Releses causing problems in truss design

Anonymous — Sun, 14 Jan 2018 21:47:23 GMT

Hello,
I am a Civil Engineering student. Forgive me any possible ignorance. I just started using Robot software to design a truss and came across a problem. I was always thought that all joints of the truss must be pinned. Hence while designing truss in Autodesk Robot(structure type: 2D Frames) I leave all joints released. However, it causes instabillity of the design(Type 1). Apparently I need to delete some of the releases. And that raises a few questions that I need to ask to be able to design that truss:
1) Can I delete the releases? How can my truss have firm connection if the theory of truss design says that all joints must be pinned so that bending moment isn't transferred? How will this affect my construction? How does it correspond to real life structure(what does it change if I would be really building designed truss?)
2) What is the difference between pinned-pinned release and pinned-fixed release? Which one should be used in what situations? How does it correspond to real life structures?
3)Why does structure type "2D trusses" doesn't have freedom in RZ dimension? Is it like bending moment wouldn't be happening at all? What's the point of such design if such situation isn't possible in real world since bending moments almost always appear in any complex structure?

Re: Releses causing problems in truss design

Anonymous — Mon, 15 Jan 2018 03:25:28 GMT

Welcome my friend

In order to avoid instabilities in the analysis of trusses its recommended to use trusses from library (picture below), but If you want to model a truss from scratch I suggest to model chords as continuous members and to define releases only in diagonals and verticals. If you define releases in chords you are providing a mechanical instability so thats why you got Type 1.

1) Can I delete the releases? How can my truss have firm connection if the theory of truss design says that all joints must be pinned so that bending moment isn't transferred? How will this affect my construction? How does it correspond to real life structure(what does it change if I would be really building designed truss?

The basic of trusses is that the joints are pinned but that's just a mathematical model and usually we ignore bending moment. And that is because all the members that converge in the joint, do it in the same point(concentric), so you can assume joint as pinned.

2) What is the difference between pinned-pinned release and pinned-fixed release? Which one should be used in what situations? How does it correspond to real life structures?

Basically the difference is about the capacity of the joint to transfer moment (rigidity of the joint). For example, if you have a steel beam-column connection you could fixed it adding some extra plates that make it more rigid. It's all about the rigidity.

3)Why does structure type "2D trusses" doesn't have freedom in RZ dimension? Is it like bending moment wouldn't be happening at all? What's the point of such design if such situation isn't possible in real world since bending moments almost always appear in any complex structure?

I already answer this in the other two.

I recommend you to watch webinars recorded by Technical Support Specialists of RSA https://forums.autodesk.com/t5/robot-structural-analysis-forum/robot-structural-analysis-webinars/td-p/5523337

Best regards,

Cesar

If I helped to answer your question accept this as a solution. ty.

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Releses causing problems in truss design

Re: Releses causing problems in truss design