Wendel Bollman was born in Baltimore of German parents in 1814. His
father was a baker, who in the same year had aided in the city's defense
against the British. Wendel's education, until about the age of 11, was
more or less conventionally gained in public and private schools in
Baltimore. He then entered into informal apprenticeship, first to an
apothecary in Sheperdstown, Virginia (now West Virginia), and then to
one in Harpers Ferry. In 1826 or 1827 he became ill and returned to
Baltimore for cure. From that time on his education was entirely
self-acquired.
[Illustration: Figure 3.--TRUSSED BEAM.]
It is of interest, in light of his later career, to note that on the
Fourth of July 1828, he marched with other boys in a procession that was
part of the Baltimore and Ohio Railroad's cornerstone-laying ceremony.
Shortly afterward, he apprenticed himself to a carpenter for a brief
time, but when the work slacked off he obtained work with the B. & O.
The right-of-way had been graded for about five miles by that time, but
no rail was down. The boy was at first given manual work, but soon
advanced to rodman and rapidly rose as he gained facility with the
surveying apparatus. In the fall of 1829 he participated in laying the
first track. As his mother was anxious that he continue his education in
carpentry, he left the railroad in the spring of 1830 to again enter
apprenticeship. He finished, became a journeyman, helped build a
planter's mansion in Natchez, and returned to Baltimore in 1837 to
commence his own carpentry business. The next year, while building a
house in Harpers Ferry, he was asked to rejoin the B. & O. to rebuild
parts of its large timber bridge over the Potomac there, which had
fallen victim to various defects after about a year's use.
[Illustration: Figure 4.--SIMPLE BEAM of 50-foot span with three
independent trussing systems. Bollman's use of this method of support
led to the development of his bridge truss. This drawing is of a
temporary span used after the timber bridge at Harpers Ferry was
destroyed during the Civil War. (In Baltimore and Ohio Collection,
Museum of History and Technology.)]
Shortly after the Harpers Ferry bridge reconstruction, Bollman was made
foreman of bridges. It is apparent that, on the basis of his practical
ability, enhanced by the theoretical knowledge gained by intense
self-study, he eventually came to assist Chief Engineer Benjamin H.
Latrobe in bridge design. He later took this work over entirely as
Latrobe's attentions and talents were demanded in the location and
extension of the line between Cumberland and Wheeling.
[Illustration: Figure 5.--BOLLMAN'S ORIGINAL PATENT DRAWING, 1851. (In
National Archives, Washington, D.C.)]
The B. & O. did not reach its logical destination, Ohio (actually
Wheeling, West Virginia, on the east bank of the Ohio River) until 1853.
In the years following Bollman's return to the railroad, the design of
bridges was an occupation of the engineering staff second in importance
only to the location of the line itself. During this time Bollman
continued to rise and assume greater responsibilities, being appointed
master of road by Latrobe in 1848. In this position he was responsible
for all railroad property that did not move, principally the
right-of-way and its structures, including, of course, bridges.
The recognition of Bollman's abilities was in the well-established
tradition of the B. & O., long known as America's first "school of
engineering," having sponsored many early experiments in motive power,
trackwork, and other fundamental elements of railroad engineering. It
furnished the means of expression for such men as Knight, Wright,
Whistler, Latrobe, and Winans.
[Illustration: Figure 6.--PLAN OF HARPERS FERRY BRIDGE as built by
Latrobe. The second Winchester track was later removed.]
Of these pioneer civil and mechanical engineers, some were formally
trained but most were self-taught. Bollman's career on the B. & O. is of
particular interest not only because he was perhaps the most successful
of the latter class but because he was probably also the last. He may be
said to be a true representative of the transitional period between
intuitive and exact engineering. Actually, his designing was a composite
of the two methods. While making consistent use of mathematical
analysis, he was at the same time more or less dependent upon empirical
methods. For years, B. & O. employees told stories of his sessions in
the tin shop of the railroad's main repair facility at Mount Clair in
Baltimore, where he built models of bridges from scraps of metal and
then tested them to destruction to locate weaknesses. It seems most
likely, however, that the empirical studies were used solely as checks
against the mathematical.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
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.