truss design, instability and displacement
If it´s a plane truss you don´t need to define truss bars , just choose the correct structure type when opening Robot.
Rafael Medeiros
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Can you send your model?
Rafael Medeiros
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Received your file . Your answers
1- It´s "shell" structure type , so all DOFs are available for the bars nodes . So ,for the plane trusses you have on your structure , at least the chords cannot be truss bars!!! . The top chords are linked to the diagonals and the bottom chords have also the diagonals and the beams that transfer the loads to the trusses attached . So ,they cannot be truss bars!!
I made the chords work as normal bars and all the instabilities warnings (and consequent huge displacements) are gone. Instabilities type 1 and 2 cannot be ignored. Watch the webinar about it (and all the others) on Build your Robot Structural Analysis IQ - YouTube
2- No more huge displacements , But your wind load load is giving 212mm displacement on top node. probably due to the lack of horizontal truss linking the trusses top chords . Bucking will be a big factor when designing the top chords
3- Moving loads (alone or in a combination) generate 2 additional load cases(+/-)which are envelopes for all generated moving load components .
Other than that ,I think your moving load step should be 0,5m instead of 1m and your route should be extended in 2 meters so You can "see" your vehicle entering the bridge.
File corrected , as I think it should be , sent directly to your email
Rafael Medeiros
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1- There's no ideal truss on the real world. Nobody will build it using a pinned conection on each truss node. And even if they do , you would have loads on midspan due to selfweight and wind at least ruining the " ideal truss"
2- For the out of the truss plane buckling , your top chord is totally free , unrestrained. So you will have a huge buckling length and you will have to use a pretty big section to be obbeying the design codes. And it will be worse because the top chord will always be in compression
2- For the out of the truss plane buckling , your top chord is totally free , unrestrained. So you will have a huge buckling length and you will have to use a pretty big section to be obbeying the design codes. And it will be worse because the top chord will always be in compression
Rafael Medeiros
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1- By normal bar , I meant a bar without advanced properties . Beam , columm , they are all normal bars
2- Yes ,You should get aproximately similar results
3- Not sure of what is the question , but yes forces in the chords are a lot greater than in the diagonals
4- check the excelent steel design webinar (3rd webinar) on Build your Robot Structural Analysis IQ - YouTube
You will have to set code paramenters for each of your bars through here.
Rafael Medeiros
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1- Don´t know what you mean by " controlled section" , But in Robot all bars are born "fixed" with all DOFs restrained . The results for a 10m bar is the same for 10 x 1m bar .Dividing a bar in several pieces doesn´t make any difference on the results regarding internal forces and displacements.
2- On a 3d structure type enviroment , You wouldn´t be able to perform any kind of analysis(including buckling analysis), setting al bars of the truss as "truss bars" . Structure would be unstable and you would get several instabilities warnings as you did receive.
On an "ideal" 2d truss , yes , all nodes would be , by definition ,restrained on the transversal (out of plane) direction and the chord buckling lengths would be the length of the bar between 2 nodes.
4 ,5 ,...- For the top chord I think that conservatively you should consider the length of each bar for the in plane buckling (Ly) and the whole arc length(41,06m) for the out of plane buckling length (Lz).
for the diagonals , between 0,7 and 1 as K factor .
For a global buckling analysis ,you can do a buckling analysis. I did it considering only the self weight load case . The 1st buckling mode is of course the expected one (Top chord lateral sway)and gives a 2,76 critic coefficient.
Rafael Medeiros
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1- Both . You have to check all structure bars against the code of your preference . And you can use the Robot buckling analysis ,for example , find a more precise out of plane buckling length for the chord bars.
I suggested the whole chord lenght (41.06m) ,but seeing the buckling analysis results , it is giving around 16,3m for Lz in the 2nd mode , which is the mode cleary affects only the top chord. And seeing the deformed shape of this mode with its double curvature ,it appears that this value for Lz is not out of reality
Not very familiar with buckling analysis though . Maybe Artur ,Rafal or Pawel , could give us a hint if in this case the Lz value is a good way to determine a trustable out of plane buckling length.
4 - For moving load you probably should create a case from a component and combine with your other vertical loads. There is no point analysing buckling for a moving load case alone.
Rafael Medeiros
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yes , for steel design you don´t need to "create a case from a component".
For buckling , probably it will be the component that gives the chord its highest compression force
Rafael Medeiros
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unmark everything on "advanced bar properties" , select/list the bars and click apply
Rafael Medeiros
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