time history analysis on a bridge pier

[enrico tubaldi]

Hello,
I am trying to do some time history analysis on a reinforced concrete bridge pier made of fiber section dispbeamcolumn elements at the base and elastic elements at the top
and comparing the results with a pushover analysis.
Applying at the top of the pier a force which grows in a linear manner (displacement control) i found out that the top
of the pier has a vertical displacement in addiction to the horizontal one.This is probably due to the eleongation of the center line after fessuration of concrete.
Moreover the axial force in the pier grows significantly (6000 kN) even if i didn't apply any vertical load.
I have also used another fiber element based program called "Seismostruct" to compare the results and the axial force is almst null(10 kN).
It seems therefore the equilibrium condition it is not satisfied in Opensees.Could you please explain me why?Shall i use forcebeacolumn elements instead?
Thank you very much in advance for your help.
ENRICO TUBALDI, UNIVERSITA' POLITECNICA DELLE MARCHE

[silvia]

First of all, yes, you should use a single force-based element.
Since you are doing a cantilever column, you should look at the examples manual, it has exactly what you need.

OpenSees does maintain equilibrium, by definition. what kind of axial load are you imposing on the pier? what kind of boundary conditions are you using?
you should compare your model to the one in the examples manual.

[CMPeng]

Maybe you have imposed the load in the wrong direction.

check your script !