Rigid Link Transmittal of Axial load

[Bridge_Man]

Hi,

I found what I think a mistake in a model am working on. I have a tank supported by both a latticed tower and a riser that feeds it water.

I previously had the tower leg connect into the ring girder and then get connected via a rigid link to the shells of the tank, but afterwards I thought this could probably be neglecting the axial rigidity of the tower leg, so I connected the leg directly to the shells which the actual detail do. I found a big difference in tower legs' reaction after doing so, and the behaviour overall became much more realistic.

why didn't the rigid link do what it was supposed to do ? am still wrapping my head around this.

old (Rigid link connect the tower leg to the tank) - New (tower leg directly connected to the tank)

Thanks !

 

[rb1957]

it's bad for me to read your model, but it looks like in the old model the stiffness for vertical load (into the leg) depends on the bending stiffness of the shells. if I'm right then the old model will show vertical displacement between the two rigid element nodes.

what FE code are you using ? In NASTRAN there are two types of rigid elements, one constrains the geometry to stay fixed (which I think adds stiffness to the model) and the other doesn't add stiffness (so the rigid element loads move relative to one another depending on the stiffness of the structure.

clear as mud ?

another day in paradise, or is paradise one day closer ?

 

[Bridge_Man]

rb1957, Brother (or sister not sure), you are right on. there is indeed vertical displacement between the 2 end nodes. this shouldn't be the case which indicates the old connection is wrong.

I agree that the old model leg is depending on the bending stiffness of the shells.

Thanks !

 

[rb1957]

right on, man !

Instead of a rigid element you want a constraint equation, constrain the vertical deflection of the top of the post (in the old model) to be equal to the corresponding tank node displacement.

another day in paradise, or is paradise one day closer ?

 

[Bridge_Man]

yes, either that (a rigid constraint), or what I actually did which is changing the end node location of the leg to be the node on the shell.

Thanks !

 

[rb1957]

and you can live with the minor change in strut axis, 'k.

one thing to think about, would be treating as an offset load when calc'ing the column allowable ?

another day in paradise, or is paradise one day closer ?