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Re: reducing rod deflection with tension

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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. 
 
. > Thanks in advance for your help.
 
. > Best regards,
 
. > Ken
 
. > Kenneth S. Peoples, P. E.
. > Lehigh Valley Technical Associates, Inc.
. > 1584 Weaversville Road
. > Northampton, PA 18067-9039
. > Phone: (610) 262-6345
. > Fax: (610) 262-8188
. > e-mail: kpeoples(--nospam--at)lvta.net

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