Robot Model Audit

[Anonymous]

Hi Guys

Please can one of the proficient Robot users in this forum browse through my model and comment on anomalies or discrepancies that exist. Being a new user to Robot, and the only user in our office, I have undertaken to learn the software and prove to the hierarchy that Robot is the way to go!

Unfortunately, in order to have a valid case, I need to be able to produce a model that's structurally correct. My model at the moment returns no warnings or errors and is geometrically sound. 

However, when viewing the analysis results the column moments in MY direction differ considerably and I have no idea why. The results which are expected to be quite similar, differ significantly.

This model is a Vierendeel Truss Pedestrian Bridge. I am quite sure the error lies in something intricate which I haven't accounted for. Please can one of you take a look and comment/advise.

It would be much appreciated!

Thanks

[Rafacascudo]

   That´s because your collums have not the same bar local direction. On Diagram for bars ,all forces are shown in terms of bar local axes. So You must be seeing this below and comparing , but in fact , half of your columms have different local directions.

Setting them all on the same direction on Geometry/properties/gamma angle, you get this below

Other than that you have some problems on your meshed slab.

Watch webinars 1 and 2  about good meshing practices on Build your Robot Structural Analysis IQ - YouTube

Also , on the picture above , are those the main directions you want for your orthotropic slabs on the inclined section???

[Anonymous]

Thanks @Rafacascudo!

Your expert opinion is much appreciated, Thank you very much for your time.

The local direction of the bars is most likely what is causing it. I will amend it and run the design module again. Will this affect the direction of major & minor axis in the columns that the design module considers in the code checking? I was getting unrealistic section sizes due to the high bending moments that the software reported in the columns weaker axis. Will update and run the code checking again and see if this helps.

In terms of the orthotropic slabs, my intention was to model it as 1 way spanning between the floor joists at the bridge deck level. This is indicative of a composite deck that will behave in a similar fashion. Is this not what is shown in the model??

[Rafacascudo]

Aren´t the columms all round section bars?? There´s no major or minor axes!

For the orthotropic slab , I thought that maybe it would be better to span it perpendicular to the edge beams on the inclined part of the bridge.

[Anonymous]

Hi @Rafacascudo

I mistakenly referred to the Vertical members of the truss as columns. Hence, in my reply earlier today I was speaking of exporting to the Steel design module. Apologies for that! I am happy with the RC columns and the cantilever beams that support the bridge.

My main concern in this model is when you view the moments about My for the Truss Verticals, you will notice excessively high values for those near the inclined section of the bridge. I have tried releasing certain degrees of freedom, but have not been too successful. I can't explain why the Vertical that is governing the design, viz bar number 1170, has such a high variance in moment to the rest as per the snap shot attached.

With regards to the composite deck, Do you mean spanning it 11m across the width of the bridge?

Thanks again for your help!