Braced Frame analysis fails even with Elastic elements

[IsmBaha]

Hello Frank,

I am having a problem with modeling braced frames (Inverted V CBF). I crated a model using nonlinear fiber elements for beams, columns and braces. I didn't model the gusset plates yet b/c the model is diverging after big forces excite the system when I model it as pinned braces under EQ excitation.

I tried many things, I couldn't get the model to pass divergence problem once it is excited by a significant excitation.

Finally I made everything linear, I removed any complicated elements materials and fiber sections. I created a braced framed that consists of only elastic beam column elements. I used corotational transformation for braces and Pdelta for others. I defined a L/1000 initial imperfection on braces.

Even with the elastic building, analysis starts failing after EQ reaches about its half PGA, not even at its peak, of the time history and I can see that brace buckles and has large deformations, so I shouldn't be surprised that nonlinear model is diverging when the elastic system analysis fails b/c even the elastic system starts failing.
Could the elastic system be failing because of the Pdelta and corotational transformation geometric nonlinearities? Can I have your insights about it?

I wish I could copy the code here but it d be very long to paste on a web page. I really appreciate your help.

Sincerely,
Baha

[fmk]

it can .. the model is is nonlinear geometry with linear material relationship .. that still makes it a nonlinear problem .. though to fail it must be undergoing some large displacements