Figure 7.—Recent model of Bollman’s Winchester span. Only two of the three lines of trussing are shown. The model is based on Bollman’s published description and drawings of the structure. (USNM 318171; Smithsonian photo 46941.)
In the period when Bollman began designing—about 1840—there were fewer than ten men in the country designing bridges by scientifically correct analytical methods, Whipple and Roebling the most notable of this group. By 1884, the year of Bollman’s death, the age of intuitive design had been dead for a decade or longer.
Figure 8.—The Baltimore and Ohio Railroad’s Potomac River crossing at Harpers Ferry, about 1860. Bollman’s iron “Winchester span” of 1851 is seen at the right end of Latrobe’s timber structure of 1836, which forms the body of the bridge. (Photo courtesy of Harpers Ferry National Historical Park.)
The B. & O. was in every way a truly pioneer enterprise. It was the first practical railroad in America; the first to use an American locomotive; the first to cross the Alleghenies. The spirit of innovation had been encouraged by the railroad’s directors from the outset. It could hardly have been otherwise in light of the project’s elemental daring.
The first few major bridges beyond the line’s starting point on Pratt Street, in Baltimore, were of rather elaborate masonry, but this may be explained by the projectors’ consciousness of the railroad’s significance and their desire for permanence. However, the aforementioned economic factors shortly made obvious the necessity of departure from this system, and wood was thereafter employed for most long spans on the line as far as Harpers Ferry and beyond. Only the most minor culverts and short spans, and those only in locations near suitable quarries, were built of stone.
In addition to the economic considerations which prompted the company to revert to timber for the major bridges, there were several situations in which masonry construction was unsuitable for practical reasons. If stone arches were used in locations where the grade of the line was a relatively short distance above the surface of the stream to be crossed, a number of short arches would have been necessary to avoid a very flat single arch. In arch construction, the smaller the segment of a circle represented by the arch (that is, the flatter the arch), the greater the stress in the arch ring and the resulting horizontal thrust on the abutments.
Figure 9.—Bollman skew bridge at Elysville (now Daniels), Maryland, built in 1853-1854. (Photo courtesy of Maryland Historical Society.)
The piers for the numerous arches necessary to permit an optimum amount of rise relative to the span would have presented a dangerous restriction to stream flow in time of flood. By the use of timber trusses such crossings could be made in one or two spans with, at the most, one pier in the stream, thus avoiding the problem.