So I don’t want to imply that the Beirut disaster didn’t result in any casualties, because that’s obviously not true. But I was also thinking about the asteroid that hit Chelyabinsk which resulted in a lot of broken windows but not a great deal of injuries.
In: Physics
First of all humans are squishy while windows are fragile. A shockwave from an explosion will have a lot of force behind it but will not be able to push very far. If you push a human a few millimeters they will just absorb the hit without any issues but if you push a pane of glass the same amount it will shatter into a thousand pieces. So humans can be much closer to an explosion without receiving fatal injuries then rigid structures like glass or even concrete. Another reason is that windows are much bigger then humans, especially if you consider that a window is set in a wall. When a shockwave hits a human some parts of the shockwave will hit around the human and go around the back pushing from both sides. And if you have ever tried to punch a hole in a piece of paper by pushing it from both sides you know that this does not do much damage. However with a window the shockwave will hit the walls and never really gets into the room to support the window from behind. You might notice in a few of the images after shockwaves that windows that were left open are more likely to be intact as the shockwave was able to go around them.
Shockwave is a pressure difference, so it works based on surface area.
Let’s say an explosion is quite far away and it only created a pressure difference of 1% of regular atmospheric pressure where your window is. Now for a human that’s almost nothing dangerous, equivalent of going 100m up, only felt in the ears as if a plane takes off.
But even that difference will push on a window of 2 square meters with 200 kg, enough to break many of them.
* Glass is hard.
* It can’t flex very much before it breaks.
* Humans are soft.
* They can flex quite a lot before they break.
It all comes down to the overpressure and local construction
When there’s an explosion, the shockwave that you see propagating outward is a sudden increase in pressure from the ambient, how big of an increase in pressure it is determines what damage it will do. Lots of research was done into what overpressure thresholds have what effect during the nuke testing in the 60s
Windows will shatter at just 1 psi of over pressure
Brick walls will break at 2 psi of over pressure, but fatalities from the blast are unlikely
Non concrete buildings will collapse at 4 psi of over pressure, fatalities will occur at this level from blast effects
Nukes target their blast altitude to give the largest 5 psi overpressure which takes out any non-reinforced buildings and produces widespread fatalities
The level at which windows shatter is actually really low, and since the Chelyabinsk meteor detonated sooo high up, there weren’t any parts of the city that experienced really high pressure unlike in Beirut where a couple blocks could be the difference between 1 psi with broken windows and 4 psi with collapsed buildings.
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