The principal timber bridges as far west as Cumberland were of Latrobe’s design. These were good, solid structures of composite construction, in which a certain amount of cast iron was used in joints and wrought iron for certain tension members. They were, however, more empirical than efficient and, for the most part, not only grossly overdesigned but of decidedly difficult fabrication and construction.
What is interesting about the Latrobian timber trusses, however, is the effect they appear to have had upon Bollman’s subsequent work in the design of his own truss. This effect is evidenced by the marked analogy between the primary structural elements of the two types. The Latrobe truss at Elysville (fig. 2) was only partially a truss, inasmuch as the greater part of the load was not carried from panel to panel, finally to appear at the abutments as a pure vertical reaction, but was carried from each panel (except the four at the center) directly to the bearing points at the piers by heavy diagonal struts, after the fashion of the famous 18th-century Swiss trusses of the Grubenmanns. It was a legitimate structural device, and the simplest means of extending the capacity of a spanning system. However, it was defective in that the struts applied considerable horizontal thrust to the abutments, requiring heavier masonry than would otherwise have been necessary.
It is quite likely that Latrobe did not have absolute confidence in the various pure truss systems already patented by Town, Long, and others, and preferred for such strategic service a structure in which the panel members acted more or less independently of one another. It will be seen that, similarly, the individual panel loads in Bollman’s truss were carried to the ends of the frame by members acting independently of one another.
The Bollman Truss
There had never been any question about the many serious inadequacies of wood as a bridge material. Decay and fire risk, always present, were the principal ones, involving continuous expenditure for replacement of defective members and for fire watches. It was, in fact, understood by the management and engineering staff of the B. & O. that their timber bridge superstructures, though considered the finest in the country, were more or less expedient and were eventually to be replaced. In this regard it is not surprising that Latrobe, a man of considerable foresight, had, at an early date, given serious thought to the possible application of iron here.
Figure 10.—Potomac River crossing of the Baltimore and Ohio at North Branch, Maryland, built in 1856. There are three Bollman deck trusses. (Photo courtesy of Baltimore and Ohio Railroad.)
Figure 11.—The Fink truss. (Smithsonian photo 41436.)