SAP2000 Applying a moment to a solid element 1

[pjlefeb]

Hi all,

I am modeling the elastomeric bearing pad of a highway bridge. I am trying to apply a moment at the top-center of a rectangular solid element. The mesh is actually 2x2x9 length by width by depth.

When I apply a moment load at that top center node and run the analysis, I get a rotation of 88 million radian, obviously false. Also, the rotation only shows up at that node and only in the direction it was applied; all other rotations and deflections at every other node is zero.


It seems that SAP thinks there is zero rotational stiffness at the nodes, but I don't know why.

Some things I have checked:

1. rigid body motion is restrained by fixity at all nodes at the base.
2. point loads give me the deflections, deflected shape, and stress distributions I would expect, so it doesn't seem to be a connectivity problem.
3. If I apply an equivalent couple, I now get the deflected shape I expect, but the rotations at all nodes are zero.
4. I tried another node and saw the same results.


Anyone have any advice for me?

Thanks,

Paul

 

[WillisV]

The nodes for SAPs solid elements do not have individual rotational degrees of freedom. The only way to apply a moment is to apply it as a force couple to the nodes. Individual nodal rotations will be zero as they are not active d.o.f.

 

[pjlefeb]

Willis,

Thanks for your response. The model definitely agrees with what you suggest is going on. Just out of curiosity, is this typical of most FE packages when modeling solid elements?

Anyway, if I needed to get rotations of a node, it seems that the only way to do that would be to back calculate them based on the change of geometry. What do you think?

 

[WillisV]

I am not familiar with a wide aray of FE packages so I can't answer that. Regarding the nodal rotations - I fail to see why you really need need them, but yes you could figure out whats occured with geometry.

 

[pjlefeb]

Thanks Willis.

Regarding those nodal rotations-if you fail to see why I would need them, the best way to find out is to ask!