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

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

(OP)
I am designing a jib. There is a pair of 3/4" x 20' long round bars as tension members to support the wide flange that the trolley runs on. RISA says that these round bars will deflect 13 inches in the center. This might be the case if there were no tension but in reality they don't deflect at all on the jibs we have like this here already. The thing less trustworthy is that the gravity deflection doesn't change when I place the load on the end of the jib and the round bar member has a large tensile load. I expect that the deflection would decrease with a tensile load.
Replies continue below

### RE: 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.

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

(OP)
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!

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

(OP)
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.

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

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.

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

(OP)
I like it! Thanks!!

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