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Re: big dig structural failure - epoxy anchors overhead supporting gravity

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On Jul 15, 2006, at 4:15 PM, S. Gordin wrote:

Now, if I place a sandbag on it - the vibrations will be much less without adding any rigidity to the system, just because of the increased mass.
Again, I don't see how that works, except if the decreased frequency due to the added mass is enough less than the frequency of the excitation of the fan. And you could get the same effect if you were to stiffen the deck, say by adding a support.

The frequency of a flat slab carrying only its own weight is proportional to the thickness and inversely proportional to the square of the span. You get more bang for the buck by adding supports than you do from adding thickness. Adding thickness means using stronger supports, but adding supports decreases the load per support.

If those panels would be made of, say, steel decking, they will vibrate; fill the deck with concrete - for all practical reasons, they won't.
I wonder if the purpose is to use the mass to hold them in place against uplifting from the pressure wave. Strictly speaking that isn't vibration, it's rigid body motion. Otherwise I think I'll stick to my guns.

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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