I am learning about shearing stress, and I am a little confused about the area of projection mentioned in my book. When it introduces it, it shows a plate with a rivet through it. The plate is of thickness t, and the diameter of the rivet, d. It shows the plate and the rivet cut in half by a plane, and reveals the rectangular area exposed as the rivet is cut, and claims that A = dt. I am trying a practice problem where the same type of thing is going on, and asks me to find the shearing stress between the plate and the shaft. However, the solution states that A = pi*d*t. In this case, is it cutting straight through the shaft, or is it the area of half of the hole the shaft creates? Basically, why is pi included here?