# Stiffness Matrix Question

Pooty
So, I am trying to assemble the Stiffness Matrix in my Finite Element Analysis course (Structures) and I keep coming out with a stiffness matrix that is not symmetric. I learned that for any neutrally stable structure, the stiffness matrix must be symmetric. Are there special cases that I am not taking into account that may allow for it to be antisymmetric? Thanks.

Also, this might seem sort of redundant but I actually look at these two definitions in a different light. Maybe someone could shed some light on the definition of a stiffness coefficient for me but I was given 2 definitions.

Kij

Definition 1: The Force at i caused by a unit displacement at j
Definition 2: The Force at i required to cause a unit displacement at j

If someone is really good at setting up stiffness matrices I would love to post what I got.

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pongo38
Look at the Muller-Breslau theorem,(that the deflection at A due to a force P at B is equal to the deflection at B due to the same force P at A) and maybe that will reveal (1) the symmetry of the stiffness matrix and (2) the definitions you give follow from each other? That's just a suggestion for you to consider. I may be wrong though.