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In determining the shear stress in a beam, the dimension b is not always measured parallel to the neutral axis. The beam sections shown in Figure [2-23] show how to measure b in order to compute the static moment Q. Always remember that it is the tendency of the shaded area to slide relative to the unshaded area, which causes the shear stress. Thus, the shear stress at any section is always obtained by selecting the minimum b for that section.
Another useful idea in the analysis of beams is the concept of shear flow. By merely removing the dimension b from Equation [2-35] we get the shear flow q as
q = V * Q / I [2-40]
where q is in force units per unit of length of the
beam at the section under consideration. Basically,
the shear flow is simply the shear force per unit
length at the section defined by y = y1. When the
shear flow is known, the shear stress is determined
by the equation
tau = q / b [2-41]
![Figure [2-23]](fig0223.GIF)
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