# Area - Local Axes

Form:  Area Local Axis, Area Advanced Axis

Each Shell object (and other types of area object) has its own object local coordinate system used to define Material properties, loads and output. The axes of this local system are denoted 1, 2 and 3. The first two axes lie in the plane of the object with a user-specified orientation; the third axis is normal. It is important to understand the definition of the object local 1- 2-3 coordinate system and its relationship to the global X-Y-Z coordinate system. Both systems are right-handed coordinate systems. The user defines the local systems that simplify data input and interpretation of results. In most structures the definition of the object local coordinate system is extremely simple. The program also provides advanced options to define the orientation of the tangential local 1 and 2 axes, with respect to an arbitrary reference vector when the object coordinate angle, ang, is zero. If ang is different from zero, it is the angle through which the local 1 and 2 axes are rotated about the positive local 3 axis from the orientation determined by the reference vector. The local 3 axis is always normal to the plane of the object.

1. Select one or more area objects to be assigned the same local axes.

2. Click the Advanced > Assign > Areas > Local Axes command to access the Area Local Axis form.

3. Type in a value in the Angle in Degrees edit box. This is an angle by which the local axis 2 of the object will be rotated around the local axis 3. Local axis 3 is normal to the surface of the area object. By default, local axis 2 is always in the 3-Z plane (and lies in the plane of the area), except if the object is horizontal and then it is parallel to the global Y axis. The definition of the local axes follows the right-hand rule. The angle is measured counter-clockwise as positive if local axis 3 is pointing towards you.

4. Check the Advanced Axes check box and click the Advanced button to display the Area Advanced Axis form. Use the Area Advanced Axis form to refine the specification of the local axes for the selected object(s).

• Local Axes Type - When the Default option is selected, the other areas of the form are not available and no further input is required. When the Advanced option is selected, the input required on the form changes depending on the option selected in the Plane Reference Vector Specified by area of the form.

• Local Plane - Select the local plane for the area object from the drop-down list.

• Plane Reference Vector Specified by - Choose one of the following options to specify how the plane reference vector will be specified:

• Coordinate Direction - When this option is selected, the form will display a Coord System drop-down list and a Plane Coordinate Direction area with Primary and Secondary Dir (Direction) drop-down lists. Select the coordinate system for the reference vector and then choose a coordinate direction from each drop-down list to define the reference vector that is parallel to the Plane selected in the drop-down list in the Local Plane area of the form.

• Two Joints - When this option is selected, the lower portion of the form will display a Two Joints for Plane Reference Vector area with Plane Joint 1 and Plane Joint 2 drop-down lists. The lists consist of the joints included in the model and a None selection. Choose the two joints to be used to define the reference vector that is parallel to the Plane selected in the drop-down list in the Local Plane area of the form.

• User Vector - When this option is selected, the form will display a Coordinate System drop-down list and a User Plane Reference Vector area with edit boxes for Plane Vector X, Plane Vector Y, and Plane Vector Z. Select the coordinate system for the reference vector and then enter coordinate values in the edit boxes to define the reference vector that is parallel to the Plane selected in the drop-down list in the Local Plane area of the form.

Note: The positive direction of the local axis 3 is determined by the definition of the area object connectivity. See Sign Convention for more information.