Define one RC element with two sections.

[mohsenrock]

Hi,
I have a reinforce concrete beam in my RC model that should be defined with two different sections because of the different distributions of the steel reinforcement along the element. I have defined 2 fiber sections and I am using dispBeamColumn to define the elements. In order to construct my model. First I defined the beam element with just one disp element and assigning one section to it. Then I define an additional node in the position of the middle of the beam and then I defined two elements but in this case with equal sections. But the results are completely different than the case of one element and are showing much higher values for the displacements. Do I have to consider additional commands when having two sections for one element? what causes this problem.

Thanks

[fmk]

the problem is not due to the differing section types but more likely a consequence of the fact that for nonlinear analysis to get good apprx of the curvature distribution along a nonlinear disp beam element you have to add more elements. in your case you need to compare the effect of differing types of sections along the length using models with the same number of elements per span.

[imadelkhouri]

Hello Mohsen,

I am dealing with a similar issue and I am not sure why i am getting more displacement for the "one element" frame rather than the frame with 2 elements.
Did you manage to solve the problem, would you mind if we can share some code to overcome this problem?

Thank you.

Regards,
Imad

[selimgunay]

This is a consequence of the formulation of the displacement beam-column element. It considers linear curvature displacement along the element length, therefore using a single displacement element is not correct for inelastic analysis.

1. You would get same results from one and two displacement based element only when the response is within the linear elastic range.

2. For the general case, you should use multiple displacement based elements (probably 4 or 5) or a single force based element. Please see the below link for more information on this matter.

http://opensees.berkeley.edu/wiki/index ... ed_Element

[selimgunay]

typo: linear curvature distribution