Modeling braced frame with truss element

[amy]

Hi everyone.
I am trying to model a one bay one storey steel concentrically braced frame. The beam and column joints are moment restrained, modelled with inframeFB element. The bracings are pin connected modelled with the inelastic truss element. I divided the bracing into four elements and introduce initial imperfection to model buckling. First I wanted to do the Eigen value analysis. But it didn't work. Gives the message nodes connected with the bracing has zero diagonal term.
I believe this is the stability problem. It will be grate help for me if anyone can tell me why I am getting this problem and how to overcome this.

Thanks a lot
Amy

[huffte]

I am wondering if the problem is the discretization of the bracing member into 4 elements. This would result in unstable hinges where braces connect to other braces only. You may have to employ a combination of inelastic truss elements with frame elements. I see why you have done the discretization - an attempt to model buckling. I do not believe you can have only truss elements framing into any given node. Incidentally, I do not believe that the eigen analysis particular to SeismoStruct can be made to give you buckled shapes, only dynamic mode shapes. I could be wrong here, but I believe the equations being solved in a buckling eigen-analysis are not the same as those being solved in a dynamics eigen-analysis. Hope this helps and best of luck to you in your modeling.

[amy]

Thanks for your reply. If I don't discretize the bracing with truss element still it gives the same warning ' Zero diagonal term'. I don't know how to solve it. suggestion will be appreciated.

Again thanks for your help.

[huffte]

Can you elaborate on your model. Do you have at least one frame element at every node?

[amy]

I am attaching the model for you. Please let me know your suggestions.
Thanks
Amy
https://www.dropbox.com/s/8cye3hozprdh5 ... e.spf?dl=0

[huffte]

Can you e-mail the model to me? I cannot access dropbox at work. The file appears to be quite small.

[amy]

Sorry I can not attach the file. I tried to send email with attachment. But there is no attachment option for me.

[huffte]

Hi amy. Note that with node 19, you do in fact have only truss elements framing into the node. So, strictly speaking, there is a zero diagonal term for rotation about the global y-axis at this node. Hence, the warning from SeismoStruct. Having said that, I note that the model still runs fine for both an eigenvalue analysis and a pushover analysis. Of course, you will have to examine the results for yourself and ensure that they make sense for the behavior you seek to mode. I wuls note that you have the global mass settings to ux, uy, and uz, while I think your intention may be to model plane frame behavior, which would correspond to ux, uz, and ry degrees of freedom. Best of luck.

[amy]

Thanks a lot sir. I corrected the mass. I can not get time period from Eigen value analysis with the same model. It gives me imaginary null value. I am using version 6. May be this is the reason.
My goal is to model the buckling of bracing. So I think I can not use truss element. So I modelled the bracing with infrmFB element with 4 divisions. I introduce initial imperfection. As the bracing is pin connected , I modelled with infrmFB element with end release M2a at the ends. The problem is now when I conduct push over, I get moment in the bracings. I believe there shouldn't be any moment in the bracing as it is pin connected. Here is the model for your look. I also sent it to your email The model even can not apply the full load because of convergence difficulties due to buckling. I know I have to play with the convergence criteria. .

https://www.dropbox.com/s/730tvhnsgq2p0 ... e.spf?dl=0

Thanks
Amy

[huffte]

I was able to get eigenvalues and pushover results with the original model using version 7, amy. I had the same problem you did with the second model - the one with inelastic FB elements possessing moment releases. The use of releases on inelastic FB elements is clearly troublesome, as indicated my the message SeismoStruct presents upon running the model. I wonder if v7 might solve your problems with the original model? Best of luck and happy modeling, amy.