By the late 1850's iron was well established as a bridge material
throughout the world. Once the previous fears of iron had been stilled
and the attention of engineers was directed to the interpretation of
existing and new spanning methods into metal, the Bollman truss began to
suffer somewhat from the comparison. Although its components were simple
to fabricate and its analysis and design were straightforward, it was
less economical of material than the more conventional panel trusses
such as the Pratt and Whipple types. Additionally, there was the
requisite amount of secondary metal in lower chords and braces necessary
for stability and rigidity.
A factor difficult to assess is Bollman's handling of his patent, which
was renewed in 1866. There is sufficient evidence to conclude that he
considered the patent valuable because it was based upon a sound design.
Therefore, he probably established a high license fee which, with the
truss's other shortcomings, was sufficient to discourage its use by
other railroads. As patron, the B. & O. had naturally had full rights to
its use.
An additional defect, acknowledged even by Bollman, arose because of the
unequal length of the links in each group except the center one. This
caused an unevenness in the thermal expansion and contraction of the
framework, with the result that the bridges were difficult to keep in
adjustment. This had the practical effect of virtually limiting the
system to intermediate span lengths, up to about 150 feet. For longer
spans the B. & O. employed the truss of another of Latrobe's assistants,
German-born and technically trained Albert Fink.
The Fink truss was evolved contemporaneously with Bollman's and was
structurally quite similar, being a suspension truss with no lower
chord. The principal difference was the symmetry of Fink's plan, which
was achieved by carrying the individual panel loads from the panel
points to increasingly longer panel units before having them appear at
the end bearings. This eliminated the weakness of unequal strains. The
design was basically a more rational one, and it came to be widely used
in spans of up to 250 feet, generally as a deck-type truss (see fig.
11).