topic Re: Foundation beam Elastic ground in Robot Structural Analysis Forum

Foundation beam Elastic ground

Anonymous — Thu, 07 Mar 2019 11:34:25 GMT

I am working with a Foundation beam, and I applied the elastic ground to it.

On this beam I have different loads, some gravitational loads and some lifting loads caused by the wind.

I made a simplified model to see what the changes are depending on how I define the elasic ground. I only wrote a linear load in Z of -10 KN/m, and a load of Z +10KN/m, and used the default elastic ground values.

My question is, when I go to check results and click in the bar diagrams in NTI and check on the KZ reaction, why do I get a reaction on the ground when I am looking at the uplifting load (+10KN/m).

I don't understand quite well how the Unidirectional tab of the elastic ground works, so I dont know if I should activate one of those options, and the literature and forums I have found are really really limited and i could not find the answers.

Re: Foundation beam Elastic ground

Romanich — Thu, 07 Mar 2019 11:46:56 GMT

Hi @Anonymous ,

Have you searched the forum? I hope this thread may help:

https://forums.autodesk.com/t5/robot-structural-analysis-forum/beam-on-elastic-foundation-winkler/m-p/8549024#M73650

Re: Foundation beam Elastic ground

Anonymous — Thu, 07 Mar 2019 13:00:35 GMT

Thank you for your quick reply,

It was actually very helpfull!

I have 3 concerns,

I changed the supports and the elastic ground definition, as the thread recomended, then I created a nodal load case, with a load going upwards and another going downwards because I wanted to see the soil reaction, and think it's perfect how it reacts (only on the downards load).

But I am not so sure about the values of the deformation of the beam on the upwards load, I think the deformation is too small and the fact that there is no displacement at the end of the beam even if I have no support there, does it has to do with the active unidirectional Kz +Uz?

Also i have 2 errors of non linear convergence as you can see on the screeshot, on the case number 2, I asumed it was because there is only an uniform upwards load of +10KN.

And I didn't have any supports to block that reaction, and so I applied a roller and I had the same errors.

And this also aplies to the point loads (Load case 3).

I would like to fix this errors, and also i would like to ask for a detailed explanation of the unidirectional uplifting similar to this, wich I found so nice and clear about the supports. (because I dont understand if they are blocking it or allowing it?)

"In a 3D model you have 6 degrees of freedom: 3 translations UX UY UX and 3 rotations RX RY RZ. To calculate a structure (whatever it is including a beam on elastic soil) you have to provide restrains for all of them as otherwise it will either move way or spin.

When you define UZ elastic soil you blocked two of them (UZ and RY) but still you need to take car of the others. If you define UX and RX support in one node you dealt with the next two. Now you have to think about horizontal displacements and rotation about the global Z which means either UY support in two nodes (or UY and RZ in one) or UY elastic soil along the beam"

Thank you very much in advance!

Re: Foundation beam Elastic ground

Rafacascudo — Thu, 07 Mar 2019 19:29:22 GMT

Uplifting direction is according to the bar local directions.Then if the bar local z is the same of global positive z direction , you have to set uplift z+.

There´s no point on seeking convergence for single cases other than self weight. Other cases like wind ,will never act alone.

SW and combinations are the cases that you should take care. Set the other cases as auxiliary to save analysis time