hypothetical modeling a 3 guywire mast in RISA

[adamewood]

Per the subject title, what we have is a 20ft tall mast on a hospital roof with 3 guy lines attached near the top. They are not spaced equilaterally, but let's assume they are, and each guy is of varying length and vertical slope since they attach to accessible column stub-ups on surrounding lower roofs (about 40ft elevation from top of mast to top of lower roof, or 20ft below the base of the mast). Technically, this mast is a vent stack, but for purposes of this problem, assume that MEP has provided you with a giant self-supporting plastic noodle that can't buckle but will tip over if someone breathes on it.

Now say you were given the task of modeling this system in RISA. You follow the proper RISA protocol for cables by setting their density to 0, making them T only members, and giving each just a little pretension... however you soon realize that it's first order statics which RISA is having trouble with: Say the wind load, acting along the building X or Y axis, happens to be within 30 degrees (say it's at 29.99) of one of your equilateral tension-only guy-lines; you end up with the two guys over 90 degrees away loaded in compression (so they fall slack), and then just the one tension guy holding everything up. And with your wind acting at 29.99 degrees out-of-plane, RISA will only allow that guy to support 0.866 of the load in-plane, so the 1/2 wind load normal to this goes unsupported, resulting in at best, RISA grounding you by locking your joints, or at worst, RISA giving you the boot for causing an "unhandled exception".

To avoid such a fate and retain your reputation as a RISA master-modeler, what do you do to get this model to behave realistically (giant noodle MEP mast notwithstanding)?

 

[rb1957]

run with rods, and apply pre-tension as a hand calc. pre-tension should be 1.5* max compression. max tension is pre-tension + max tension. Then the cables are always in tension and so no one is "cheating". Or maybe apply pre-tension to the rods in the model.

another day in paradise, or is paradise one day closer ?

 

[3DDave]

I read the RISA documentation on simulating cables - they are just beam elements assigned negligible transverse resistance and assigned Tension-only (RISA also lists Compression-only and some other "E" property that I've now forgotten) property. The zero mass requirement is because RSIA-3D (per the docs) doesn't evaluate the catenary as having a way of applying additional tension so dividing the weight by the negligible rate in the transverse direction would produce (per RISA) stupidly large and unrealistic displacements of the beam element.

I'd suggest contacting RISA at this point. Their literature and online documentation suggest this should be no different than any other beam element subject to the specified conditions. It would not be the first time the documentation left out a critical step.

Also, there's a specific RISA forum here ->

https://www.eng-tips.com/threadminder.cfm?pid=1419&page=1

How To Ask Questions The Smart Way

http://www.catb.org/~esr/faqs/smart-questions.html#intro

 

[1503-44]

https://engineerexcel.com/catenary-cable-tension-in-excel/