Occasional Electronic Publication No. 1 - Society for Industrial ...

Introduction:
American Truss Bridge Configurations
The Evolutionary Process
The picturesque truss bridges of the mid- to late-nineteenth century that dominated the American
transportation system of roads and rails are slowly and inexorably becoming rare phenomena. Time
and traffic have simply worn them out. While a tiny handful have been, and are being, preserved, the
cost of doing so has usually resulted in their replacement with a new structure capable of handling
the increased loads of modern trucks and trains. Steel plate-girders and post-tensioned concrete
assemblages are today's means of bridging short- to modest-length crossings. Where steel trusses
still sometimes constitute a viable economic solution, variations on only a handful of configurations
are now seriously considered and ultimately used – mostly the Warren truss with its repetitive
inverted “Vs,” or variations of the Pratt truss with its tensile diagonals contained within a rectangular
panel.
The hundreds of patented solutions proposed during the nineteenth century lay ignored in the
archives, items of intellectual curiosity with no current practical value. Culled from these hundreds
of inventions, only several dozen configurations ever proved to be of value, and then for relatively
short periods of time. A progression of new construction materials permitted and inspired new
forms. The forms that made sense in wood were replaced by new and different configurations made
possible by the use of cast and then wrought iron for some, and then all, of the truss parts. Steel
brought further possibilities.
Perhaps providing an even greater impact on truss development was the dissemination and dispersal
of a scientific understanding of what was actually happening to the various configurations as they
strained to carry their imposed loads. The intrinsic, trial-and-error methods employed by local
carpenters and village smithies were slowly replaced by more sophisticated understandings. In 1835
Rennsselaer Polytechnic Institute started issuing degrees in engineering. In 1847 Squire Whipple
published the first book in America that provided meaningful information as to how truss members
dealt with loads – just in time to help cope with the outburst in railroad construction. Herman Haupt
added his text four years later. But it took time for the average builder of bridges to acquire the
education and ability to understand these texts. Thus proposals were made that seem wild, even
comical, in light of today's knowledge.
Stretching over a period of a hundred years, American truss builders sought ways to span greater
distances at the lowest possible cost. Initially labor was cheap and the cost of the then-new material,
iron, high. Thus the thrust of new ideas concentrated on using the minimum amount of material to
get the job done. Compared to contemporary European crossings of similar spans, American pinconnected
trusses were marvels of lightweight delicacy. The price of this minimalist design
approach was a non-stop replacement of structures in order to cope with the ever-increasing loads
bridges were required to carry. The over-designed, often ponderous, more rigid, riveted European
plate-girder and Warren-truss crossings, while carrying higher initial costs, possessed far longer life
expectancies.
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