Shear stress in thin wall beams

[princejan7]

Homework Statement

http://postimg.org/image/9va37id55/
The question is to find the shear stress at the points A, B and C. The solution uses the sectioning planes aa, bb and cc to do so.

For example, at C, how should I know whether to use a horizontal or vertical sectioning plane. Also, how should I know which portion of the beam i would have to consider as A', the bit to the left or right of the sectioning plane ?

Homework Equations

Shear Stress(Flange) = q(Flange)/thickness(Flange)
where q(flange) = VQ(Flange)/I

Shear Stress(Web) = q(Web)/thickness(Web)
where q(Web) = VQ(Web)/I

V is the transverse shear force
Q = A'y" is the first area moments of the area A' with respect to the neutral axis

The Attempt at a Solution

 

[SteamKing]

Homework Statement

http://postimg.org/image/9va37id55/
The question is to find the shear stress at the points A, B and C. The solution uses the sectioning planes aa, bb and cc to do so.

For example, at C, how should I know whether to use a horizontal or vertical sectioning plane.

Generally, you have the highest shear stresses acting on the thinnest material.

For example, the shear stress τ = VQ / (I t), so for a given shear force V, the smaller t is, the higher the stress τ.

Also, how should I know which portion of the beam i would have to consider as A', the bit to the left or right of the sectioning plane ?

The shear stress will be zero at any free ends of the section. Start from the nearest free end and work toward the section plane at which you wish to calculate the shear stress.