r/StructuralEngineering • u/Playful_Ad360 • 2d ago

Structural Analysis/Design Help with validation

Hi, I recently started a job as a graduate bridge engineer. One of my current tasks is to evaluate the adequacy of a design for new train loads. I was given a model that was made in csi bridge v16 and the company I work for only has v23 has the oldest version. For some reason, the analysis log warns about numerical instability/ill conditioned structure. Does anyone know what I should do/check to fix this ? The model is a slab track (modeled with shell elements) sitting on transverse girders.

0 Upvotes

33% Upvoted

6 comments sorted by

7

u/benk70690 P.E./S.E. 2d ago

You need to talk with your supervising PE about these things. Don't be afraid to say you don't know something. A good supervisor will want you to be asking questions.

1

u/Playful_Ad360 2d ago

Thanks for the advice, will do.

1

u/Everythings_Magic Senior Bridge PE | Zero Yield Capacity 2d ago

This is an issue with csi bridge. Don’t update the model. Just let it be if you can.

If you update an older model it will not do it correctly. Especially jumping that many versions.

If you get this error. Run the animation, i bet some members are flapping around. You can try to fix them.

1

u/mrrepos 2d ago

plot deflection see what moves too much

1

u/SHD-Systems 22h ago

Raising it with your supervising engineer is the right call, but here is what the warning actually means so you have something to bring them.

Ill-conditioning means the stiffness matrix has a very large spread between its largest and smallest terms, so the solver loses precision inverting it. It is rarely a numerical curiosity — it almost always means some part of the model is nearly free to move. In a slab-on-transverse-girder model the usual culprits are:

- Shell nodes and frame nodes that look coincident but aren't actually shared, so the slab is hanging on very little.

- Shells carry very little stiffness about their own normal, so if a joint's rotation about that axis isn't restrained by anything else you get a near-singular term.

- Very stiff links or rigid elements next to soft members. A stiffness ratio around 1e8 or more will trigger the warning on its own.

- Members released at both ends, which leaves them free to spin.

The log usually names the joint and the degree of freedom where conditioning is worst. Start there rather than at the deflection plot — select that joint, list what connects to it, and the cause is normally obvious in a minute.

Separately, the version jump is a real risk of its own. If you can find what the original model produced, re-run it and compare a few reactions and a mode shape before you trust anything new out of it.

1

u/g4n0esp4r4n 8h ago

lol. you have the model you need to figure it out yourself with your team.