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Re: real life seismic house test

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On Nov 15, 2006, at 8:40 AM, Andrew Kester, PE wrote:

So here is the pessimist in me coming out, so forgive me beforehand. But anytime you try and replicate a real life scenario in a lab, the results can be just slightly off the real thing. Britain made the first commercial jetliner and did all of their pressure tests in a lab. After 2 crashes killing all the passengers on board, because of stress failures at square windows due to real life compression/decompression, the company eventually quit making planes..
You're bang on about testing. I've been presented with a lot of test results supposedly relating to FEA and the most common question I was ever asked was, 'We didn't (do, measure, consider, incorporate, keep track of, connect, etc) that--is that important?' The second most common question was, 'We had to fiddle around with the test fixture a couple of times during the test--is that important?' Also it's not common to consider degradation over time during tests. I ran into that with some very high tech tankage for high pressure gas. They'd done all kinds of burst testing

As far as deHavilland (designer of the Comet I) there were actually 3 crashes relating to fatigue cracking around the windows. The explosive decompression occurred when the cracks reached a critical size according to the principles of fracture mechanics. At the time fracture mechanics wasn't commonly considered in design--people still thought fatigue occurred when metal 'crystallized,' and that local yielding would mitigate stress concentration. Like with the Liberty ship fractures a decade earlier and the moment connections that broke during Northridge. de Havilland didn't exactly quit making airplanes. The company was bought by Hawker-Siddley and eventually incorporated into British Aerospace. The Canadian and Australian subsidiaries were bought by Bombardier and Boeing. Still plenty of de Havilland designs flying only under the new brands. If you've ever had the good fortune to get close to a de Havilland Mosquito at an air show, they really look timeless, like a Duesenberg automobile or the Chrysler building.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)

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