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Re: reducing rod deflection with tension force

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I am not sure if this will help but I had a similar situation with the design of a king post truss.  My issue was to get near zero deflection of the top beam.  I made a simple 2-D model in RISA and applied a negative temperature to the rod.  This in effect provided the post tensioning of the rod and then the computer provided all the forces/deflections in the other members.  By trial and error on the temperature I was able to simulate the deflection I was shooting for.  Note that as you put tension in the rod the beam will deflect upward and the compression in the beam increases which needs to be incorporated in the design.

Thomas Hunt, S.E.
ABS Consulting

"Ken Peoples" <kspeoples(--nospam--at)>

08/28/2003 07:00 AM

Please respond to

"Seaint" <seaint(--nospam--at)>
reducing rod deflection with tension force

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,
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