Shear stress in any fiber is given by the equation
τy = V A’ Y’ /I B
τy is the shear stress in a fiber at distance y from the centroid axis
Normally V will be the absolute maximum shear force found from the shear force diagram ( Or V is the shear force at section being considered along the length of the beam and is found from the shear force diagram).
A’ is the area of cross section above the fiber being considered at distance y from the centroid axis
Y’ is the distance of centroid of area A’ from the centroid axis of the entire cross section
I is the moment of inertia of the entire cross section about the centroid axis
B is the width of the fiber at distance y from the centroid axis
1. There are longitudinal as well as vertical shear stresses. These are variable stresses. These are maximum at the centroid axis and zero at the outermost fiber.
2. These are complementary of each other. One of these is applied while the other is induced.
3. Both are equal in magnitude but opposite in nature i.e. one has a rotational and other has an anti-clockwise rotational tendency.
4. Both vary in a parabolic manner.
5. Shear stress is maximum at the centroid axis and zero at the extreme fibers while bending stresses are maximum at the extreme outer fibers and zero at the centroid axis. It is true in case of symmetrical sections like square, rectangular and circular.
6. Therefore, a beam can be designed independently on bending and shear stress basis.
7. Shear stress is maximum at half the height in case of a triangular section and not at the centroid axis. It is an exception.
8. For rectangular, circular and square sections, maximum bending stress becomes the maximum principal stress.
9. However in case of an I section, maximum principal stress occurs at the junction of web and flange.
10. The ratio of τmax/τav is as follows:
|Shape of cross section||Rectangular||Circular||Square|