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Glass Cracking

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Here's something that I hope one or more of you can give me an idea of what 
is happening.

Two windows in a 2-story frame residence, one on the second floor, the other 
on the first floor.

Window on the second floor is on the east side of the house.

Window on the first floor is facing west, and under a porch overhang.

Windows are double glazed, insulating glass.

Interior pane on the window on the second floor has a parabolic crack similar 
to the shape of the SEA logo.  Crack started at the left jamb, proceeded up 
to the right, curved around and came back to the sill near the right jamb.  
Homeowner watched the crack develop and in 3-hours (8:00am to 11:00am) it 
progressed about 8-inches around the perigee.  (The homeowner marked the 
times on the glass.)  The exterior pane is undamaged.

The owners report that the same type of parabolic crack occurred in the first 
floor window, with the interior pane cracking, not the exterior pane.  (The 
first floor window has been repaired.)

The house is about 2-3 years old and there is no visible sign of distress in 
the house.

Any thoughts, anyone???

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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