Issue using truss elements

[Anonymous]

I created a simple model using beams and columns to form a new station structure and canopy, with plan bracing at roof level to transfer wind loads to the elevation bracing. I used truss elements in Robot to ensure the bracing only took axial load. See attachment of part of the structure. I found that I had a weird bending moment/ deflection diagram whereby the beams with bracing members connected had double the values as the other beams (141.2 vs 70.6) and was expecting all beams to have a bending moment of the average of these values. Having checked my model thoroughly I realised that this issue was occurring due to the plan bracing members and couldn't think of any explanation as to why this would happen - load being applied vertically should not be taken at all by horizontal axial only members. I exported the model to Tekla to double check my logic and the software gave me the bending moments and deflections as expected (i.e. the same values for all the beams). Could someone please explain why this has occurred in Robot and how to ensure I get the correct results?

Thanks

Ian

[Muhammed.OPERA]

Hi @Anonymous

For the matter of double value for each element, make sure that you display the diagrams in one of your defined load cases (not simple cases or combinations).

For checking upon your values , please attach your model.

[Anonymous]

The screenshot was a loadcase with area load. I've attached the file.

[Muhammed.OPERA]

Hi @Anonymous 

I noticed there's many instability warnings, of course as a result to the releases that you made.

I recommend to you that webinar (that's very useful in that case) :

https://www.youtube.com/watch?list=PLY-ggSrSwbZqow_60fiqJwS69mg1nQMzk&v=5CCLzG12JQ4

and you better check your cladding local direction and it's load distribution.

I hope that helps.

[Anonymous]

No, that doesn't help. I can easily fix the releases and the panel distribution is fine, please take out the truss elements forming plan bracing in my model and you'll see you get equal moments in each beam. The releases and the load distribution is not the issue, the truss elements are causing my moments to be incorrect which should not be the case and does not happen in Tekla or if you were to calculate moments by hand. Please only respond with helpful advice answering my question.

[Artur.Kosakowski]

Hi @Anonymous

Try to block RZ in the supports to prevent columns rotation about their own axes due to beams' releases. 

@Muhammed.OPERA  thank you for trying to help. I do appreciate this.

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

[Artur.Kosakowski]

Hi @Anonymous

Do you still need any additional help with your model or has your question been answered? If the later, would you mind marking the solution so that others could find it faster? Thank you.

[Anonymous]

Hi Artur, no it didn't. I just fundamentally want to know why the truss elements (axial only bracing on plan) are causing my bending moments to be different in my primary beams. I have attached a very simple model to illustrate my issue. Thanks, Ian

[Artur.Kosakowski · Accepted solution]

Hi @Anonymous

I assume you don't want the load from cladding to be distributed to bracing, do you? 

When you apply uniform load to truss bar it is replaced by end nodal forces (e.g. for self-weight).

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

[Anonymous]

Hi Artur, no, I don't, but this shouldn't be happening as the truss elements are axial load only, so why are they taking vertical loads when they are horizontal and axial only? I can't see the screenshot well enough to know how I stop this happening, as the obvious way of stopping members taking shear forces is to make them axial only, which obviously doesn't work in Robot.

[Artur.Kosakowski]

Hi @Anonymous

I don't, but this shouldn't be happening as the truss elements are axial load only, so why are they taking vertical loads when they are horizontal and axial only?

This is as design to allow you to define such load e.g. when they are used to model a truss and also to include their self-weight in calculations. The difference comparing to the \standard' bar element is that such load is converted into nodal loads so that there is no bending in such elements.

I can't see the screenshot well enough to know how I stop this happening,

You can click on the picture to enlarge it.

as the obvious way of stopping members taking shear forces is to make them axial only, which obviously doesn't work in Robot.

There is a difference between tension/compression only (no shear, no bending) and truss bars (no bending).

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.