Modeling a rigging wire rope in RISA 3

[Structural noob]

Hello folks!

I was wondering if there is a way to model a axial only member in RISA such that it would take no moment. I'm trying to model a wire rope that is picking up a frame at 4 points and would prefer it to be an axial only (tension only) member. Right now I have modeled it as a tension only member but I am still seeing pretty high moments in the wire rope which for some reason don't show up in the detailed report that RISA generates - see RISA snapshot here ->

Is the moment due to P-delta irregularities? I do get a warning from RISA which says "Could not obtain accurate results of sectional deflections and moments with Geometric Nonlinear Stiffness P-delta method. Please consider adding internal nodes or subdividing the member" When I add a couple of nodes to the four wire rope members the moment goes away. But I was just curious if there was a way to model Axial only members in RISA so as not to add any internal nodes.

 

[TLHS]

I'm not familiar with the down and dirty of RISA, but some software won't do tension only members with some types of non-linear analysis. Might want to check if that's the case here. If they're not aligned in a way where they could take compression, what if you model it as a truss member instead?

 

[azcats]

Is the moment all due to self weight? I've used a weightless material property for tension only rods before when the model acts up.

 

[azcats]

[URL unfurl="true"]https://www.eng-tips.com/viewthread.cfm?qid=293208[/url]

Our resident Risa expert (expat?) JoshPlumSE explains that pretty well in the above thread.

 

[JoshPlumSE]

One of the key ideas here is that RISA does not have a true "cable" member. So, anytime you want that type of behavior you are using a "frame" member to approximate that behavior.

Some words of caution:
a) The first thing (which is mentioned in that other thread) is that you should probably use a material with 0 density so that self weight doesn't cause weird moments and deflections.

b) You really only want to model cables that can be adequately represented by a STRAIGHT LINES. Like an guy wire or such. Why is this? Anytime the "cable sag" effect will be significant the cable behavior will be NON-LINEAR which RISA can't really handle.

c) For Guy wires and other straight cables that require pre-tension to remain straight, it is a good idea to add a negative thermal load to create the pre-tension. The RISA help file discussion on thermal loads should have some discussion on how to calculate this load.

Disclaimer:
I was the head of tech support and training and such for much of the 16 years that I worked for the old version of RISA (before they were bought by a software conglomerate). While I still consider myself something of a RISA expert, I have not used the program very much since 2018. I'm sure they've added new features since then. If they've added a cable element, then I will have very little knowledge of it's development and testing. My comments refer specifically to the way the program worked when I was there.

Also, I now work for one of RISA's main competitors (CSI / SAP2000). The analysis capabilities of the CSI programs generally have a lot more non-linear capabilities than RISA (including cable elements). Though I am still learning about much of our non-linear features. Regardless, I have some personal and professional reasons to be biased on this subject (RISA and non-linear features). Therefore, please understand that my comments about RISA may include my bias and may be a bit out of date.