derived in Sec. 5.4 also applies to composite beams, provided that Q and I
are calculated using the transformed section and b is the width of the original
cross section.
To arrive at this conclusion, consider a composite beam with the cross
section shown in Fig. 9.3(a). The shear stress t acting on the horizontal section
a-a can be obtained from equilibrium by using the free-body diagram in
Fig. 9.3(b). The free body is bounded by two cross sections a distance dx apart
and the section a-a. The equilibrium equation F=0 yields (I),
or