been minimal as shear stress was only induced by one of the adjacent members of the chord. 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. Neither the joint committee, nor the coroner's jury, nor any of the engineers employed by them identified the flaw in the angle block casting. In retrospect, Gasparini and Fields say, the point where the lug transitioned to the main body of the angle block was extremely problematic. The solidification rates of the molten iron were so different, this was a likely site for the formation of cracks or voids. Poor construction caused overwhelming thrust load Poor construction of the bridge's diagonals worsened the stresses placed on the lugs on the angle blocks. Howe trusses rely on prestressing of braces and counter-braces to improve the way the bridge carries load. Tightening the nuts on the vertical posts (prestressing) puts the verticals in tension (stretches them). If the diagonals are already closely fitted to the angle blocks, prestressing compresses the diagonals. This allows them to carry more load. The Ashtabula Bridge diagonals, however, were only loosely fitted to the angle blocks. Prestressing brought the diagonals into a relatively close fit with the angle blocks, but did not put the diagonals in compression. The problem was worsened because shims were used to fill the space between the diagonal bearings and the angle blocks. The history of the bridge indicates that some of these shims had come loose over time and fallen away. The loss of shims induced uneven loading, as the more tightly connected diagonals absorbed load before the loose ones did. Åkesson points out that the shims themselves may even have created unequal pressure points between I-beams and the