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 lugs, subjecting the lugs to bending forces as well as shear forces. With the diagonals not carrying the load they were intended to carry, extra stress was placed on the chords. Unequal loading of the angle blocks worsened the metal fatigue.