2D Frame-Torsional Spring-Frame element model 1

[mlevett3]

I am trying to come up with the stiffness matrix for a 2D system of a frame-torsional spring-frame FE model to grow into a full 3D auto body "stick" model of frames and springs. In the past I've substituted the spring with a stubby beam of equivalent torsional stiffness for 2D, but I'd rather program explicit torsional constants in now. I think that will be easier to work with in 3D. I guess the spring is a type of "connection element"? I've never studied those. I'm figuring that the beam elements share translational DoFs, but not rotational DoFs, since I'm splitting them up with a rotational element.

My current approach is attached. When I program that, clamp the left end, and solve with a vertical force at the right end the results make no sense. If I make the beams really stiff so most deformation happens on the spring, the energy of the flexible spring should almost match the work done by the force. But it doesn't. Can someone point out what I'm doing wrong or point me to a resource that could explain connection spring elements between beams?

 

[rb1957]

1) read up on freedom releases (since you want to release the rotational freedoms betwen the two beams, yes?)
2) mesh a short beam element between the two "real" beams, with a very low MoI
3) use a CBUSH to join the two beam elements

another day in paradise, or is paradise one day closer ?

 

[mlevett3]

I'm doing this by hand, so what ended up working for me was to connect the frame nodes with a zero length, zero mass, extremely stiff connecting springs for both translational degrees of freedom and a flexible torsion spring with user inputted stiffness. Similar to the approach in Zuo, Li, Xu, Xuan, and Na (2012).