Principal stresses in a rectangular tube

Constructing the shear diagram

Considering the segment
from x=3 m to x=10 m

     area under load
                diagram =  -14.00 kN
         concentrated
                   forces = 0.00 kN
     total shear force
                  change =  -14.00 kN

         at x=3 m,    V=     +8.00 kN
     total shear force
                 change        =     -14.00 kN
           at x=10 m, V=  -6.00 kN