Robot Structural Analysis Help

[Anonymous]

Hi all, I'm pretty new to robot and started to encounter a few problems that I cannot solve. 

It's a simple project, just a frame for a acoustic roof that will be suspended from cables. I'm designing the steel structure that will support a pulley system and some other loads. I can't seem to get an equal distribution of shear force and bending moments and the structure in plan is symmetrical. The results appear to be all over the place. If I provide further details and the robot model, could someone give me a hand solving it? 

Thanks 

[Simau]

Send your model

[Anonymous]

Hi, thanks for the reply. I’ve provided further information and the model.

Structure

Steel 355 UB – Hot UK Rolled Sections

Designed to Eurocode

Assumed that all supports are simply supported

From the image you can see that there are 4 beams spanning in the x-direction and smaller beams spanning perpendicular between these long beams. Although I have showed pinned supports, these will actually be cables suspending the structure. I don’t really require the reaction forces at these points, all I’m trying to find out is the location and size of maximum/minimum bending moment and shear force. I have applied the loads, there will be an area load that I'm going to add later. If there is any further information you need please ask, and thank you so much for your help.

[Simau]

You can use these graphical tools

Clik on the bare (bares) you want information,

[Anonymous]

Thanks for the reply, I can run the results and get a graphical representation, but the results don't seem to be correct. Not sure why the shear force is not symmetrical if the structure and applied loads area. 

[Simau]

Give some examples

[Rafacascudo]

You have instability type 2 warning which cannot be ignored. Watch webinar 16 on Build your Robot Structural Analysis IQ - YouTube 

Inspecting the deformed shape and Looking closer your bar 5 has its ends disconnected from bars 8 and 9

Connecting the bar 5 and using only the necessary releases , you will get the diagrams you want

Corrected file attached

[Anonymous]

Hi Rafael, I appreciate the time you have taken to fix the model. 

I would like to ask how did you solve the instability type two, and what effect did it have by changing the release to Ry-Ry. 

Thanks 

Zehshan 

[Rafacascudo]

Connecting bar 5 to bars 8 and 9 and also you don't need to release moment on bars ending on a pinned support.

[Anonymous]

Hi Rafael, I don't quite understand what you mean. Most beams now have release in Ry - Ry which means that they are free to twist. Is that correct? If they are changed to pinned-pinned, does this  mean they can rotate at the ends in all three axis, which is what we want. You mentioned that you don't need to release on bars ending on pin supports, however most of these bars are connecting to other beams. 

Thanks 

[Anonymous]

Hi Rafael, I understand what you have done, releasing the beams in the y plane makes this a pinned structure. I would just like to ask, why is it that the longer beams have to be set to fixed-fixed (N/A). 

Thanks 

[Rafacascudo]

because ,as I said before ,you don´t need releases on bar nodes that end on a pinned support. Rotations are alrealy released!!!

[Artur.Kosakowski]

Hi @Anonymous

I'd recommend watching the recording of the Instability warnings during model analysis Robot webinar. 

If one or more of these posts (especially these made by @Rafacascudo (thank you) ) answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.