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

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adding mass to "reduce" vibrations seems counter intutitive; since for most of us dynamic types....adding mass means lowering the system natural frequency, making the system softer

That said, I have worked on vibration problems where adding mass (& thus lowering the Fn) has helped reduce system repsonse to forced input (random mutliple foot falls)

My boss (who at this point will remain nameless since I'm going to probably misquote him) called it "mass damping"

The reason (I think)  why it worked was the mass was increased to the point where the mass intertial effects were so large that the input forces had little effect.

Add more mass to a diving board system & the diver's jump will less effect.

hope this helps......my explanation might not be great but I've seen the results of adding mass.....vibrations were reduced.

cheers
Bbo

On 7/15/06, Mark D. Anderson PE <mark(--nospam--at)alaskaengineer.com> wrote:
If it helps to clarify a possible source of the difference in viewpoint
between Steve and Chris, it appeared that in Steve's analogy (the fan) he
was referring to a forced vibration problem, and Chris' focus is on free
vibration response.

Whether the original Big Dig issue concerned forced or free vibration
response is beyond me...

Mark D. Anderson
Anchorage


----- Original Message -----
From: "Christopher Wright" < chrisw(--nospam--at)skypoint.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Saturday, July 15, 2006 1:34 PM
Subject: Re: big dig structural failure - epoxy anchors overhead supporting
gravity



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)skypoint.com   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania
1864)
http://www.skypoint.com/~chrisw/


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