COVERED BRIDGE TERMINOLOGY
While researching the covered bridge topic we chose
for this issue of The American Antiquities Journal, I was having
difficulty understanding a lot of the terminology used in describing
covered bridges. So in order to assist our readers in understanding
those terms, I have compiled a list of terminology and descriptions.
One of the first, and most used, terms in describing
a covered bridge is the word "truss." Truss is a common and
general word used in the construction trade meaning a structural
framework that provides support. We usually see trusses used on the
roofs of houses, however they are used in many different applications.
Trusses are typically (but not necessarily) composed of triangles
because of the structural stability of that shape and design. A triangle
is the simplest geometric figure that will not change shape when the
lengths of the sides are fixed and stress is applied.
One way that a covered bridge is identified and
described is by the type of truss used in its construction. Truss design
determines how long a span the bridge can have and how complex it is to
build. Local residents can often construct simple designs that cross
small streams. More complex and multi-span bridges require more
experienced builders. Types of trusses are usually named for the men who
design them.
The oldest covered bridge design is the kingpost. It
is the simplest, most common, yet most limited (40-ft span) type of
truss construction. The kingpost truss, however, can be used in
multiples to span longer lengths.
The queenpost truss forms an elongated, topless
triangle with support posts at each end. Also a simple design, it allows
for greater bridge lengths than the kingpost.
Theodore Burr of New York patented the Burr Arch
truss in 1804. It incorporates reinforced arches that tie directly into
the bridge abutment (the concrete or rock pillar that supports the ends
of a bridge at the shoreline) with a series of triangular support posts.
It allowed bridges to span lengths over 100 feet for the first time.
Colonel Stephen H. Long developed what became know as
the "X" truss.
Connecticut architect Ithiel Town patented his truss
of crisscrossed diagonals or lattice in 1820.
Builder William Howe introduced iron into wooden
truss design by substituting adjustable iron rods for the vertical
members of Long’s truss.
Robert W. Smith developed trusses in three variations
of his basic design in 1867 and 1869.
Rueben L Partridge received a patent for a design
that was very similar to Smith’s truss.
Horace Childs’ truss simply added diagonal iron
rods to a multiple kingpost design.
Two Englishmen patented a truss using isosceles
triangles and called it the Warren Truss.
There is also a Warren Truss Plus Arch.
As early as 1805 German designer extraordinaire,
Lewis Wernwag constructed a single span bridge across Pennsylvania’s
Schuykill River, a span of 340 feet. The bridge was so enormous that
rumors spread all across Philadelphia that the bridge would collapse as
soon as the scaffolding was removed. Thousands of people lined the river
to watch the catastrophe. To demonstrate faith in his project, Wernwag
removed the supports himself. The bridge proudly stood for 26 years
until it was destroyed by fire.
At least two covered bridges make the claim of being
the first built in the United States. Town records for Swanzey, New
Hampshire, indicate their Carleton Bridge was built in 1789, but this
remains unverified. Philadelphia claims a bridge built in the
early 1800s on 30th Street and over the Schuylkill River was the first,
noting that investors wanted it covered to extend its life
According to Covered Bridges Today (Daring
Books, 1989) by Brenda Krekler, as many as 12,000 covered bridges once
existed in the United States, but that number dropped to under 1500 by
the 1950s. The National Society for the Preservation of Covered Bridges
was formed in 1950. |