Cables in Robot - strange deformations

[Anonymous]

Hello everyone,

We are trying to calculate suspension bridge in RSA 2014. The main idea of the structure - it has span (two beams) which are joined together with girders.

This strucuture supported with main cables (like in classic bridge) and  secondary cables, which going from main cable to girders. The structure of the bridge is shown on the first picture below.

Unfortunatelly, we are getting quite strange deformations of the structure on the first load (selfweight) - I'm showing the most strange place in red rectange on the second picture. 

Can you help me please? Or may be you can explain what's wrong in our model? Thank you in advance!

[Artur.Kosakowski]

Mind the scale factor

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[Anonymous]

Hello, Artur,

Thank you for your help. But I can't agree with you. 

I've tried to analyse this structure more detaily and exported Nodes Deflections into Excel. Surprisingly I've obtain follow graphic (please, see screenshot image0.png). 

It's very strange for me that first node from support (on each sides) has an uplift. In my mind it against common sense. Can you comment this effect? May be something was modelled incorrectly?

Also, I've modified this file, activated "P-delta" option and removed needless supports. But results are strange in any case...

Thank you!

[Artur.Kosakowski · Accepted solution]

I think it is OK for the defined geometry of the model. Try to increase the length of the first posts (e.g. change Z coordinate of nodes  241to244  from 3.77 m to 3.8 m. For the original geometry the force in the cables stretch these posts which are just bars of 0.6 mm diameter. If you replace hem with e.g. 6 cm ones (with no other change in the geometry) the effect you descried will disappear as well.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Dear Artur,

Thank you for your help! I think you are right and Robot calculates correct values of deformation.

I have checked this task in Ansys and got the same deformations (please, see attached image). Unfortunatelly, I can't change the geometry and sections of the structure, because task for now is to check the design of this structure.

I think that the reason for untypical behaviour of this structure is that span beams of the bridge is quite stiff and cables aren't prestressed; so they are can't "turning on" properly on selfweight load.

Thank you again for your help!