The critical issue, Åkesson says, is that the broken lug on the southern angle block atop the joint between the second and third panel had a void. The void itself weakened the integrity of the block. Voids also encourage the formation of large grains and can accumulate impurities like slag, both of which also increase the brittleness of iron. The void also worsened stress on the block through stress concentration effect. Gasparini and Fields conclude that this void, combined with metal fatigue, caused the lug to fail. Metal fatigue was an issue in cast and wrought iron which only a few metallurgists and engineers were aware of in the 19th century. The failure of this lug on this angle block caused the entire bridge to fail.