Earth-quake-- windows shatter--why? 2

[kashyap]

In case of an earthquake the windowpanes shatter first. I do know that this is related to resonance, but that would mean, the frequency of excited vibration(due to earth-quake) matches with the natural frequency of glass window. But again i find this explanation incomplete, can anyone elaborate this and explain further.

 

[Hatch]

My guess is that glass is brittle and does not like to be bent in any direction (it shatters). During an earthquake buildings undergo large displacements and the window frames will be distorted which will break the glass. C. Hugh (

http://www.Hatch.ca)

 

[electricpete]

I agree glass is brittle. Since you ask about resonance (frequency response), it's worth mentioning that the brittleness does depend on the frequency....if you tried to move the glass slowly over a period of years, it would be very ductile (it flows). With normal frequencies in the hertz range, glass acts brittle.

 

[steeno]

How about this:
(I know nothing about earth quakes, we only have tiny
tiny ones and they are by the way mostly reported nightly
- needs silence to be recognized here in Danmark, Europe
so this is just guessing)

Lets assume that the earth quake begins with a huge mass
being moved abrubtly - this could create a "delta shape"
vibration - i.e. momentarily high energy.

If you freq analyse that f.ex by FFT you will have
a flat freq distribution up to some high freq of the
emitted sound.

And thereby you have also energy at the eigenfrequencies
of the window panes. These may somehow act as resonance
circuits with a high gain. Thereby your first notice of
the quake would be the window panes sounding.

There is probably also acoustic energy at all other
frequencies, only you will only notice them later as
the quake generates new and larger "delta pulses".

 

[kashyap]

Hi Steen,

u'r explanation seems to be very much convincing.
Thanks for the efforts. It would have been of much more help, had u qouted some authentic source (Books/Website......).

thanks again,
with best regards,
kashyap

 

[sms]

I doubt very much that the breakage is related to resonance of the window. I am fairly convinced that it is related to deflection of the window frame. As was noted earlier, glass is brittle, and if the window frame deflection is high it will break. I had an experience with this a few years ago, the foundation of my house was settleing very badly on one corner. Cracks were developing in both the brick work outside and the plaster inside the house. The door frame in this part of the house had deflected to the point that I could no longer properly close the door. One night while sleeping, I was awakend to the sound of a window breaking. I thought someone had thrown a rock or something,the weather was fine, no wind. I found that the window had simply poped under the stress of the frame deflection of the window frame. At that point I called in a foundation repair company and had the house leveled. I watched them do the work and was shocked when they had to raise that bad corner of the house about 4 inches to bring it level. Afterward my door worked again, and I have had no problems with self breaking window.

 

[kashyap]

SMS,

now iam really perplexed as to what is right after looking at u'r explanation. But now it reminds me of a different problem altogether,i shall first explain it.

Musicians in opera break glasses while playing some stringed instruments, most of them might have heard about it. Well, now i would like to ask, wether is this related to resonance.

happy thinking,
regards,
kashyap

 

[sms]

Why yes! It certainly is! But you must understand that resonance is not a problem in itself, all material has a resonant frequency. The resonant frequency of a wine glass is pretty high. If a forcing frequency is close to the resonant frequency then you get high vibration of the mechanical system assuming there is little damping in that system. The high vibration in the wine glass breaks it, since there is little damping in the glass.

The forcing frequency in a earthquake is very low frequency less than 1 Hz for the most part. The resonant frequency of a glass window pane, is likely in the Kilo Hertz range. That is why I believe it is the frame deflection that breaks the glass. Now it is conceivable that the resonant frequency of the building or wall is resonant around the earthquake forcing frequency, and thus the wall deflections are very high causing high window frame deflection and breaking the glass.

For some perspective, the middle C on the piano is 261.63 Hz,the middle A on the piano is 440 Hz, high C on the piano is 4186 Hz. The opera singers who break wine glasses are probably singing in the area of a high C, or maybe even an octave above that, the high frequency response of my ears is not good enough to make it out. That is why I stick with the guitar, where you are playing an octave or two lower for the most part.