Driver Killed by Manhole Cover

[Ben]

Firstly, condolences to the victim's family.

Reading only what's in the story, I'm trying to visualize how the manhole cover both came loose and went airborne high enough to go through her windshield. The story doesn't say if it was smacked by oncoming traffic or by someone in front of her in her lane. It almost seems like it would have had to be hit by oncoming traffic to do this. Even then, it's a fairly flat piece of metal on a flat surface, so it seems like it would be bizarre circumstances for it to go airborne vs just being run over. From her lane, I can only visualize that it somehow went flying via a spinning wheel from the vehicle in front of her, but that seems like a one in a million thing.

My brain is not computing scenarios. Maybe the article is wrong or leaving out info and the thing was somehow not laying flat, which would provide contact surface to send it flying. It also doesn't specify if the cover was just loose in its mount, or completely removed from the manhole (but an open manhole would probably cause a worse accident when somebody's tire intersected it).

How do you guys think it happened?

http://www.foxnews.com/us/2016/02/13/manhole-cover-crashes-into-suvs-windshield-killing-driver.html?intcmp=hpbt3

[French G.]

Another vehicle went over it while the cover was partially over the hole? Tipped part of the circle into the hole? That would have gotten the cover off of flat and put enough energy into it to bounce off the underside of the departing car or bounce off the pavement.

[230RN]

All sympathy for the family and friends.

The officials are puzzled, too, and they're right there on top of the situation.

I suppose some sudden pressure wave in the storm drain (methane or natural gas leak explosion, trick of wind, sudden release of water into the system, somebody dumped flammables into the sewer, etc.) lifted the cover off slightly, uncentering it for the next vehicle, or blew it upward altogether as Ms. Clavette approached.  I believe that's not unheard of.  Recall that workers in these tunnels use big blowers to keep fresh air blowing down the holes.  Guessing 30" in diameter, it would only take about 3.5 psig overpressure to lift one at 200 lb weight if my arithmetic is correct.  Less if they're bigger in diameter than that.

Also, I understand that theft of them was pretty rampant in NYC a while ago for sale as scrap metal.  Maybe someone tried that in Boston and found that 200 lb was too much for him and left it sitting askew, so the next car that came along threw it upward into Ms. Clavette's path.  (ETA, similar to the above post --I often hear them go clank-clank, clank-clank as I drive over them.)

Terry  

[brimic]

These happen much more often than anyone would expect. I heard of another manhole cover launch a few months back.
About 15 years ago my coworker's dad drove over one in a brand new F150 just as it blew. It flipped the truck end over end.

[zxcvbob]

askew 

Gesundheit.

[French G.]

Could be a case of exploding manholes. I remember when there was flooding in Norfolk, Va and this somehow got electrical fires going in the underground services. Covers were getting to about 3-4 story elevation. Perfectly happy to see that on the news and not up close.

[never_retreat]

Your talking 150+ pounds of steel for a standard cover. I car did not just flip it up in the air.
I betting it fell of the back of another truck and there trying to keep it quiet.
I was the passenger in a plow truck that hit a raised manhole. It was a big old mack and came almost to a dead stop. The cover was only thrown about 50' down the road.

[230RN]

Gesundheit.

I look askance at your remark. 

brimic said,

About 15 years ago my coworker's dad drove over one in a brand new F150 just as it blew. It flipped the truck end over end.

Kinda-sorta fits in with the exploding gas.  Maybe a spark occurred from the impact, setting off the sewer gases.  I wonder if they're going to sample the air down there in the instant case.

I'm betting methane (lotsa bio-decay-type stuff going on down there) or volatile flammables dumped in the storm drain.

Terry, 230RN

[sumpnz]

I suppose it's possible, if rather unlikely, that some exotic car that produces a LOT of downforce drove over it and sucked it out of the road.  I know when they run Indy Car races on street circuits they actually weld the manhole covers in place as they'll go flying otherwise.

[Firethorn]

I suppose it's possible, if rather unlikely, that some exotic car that produces a LOT of downforce drove over it and sucked it out of the road.  I know when they run Indy Car races on street circuits they actually weld the manhole covers in place as they'll go flying otherwise.

Tested and failed by the Mythbusters.  Not even a F1 racer can actually pop the cover.  They weld them more out of paranoia.

[brimic]

http://www.dailymail.co.uk/news/article-3199591/Terrifying-triple-manhole-cover-explosions-rock-Manhattan-block-send-heavy-metal-covers-flying-15-feet-air.html
http://www.nbclosangeles.com/news/local/Underground-Electrical-Fire-Reported-in-Long-Beach-315580761.html

A video to put this into perspective: http://www.cinemablend.com/television/Watch-Manhole-Cover-Explode-Shoot-Air-Live-TV-71290.html

[birdman]

Guessing 30" in diameter, it would only take about 3.5 psig overpressure to lift one at 200 lb weight if my arithmetic is correct.  Less if they're bigger in diameter than that.  

Your math is wrong.  30" diameter = 225pi square inches (706), so 0.28psi for 200lbs.

Tested and failed by the Mythbusters.  Not even a F1 racer can actually pop the cover.  They weld them more out of paranoia.

Yes, not because the average delta pressure across the car isn't enough (given an F-1 car is about 4.6x1.8m, and about 75% of that at least is downforce generating, that's ~10,000sqin, and 3+g of downforce in a 1550lb car is an -average- downforce of 0.45psi, more than enough to lift a man-hole cover, with ~0.5g of excess, which, given about 3" of acceleration room, or theoretically, enough to achieve about 6" of peak height)
It doesn't lift because the 0.45psi isn't the relative pressure under the car, it's the difference between the static pressure on top and on the bottom, and the bulk of it is an increase on the top, rather than a decrease on the bottom, hence, no manhole cover lift.

The is somewhat intuitive given the camber on the wings--steeply cambered wings have an effective increase in pressure on the concave side that is much greater than the decrease on the convex--as a pressure increase in a concave side is stable, while a decrease on the convex side can stall/have massive boundary layer separation.

Further side note, since the man-hole would have to clear the car if it did achieve lift-off, the car would have to be moving at least 40mph for 3" of clearance, or 120mph for 1".  So given that the maximum downforce occurs at high speed, even for extreme cases, if it did lift, it wouldn't hit the car.

Even further side note--because the pressure aspect is reversed on a sucker-car, a fully skirted car with or without an active drive fan -could- suck a manhole cover up.

Even further...given a thockness of ~1", a manhole cover has an areal mass of 0.29psi...so that is the value to lift it, regardless of diameter.

Even even further...a wind-speed of 140mph creates 0.29psi of stagnation pressure--and if it caps a tube, and the incoming stream is 1/2 the area, only 100mph is needed, meaning 4 3" diameter leaf blowers, each with a speed of 140mph could blow a 12" manhole cover off from underneath.

Okay, I'm leaving this rabbit hole.

[230RN]

Quote from: 230RN on February 13, 2016, 07:37:03 AM
Guessing 30" in diameter, it would only take about 3.5 psig overpressure to lift one at 200 lb weight if my arithmetic is correct.  Less if they're bigger in diameter than that.  

Your math is wrong.  30" diameter = 225pi square inches (706), so 0.28psi for 200lbs.

You're right, thanks!   Even better for the thesis --less overpressure to lift one.  What I dood was 706/200 instead of 200/706.  That's why I always add the "arithmetic" disclaimer.  Let that be my worst mistake of the month.

Okay, I'm leaving this rabbit hole.

Apun your leaving, I just wanted to thank you again.

Terry, 230RN

[birdman]

You're right, thanks!   Even better for the thesis --less overpressure to lift one.  What I dood was 706/200 instead of 200/706.  That's why I always add the "arithmetic" disclaimer.  Let that be my worst mistake of the month.

Apun your leaving, I just wanted to thank you again.

Terry, 230RN

Not leaving .net!  Just that intellectual digression

[Ron]

Okay, I'm leaving this rabbit hole.

If the cover won't lift is it still a hole?

[230RN]

Not leaving .net!  Just that intellectual digression.

I got that, I just thought you were making a pun...rabbit holes, manholes...

I couldn't leave that uncovered, hence my "Apun your leaving..."

And I can't avoid the question about "if the cover is stuck on, is it still a hole?"  No, it's a void.

"Let's chloride, Chemosabe!"

"Right, Tonto...  Hi-yo Argentum, awayyyyy!"



Terry

[brimic]

 

[HeroHog]

Were all those calculations done on a manhole cover in a cylinder or in open air or at least with a 1 - 2 " air gap around the circumference? And even if in a cylinder, was that with a perfect seal or were the leaf blowers, air just blowing up in an open ended cylinder? How much height/velocity would those pressures result in?

[HeroHog]

This is what always amused me in college. Classroom examples relied on "perfect world" conditions that don't exist in The Real Worldâ„¢ and those kids were clueless when they had to actually apply their shiny new learnin out in it.

[birdman]

Were all those calculations done on a manhole cover in a cylinder or in open air or at least with a 1 - 2 " air gap around the circumference? And even if in a cylinder, was that with a perfect seal or were the leaf blowers, air just blowing up in an open ended cylinder? How much height/velocity would those pressures result in?

Gap/seal almost doesn't matter, you could have a substantial gap and lose only a little due to restrictive flow.

This is what always amused me in college. Classroom examples relied on "perfect world" conditions that don't exist in The Real Worldâ„¢ and those kids were clueless when they had to actually apply their shiny new learnin out in it.

And that's why real world engineers always add margin...which I did. :)

[HeroHog]

Still, just LIFT or Launch/Accelerate to a height that would do the damage indicated. The pressure would have to be significantly greater I would assume.

Multi part questions require multi part answers, -10 for Gryffindor.

[birdman]

Still, just LIFT or Launch/Accelerate to a height that would do the damage indicated. The pressure would have to be significantly greater I would assume.

Multi part questions require multi part answers, -10 for Gryffindor.

Not really sure what you mean.

[230RN]

Oh, come now, HeroHog.  Just rough calcs to see the possibilities.  In my example, I just guessed at the 30 inches diameter by eyeballing the distance between my two hands held apart about what I figured a manhole cover would look like.  I don't actually know the diameter.

That's how I came up with my inversion error of "3.5 inches per square pound." <  Dumb me.

Thanks again to birdman for catching that.  ( 1/3.5 = 0.28)

We're just looking at some rough numbers to get an idea of the minimum pressure required to lift a guessed-at manhole cover.  A quarter of a pound absolute (0.28 psia per birdman's correction) is not a lot of pressure.  A man can generate about one psia just with his lungs.

In a classroom situation, maybe a lot of students wouldn't "get" the idea that this is how to figure out a problem like that as a first approximation.  Perhaps as a place to start to design the measuring apparatus to test it all out empirically.

Oh, and the velocity required to get something to go up 50 feet is v = SQRT (2*g*50) if I recall correctly... birdman, check my memory on that, please?

But that's only for spherical chickens in a vacuum.  For disc-shaped manhole covers in air, you're on your own.

Terry

[HeroHog]

We're just looking at some rough numbers to get an idea of the minimum pressure required to lift a guessed-at manhole cover.  A quarter of a pound absolute (0.28 psia per birdman's correction) is not a lot of pressure.  A man can generate about one psia just with his lungs.

But that is exactly my initial point, that calculation involves a perfect frictionless seal in a sealed cylinder does it not? If so, and I am betting it does, there are HUGE losses when you are talking a plate on a poorly "sealed" surface that instantly vents to atmosphere and does not remain perfectly horizontal in flight making such guesstimations several orders of magnitude off at best.

[French G.]

All this math, can we find the cover that blew off the shaft of the nuclear test?