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Re: Very short load duration

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On Jul 8, 2009, at 8:43 AM, Jim Carey wrote:

I have a steel duct that is to be designed for, among other things, a “severe pressure pulse” load that lasts 10 milliseconds.
The rule of thumb is that a suddenly applied load causes twice the deformation of the same load applied statically. Consequently the stress is twice that of the stress from static application. The ground rules for the rule are that the ramp up time be less than than 10% or so of the system period, that the dynamic response have roughly the same shape as the static response and that you ignore damping. The first is true by hypothesis; the second is normally true for flat panels, but not necessarily for short cylinders with very thin walls and the third is conservative, but not grotesquely so. The basis for all this is the assumption that one vibration mode dominates so your duct will respond like a single-degree-of-freedom system and you don't have wave propagation issues.

If your client wants a reference, try Harris and Crede, _Shock and Vibration Handbook_.

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



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