Bollman resigned from the Baltimore and Ohio in 1858 to form, with John
H. Tegmeyer and John Clark, two of his former B. & O. assistants, a
bridge-building firm in Baltimore known as W. Bollman and Company. This
was apparently the first organization in the United States to design,
fabricate, and erect iron bridges and structures, pioneering in what 25
years later had become an immense industry. The firm had its foundation
at least as early as 1855 when advertisements to supply designs and
estimates for Bollman bridges appeared over Tegmeyer's name in several
railroad journals (see fig. 12).
Bollman's separation from the B. & O. was not a complete one. The
railroad continued its program of replacing timber bridges with Bollman
trusses, and contracted with W. Bollman and Company for design and a
certain amount of fabrication. There is some likelihood that eventually
fabrication was entirely discontinued at Mount Clair, and all parts
subsequently purchased from Bollman.
The firm prospered, erecting a number of major railroad bridges in
Mexico, Cuba, and Chile. Operations ceased from 1861 to 1863 because of
difficult wartime conditions in the border city of Baltimore. Following
this, Bollman reentered business as sole proprietor of the Patapsco
Bridge and Iron Works.
[Illustration: QUINCY BAY BRIDGE
Figure 17.--CHICAGO, BURLINGTON AND QUINCY RAILROAD BRIDGE over Quincy
Bay (branch of the Mississippi River) at Quincy, Illinois. The pivot
draw-span was formed of two Bollman deck trusses supported at their
outer ends by hog chains. The bridge was built in 1867-1868 by the
Detroit Bridge and Iron Co., Bollman licensee. (Clarke, _Account of the
Iron Railway Bridge ... at Quincy, Illinois_.)]
The most noteworthy of Bollman's works in this period was a series of
spans at Harpers Ferry. The B. & O.'s timber bridge had been destroyed
by Confederate forces in June 1861, and the crossing was thereafter made
upon temporary trestlework. This was a constant source of trouble, with
continuing interruptions of the connection from high water, washouts,
and military actions. The annoyance and expense of this became so great
that the company decided to risk an iron bridge at the crossing. In July
and August 1862, two sections of Bollman truss, spans no. 4 and no. 5
were completed. As this occurred during the time when W. Bollman and
Company was inoperative, the work was produced at Mount Clair to
Bollman's design and, undoubtedly, erected under his supervision. Five
weeks later, on September 24, these and Bollman's famous Winchester span
of 1851 were blown up by the Confederates, and the line's business was
again placed at the mercy of trestling.
The spirit of the B. & O. administration indeed seems to have been
unshakable when, in the face of such heartbreaking setbacks, it
determined to again bridge the river with iron, even at the height of
the hostilities. In November, span no. 5 was erected, and by April 1863
nos. 3, 4, and 6 also. These were the four straight spans in midriver
between the "wide" (or "branch," or "wye") span and the span on the
Maryland shore over the Chesapeake and Ohio Canal (see fig. 13).
Although the wood floor system of these spans was burned for strategic
reasons by U.S. troops later in 1863, they survived the war.
In 1868 the remaining trestlework was replaced with Bollman trusses.
This magnificent structure served the railroad until 1894 when the
right-of-way was realigned at Harpers Ferry. However, the half used by
the common road remained in use until carried away by the disastrous
flood in 1936. The piers may still be seen.
During the prewar years, Bollman evolved a structural development of
most profound importance, which is usually associated with the Phoenix
Iron Works and its founder, Samuel J. Reeves. In the erection of a high
trestlework viaduct for the Havana Railroad, Bollman apparently became
concerned with the tensile weakness of cast iron when applied in long,
unsupported columns. Although a column is normally subjected to
compressive stresses, when the slenderness ratio--that is, the length
divided by the radius of gyration of the cross section--becomes great, a
secondary bending stress may be produced. If this stress becomes great
enough, the value of the tensile stress in one side of the column may
actually exceed the principal compressive stress, and a net effect of
tension result.
[Illustration: Figure 18.--OHIO RIVER CROSSING of the Baltimore and Ohio
at Benwood, West Virginia, completed in 1870. Bollman deck trusses were
used in the approaches on both sides. (Photo 693, Baltimore and Ohio
Collection, Museum of History and Technology.)]
[Illustration: Figure 19.--PATAPSCO RIVER crossing of the Baltimore and
Ohio between Thistle and Ilchester, Maryland. (Photo 695, Baltimore and
Ohio Collection, Museum of History and Technology.)]
As already mentioned, the few available rolled-iron shapes were of
relatively small area and quite unsuitable for use as columns unless
combined and built up in complex fabrications. The normal practice at
the time was to use cast compression members in iron bridges and
structures, with their sectional area so proportioned to the length that
a state of tension could not exist. In the case of long members, this
naturally meant that an excessive amount of material was used.
[Illustration: Figure 20.--TWO VIEWS OF BOLLMAN-BUILT "water-pipe truss"
that carries Lombard Street over Jones Falls in Baltimore. Built in
1877.]
Bollman was conscious of the problem from his experience with the
stretchers and struts of his truss, and he must have been aware of the
great advantage which would be obtained by a practical method of forming
such members in wrought iron, the tensile resistance of which is
equivalent to the compressive. He eventually developed the forerunner of
what came to be known as the Phoenix form by having special segmental
wrought-iron shapes rolled by Morris, Tasker and Company of
Philadelphia, these shapes being combined into a circular section with
outstanding flanges for riveting together. The circular section is
theoretically the most efficient to bear compressive loading. A column
of any required diameter could be produced by simply increasing the
number of segments, the individual size of which never exceeded
contemporary rolling mill capacity (see fig. 16).
The design exhibits the inspired combination of functional perfection
and simplicity that seems to characterize most great inventions.
[Illustration: Figure 21.--THE HARPERS FERRY BRIDGE toward the end of
its career, carrying a common road over the Potomac. The westernmost
line of trussing and span no. 1 had been removed long before. View
through the Winchester span looking toward Maryland in 1933. (_Photo
courtesy of Harpers Ferry National Historical Park._)]
It may have been because he had no facilities for rolling that Bollman
communicated his idea to Reeves, although this seems illogical. At any
rate, Reeves and his associates patented the system extensively, and the
Phoenix column was eventually employed to the virtual exclusion of
cast-iron and other types of wrought-iron columns. By the end of the
19th century it began to pass from use, as mills became capable of
producing larger sections with properties relatively favorable to column
use and more adaptable to connection with other members.