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RE: Column with unknown axial load

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Gregory:

OK. Assume that the frequency of the column having zero axial stresses is 25 Hz. A 10 Hz measure indicates that the column is in compression. A 60 Hz measure indicates that the column is in tension. But, how to know that zero stress occurs at 25 Hz, since the column is framed?

Jorge Jimenez, PE

From: gregory szuladzinski [mailto:ggg(--nospam--at)bigpond.net.au]
Sent: Friday, March 04, 2011 5:41 AM
To: Sseaint Org
Subject: Column with unknown axial load

 

Reply:

 

One of the ways to test is to measure frequency of lateral vibrations.

Tension will increase, compression decrease.

(One of the ways to determine elastic buckling is to say that zero frequency

is achieved at that state.)

The formula is quite simple and can be found in many sources.

 

An easy task would be if no other column was directly above this one.

You could then place two different weights above and measure two different

frequencies. This would define the whole range including no-load point.

 

Sincerely

Gregory from Oz

 

 

 

Question:

 

From: "Jorge Jimenez" <jajimenez(--nospam--at)onelinkpr.net>
To: <seaint(--nospam--at)seaint.org>
Subject: Zero stress column

I have a column resisting heavy loads in an industrial warehouse. The =
lower
part of the column should be removed to allow a projected circulating =
space.
I am designing a reaction frame to carry out the load from the upper =
part of
the column, with an special movable connection intended to reduce the
vertical movement above the column cutting point in the moment of the
cut-off of the lower part of the column. The perfect moment for the =
cutting
of the columns is when compression stresses become near to zero. It is =
not
easy to estimate closely the existing load on the column. I am wondering =
if
it's possible to detect the change from compression to tension in the =
column
with not invasive tests. Something able to detect any special =
characteristic
of the steel internal structure being different in compression and in
tension. Any help?

 


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