I recently took a quiz for my Matrix Methods of Structural Analysis Class, in which I encountered the following MCQ. My answer of '6' was justified, in my opinion, by the size of the stiffness matrix for a general 3-D beam element, which is 12x12 (6 for each node). I am pretty sure I am correct but I would like confirmation on whether to approach my instructor with this.

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  • $\begingroup$ Why are you not sure of your answer? Have you checked your notes? If you are sure then contact your instructor - easy enough to correct a question. $\endgroup$
    – Solar Mike
    May 14, 2020 at 8:12
  • $\begingroup$ I have gotten lazy during online classes, so no notes. Also he hasn't introduced 3D elements yet. $\endgroup$
    May 17, 2020 at 17:30

1 Answer 1


Six seems to be the correct answer: 3 translational DoFs $\left(u_x\right.$, $u_y$, $\left.u_z\right)$ and 3 rotational DoFs $\left(\phi_x\right.$, $\phi_y$, $\left.\phi_z\right)$

You can even have a 3D beam element with 7 degrees of freedom. The 7th is for warping


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