Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

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

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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/


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
*
*   This email was sent to you via Structural Engineers
*   Association of Southern California (SEAOSC) server. To
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
*   send to the list is public domain and may be re-posted
*   without your permission. Make sure you visit our web
*   site at: http://www.seaint.org
******* ****** ****** ****** ******* ****** ****** ********



******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
* Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********