitude. This is of course a major topic of research in my department and I can go into as much detail as might like, but wind-driven scour is basically only a problem in desert environments, and then over a time scale of years including major storms.

[LaCroix]

Biggest factor of houses being picked up is their minuscule density in comparison to area - the house is basically a huge sail attached to the foundation. Same foundation will only stay airborne for a few moments after the house disintegrates from the actual picking up.

Same problem for vehicles and everything but the motor/drive and frame.

A small bunker, on the other hand, has a much smaller surface for forces to act on, and a density that makes it almost impossible to pick it up - let's say you have a 3.3x3.3m Bunker, 10m² area, and make an 1m foundation - that's about 24 tonnes only for the concrete of the foundation (discarding rebar and other weights), and about half as much for the rest of the structure (~1 foot thick, 2m high concrete walls and and equally thick roof) - making it roughly a cube of 3.3m sides with 36 tonnes plus change of weight.

That's an object a third of the size but more than half the weight of an M1 Abrams burried in the ground to it's tracks. Or a new definition of an unmoveable object...

[Sea Skimmer]

If something costs $1 million and you only have $10 thousand then it might as well be impossible. You're saying physically impossible and I'm saying fiscally impossible. Is that clearer, now?

Its certainly not what I would assume from your previous posts. In any case if were talking about a school a million bucks paid off on a bond over twenty five years is something not unreasonable to do. Shelters of any form get cheaper as size increases. If you just wanted to protect a small family nothing like that kind of money is involved for this kind of protection if your afraid of flooding.

You realize that scour is governed by a complicated set of factors including viscosity that make it much worse in water than in air unless the surface material of the solid being affected is incredibly loose like desert sand, right? Even a highly particulate tornado hovering over a town for a month (!!) like in your example would probably only drive surface scour to something like 6mm, not meters, because of the differences in density and viscosity more than cancelling out a rough doubling of wind speed--by a few orders of magnitude. This is of course a major topic of research in my department and I can go into as much detail as might like, but wind-driven scour is basically only a problem in desert environments, and then over a time scale of years including major storms.

You know I've been looking at a bunch of videos of ground scour from twisters, and it appears that the only reason scour is happening is because the grass turf is providing a lot of drag area, allowing itself to be pulled up. Once the turf comes up, nothing else follows, and the turf itself doesn't seem to even leave the immediate area of uprooting, looks like it generally balls up on itself, probably a side effect of it being unevenly removed.

[Simon_Jester]

You realize that scour is governed by a complicated set of factors including viscosity that make it much worse in water than in air unless the surface material of the solid being affected is incredibly loose like desert sand, right? Even a highly particulate tornado hovering over a town for a month (!!) like in your example would probably only drive surface scour to something like 6mm, not meters, because of the differences in density and viscosity more than cancelling out a rough doubling of wind speed--by a few orders of magnitude. This is of course a major topic of research in my department and I can go into as much detail as might like, but wind-driven scour is basically only a problem in desert environments, and then over a time scale of years including major storms.

So is this idea of tornadoes ripping pavement off the ground or digging up a building's foundations and throwing it into the air a myth, then? Or is some force other than scour at work?
_____________________

Anyway, is it physically possible to build above-ground tornado shelters to withstand the 'ultimate' in tornado intensity? I'm sure it is. Would anyone actually do this? Probably not, since the mean time between EF5 tornadoes for any given place, even in the Midwest, is measured in centuries if not millenia. If we were lavish in the way we fund construction you might get million dollar reinforced shelters in schools that could laugh off puny cyclones. But school construction budgets usually aren't that lavish, and so you get people shrugging and saying "eh, just have the kids huddle in the central hallway like they would at home, they'll be fine."

That may change now, though I doubt that even this will make Oklahoma willing to spring a milllion dollars a building to build useful anti-EF5 bunkers in its schools.

[Sky Captain]

So is this idea of tornadoes ripping pavement off the ground or digging up a building's foundations and throwing it into the air a myth, then? Or is some force other than scour at work?

Given high enough wind speed it is certainly possible to rip asphalt from the ground. I remember a Mythbusters episode where they used parked jetliner to blow school bus and car into the air. When pilot increased thrust to max jetblast also ripped off large pieces of asphalt.

[Sea Skimmer]

Yeah it did, and the asphalt was clearly already old and cracked off its subgrade, so you get drag loading at the cracks that lets it be lifted as sheets that nothing but gravity was ever holding down in the first place. Its like blowing away thick sheets of plywood at that point, and rather different than scouring out packed earth which has to be stripped away bit by bit.

[Napoleon the Clown]

http://en.wikipedia.org/wiki/1985_Unite ... o_outbreak They strip asphalt off the roadbeds. It's documented. EF5 does shit that makes no sense. These aren't farmers saying "It done ripped up the road!" It's people coming in and surveying the damage going "holy shit this ripped up the road."

Once you hit EF5 strength you shouldn't try to compare it to anything you're familiar with unless you yourself are familiar with that size of tornado. Don't poo-poo shit meteorologists have actually confirmed themselves and will tell you "This happens."

[MrDakka]

http://en.wikipedia.org/wiki/1985_Unite ... o_outbreak They strip asphalt off the roadbeds. It's documented. EF5 does shit that makes no sense. These aren't farmers saying "It done ripped up the road!" It's people coming in and surveying the damage going "holy shit this ripped up the road."

Once you hit EF5 strength you shouldn't try to compare it to anything you're familiar with unless you yourself are familiar with that size of tornado. Don't poo-poo shit meteorologists have actually confirmed themselves and will tell you "This happens."

Pic from wikipedia article source.

It looks like the tornado rips up asphalt in chunks like Skimmer says and not by scouring (which I'm assuming is like sandblasting on steroids)

[LaCroix]

I'd also propose that this is more a case of backpressure building up on an exposed edge somewhere on the road, with a resulting cascade of catastrophic failure once the first small bit of pavement gives, exposing a new edge to the wind. You can see that the gravel bed underneath is pretty much still there, only the slates of asphalt have been taken.

Also, it pretty much started at the edge of the road (in this picture's lower right corner), and peeled the asphalt away right to about the middle of the road. Since roads are usually made with two strips of tarmac, joined in the middle, and are slanted to their edges to let water run off, you either had one side poorly made, or the slant resulted in a unfavorable angle of attack for the wind to the exposed edge still left.

If it were some suction related thing, you would expect a trench to be dug.

[The Duchess of Zeon]

Yes, I'm not disputing this happens, I'm saying it's NOT SCOUR, which is a very big thing. Breakwaters and coastal buildings can just get destroyed during storms without being undermined. Nuclear detonations can rip up pavement, too (in fact the intactness of pavement in the subdivisions was the big sign it wasn't a nuclear event).

Clown:

The blithering moronitude of "EF5 does shit that makes no sense" is such that you should go become a docent at the Creation Museum. It's just high velocity particulate matter moving in a coherent wave form. It isn't magic, it can be modelled with fluid dynamics, and a surface structure which is impervious to 99.9% of effects resulting from it can certainly be constructed. In fact I am pretty sure that you could just build a dome-home out of 1-meter thick prestressed concrete if you were really that worried, and then walk outside and take pictures of the neighbour's wrecked McMansion a few minutes after it passes. The main thing would be just to use translucent concrete instead of windows and make sure there aren't any exposed ledges. If you were really paranoid, had a million bucks, and didn't want your house wrecked, I'm pretty sure it could be built, let alone a surface shelter-in-place inside of a normal house (battering effect of collapsing walls would have to be accounted for, of course).

Bottom line: EF5 is energy being delivered to a target, like a nuke. There is nothing magical about it, nukes also strip asphalt off roadways, THAT'S documented too, and meteorology is a science, which I am therefore respecting and treating like one--not worshipping like King Jeebus as you apparently do. Go to the Creation Museum, because you are talking like you belong there.

[The Duchess of Zeon]

I'd also propose that this is more a case of backpressure building up on an exposed edge somewhere on the road, with a resulting cascade of catastrophic failure once the first small bit of pavement gives, exposing a new edge to the wind. You can see that the gravel bed underneath is pretty much still there, only the slates of asphalt have been taken.

Also, it pretty much started at the edge of the road (in this picture's lower right corner), and peeled the asphalt away right to about the middle of the road. Since roads are usually made with two strips of tarmac, joined in the middle, and are slanted to their edges to let water run off, you either had one side poorly made, or the slant resulted in a unfavorable angle of attack for the wind to the exposed edge still left.

If it were some suction related thing, you would expect a trench to be dug.

And I wouldn't really expect it on an object which had very little exposure above the surface. I mean, if a tornado could scour away a road for being a couple feet above the surface, why aren't hills regularly lowered several dozen feet by tornadoes? After millions of years of the same climatic regime encouraging their formation, you'd expect Oklahoma to have blatantly recognizable multi-mile long perfectly flat strips criss-crossing it like impact craters. It's just ridiculous, what people are saying about the capability of tornadoes in this thread. They're being treated like magical demons.

[Sea Skimmer]

I'm sure 1m thick concrete is a big margin of over protection, the proto TAB-V aircraft shelter I referenced had only a 1.8mm steel liner, stock corrugated sheet steel, covered over with 18in concrete cover (4000psi at 28 days, nothing special at all) with a layer of low grade rebar holding that onto it. Meant to be cheap as it could be. Not only could it withstand a shelter adjoining it being blown up and thrown into it as chunks of debris by nine thousand pounds of bombs, it also fully contained the internal explosion of four 25lb sidewinder warheads with the door closed. So tornado level negative pressure sure wont matter unless you mess up on the door. Said shelter was also tested against a 630 ton ammonia nitrate bomb at 600ft to simulate an nuclear attack. Over 30psi peak hit the thing, around 700mph wind, and its only foundation, built in the open desert, was the floor slab poured directly on the native earth. Oh my lookout if the twisters can hit mach .9!

A paper aircraft shelter with 4-6ft thick (set by conventional weapons protection requirements) reinforced concrete was expected to withstand 250psi overpressure, besides minor stuff like direct hits from 500kg SAP bombs exploding after partial penetration, and that was with arch spans wide enough to hold an F-111 with its wing fully spread. It had a slightly deep foundation, basically a pyramid of concrete but this was set completely by the need to hold the weight of the arch. 250psi = about 1,800mph wind speed. So if we get mach 2+ tornadoes we may really want that meter thick cover.

[Sea Skimmer]

Oh stupid me, I didn't read that chart right, 250psi dynamic pressure is about 1,800psi. 250psi of over pressure from the nuke will involve more like 2,250mph maximum wind. This is fast approaching mach 3 at sea level.

[Simon_Jester]

Can I get a price tag on that?


I think I can say, without fear of contradiction, the following:

1) An EF5 tornado wreaks destruction that is hard to fully understand unless one is familiar with damage caused by similar phenomena- and just about the only thing in the world 'like' an EF5 tornado is overpressure from huge explosions.

2) Strictly speaking, one can build above-ground shelters capable of withstanding an EF5 tornado. I don't think anyone is really disputing this.

3) However, such shelters are relatively expensive, NOT a normal part of residential or commercial architecture, and hard to retrofit into existing buildings.

4) Because of (3), very few such shelters exist in random Oklahoma suburbs. Nor are such shelters likely to be constructed en masse. They exist and are possible, but aren't really relevant to the problem of "how do we keep people in Oklahoma from dying in tornadoes," unless we get really hardcore about building tornado shelters.

[Sea Skimmer]

Somewhere like 1.4 million bucks in 1985, but that's including stuff you don't need like giant roller doors, they are hugely expensive which is why many HAS you see around the world just don't have them, fire suppression, and the fact that its a shelter big enough to hold an F-15 with the engines running.

http://www.ausairpower.net/USAF/Kadena- ... 040-1S.jpg

This is actually a slightly improved version with thicker concrete to better resist conventional weapons attacks, but same general size, around 80ft long. The bracing on the door is actually to keep the door from bounding off from the rebound effect of a nuclear blast striking it head on and allowing leakage. Obviously more then a little tall to make sense for say a school shelter unless you built floors into it. But then its huge size also makes it a huge target for the relevant massive blast loading. An actual above ground tornado shelter would just be built lower and longer, which is more or less exactly how a lot of surface built air raid shelters were made in WW2 anyway. Its also possible to scale these shelters sizes massively up if you want and not loose protection, a handful were built to hold U-2s in the UK. Door span was around 120ft.

And yes, certain people blatantly were trying to argue that aboveground shelters were not possible. The issue is really not one of cost anyway, its just the general US mentality of civil defense is duck and cover nonsense at work, followed by people howling when a disaster occurs, and then dropping back into a cycle of not caring. it happens over and over again. Hell even a very weak shelter is damn useful in an EF5 tornado anyway, the damage path of the maximum winds is far smaller then the radius over which wooden houses will be totally destroyed.

[Simon_Jester]

Somewhere like 1.4 million bucks in 1985, but that's including stuff you don't need...

An actual above ground tornado shelter would just be built lower and longer, which is more or less exactly how a lot of surface built air raid shelters were made in WW2 anyway...

Agreed. So, price tag estimate for a tornado shelter capable of holding several hundred children would be, what... I'm going to just vaguely eyeball it at a few hundred thousand dollars? Somewhere between 200 and 500, that is?

And yes, certain people blatantly were trying to argue that aboveground shelters were not possible.

Personally I think that if they were then, they aren't now, in light of what's actually been demonstrated. Or at least I hope not now that the issue has been laid out and people have had time to think.

If I'm wrong, then I guess I'm just plain wrong.

There's a reason someone might come to this conversation thinking "it's impossible," when in fact that is not true- because few if any such reinforced, EF5-proof shelters exist. On page two, you have Broomstick talking about Anderson shelters. By the standards of tornado shelter that actually exist, relying on something like an Anderson shelter is apparently considered realistic. It was a good shelter choice during the Blitz, against conventional weapons typically no larger than 1000-pound bombs, too.

By the standards of surviving nuclear blast, it's comical.

And it looks like to survive an EF5, you need something at least built to the same approximate scale as for a hardened installation designed to survive multiple psi of overpressure.

So if your experience is with real anti-tornado shelters because you live in the Midwest, and you are NOT familiar with work done on hardening aircraft revetments against nuclear attack, you might very well think "tornadoes are so destructive they can destroy any above ground shelter." Because your definition of "above ground shelter" includes things like the Anderson shelter, or some of the 1950s civil defense 'fallout shelters,' but not anything hardened against serious overpressure.

The issue is really not one of cost anyway, its just the general US mentality of civil defense is duck and cover nonsense at work, followed by people howling when a disaster occurs, and then dropping back into a cycle of not caring. it happens over and over again.

I totally agree. This is going to be doubly true in a place like modern Oklahoma, where I suspect that the current crop of government leaders will be back to reflexively opposing any kind of infrastructure spending including disaster hardening within a matter of months.

Hell even a very weak shelter is damn useful in an EF5 tornado anyway, the damage path of the maximum winds is far smaller then the radius over which wooden houses will be totally destroyed.

I totally agree.

[Napoleon the Clown]

Yes, I'm not disputing this happens, I'm saying it's NOT SCOUR, which is a very big thing. Breakwaters and coastal buildings can just get destroyed during storms without being undermined. Nuclear detonations can rip up pavement, too (in fact the intactness of pavement in the subdivisions was the big sign it wasn't a nuclear event).

Clown:

The blithering moronitude of "EF5 does shit that makes no sense" is such that you should go become a docent at the Creation Museum. It's just high velocity particulate matter moving in a coherent wave form. It isn't magic, it can be modelled with fluid dynamics, and a surface structure which is impervious to 99.9% of effects resulting from it can certainly be constructed. In fact I am pretty sure that you could just build a dome-home out of 1-meter thick prestressed concrete if you were really that worried, and then walk outside and take pictures of the neighbour's wrecked McMansion a few minutes after it passes. The main thing would be just to use translucent concrete instead of windows and make sure there aren't any exposed ledges. If you were really paranoid, had a million bucks, and didn't want your house wrecked, I'm pretty sure it could be built, let alone a surface shelter-in-place inside of a normal house (battering effect of collapsing walls would have to be accounted for, of course).

Bottom line: EF5 is energy being delivered to a target, like a nuke. There is nothing magical about it, nukes also strip asphalt off roadways, THAT'S documented too, and meteorology is a science, which I am therefore respecting and treating like one--not worshipping like King Jeebus as you apparently do. Go to the Creation Museum, because you are talking like you belong there.

I'm so glad that we don't have to be excessively literal with you. It makes things so much easier. (That's sarcasm by the way, I'm not sure you can catch anything but the most literal interpretations.) See? I can attack people themselves, too, instead of attacking the idea. It gives me credibility, especially when the person I'm talking to is someone I haven't been trying to get through to for a while. Isn't it nice to have a nice, mature discussion?

Do I really need to say that you can figure it out with the right education? Really? A layperson will look at EF5 damage and wonder how it's possible. Not everyone is into fluid dynamics enough to go "Oh, this is how it could do it."

Nowhere have I said it's impossible to withstand an EF5, just that it isn't financially feasible to make something of reasonable size that can do it. Fuck, I gave examples of things that can get an EF5 to the face and not even care. But I contradicted you once so you had to fly off the handle. Nothing new here.

[madd0ct0r]

You realize that scour is governed by a complicated set of factors including viscosity that make it much worse in water than in air unless the surface material of the solid being affected is incredibly loose like desert sand, right? Even a highly particulate tornado hovering over a town for a month (!!) like in your example would probably only drive surface scour to something like 6mm, not meters, because of the differences in density and viscosity more than cancelling out a rough doubling of wind speed--by a few orders of magnitude. This is of course a major topic of research in my department and I can go into as much detail as might like, but wind-driven scour is basically only a problem in desert environments, and then over a time scale of years including major storms.

I'd love for you to go into more detail - this is something i've spent a while looking at, but the compressibility of air (especially) means my mental models aren't up the scratch.

[Broomstick]

I'm sure 1m thick concrete is a big margin of over protection, the proto TAB-V aircraft shelter I referenced had only a 1.8mm steel liner, stock corrugated sheet steel, covered over with 18in concrete cover (4000psi at 28 days, nothing special at all) with a layer of low grade rebar holding that onto it. Meant to be cheap as it could be. Not only could it withstand a shelter adjoining it being blown up and thrown into it as chunks of debris by nine thousand pounds of bombs, it also fully contained the internal explosion of four 25lb sidewinder warheads with the door closed. So tornado level negative pressure sure wont matter unless you mess up on the door. Said shelter was also tested against a 630 ton ammonia nitrate bomb at 600ft to simulate an nuclear attack. Over 30psi peak hit the thing, around 700mph wind, and its only foundation, built in the open desert, was the floor slab poured directly on the native earth. Oh my lookout if the twisters can hit mach .9!

Wonderful.

Just a few problems:

1) virtually no one in the lower middle class is going to be afford this sort of shelter. They don't have the money readily available to pay for materials and labor. They don't have the space to add this to their current residence or land. While I'm sure you can find exceptions in the rural regions, such people tend to already have shelters as opposed to blue collar subdivisions, which is what Moore was.

2) virtually no one wants to live in fugly half-done of concrete. In order to provide EF5 protection of this sort to the at-risk population you're talking about replacing every conventional residence between the Appalacians and the Rockies. Who the fuck is going to pay for that?

[Sea Skimmer]

Who the hell said anything about replacing every residence with one of these? The only specific application I pointed at was a school shelter, at which point you'd be thinking in terms of build the gym like this if you so opted for a large design. The main point was that your and others insistence that shelters must be underground was simply false, and that said shelters need not be massive bunkers either.