well the cut point would be at the joint

Q=AY = area x distance from center to the centroid of the object.

the area would be the area of the joint since that is the point that would be used to determine the point of shear.

for joint A the distance from the centroid, 100mm - 10mm would be the Y

for joint b the y value would be 80mm

then you have Q and use the equation T=VQ / It to find the shear.

then double the number you got and that should take care of all four joints.

I think you could do the problem by using the bottom of the beam as the datum but then you would need the parrallel axis theorum

very similar to sample problem 6.3

hope that helps

- 95 Mystique LS - Zetec/5spd - 99 Contour SeVT Sport - Duratec/5spd Official NE-CEG Contour/Mustang Family