Local Axes in frame (SAP2000)

By default the local 1-axis of a frame object extends from the i-end of the element to the j-end. The default orientation of the local 2 and 3 axes depends on the frame-type (column, beam or brace) and in some instances the orientation of the frame element itself.

Note:  Use the View menu > Set Display Options command to access the Set Building View Options form; then check the Shell Local Axes and Frame Local Axes check boxes in the Object View Options area 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 white and local axis 3 is blue, always. This is the same order as the colors of the American flag: red, white and blue. Another way to display local axes orientation on the screen is to check the Extrusions check box in the Special Effects area of the Set Building View Options form.

The default local axes orientation for frame objects is different for

  • Vertical Frame Objects. For vertical frame objects the local axes have the following default orientation:
  • Local axis 1 is along the frame object. The positive local 1-axis is in the same direction as the positive global Z-axis, upward.
  • Local axis 2 is perpendicular to the frame object. The projection of the positive local 2-axis onto the global X-axis is in the same direction as the positive global X-axis.
  • Local axis 3 is perpendicular to the frame object. The direction of the positive local 3-axis is determined from applying the right-hand rule using the directions of the 1 and 2 axes described in the preceding bullets.
  • Horizontal Frame Objects. For horizontal frame objects (parallel to the global X-Y plane), the local axes have the following default orientation:
  • Local axis 1 is along the frame object. The projection of the positive local 1-axis onto the global X-axis is in the same direction as the positive global X-axis. If the frame object is parallel to the global Y-axis such that there is no projection onto the global X-axis, the positive direction of the local 1-axis is the same as the direction of the positive global Y-axis.
  • Local axis 2 is perpendicular to the frame object. The positive local 2-axis points in the same direction as the global Z-axis, upward.
  • Local axis 3 is perpendicular to the frame object and is horizontal. The direction of the positive local 3-axis is determined from applying the right-hand rule using the directions of the 1 and 2 axes described in the preceding bullets.
  • Other (neither vertical nor horizontal) Frame Objects. For line objects that are not vertical and are not horizontal, the local axes have the following default orientation:
  • Local axis 1 is along the frame object. The positive local 1-axis has an upward sense. The projection of the positive local 1-axis onto the global Z-axis is in the same direction as the positive global Z-axis, upward.
  • Local axis 2 is perpendicular to the frame object. The local 1-2 plane is vertical. The positive local 2-axis has an upward sense. The projection of the positive local 2-axis onto the global Z-axis is in the same direction as the positive global Z-axis, upward.
  • Local axis 3 is perpendicular to the frame object and is horizontal. The direction of the positive local 3-axis is determined from applying the right-hand rule using the directions of the 1 and 2 axes described in the preceding bullets.

Redefine the orientation of the local 2 and 3 axes of a frame object by rotating them about the local 1-axis as follows:

  1. Select the frame object.
  2. Select the Assign menu > Frame > Local Axes command to access the Frame Assignment – Local Axes  form. Choose the desired parameters for the bullet items that follow step 3.
  3. Click the Apply button. Any previously assigned local axis will be replaced by this assignment.

    When the Apply button is used, the Frame Assignment – Local Axes form will remain open until it is closed by clicking the Close button. This allows selection of another frame object(s), to which a different assignments 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.
  • Angle option and edit box. Rotate the local 2-axis by the specified angle (in degrees) from its default position. When the 1-axis is pointing towards you, a positive rotation is counterclockwise, that is, the right-hand rule applies. (See sketch below.)
  • Rotate by Angle option. Rotate the local 2-axis by the specified angle (in degrees) from its current location (not necessarily its default position). When the 1-axis is pointing towards you, a positive rotation is counterclockwise; that is, the right-hand rule applies. (See the sketch above.)
  • Orient with Grid System – Applies to Vertical Frame Objects Only options. Note that the four options on the form — Vertical frame object major direction is XVertical frame object direction is YVertical frame object is RadialVertical frame object is Tangential — have no effect unless the selected element is a vertical frame object. If more than one grid system has been defined in the model, first specify the grid system by selecting it from the drop-down list.

The vertical frame object major direction is the same as the local 2-axis direction (which is also the same as the minor axis).  Each of the options sets the Local 2 axis in the X, Y, Radial, or Tangential direction, depending on the option selected.  Loads acting in the major direction cause M3 bending and V2 shear. In a wide flange member, this corresponds to bending resisted by the flanges and shear resisted by the web.

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