and the intensity of stress of tension or compression is
f_t = M/A_th,
f_c = M/A_ch.
If A is the area of the plate web in a vertical section, the intensity of
shearing stress is
f_x = S/A
and the intensity on horizontal sections is the same. If the web is a
braced web, then the vertical component of the stress in the web bars cut
by the section must be equal to S.
[Illustration: FIG. 38.]
21. _Method of Sections. A. Ritter's Method._--In the case of braced
structures the following method is convenient: When a section of a girder
can be taken cutting only three bars, the stresses in the bars can be found
by taking moments. In fig. 38 m n cuts three bars, and the forces in the
three bars cut by the section are C, S and T. There are to the left of the
section the external forces, R, W_1, W_2. Let s be the perpendicular from
O, the join of C and T on the direction of S; t the perpendicular from A,
the join of C and S on the direction of T; and c the perpendicular from B,
the join of S and T on the direction of C. Taking moments about O,
R_x-W_1(x+a)-W_2(x+2a) = Ss;
taking moments about A,
R3a-W_12a-W_2a = Tt;
and taking moments about B,