topic Re: Incorrect displacements in Robot Structural Analysis Forum

Incorrect displacements

Anonymous — Fri, 19 Mar 2021 11:04:09 GMT

Hi, I am modelling a simple frame. What I cannot understand are displacements of 5 bars hanging from the main beam from the 4th load case "V1". It gives me over 34 meters of node displacement. Can somebody help me?

Re: Incorrect displacements

Simau — Fri, 19 Mar 2021 12:16:43 GMT

@Anonymous

Not easy to answer with screenshots only.

Share your model (zip it before)

Re: Incorrect displacements

Mohammed_Ata — Fri, 19 Mar 2021 12:17:54 GMT

Surely, there is a missed step. Upload your file for check

Re: Incorrect displacements

Rafacascudo — Fri, 19 Mar 2021 13:20:11 GMT

you probably ignored an instability warning?

no warnings

Anonymous — Fri, 19 Mar 2021 13:55:22 GMT

There are no warnings during the calculations.

Re: Incorrect displacements

Anonymous — Fri, 19 Mar 2021 19:03:51 GMT

I attached the file.

Re: Incorrect displacements

Simau — Fri, 19 Mar 2021 20:36:03 GMT

@Anonymous

Didn't see any problem in your model.

Yes indeed weird displacememnts for nodes 6 13 14 15 16 under case 4.

Re: Incorrect displacements

Stephane.kapetanovic — Sat, 20 Mar 2021 06:04:26 GMT

your beam has no torsional inertia. Use a truss or create a compound section.

Re: Incorrect displacements

Anonymous — Sat, 20 Mar 2021 09:05:14 GMT

Torsional inertia is almost identical with given on a data sheet.

Re: Incorrect displacements

Simau — Sat, 20 Mar 2021 10:08:25 GMT

There is no instability reported

So what is weird is that the displacement is 10 times greater than the length of the bar if it was horizontal

Re: Incorrect displacements

Anonymous — Sat, 20 Mar 2021 10:16:42 GMT

So I can assume that this is some kind of Robot error? I will try to model this structure again so we can be sure that this is a problem caused by the software.

Re: Incorrect displacements

Mohammed_Ata — Sat, 20 Mar 2021 14:08:20 GMT

What type of sections used! I think the section are wrongly defined

I tried to change the section to concrete 450*450 and the displacement seems to be reasonable! (see the pic)

Please check your sections!

Mohammed Ata

Structural Design Engineer

Autodesk Expert Elite

Re: Incorrect displacements

Stephane.kapetanovic — Sat, 20 Mar 2021 20:55:37 GMT

You have chosen a compound section and its calculation only adds the torsional inertias of each member instead of creating a complete section having an inertia as tubular