true truss system consists of massive timbers assembled in a triangle.
This is the only
two-dimensional figure that cannot be distorted under stress. Each
bridge consists of two truss systems, one on each side of the structure.
The line drawings that follow illustrate the basic pattern of each
truss. There have been many variations to these designs depending
upon locale, builder, and materials available. The heavy, solid
lines in the drawings represent solid timbers. Fine dotted lines
represent the exterior shape of the side. The heavier dash lines
represent metal rods.
is the oldest truss design used in bridge construction, initially
used under the roadway rather than above. It consists of a stringer,
a Kingpost (vertical beam), and two diagonals and is used primarily
for the short spans of approximately twenty to thirty feet. Pennsylvania
has eleven bridges with this truss. Eight of them are located
in Washington County, two in Greene County, and one in Jefferson
County, which is not a truly historic covered bridge but is listed
in the World Guide to Covered Bridges.
Multiple Kingpost design was developed to span longer distances,
frequently up to one hundred feet. The design consists of one
kingpost in the center with several right angle panels on each
side of the center. There are fifteen remaining multiple kingpost
structures scattered throughout the state in eight different counties.
Queenpost Truss system followed the kingpost in design chronology.
It was also used to span long distances, frequently up to 75 feet.
The queenpost truss is really an explansion of the kingpost design
because of an additional rectangular panel in the center which
was placed between the two triangles that originally faced the
center vertical kingpost timber. The upper horizontal member of
that rectangle, however, had to be placed below the horizontal
upper chord of the exterior side framework. Frequently, additional
diagonal timbers were placed between the corners of the central
rectangle. There are still 38 true queenpost trusses remaining
in Pennsylvania today, located in or between 12 different counties.
Seven queenpost truss systems are used in conjunction with kingpost
structures, and the Landisburg Bridge, spanning Shermans Creek
in Perry County, has two queenpost trusses used in conjunction
with a single Burr arch truss.
One of the earliest and most prominent bridge builders in our
country was Theodore Burr from Torringford, Connecticut. His career
began in New York where he built a bridge spanning the Hudson
River in 1804. Burr's truss design soon became one of the more
frequently used sytems. The Burr arch truss, as the design became
known, used two long arches, resting on the abutements on either
end, that typically sandwiched a multiple kingpost structure.
There are more bridges in Pennsylvania using the Burr truss design
than the total of the other truss designs-123 located in or between
thirty different counties.
Town truss was named for its originator, Ithiel Town, who also
came from Connecticut. He designed and built his first bridge
in 1820. His design is somtimes called the "lattice truss"
and a glimpse at the pattern formed by its members readily explain
the nickname. In some areas it became very popular because it
used smaller dimension lumber than other trusses, required a limited
amount of framing and hardware, could easily be built by unskilled
laborers, and could span distances up to 200 feet. The heaviest
concentration of Town truss structures is in Bucks County, where
every remaining historic span-twelve of them- is of this design.
Twenty Town truss systems are still standing in the state and
are located in seven different counties.
William Howe of Massachusetts patented the Howe truss design in
1840. It is really an elaboration on the multiple kingpost design
where by two heavy metal rods are substituted for the vertical
timbers. There are also variations on this pattern that add a
second diagonal timber to the original single diagonal of the
multiple kingpost and/or another diagonal timber running in the
opposite direction between the vertical rods. Some accounts indicate
that the Howe design provided a bridge that was stronger than
the all-wood structure; as a result, it became the forerunner
of iron bridges. There are 124 Howe truss spans in the United
States today. Pennsylvania, however, can claim only five of the
Howe truss bridges, located in five different counties.
J. Smith of Tippecanoe City, Ohio, developed four truss systems.
None became very popular, and only one of them was used in Pennsylvania-the
Smith truss, type 2. According to the information available, it
was also the only one of the four Smith truss types to receive
a patent. This was granted in 1869, one year after he completed
the one remaining bridge in Pennsylvania-Kidd's Mill Bridge in
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