Modeling Concentric Braces

[salehi]

Dear Vensa,
I am modeling a 2D concentric braced frame considering material and geometric non-linearities and buckling of compressive elements. Now I'm wondering how I can model compressive elements -especially braces- considering buckling. Although I've already skimmed prior posts, but I'm not sure about what I'm going to program yet! The main ideas I've got now are these:
1. I should use 'corotational' geometric transformation;
2. I should divide braces into 2~4 nonlinear force-based beam-column elements;
3. I should fix the ends of the braces (it's better than pinned ends);
4. I should use two very laterally stiff elastic beam-column element at the two ends of braces in order to model gusset plates.
I hope I've been right with the above points. In fact, my problem is about the 4th one: supposing I'm going to divide braces to 2 nonlinearBeamColumn element, shall I have each brace including 4 subdivisions, i.e. 2 elastic and 2 nonlinear? How should I define the elastic ones' strength and geometric properties? I'll be so grateful to you if you provide me a quite detailed reply .

[salehi]

Dear all,
Could anyone possibly help me, please? Thanks in advance

[vesna]

If your model is 2D you should also consider inputing a rotational hinge between elastic stiff element and nonlinear beam column element and introducing geometric imperfection at the mid-node of the brace to induce buckling. I believe that there is a paper by Roader where he proposes a value of the stiffness of the rotational spring. Another good reference is PEER 2008/08.

[salehi]

Thanks for your useful guidance, but could you explain more about "imperfection at the mid-node", please? I'm just wondering how I should define that imperfection?
Also, do you mean that I should define rotational springs at the hinges connecting middle parts to elastic stiff elements?!

[vesna]

1. look at the report I mentioned in my earlier post to understand how to define imperfection

2. rotational hinges should be defined at the braces ends (where it meets gusset plate)

[salehi]

OK... I'll read it as soon as possible. Thanks a lot

[sirvan]

Dear Vesna
i modeled Bracing as what is suggested in PEER 2008.i m modeling a 15-story building.there are 2 braced bays.and each bracing is devided into 22 elements.but unfortunately after story 10 the following error appears:
":failed to get copy of a material"
is there any limitation in use of number of NonlinearBeamColumn element?
thanks in advance

[vesna]

name of your uniaxial material is not properly defined.

[sirvan]

i have used just a fatigue material and a uniaxial material for all elements.

[fmk]

if you used this material on lower floors the probable reason for this is that you are running out of memory. how many fibers per section and how many sections per element are you using??