2D model using beam elements

Hi All,
I have managed to construct 2D axisymmetric model (CAX4R) of a steel 
rod embedded in concrete. I am obtaining the natural frequencies of 
the model. In order to compare the model i have constructed another 
model 2D planar to use beam elements to model the steel rod since 
beam elements can't be used in a 2d axisymmetry model. But for some 
reason the natural frequencies i get from the 2D planar model are 
way off. The first mode i got from the axisymmetry model is 887Hz 
(analytical model shows 917Hz) which is quite close but for the 2d 
planar model the first mode predicted is 10Hz. The elements for the 
the concrete i have specified as CPE4R (plain strain element).

Can someone advice me on this? Am i using the wrong element type or 
something...

kind regards,
Ravi








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