Often, a beam is loaded by forces that lie in a plane that coincides with a plane of symmetry
of the beam, as depicted in Figure 7.7. In this figure, the y axis is an axis of symmetry
for the cross section; it is a principal axis. Hence, if axes (x, y ) are principal axes for the
cross section, the beams in Figures 7 . 7 a~n d b undergo symmetrical bending, that is, bending
about a principal axis of a cross section, since the moment vector in Figure 7 . 7 ~an d
the force vectors in Figure 7.7b are parallel to principal axes. We further observe that since the
shear center lies on the y axis, the plane of the load contains the axis of bending of the beam