Metal truss bridges first appeared in the United
States in 1840, and are still being used today. The metal truss bridge
was the most common bridge type built between the 1870s and 1925. In
1999, when this project began, there were thirty metal truss bridges,
dating from 1887 to 1942, remaining in Bradford County,
Pennsylvania. Now, twenty-four of those metal truss bridges
Beginning in the mid-nineteenth century most of the
bridges in Bradford County were constructed from wood (Watson 200:1).
Natural causes, such as fire, flooding, and ice, damaged many of the
wooden bridges. Bradford County Commissioners naturally turned to metal
truss bridges to replace damaged wooden bridges. Most of the countyís
metal truss bridges were built in the twentieth century, and
half of those were erected in the centuryís first decade.
Metal Truss Bridges
The truss form dates to ancient times. In America,
wood truss bridges date to at least the early nineteenth century. Wood
was readily available, and a more trusted material for bridge
construction. A transition to metal trusses began in 1840, when William
Howe patented a composite truss with wooden and wrought iron members.
All-metal truss designs appeared within just a few years, but they were
not widely used until after the Civil War, when they were embraced by
the rapidly-expanding railroad industry. Iron had become more available,
and the safety standards for building metal truss bridges had improved.
Truss bridge technology transferred to the nationís road and highway
system in earnest in the 1870s (Comp and Jackson 1977:1-3).
Until the 1920s, metal truss bridges faced little
competition from other bridge types. The metal trusses were stronger and
more rigid than wooden bridges, and they were fire resistant. Due to
competition among dozens of bridge firms then in operation and
standardization of bridge designs, metal truss bridges were relatively
inexpensive and easy to construct. Steel beam technology and reinforced
concrete technology had not yet fully developed, so that metal truss
bridges were the best way to span great distances.
A metal truss bridge uses many small pieces, or
members, to make a beam long and strong enough to span distances. The
individual components are connected in a series of triangles. Each truss member is either in compression or tension, and
connections at the panel points can be either pinned or riveted.
Diagram taken from David Weitzman's book "Windmills,
Bridges, and Old Machines: Discovering Our Industrial Past"
Most metal truss bridges constructed prior to 1900
used pinned connections. In a pin-connected bridge, a cylindrical metal
bar joins the truss members together. This connection
method required holes in the ends of each member that were then aligned
together, so that the pin could be driven through all the holes to form
a structural connection. Pinned connections were popular because they
allowed the rapid erection of the trusses and made it easier to analyze
the stresses in the truss members. However, they were susceptible to
loosening, especially under the shaking caused by fast-moving vehicular
loads (Jackson 1988:24)
Fueled by improvements in pneumatic field riveting
equipment, builders of metal truss bridges made the transition from
pinned connections to riveted connections around 1890 (Simmons 1997).
Riveted connections also benefited from stronger steel,
developed in the closing decade of the nineteenth century.
Where pin connections permitted stress points to move,
the new stronger, riveted steel could accept the stress without
Concrete arches and beams would eventually supplant metal
truss bridges as common waterway crossings.
By the turn of the twentieth century, concrete had been
experimentally used in several waterway crossings.
By the advent of World War II, very few metal truss
bridges were built; most were concrete.
Concrete had the advantage of being more
addition, most concrete bridges were deck structures, that is,
the deck, or roadway, lay on top of the arch or beams.
Vehicle crashes into trusses could cause the bridge to
structures largely eliminate such a problem.
In the latter half of the twentieth century, very few
truss bridges would be built.
Because the existing truss bridges were often built for
traffic either before or in the early stages of
fossil-fuel-powered vehicles, many have become to narrow or
unable to carry modern loads.
In addition, their many exposed metal parts result in
corrosion problems, further weakening them.
As a result, metal truss bridges, once ubiquitous, are
now becoming a rarity on the landscape.
site was produced as mitigation for the replacement of the historically
. Mitigation is required as
a result of Section 106 of the National Historic Preservation Act of
1966, as amended.