Vermont's collection of metal truss bridges is rather large and quite diverse. A number patents introduced during the 19th century provided a variety of truss designs, many of which were widely used in Vermont. The state's first iron bridge was likely constructed in Bennington before 1866 and maybe as early as 1859. But few iron bridges existed in Vermont prior to 1870. The railroad began experimenting with iron bridges around 1870, due to the increased speed and weight of the locomotives over timber bridges. The use of iron spread slowly throughout the remainder of the 19th century. Two tragic railroad bridge accidents in 1886 and 1887 prompted a more widespread acceptance of iron as a superior building material. By 1900, iron bridges were numerous but not a dominant feature of rural Vermont. The major building phase for Vermont's metal truss bridges fell between 1892 and 1927, and a large number, all steel, were constructed after the 1927 flood. During the 1930s, the metal truss fell out of favor, and by WWII, few were being constructed.
Pony, Through, or Deck Trusses
Metal trusses can be categorized into three types. A pony truss has two sides that are elevated above the road but are not connected at the top by horizontal members, as seen in the image on the left. A through truss rises above the road and is connected by horizontal members at the top, as seen on the right, allowing one to pass through the structure. A deck truss supports the bridge only from below the road and is not visible when crossing over.
Caleb Pratt patented an iron and timber truss bridge design in 1844, and it became quite popular for Vermont bridges after the Civil War. Pratt incorporated wooden verticals and iron diagonals. Three examples are shown below.
A second iron truss patent was issued to James Warren and Willoughby Monzani in 1848. The Warren truss employed only diagonal members that were set in opposite directions, forming a series of diagonals. The Warren truss was widely used in Vermont bridge construction.
Also called multiple-intersection trusses and triangular-lattice trusses, the riveted-lattice truss employs a series of diagonals. The most common riveted-lattice truss bridges are double and triple-intersection. Double-intersection truss bridges, which have two sets of diagonals, were extremely common on highways and railroads in Vermont. Triple-intersection trusses, which have three sets of diagonals, were also common on railroad bridges. Riveted-lattice truss bridges with four sets of diagonals were limited to important crossings and today are quite rare. The two top images below illustrate double-intersection trusses, while the bottom image has four sets of diagonals.
Charles H. Parker, a mechanical engineer for the National Bridge and Iron Works, patented an evolved iron truss in 1870. The Parker truss utilized curved upper chords. A handful of these structures were erected in Vermont in the late 1860s and early 1870s. Parker's name was later associated with another truss type that used polygonal upper chords. Parker's patent curved truss is illustrated on the left, and the Parker Truss polygonal design is illustrated on the right.
Baltimore and Pennsylvania Trusses
Modified Pratt and Parker trusses were patented in 1871 and 1875, respectively. The new designs incorporated struts and ties into the old Pratt and Parker trusses. A few were constructed in Vermont. A Baltimore truss is shown below.
William O. Douglas, from Binghamton, New York, obtained patents in 1878 and 1884 for lenticular truss designs. Lenticular trusses consist of both upper and lower curved chords, giving the bridge the shape of a lens (hence the name lenticular). This bridge type gained popularity during the early 1880s, and a number were constructed in Vermont. Only one lenticular bridge remains in Vermont at Highgate Falls.
Cantilever Truss and Continuous Truss
Innovations in bridge design in the late 19th and early 20th centuries led to the construction of long span bridges over large bodies of water. An excellent example is the Champlain Bridge between Chimney Point in Vermont and Crown Point in New York that opened in 1929.
Many covered bridges and iron truss bridges washed away during the 1927 flood. The need for improved bridges was evident in the flood aftermath. Steel truss bridges were constructed with heavier components, making them easily distinguishable from the lighter-weight pre-flood bridges. Bridge construction was also standardized. Warren pony trusses with straight upper chords were employed over spans of 60 to 100 feet. Parker through trusses were occasionally used on heavily traveled sites. Pratt through trusses were standard for spans between 100 and 160 feet. Parker through trusses were used on spans greater than 160 feet. Notice the increased size of the truss components in the images below.