1.3.12. Global and local axes

1.3.12.1. Global and local axes for beams

RamSeries models are created taking as a reference a global axes system \(XYZ\) which is unique for the entire problem. However, beams must have its own local axes reference system, denoted \(X’Y’Z’\), in order to:

  1. Refer section properties like for instance height, thickness and inertias.

  2. Define local loads which are specified and referred to the local reference system of the beam.

  3. Beam results visualization: the resulting forces and moments over a beam are referred to its local reference system. Strength results over the beams are also referred to this local axes.

Note

The main property of the beam’s local reference system is that the local \(X’\) axis axe must coincide with the longitudinal axis of the beam.

There exist different ways of defining local axes in RamSeries. Next, the most common ones are described briefly:

  1. Default. The program assigns automatically different local axes to each beam by using the following criteria:

    • \(X’\) axis is directed along the longitudinal direction of the beam

    • If \(X’\) axe has the same direction than the global \(Z\) axe, \(Y’\) axe is assigned the same direction than the global \(X\) axe. If not, \(Y’\) direction is computed as to ensure it is horizontal (i.e. orthogonal to \(X’\) and \(Z\)).

    • \(Z’\) axe is finally computed as the cross product of \(X’\) axe and \(Y’\) axe. It will try to point to the same sense than the global \(Z\) (dot product of \(Z\) and \(Z’\) axes will be positive or zero).

Note

The intuitive idea is that vertical beams have the \(Y’\) axe in the direction of global \(X\). All the other beams have the \(Y’\) axe horizontal and with the \(Z’\) axe pointing up.

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

  2. Automatic alt. Similar to the previous one but an alternative proposal of local axes is given. Typically, the 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.

  3. User defined. Finally, the user can create different ‘named’ local axes systems invoking the GiD command Data >> Local axes >> Open and using the different methods available in the pop-up window. The names of the defined local axes will be added to the list from which local axes can be further choosen during the assign process.

Note

RamSeries tries to correct the local axes system if the local \(X’\) axe does not point to the direction of the beam. It will fail if local \(X’\) axe is orthogonal to the direction of the beam.

Note

The final local axes system for every beam can be visualized in the post-process stage. It is convenient to check the correctness of these systems after calculation is performed.

1.3.12.2. Global and local axes for shells

RamSeries shell models are created as well taking as a reference a global axes reference system denoted \(XYZ\) which is unique for the entire problem. However, as in the case of beam elements, every shell in the model must have its own local axes system denoted by \(X’Y’Z’\) in order to:

  1. Refer to the local reference system all the shell’s section properties like for instance the orthotropic material properties

  2. Define all those load conditions denoted as local (i.e. that have the prefix local)

  3. Proper visualization and interpretation of the results obtained over the shells. In particular, strength results over the shell are referred to its corresponding local reference system

  4. Dimensioning and scantling of steel in shells is based on the \(X’\) and \(Y’\) directions

Note

The main property of this local axes reference system is that the local \(Z’\) axe coincides with the the normal of the shell’s element

There exist different options for the definition of local axes in shells. The most common ones are described in what follows:

  1. Default. The program assigns different local axes to each shell using the following criteria. Being \(\hat{\boldsymbol{n}}\) the unitary normal to the shell element, \(\hat{\boldsymbol{u}}\) the vector \((0,1,0)\) and \(\hat{\boldsymbol{v}}\) the vector \((0,0,1)\), then:

    • \(Z’\) axe has the direction and sense of \(\hat{\boldsymbol{u}}\).

    • If \(n_x < 1/64\) and \(n_y < 1/64\), then \(X’\) axe will be in the direction of the cross product of \(\hat{\boldsymbol{u}}\) and \(\hat{\boldsymbol{n}}\) \((\hat{\boldsymbol{u}} \times \hat{\boldsymbol{n}})\).

    • If not, \(X’\) axe will be in the direction of the cross product of \(\hat{\boldsymbol{v}}\) and \(\hat{\boldsymbol{n}}\) \((\hat{\boldsymbol{v}} \times \hat{\boldsymbol{n}})\).

    • Finally \(Y’\) will result from the cross product of \(Z’\) axe and \(X’\) axes.

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.

  1. 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.

  2. 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.

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

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

RamSeries 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

The final local axes system for every shell element can be visualized in the post-process stage. It is convenient to check the correctness of these systems after calculation is performed.