Local Axes - Shell Objects

Form: Shell Assignment - Local Axes

In ETABS, the orientation of the 1- and 2-axes and the positive direction of the 3-axis depend on the type (orientation) of the shell object.

Note:  Use the View menu > Set Display Options command to access the Set View Options form; then check the Local Axes  check boxes under Fame Assignments and Shell Assignments on the Object  Assignments  tab of the form to display both shell and frame object axes. When local axes are displayed on the screen, local axis 1 is red, local axis 2 is green, and local axis 3 is blue, always.

The default local axes orientation for shell objects is as follows: The local axis 3 is always normal to the plane of the shell object. This axis is directed toward you when the path of the joints defining the shell object appears counterclockwise, as shown in the figure to the right.The default orientation of the local 1 and 2 axes is determined by the relationship between the local 3 axes and the global Z axis follows:

• The local 3-2 plane is taken to be vertical; that is, parallel to the Z axis.

• The local 2-axis is taken to have an upward (+Z) sense unless the element is horizontal, in which case the local 2-axis is taken to be horizontal along the global +Y direction.

• The local 1-axis is always horizontal; that is, it lies in the X-Y plane.

The object is considered to be horizontal if the sine of the angle between the local 3-axis and the Z-axis is less than 10-3. The local 2-axis makes the same angle with the vertical axis as the local 3-axis makes with the horizontal plane. This means that the local 2-axis points vertically upward for vertical objects.

Modify the direction of the local axes of a shell object as follows:

1. Select a shell object.

2. Click the Assign menu > Shell > Local Axes command to access the Shell Assignment - Local Axes form.

3. Select the appropriate option:

   • Rotation Angle from Default Orientation option and edit box.  Specify an angle in degrees to modify the default local axes directions. The angle specified is the angle from the default location of the local 2-axis (not necessarily the current location) to the new location of the local 2-axis. The angle is positive if it is counterclockwise when viewed from the positive local 3-axis side of the object.

1. • Rotation Angle form the Current Orientation option and edit box. Enter a value to rotate the local 2-axis by the specified angle (in degrees) from its current location (not necessarily its default position). When the 3-axis is pointing towards you, a positive rotation is counterclockwise; that is, the right-hand rule applies (see above sketch).

   • Normal to Selected Beam option.  To use this option, first select a frame as well as a shell object and then click the Assign menu > Shell > Local Axes command. When this options is selected, ETABS rotates the Local 1-axis of the shell object to be perpendicular to the selected frame's local 1-axis.

4. Click the Apply button to complete the assignment.

5. When the Apply button is used, the Shell Assignment - Local Axes form will remain open until it is closed by clicking the Close button. This allows selection of another shell object(s), to which a different assignment can be made. If only one assignment is being made to only one set of selected objects, the OK button can be used to both apply the assignment and close the form

Important Note: Do not confuse the local axes of shell objects with those of pier and spandrel elements. They are different. The local axes of pier and spandrel elements cannot be rotated.