Principal stresses in a rectangular tube

Constructing the shear diagram

Considering the segment
from x=2 m to x=2 m

     area under load
                diagram =  0.00 kN
         concentrated
                   forces = +21.00 kN
     total shear force
                  change =  +21.00 kN

         at x=2 m,    V=     -11.00 kN
     total shear force
                 change        =     +21.00 kN
           at x=2 m, V=  +10.00 kN