TABLE OF CONTENTS Forces that cause a member to twist about a longitudinal axis are called torsional loads. Simple torsion is produced only by a couple, or moment, in a plane perpendicular to the axis.
If a couple lies in a nonperpendicular plane, it can be resolved into a torsional moment, in a plane perpendicular to the axis, and bending moments, in planes through the axis.
The point in each normal section of a member through which the axis passes and about which the section twists is called the share center. The location of the shear center depends on the shape and dimensions of the cross section. If the loads on a beam do not pass through the shear center, they cause the beam to twist. See also Art. 5.5.19.
If a beam has an axis of symmetry, the shear center lies on it. In doubly symmetrical beams, the share center lies at the intersection of the two axes of symmetry and hence coincides with the centroid.
For any section composed of two narrow rectangles, such as a T beam or an angle, the shear center may be taken as the intersection of the longitudinal center lines of the rectangles.
For a channel section with one axis of symmetry, the shear center is outside the section at a distance from the centroid equal to e(1 + h 2 A/4 I), where e is the distance from the centroid to the center of the web, h
