Once the bending starts, it weakens the part immediately and extensively. Take a thin wooden or plastic plank, set it vertically and push on it. It will resist for a while, being still straight. But once it starts bowing, it takes very little additional pressure to fold it in half or break. That is because the pieces of the plank switch from compression load (which is fairly good) to bending and stretching, which for most materials is not good at all. Just imagine pushing on a vertical glass tube. Once the first crack starts, the part is lost.
It was announced a while ago that the gusset plates were only half as thick as they were supposed to be.
http://enr.construction.com/news/transportation/archives/080115.asp