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).
[Illustration: Figure 13.--THE FOUR BOLLMAN SPANS at Harpers Ferry that
survived the Civil War. The spans were completed in 1862-1863. (_Photo
courtesy of Baltimore and Ohio Railroad._)]
It apparently occurred to Bollman that by extending the number of rods
in a longitudinal direction, this effect could be practically amplified
to such an extent as to be capable of spanning considerable distances.
He almost certainly did not at first contemplate an all-iron system, but
rather a composite one such as described. It is entirely likely that
such trussed beams, with multiple systems of tension rods, were used by
Bollman as bridging in temporary trestlework along the line as early as
1845 (see fig. 4).
It is impossible to say whether Bollman himself, or Latrobe, was struck
with the logic of further elaborating upon the system and,
simultaneously, translating the timber compression member into one of
cast iron. Cast iron would naturally have been selected for a member
that resisted a compressive stress, as it was considerably cheaper than
wrought iron. But more important, at that time wrought iron was not
available in shapes of sufficient sectional area to resist the
appreciable buckling stresses induced in long compression members. The
cost of building up members to sufficient size from the very limited
selection of small shapes then rolled would have been prohibitive.
The trussing rods, subjected to tension, were of wrought iron inasmuch
as the sectional area had only to be sufficient to resist the primary
axial stress.
The first all-iron Bollman truss was constructed over the Little
Patuxent River at Savage Factory, near Laurel, Maryland, in 1850. In the
chief engineer's report for the year 1850, Latrobe was able to state
that the truss had been completed and was giving "much satisfaction."
He went on at some length to praise the "valuable mechanical features"
embodied therein, and expressed great confidence that iron would become
as important a material in the field of civil engineering as it was in
mechanical engineering.
[Illustration: Figure 14.--THE HARPERS FERRY BRIDGE as completed after
the Civil War. It was used by the Baltimore and Ohio until 1894, and as
a highway bridge until 1936. (Photo 690, Baltimore and Ohio Collection,
Museum of History and Technology.)]
The cost of this first major Bollman bridge was $23,825.00. Its span was
76 feet. Latrobe's confidence was well placed. The Savage span and
another at Bladensburg may be considered successful pilot models, for,
in spite of a certain undercurrent of mistrust of iron bridges within
the engineering profession--due mainly to a number of failures of
improperly designed spans--Latrobe felt there was sufficient
justification for the unqualified adoption of iron in all subsequent
major bridge structures on the B. & O.
Almost immediately following completion of the Savage Bridge, Bollman
undertook the design of replacements for the large Patapsco River span
at Elysville (now Daniels), Maryland, and the so-called Winchester span
of the B. & O.'s largest and most important bridge, that over the
Potomac at Harpers Ferry. Harpers Ferry bridge, a timber structure, had
been designed by Latrobe and built in 1836-1837 by the noted bridge
constructor Lewis Wernwag. It was peculiar in having a turnout, near the
Virginia shore, whereby a subsidiary road branched off to Winchester
(see fig. 6). Only the single span on this line, situated between the
midriver switch and the shore, was slated for replacement, as the other
seven spans of the bridge had been virtually reconstructed in the decade
or so of their history and were in sound condition at the time.
The Winchester span (fig. 8), which was the first Bollman truss to
embody sufficient refinement of detail to be considered a prototype, was
completed in 1851. Bollman was extremely proud of the work, with perfect
justification it may be said. The 124-foot span was fabricated in the
railroad's extensive Mount Clair shops. It was subdivided into eight
panels by seven struts and seven pairs of truss rods. An interesting
difference between this span and Bollman's succeeding bridges was his
use of granite rather than cast iron for the towers. The span consisted
of three parallel lines of trussing to accommodate a common road in
addition to the single-track Winchester line.
The distinctive feature of the Bollman system was the previously
mentioned series of diagonal truss links in combination with a cast-iron
compression chord, which Bollman called the "stretcher." The spacing
between the chord and the junction of each pair of links was maintained
by a vertical post or strut, also cast.
[Illustration: Figure 15.--NORTH STREET (now Guilford Avenue) bridge,
Baltimore. In this transitional composite structure cast iron was used
only in the relatively short sections of the upper chord. For the long
unsupported compression members of the web system, standard wrought-iron
angles and channels were built up into a large section. The decorative
cast-iron end posts were non-structural. (Photo in the L. N. Edwards
Collection, Museum of History and Technology.)]
Much of the appeal of this design lay unquestionably in the sense of
security derived from the fact that each of the systems acted
independently to carry its load to the abutments. The lower chords,
actually nonfunctional in the primary structure, were included merely to
preserve the proper longitudinal spacing between the lower ends of the
struts. A certain lack of rigidity was inherent in the system due to
that very discontinuity which characterized its action; however, this
was compensated for by a pair of light diagonal stay rods crossing each
panel. These rods served the additional function of distributing
concentrated loads to adjacent struts much in the manner of the bridging
between floor joists in a building.
In the Winchester span the floor system was of timber for reasons of
economy. This was a very minor weakness inasmuch as any stick could be
quickly replaced, and without disturbing the function of the structure.
Bollman received a patent for his truss in January 1852, and in the same
year published a booklet describing his system in general and the
Harpers Ferry span in particular. Here, he first calls it a "suspension
and trussed bridge," which is indeed an accurate designation for a
system which is not strictly a truss because it has no active lower
chord. (The analogy to a suspension bridge is quite clear, each pair of
primary rods being comparable to a suspension cable.) Thereafter,
Bollman's invention was generally termed a suspension truss.