Postby **HarryvL** » Sun Jan 13, 2019 8:04 am

I checked and can find 3 types of spring in the manual, with the following funny naming convention. All three get defined on the *SPRING card.

SPRING1

One-noded spring.

Input:

- direction vector of spring n

- spring stiffness k

Behaviour:

F = k u,

where F = reaction force vector, k = spring stiffness, u is the nodal displacement in the direction of n

SPRING2

Two-noded spring.

Input:

- two direction vectors. One at first node and one at second. The only practical application I see is when the direction vectors at both ends are equal ... say n. In this case the direction vector is the direction vector of a spring tieing both nodes.

- spring stiffness k

Behaviour:

F = k (u2 - u1)

where F = reaction force vector on the first node and -F = reaction force vector on the second node, k = spring stiffness, u1 is the displacement of the first node in the direction of n and u2 is the displacement of the second node in the direction of n

SPRINGA

A simple spring element definition between two nodes.

Input:

- spring stiffness k

Behaviour:

F = k (L-L_0) n,

Where L_0 = original distance between first and second node, L = final distance, n = unit vector pointing from first to second node.

So as far as I can see there is no face spring in CCX and for the purpose of modelling an eleastic foundation we either use equivalent springs of type SPRING1 or define a new user element in Calculix. I will derive the elastic stiffness matrix for a 3 and 6 noded isoparametric face element that matches the 4 and 10 noded tetrahedral volume elements generated with GMSH. This can either be used to define SPRING1 elements (by lumping stiffness on the diagonal of the matrix) or in a fully consistent form (in a Calculix UEL.f subroutine).