Instability issues

[Anonymous]

We're designing a rather large steel roof structure which combines HEB1000 elements (main beams) with smaller HEA240 elements.

If we use the Automatic solver (Job preferences), quite a few instabilities occur: also instabilities of type 3, which might be the result of the relative difference in stiffness between the combined elements (Help file).

However, as we use the Multi-threaded solver - which would be more suited to deal with large structures - no instabilities are reported (not even the types 1 or 2 we know are still present); the only instability type reported is type 98 (?).

So, is it correct to use the Multi-threaded solver instead?

Also, is there a quick way to get a listing of all occuring instabilities?

We're using Robot Structural Analysis Professional 2011.

Thanks in advance

[Artur.Kosakowski · Accepted solution]

This solver by its nature is not capable of reporting instabilities with similar accuracy as other solvers available in Robot and this is why (despite being the fastest for larger models)  it is not set as the default one in the program . I would suggest to do the following actions:

1. Delete (or set as auxiliary) all load cases but case 1 (usually self-weight)

2. Run calculations for such 'limited' model using 'Automatic' solver (Sparse_M in most of the cases) and check if any instability is reported

3. If yes, investigate - find the reason - correct if necessary; run calculations again

4. If not run calculations of the complete model with the Multi-threaded solver

Hope this helps.