Hello,

I am currently studying the construction of stiffness matrices for Euler-Bernoulli beam elements with 2 nodes in 3D. In this case, there are 6 DoF (degrees of freedom) for each nodes. It gives a (12*12) matrix for a beam element ( see "theoretical_matrix.png"). I tried to understand how this matrix was implemented for B33 elementqs in Abaqus. From the theory guide, I learned that cubic beam element have 2 additional variables for axial strain, so, the elementary stiffness matrix is now (14*14) instead of (12*12). How are these terms defined in the matrix ?

Also, when i compared the 14*14 stiffness matrix generated in Abaqus with my theoretical 12*12 matrix, I could retrieve all the terms relative to bending and twisting but the the term relative to compression/traction seems different than my analytical matrix ( EA/L). Is this difference linked to the introduction of the 2 additional variables for axial strain ?