Global and Local axes

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:

1.      Refer section properties like orthotropy properties to this system.

2.      Some of the loads (that have the prefix Local) are related also to this system.

3.      Strength results over the shell are referred to this local axes system.

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:

1.      Default. The program assigns a different local axes system to the shell with the following criteria:

·        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.

2.      Automatic. Similar to the previous one but the local axes system is assigned automatically to the shell by GiD. The final orientation can be checked with the Draw Local Axes option in the GiD Conditions window.

3.      Automatic alt. Similar to the previous one but an alternative proposal of local axes is given. Typically, User should assign Automatic local axes and check them, after assigning, with the Draw local axes option. If a different local axes system is desired, normally rotated 90 degrees from the first one, then it is only necessary to assign again the same condition to the entities with the Automatic alt option selected.

4.      User defined. User can created different named local axes systems with the GiD command:

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.