The problem is to determine the shearforce Q on the hut near the ground. This is not a homework or anything like that, I'm just studying for an exam and this problem is in the book "Engineering Mechanics, Statics" By Meriam Kraige. On another forum, I found this: I understand the part in the parenthesis in the first integral, it's just the all the horizontal components of P summed. But why multiply with r? And why go from 0 to π when the wind only generates pressure against the right side of the hut, which goes from 0 to π/2?