ForceBeamColumn vs DispBeamColumn

[ozgura]

Hello,

I am not very knowledgeable about distributed plasticity so I need some advice on the beam-column elements of my model. I am modelling a BRBF where I use a corotational truss for the braces and forcebeamcolumn type elements for the beams and columns. I have rigid offsets on all the elements, i.e beams, column and braces. When I use corotational transformation and run dynamic analyses, Force based beam-column element didnt work well as the forces and deformations in the elements were not compatible. When I switched to Displacement based beam-column element, I didnt get that warning.

I can run the pushover analysis fine for both force and disp beam column elements, however, the pushover curves are somewhat different. They are the same in the elastic portion of the curve, but when the elements start yielding, DispBeamcolumn element model results in higher force or base shear values than the ForceBeamColumn element model. When I make this comparison I am basically keeping everything the same but the element type for the beams and columns. I keep number of fibers and integration points of WF sections the same as well.

So which one should I rely on? Actually, I guess I will have to go with DispBeamColumn as I am getting warning messages from the ForceBeamColumn during dynamic analysis. Note that I am using a pretty small time step for the dynamic analysis.

Any idea is appreciated.
Thanks
-Ozgur

[sama]

Is there any source to find out the exact difrences of displacement-beamcolumn vs force based beam colum. about how they distribute plasticity and ...
and wich one of them u(authors) suggest. or maybe better to ask, is there any recomandation where to use each on of them.

[vesna]

Here is a good reference which describes and compares the two elements:
Neuenhofer, Ansgar, FC Filippou. Evaluation of Nonlinear Frame Finite-Element Models. ASCE Journal of Structural Engineering, Vol. 123, No. 7, July, 1997. ISSN 0733-9445/97/0007-0958-0966. Paper No. 14157. pp. 958-966.

[vesna]

Ozgur, I recommend you to read Neuenhofer paper, so you will understand the difference between the two elements better.

[sama]

thanks