Figure 12.—Advertisement in the Railroad Advocate, August 1855. [Click here] to read transcription.
The world’s first major iron bridge, the famed cast-iron arch at Coalbrookdale, England, had been constructed in 1779. Its erection was followed by rather sporadic interest in this use of the material. The first significant use of iron in this country was in a series of small trussed highway arches erected by Squire Whipple over the Erie Canal in the early 1840’s, over 60 years later. In these, as in most of the earlier iron structures, an arch of cast iron was the primary support. The thrust of the arches was counteracted by open wrought-iron links with other wrought- and cast-iron members contributing to the truss action.
The Whipple bridges promoted a certain amount of interest in the material. In the B. & O.’s annual report for the fiscal year 1849 appears the first record of Latrobe’s interest in this important matter. In the president’s message is found the following, rather offhand, statement:
$6,183.19 have been expended toward the renewal of the Stone Bridges on the Washington Branch, carried off by the flood of Oct. 7th, 1847. Preparations are made and contracts entered into, for the reconstruction of the large Bridges at Little Patuxent and at Bladensburg which will be executed in a few months…. It is proposed to erect a superstructure of Iron upon stone abutments, at each place—with increased span, for greater security against future floods.
It is interesting to note that it was indeed Bollman trusses to which the president of the railroad had referred. How much earlier than this date Bollman had evolved his peculiar trussing system is not clear. The certain influence of Latrobe’s radiating strut system of trussing has been mentioned. As likely an influence was another basic technique commonly used to increase the capacity of a simple timber beam—that of trussing—i.e., placing beneath the beam a rod of iron that was anchored at the ends of the beam and held a certain distance below it at the center by a vertical strut or post. This combination thus became a truss in that the timber portion was no longer subject to a bending stress but to a simple one of compression, the rod absorbing the tensile stress of the combination. The effect was to deepen the beam, increasing the distance between its extreme fibers and—by thus reducing the bending moment—reducing the stress in them (see [fig. 3]).
