Principal stresses in a rectangular tube

Constructing the shear diagram

Considering the segment
from x=0 m to x=2 m

     area under load
                diagram =  -6.00 kN
         concentrated
                   forces = 0.00 kN
     total shear force
                  change =  -6.00 kN

         at x=0 m,    V=     -5.00 kN
     total shear force
                 change        =     -6.00 kN
           at x=2 m, V=  -11.00 kN