Spread Footing Coordinate System
A spread footing is analyzed and designed with respect to the footing’s local coordinate system. The origin of the footing local axis will be at the centerline of the supported column. The major axis direction is designated by an arrow on the spread footing. In the RAM Structural System the major moments are those in the direction of the major axis and not about the major axis. Thus, for an I-section the major axis is parallel to the web.
The major axis of the footing is defined as follows:
- When the footing is not rotated with respect to the column the major axis is parallel to the column web. This is the default footing orientation, which would match the column local axis orientation.
- The major direction of the footing is also denoted as the length, or l direction.
- The minor direction of the footing is also denoted as the width, or w direction.
In the last two cases there is also the issue of the column dimensions to be used when checking the critical sections for shear and flexure checks as the supported member (concrete column or base plate) sides will not be parallel to the footing sides. In this case the supported member dimension is always taken as the actual dimension. If the footing is rotated up to 45 degrees with respect to the column, the dimensions of the supported member parallel to the member major axis will be used for the dimension parallel to the footing major axis. If the rotation angle is between 45 and 90 degrees the supported member major and minor dimensions will be switched.
Note: When supported members (concrete column or base plate) are rotated with respect to the footing the following convention is used to obtain their dimensions for flexure and shear checks of the footing:
- If the footing is rotated up to 45 degrees with respect to the column, the dimensions of the supported member parallel to the member major axis will be used for the dimension parallel to the footing major axis. If the rotation angle is between 45 and 90 degrees the supported member major and minor dimensions will be switched.