The shear strength of reinforced concrete members without shear reinforcement
is related to the shear stress carried by the concrete effective shear area. For
rectangular sections the effective shear area represents the area corresponding to
the effective depth. However, for circular sections this area is not as readily defined.
Different empirical proposals have been suggested recently but no analytical solution
exists so far in literature. In this work the effective shear area of circular sections has
been derived purely analytically. The ratio of the effective shear area to gross section
area was expressed as a function of the neutral axis depth for different values of the
concrete cover. For a typical value of neutral axis depth, it was shown that the
effective shear area ranges between 0.6 and 0.8 times the sections gross area
depending on the depth of the concrete cover. Thus, an average value of 0.7 seems
to be a reasonably accurate value for design purposes and, moreover, it is in good
agreement with other recent proposals.