How the DoF of the stiffnesses K1, K2, K3, K4 , K5, K6 are defined in property PBUSH for the solver Optistruct? and How K values behave in a CBUSH element subjected to loads?

Samuele Divano

Hi Everyone,

My goal is to create a spring connection to connect two beam elements, with some stiffness in all DoF. At first, I'm analyzing a simplified model with only the spring element CBUSH with PBUSH property. Spring axis is on y axis of the global reference frame and the directional vector is oriented along x axis of the global one. Spring element is constrained at one end and subjected to loads to other end. Each load is acting in a different DoF and for each one I have created a load case, with a total of 6 load cases:

1) Fx: force on global x axis

2) Fy: force on global y axis

3) Fz: force on global z axis

4) Mx: moment around x axis

5) My: moment around y axis

6) Mz: moment around z axis

For each simulation, I'm changing one Ki at time with 1E+8, leaving the other ones equal to zero (it means Real or RIGID), in order to understand what are the load cases subjected to deformation and so the acting DoF of Ki analyzed.

for example: 

1st Analysis: K1=1E+8; K2=0; K3=0; K4=0; K5=0; K6=0 => OptiStruct simulation =>analyse displacements in each subcase in HyperView

2nd Analysis: K1=0; K2=1E+8; K3=0; K4=0; K5=0; K6=0 => OptiStruct simulation =>analyse displacements in each subcase in HyperView

and so on...

From my expectations, each Ki should contribute to deformation of maximum 2 load cases, in the DoFs where the stiffness should act. But from my results they are responsible of deformation in 3 or 4 different load cases, sometimes with strange values. After this long examination, I'm going back to my questions:

1)What are the DoF of each Ki parameter in PBUSH property?

2) What are the results of deformation in each load case, that I'm expecting to found applying a precise Ki parameter?

Thanks for your attention and time.

Adriano_Koga

For the CBUSH element, you could do it 2 ways:

 

- define an orientation vector V (given by X1, X2,X3)

 

- or define a CID (Coordinate System).

 

Using a vector, the X-axis of your element is aligned with the spring element. The Y-axis is given by the projection of V (vector product of X and V).

Using CID, you will overwrite this definition with the local system. (leave it 'unespecified' for global system)

 

Documentation for CBUSH:

https://2021.help.altair.com/2021/hwsolvers/os/topics/solvers/os/cbush_bulk_r.htm