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Can anyone explain this?

NeilC

Graduate Poster
NeilC
Joined
Sep 1, 2005
Messages
1,347
My office is in Hemel - where the massive explosion was. Although it's well over a mile away from the fuel depot the building was damaged - windows and doors shattered.

The windows in the office are double glazed - there is a 3-4" gap between the panes which is filled with a venetian blind. It's not a vacuum between the panes but it's quite well sealed.

When we came in to inspect the damage, we noticed that in all cases, it was the INSIDE window pane that was broken.

Also the main doors are glass, followed by a small hall with a matt and post boxes etc followed by free-swinging glass doors - both these were shattered but again, the outside doors were not.

Weird. Any ideas?
 
Has the outer glass got some covering on it? In our place, the outer glass has been covered with a laminate in order to reduce the sun glare.
The other alternative is that the outer glass is toughened in order to prevent people smashing it to break in.
 
Possibly the outer pane would compress the gas inside the sealed unit which would help to buffer it and prevent it bending enough to break. The inner pane has nothing to buffer it, so it bends until it breaks.
 
Yeah that's what I've come up with so far.

I toyed with the idea of negative pressure following the blast etc but this seems the most likely cause. Except for the lobby doors which have a hell of a lot of air to compress, plus they are not remotely closely fitted, having a gaps all the way around.
 
Maybe it wasn't broken by the direct shock wave, but by the building flexing due to the shockwave? Or...the shock wave hitting the roof, the roof acts as a diaphragm, compressing the air in the building, which pushes outwards on the inner window???
 
I'd go with the outside film which sounds pretty logical. Otherwise I might look at how the frame holds the glass. Could be more support on the outside.

Otherwise, again, I'd say the blinds were smashed against the inside panes and gave a sharper impact.
 
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Another point of interest is the great amount of buildings damage compared to the few, relatively minor injuries to people. OK, there weren't many people about, but there were some, and most of the accounts seem to involve someone hearing a huge bang, falling over, then picking themselves up pretty much unharmed to realise the building around them was very badly damaged. Must be some property of the shock wave.

Rolfe.
 
Hmmm... That all depends. What's your astrological sign?

Seriously, the compression of the gases between the two panes and subsequent lack of such a buffer on the other side sounds like the best explanation so far (although I am not a window physicist).

--- G.
 
jimlintott said:
Doesn't a Newton's Cradle tend to explain this?
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Possibly, by using the blinds in between the panes, but it's hard to say without seeing them.

One could speculate even more by imagining the outer pane pushing in, but slowed by the compressing air between so it doesn't break. The inner pane flexes inward due to the pressure, but then when the outer pane rebounds it "sucks" the inner one back even harder, and it breaks.

However Occam tells me that it's probably the film on the outer panes, until confirmed not present.
 
Rolfe said:
Another point of interest is the great amount of buildings damage compared to the few, relatively minor injuries to people. OK, there weren't many people about, but there were some, and most of the accounts seem to involve someone hearing a huge bang, falling over, then picking themselves up pretty much unharmed to realise the building around them was very badly damaged. Must be some property of the shock wave.

Rolfe.
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It is the shock wave. This, the way I understand it, was a gas (or vapor) explosion. This kind of explosion makes a huge, but relatively soft (slow) shock-wave. High explosives (HE), on the other hand, make a limited, but very hard (fast) shock-wave. Now, people are small and flexible, buildings are large and stiff. So a big, soft shockwave is bad news for a building, but doesn't do much damage to a person. A HE shockwave, OTOH will bounce off a building but smash a body.

In gas explosions (escaping gas exploding in a house) are notorious for demolishing buildings and sometimes leave inhabitants in the nearest tree, relatively unharmed. Things like HE hand-grenades, fired inside rooms with people in them, will leave the house standing, but you have to scrape people off the walls (sorry for that picture, but, that's how it is :().

As for the windows: Probably, as mentioned, the outer panes are more sturdy than the inner ones. They can be both thicker, and they might be hardened. Same with outer doors.

Hans
 
Also, there's likely to be different glasses used for the inner and outer panes. The outer ones tend to be relatively thick, tough and flexible (not to mention laminated), whereas the inner ones tend to be thinner and more brittle. The inner ones could also very well be "safety glass", which is actually designed to smash easily into small, relatively harmless fragments (instead of long dangerous shards). This might also apply to the inner and outer doors of your building.
 
If you want to kill people using explosives you would use shrapnel. Shrapnel will fly a long way and can easily do serious damage to a person.
 
MRC_Hans said:
This kind of explosion makes a huge, but relatively soft (slow) shock-wave. High explosives (HE), on the other hand, make a limited, but very hard (fast) shock-wave. Now, people are small and flexible, buildings are large and stiff. So a big, soft shockwave is bad news for a building, but doesn't do much damage to a person. A HE shockwave, OTOH will bounce off a building but smash a body.
Hans
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ALL shock waves travel at the same speed- the speed of sound. Slower wouldn't BE a shockwave, or even a sound.

I'd bet on different glass, inside vs out, if it werent for the doors. I'd think both sets of doors would have to be just as strong/idiot proof, while the outer layer of the windows would need to be more 'stone proof' and the inners only need to be 'paper airplane proof'. I still think it may have been some 'echo' of the shock wave, possilby downward from the roof.
 
I'm getting a bit confused here - why would a shock wave not be able to travel at a speed in excess of the speed of sound? Is this speed intrinsic to our atmosphere/mix of gases, and can't be exceeded by any kind of propagation of force?
Also, come to think of it, why does every kind of sound, regardless of strength, automatically travel at this maximum speed?

*staring into a newly opened can of worms*
 
Floyt said:
I'm getting a bit confused here - Also, come to think of it, why does every kind of sound, regardless of strength, automatically travel at this maximum speed?

*staring into a newly opened can of worms*
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I think that is a good question. Just done a quick search and cannot see a non-mathmatical answer. But see these for mathmatical answers
http://en.wikipedia.org/wiki/Speed_of_sound
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
http://www.aerospaceweb.org/question/atmosphere/q0126.shtml

This one comes close http://library.thinkquest.org/19537/Physics4.html
The speed of sound depends upon the properties of the medium it is passing through. When we look at the properties of a gas, we see that only when molecules collide with each other can the condensations and rarefactions of a sound wave move about. So, it makes sense that the speed of sound has the same order of magnitude as the average molecular speed between collisions. In a gas, it is particularly important to know the temperature. This is because at lower temperatures, molecules collide more often, giving the sound wave more chances to move around rapidly. At freezing (0º Celcius), sound travels through air at 331 meters per second (about 740 mph). But, at 20ºC, room temperature, sound travels at 343 meters per second (767 mph).
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casebro said:
ALL shock waves travel at the same speed- the speed of sound. Slower wouldn't BE a shockwave, or even a sound.
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No, shock waves are supersonic.

But don't take my word for it...

"Unlike ordinary sound waves, the speed of a shock wave varies with its amplitude. The speed of a shock wave is always greater than the speed of sound in the fluid and decreases as the amplitude of the wave decreases. When the shock wave speed equals the normal speed, the shock wave dies and is reduced to an ordinary sound wave."

http://hypertextbook.com/physics/waves/shock/

"SHOCK WAVE is a thin transitive area propagating with supersonic speed " (I like this one's animations - note how the cone around the supersonic jet trav els along with the jet ie at Mach > 1.)

http://physics.nad.ru/Physics/English/swa_txt.htm

"Sound waves in the air, whether from a whisper or a yell, travel at the speed of sound, called a, for "acoustic" speed. This speed depends on air temperature, but a is typically about 340 meters per second in "standard" air. Shock waves, on the other hand, travel faster than a, being supersonic wave phenomena. "

http://www.americanscientist.org/template/AssetDetail/assetid/48547/page/3
 
Okay, I've learned something new already today, and I've only been on the net 10 minutes.

I've sure seen aerial movies of bombardments, where the supersonic shock wave's effect is seen on the ground/trees. It is followed shortly by the dust being raised by the pressure wave, at the speed of sound. I guess I didn't know what I was seeing. So, a structure could be hit by 2 impacts: the shock wave, and then the pressure wave- assuming the explosion was of large enough amplitude to create a shock wave to begin with.

So, we have two layers of glass. One is resistant to both shock and pressure of the extant amplitudes, one layer less resistant to one or the other....which?
 
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