Need a book? Engineering books recommendations...

# Re: reducing rod deflection with tension

• To: seaint(--nospam--at)seaint.org
• Subject: Re: reducing rod deflection with tension
• From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
• Date: Thu, 28 Aug 2003 10:45:21 -0400

```Ken,

It is probably better to specify the deflection of the beam than to specify
the tension in the rod.

For example, the deflection of the beam under applied loading without the
Queen post is calculated for queen post locations/center of span.  Calculate
the load that the queen posts must apply to bring this deflection back to
what you want.  From geometry, you can calculate what force is in the rod,
but the contractor does not have to worry about that.  You just have to make
sure that the rod is big enough to take that force.

If you want to specify the force in the rod to prevent the rod from sagging
more than 1/2", you might end up with the beam looking like an arch.

HTH

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Ken Peoples wrote:

. > I have a queen post truss with 1" rods for the bottom chord.  The spacing
. > between the panel points is 22 feet.  According to our calculations, the
. > 1" rods would sag about 10" under their own weight if they are not in
. > tension and free to move at their ends.  I would like to find out how
. > much tension will need to be applied to reduce the self-weight deflection
. > to a reasonable amount - say 1/2".

. > I see from Roark's formulas - Table 12 Case 3 that if the ends can't
. > move, then the force on the ends would be about 1100 pounds and the
. > deflection of the rod would be about 1 1/2".

. > While this is helpful, this formula does not get me the force required to
. > bring the rod up that remaining 1" to get the deflection down to 1/2".  I
. > understand that it is not possible to get the deflection to zero and
. > expect that as one approaches zero the force qets quite high.  What I
. > don't want to do is to specify that the erector have to take more sag out
. > of the rod than is reasonable - thus inducing huge tension forces.

. > Best regards,

. > Ken

. > Kenneth S. Peoples, P. E.
. > Lehigh Valley Technical Associates, Inc.
. > Northampton, PA 18067-9039
. > Phone: (610) 262-6345
. > Fax: (610) 262-8188
. > e-mail: kpeoples(--nospam--at)lvta.net

******* ****** ******* ******** ******* ******* ******* ***
*   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