The model has been created related to a global axes system XYZ that is unique for the entire problem. But every shell element must have its own local axes system X'Y'Z' in order to:
The main property of this local axes system is that the local Z' axe must have the same direction than the normal of the element.
![]() |
The ways for defining local axes systems are:
· Be N the unitary normal of the shell element, U the vector (0,1,0) and V the vector (0,0,1). Then:
· Z' axe has the direction and sense of N.
· If Nx<1/64 and Ny<1/64, then X' axe will be in the direction of the cross product of U and N (UxN).
· If not, X' axe will be in the direction of the cross product of V and N (VxN).
· Y' axe will be the cross product of Z' axe and X' axe.
Note: Intuitively, this local axes system is calculated so as if element is approximately contained in the plane XY, local X' axe will point towards global X axe. If not, this X' axe is obtained as orthogonal to global Z axe and local Z' axe.
Data->Local axes->Define
and with the different methods that can be chosen there. The names of the defined local axes will be added to the menu where Local axes are chosen in the conditions window.
Note 1: rambshell tries to correct the local axes system if the local Z' axe does not point to the direction of the normal of the element. It will fail if local Z' axe is orthogonal to the direction of the normal.
Note 2: The final local axes system for every shell element can be visualized in the postprocess stage. It is convenient to check the correctness of these systems after calculation is performed.
|