Support Definition Problem

[dtarwater]

I am analyzing a bridge girder in Robot.  I have added lateral supports and creat a support definition that is fixed only in the UY direction.  When I view my results, however, the supports have a reaction in the FZ, FY, and MY directions when I am expecting only a reaction in the FY direction.  The supports also seem to lower my total moment (MY direction) because when I take them out, the maximum MY is slightly higher.

Am  I creating the support definition wrong?

I have included screenshots of the lateral brace definition, and the results both with and without the lateral braces.

Thanks

Doug Tarwater

[Artur.Kosakowski]

Is there any instability reported during calculations? If it is nonlinear analysis - has it converged?

[dtarwater]

Yes, it says there is an instability of type 3.

[Artur.Kosakowski]

This may be the possible reason. Try to check if the boundary conditions you provided are correct. Check releases, definition of other supports etc. Please attach the file if any further help is needed.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[dtarwater]

Could it be reporting this instability because there is only one support in the center of the beam?  Maybe it is slightly out of balance?  Would this cause  the instability and therefore the problems with the support?

[Artur.Kosakowski · Accepted solution]

The FEM type programs (as Robot) are not able to handle such situations. You have to change the support conditions e.g. restrain the rotation a the support (for the symmetrical beam and load the MY reaction will be 0).

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[dtarwater]

Thank you for your help!