Gravity load deflection doesn't respond to tensile force in RISA

The RISA analysis (for the most part) is based on small deflection theory. That means that the displacement of the structure does not change the stiffness matrix.

The example I like to give is the trapeze artist standing on the cable. How does the cable resist his weight? The bending stiffness of the cable is essentially zero. Instead the cable deflects and forms a V. Then the axial tension in the cable on either side of the V act to resist the trapeze artist's weight. That's the reality.

In a program like RISA, however, the initial stiffness of the cable due to out of plane bending is based solely on the moment of inertial of the cable. So, the program would show a very, very large deflection.

If your cables or rods are mostly straight and subjected to purely axial forces then your analysis may still be pretty good.... except for the self weight of the cable. Therefore, you might change the material of the cable to one with the same properties, but with zero density.... That way, the self weight of the cable doesn't cause a large deflection that makes your analysis look goofy.

Ah, very good information! Yes, it appears to be treating the tension rods as a beam in bending moment. OK, I'll stop being concerned. Thanks a lot!

Josh,
Your suggestion of changing the density of my round bar tension member is a great one. I'm not finding a way to do that or information in RISA on doing it. Can you help with this? Thanks.

Step 1:
Go to the materials spreadsheet. HRSteel tab or General tab depending on which type of material you used to define your rod.
Step 2:
Create a new line in that spreadsheet something like "A36-wgtless" that has the same properties as the material you want to use with your bar. But, set the density to zero.
Step 3:
Assign that material to your rod member or the section set used for your rod.

I like it! Thanks!!