Tension only bars - best solver ?

[Anonymous]

Hello,

I have a steel frame structure that is not very stiff. It is a sway moment frame in one direction, in the other there is no frame and it relies on the stiffness of the columns. There are quite a lot secondary pinned-pinned beams, rigid links, an horizontal diaphragm and some braces. All of this results in a poorly conditioned matrix. I only have static vertical and lateral loads.

I had to use the iterative method to calculate the structure without "Type 3" instability errors. Now I want to go a step further and define my bracing as tension only members.

I have tried several options for the solver, but I always get a non convergence error.

I have tried making inactive half of my bracing and leaving the rest as compression & tension bars, and that converges, so I know there is enough stiffness in the structure to make it stable.

I have read other posts about solving tension only bars, so I introduce a 5% relative stiffness for the releases of the problem.

Could you please advise of the best settings for the solver to get convergence for this type of nonlinear problem? I don't mind if it is very slow.

Due to confidentiality issues, I cannot share the model.

Any ideas, suggestions would be greatly appreciated.

Cheers,

V.

PS: I have read other posts about it, so I

[Artur.Kosakowski]

Hi @Anonymous

I had to use the iterative method to calculate the structure without "Type 3" instability errors

This is not the best solution as this solver is actually not good at 'catching' instability issues and you should be extra careful checking the correctness of the obtained results. If you use 'automatic' choice, what are the values of displacements and rotations of nodes (are they reasonable small) as well as the shapes of deformations (as expected) for model with no tension only elements?

If yes, you may try to set the full newton-Raphson method and calculate only load combinations (simple load cases being set as auxiliary ones) for the model with tension only elements. 

If this doesn't help - can you attach your model to the forum? 

[Artur.Kosakowski]

Hi @Anonymous

I just wanted to check with you if you still need any additional help with your model.