[Illustration: Figure 1.--WENDEL BOLLMAN, C.E. (1814-1884). (_Photo
courtesy of Dr. Stuart Christhilf._)]
_The development of structural engineering has always been as
dependent upon the availability of materials as upon the
expansion of theoretical concepts. Perhaps the greatest single
step in the history of civil engineering was the introduction
of iron as a primary structural material in the 19th century;
it quickly released the bridge and the building from the
confines of a technology based upon the limited strength of
masonry and wood._
_Wendel Bollman, self-taught Baltimore civil engineer, was the
first to evolve a system of bridging in iron to be consistently
used on an American railroad, becoming one of the pioneers who
ushered in the modern period of structural engineering._
THE AUTHOR: _Robert M. Vogel is curator of civil engineering in
the Smithsonian Institution's Museum of History and
Technology._
Wendel Bollman's name survives today solely in association with the
Bollman truss, and even in this respect is known only to a few older
civil and railroad engineers. The Bollman system of trussing, along with
those of Whipple and Fink, may be said to have introduced the great age
of the metal bridge, and thus, directly, the modern period of civil
engineering.
Bollman's bridge truss, of which the first example was built in 1850,
has the very significant distinction of being the first bridging system
in the world employing iron in all of its principal structural members
that was used consistently on a railroad.
The importance of the transition from wood to iron as a structural and
bridge building material is generally recognized, but it may be well to
mention certain aspects of this change.
The tradition of masonry bridge construction never attained the great
strength in this country which it held in Europe, despite a number of
notable exceptions. There were several reasons for this. From the very
beginning of colonization, capital was scarce, a condition that
prevailed until well into the 19th century and which prohibited the use
of masonry because of the extremely high costs of labor and transport.
An even more important economic consideration was the rapidity with
which it was necessary to extend the construction of railways during
their pioneer years. Unlike the early English and European railways,
which invariably traversed areas of dense population and industrial
activity, and were thus assured of a significant financial return almost
from the moment that the first rail was down, the Baltimore and Ohio
and its contemporaries were launched upon an entirely different
commercial prospect. Their principal business consisted not so much in
along-the-line transactions as in haulage between principal terminals
separated by great and largely desolate expanses. This meant that income
was severely limited until the line was virtually complete from end to
end, and it meant that commencement of return upon the initial
investment was entirely dependent upon the speed of survey, graduation,
tunneling, and bridging.
[Illustration: Figure 2.--MODEL OF B. H. LATROBE'S TRUSS, built in 1838,
over the Patapsco River at Elysville (now Daniels), Maryland. (_Photo
courtesy of Baltimore and Ohio Railroad._)]
The need for speed, the general attenuation of capital, and the simple
fact that all the early railroads traversed thickly forested areas
rendered wood the most logical material for bridge and other
construction, both temporary and permanent.
The use of wood as a bridge material did not, of course, originate with
the railroads, or, for that matter, in this country. The heavily wooded
European countries--Switzerland in particular--had a strong tradition of
bridge construction in timber from the Renaissance on, and naturally a
certain amount of this technique found its way to the New World with the
colonials and immigrants.
America's highway system was meager until about the time the railroad
age itself was beginning. However, by 1812 there were, along the eastern
seaboard, a number of fine timber bridges of truly remarkable structural
sophistication and workmanship.
It was just previous to the advent of the railroads that the erection of
highway bridges in this country began to pass from an art to a science.
And an art it had been in the hands of the group of skilled but
unschooled master carpenters and masons who built largely from an
intuitive sense of proportion, stress, and the general "fitness of
things." It passed into an exact science under the guidance of a small
number of men trained at first in the scientific and technical schools
of Europe, and, after about 1820, in the few institutions then
established in America that offered technical instruction.
The increasing number of trained engineers at first affected highway
bridge construction not so much in the materials used but in the way
they were assembled. In a bridge designed by a self-taught constructor,
the cheapness of wood made it entirely feasible to proportion the
members by enlarging them to the point where there could be no question
as to their structural adequacy. The trained engineer, on the other
hand, could design from the standpoint of determining the entire load
and then proportioning each element according to the increment of stress
upon it and to the unit capacity of the material.
By the time railroads had started expanding to the West there had been
sufficient experience with the half dozen practical timber truss systems
by then evolved, that there was little difficulty in translating them
into bridges capable of supporting the initial light rail traffic.
In spite of its inherent shortcomings, wood was so adaptable that it met
almost perfectly the needs of the railroads during the early decades of
their intense expansion, and, in fact, still finds limited use in the
Northwest.