Historic Bridges

Deck, pony and through truss

In addition to classifying metal truss bridges by name, their form is further distinguished by the location of the bridge deck in relation to the top and bottom chords, and by their structural behavior.

Examples of the three common travel surface configurations are shown in the truss type drawings to the left.

• In a Deck configuration: traffic travels on top of the main structure
• Pony configuration: traffic travels between parallel superstructures, which are not cross-braced at the top.
• Through configuration: traffic travels through the superstructure (usually a truss), which is cross-braced above and below traffic.

Howe truss

A Howe truss slightly resembles a Pratt truss, but the Howe diagonal web members are inclined toward the center of the span to form A-shapes. The vertical members are in tension, while the diagonal members are in compression, exactly opposite the structure of a Pratt truss. Patented in 1840 by William Howe, this design was common on early railroads. The Howe truss was patented as an improvement to other forms of covered bridge designs.

Town lattice truss

The town lattice truss was designed by Ithiel Town and patented in 1820. It consists of a lattice of web members, crossing at an angle of 45 to 60 degrees, connected by wooden pins to horizontal top and bottom cords. There are no vertical members dividing the town truss.

Baltimore truss

The Baltimore truss changed the basic Pratt configuration by adding additional, auxiliary members, like the Pennsylvania truss, but it does not have an inclined upper chord. The upper and lower chords of a Baltimore truss are parallel, like the Pratt truss. Both the Baltimore and Pennsylvania truss types were developed by engineers of the Pennsylvania Railroad in the 1870s. Both types were also used for highway bridges.

Bowstring truss

The bowstring arch truss was the iron span of choice for Iowa counties in the late 1860s and 1870s. Developed and patented by Squire Whipple in 1840, bowstrings were marketed extensively throughout the Midwest by such industry giants as the King Bridge Company and Wrought Iron Bridge Company. A small number of bowstring arches were erected in Iowa in the 1880s. However, the number dwindled precipitously by the decade's end.

Camelback truss

The cantilever truss is configured so that one or both of its end sections extend beyond the supports. The main span of a typical cantilever truss has two projecting arms extending from piers supporting a suspended span. The secondary spans are projecting arms from the anchor piers that counterbalance the main span. The cantilever truss was not widely used in the United States until the last quarter of the 19th century. By the end of that century, it was clearly understood that the longest spans were possible with truss bridges of the cantilever type. Twentieth century bridge designers used the cantilever truss frequently for spanning major rivers.

Kingpost truss

One of the simplest of truss types. Originally they were often made out of pipe or wood.

Parker truss

The Parker truss, developed by C.H. Parker, is a Pratt truss with an inclined top chord.

Pennsylvania (Petit) truss

The Pennsylvania (Petit) truss modified the Parker truss by introducing sub-struts or sub-ties, which are members of the truss acting to resist or transmit stresses, respectively. Both the Pennsylvania and Baltimore truss types were developed by engineers of the Pennsylvania Railroad in the 1870s.

Pratt truss

The Pratt truss was originally patented by Thomas and Caleb Pratt in 1844. In its earliest form, the Pratt truss was a combination wood and iron truss. The top chord and verticals acted in compression and were made of wood, while the bottom chord and inclined members acted in tension and were made of iron. This combination Pratt truss was built through the 19th century and was cited as a continued form by bridge engineers as late as 1908. The Pratt truss survived the transition to metal construction and was widely built as an all-metal truss well into the 20th century. In 1916, bridge engineer and historian J.A.L. Waddell claimed that the Pratt truss was the most commonly used truss type for spans under 250 feet.

Suspension types

The longest bridges in the world are suspension bridges or their cousins, the cable-stayed bridge. The deck is hung from suspenders of wire rope, eyebars or other materials. Materials for the other parts also vary: piers may be steel or masonry; the deck may be made of girders or trussed. A tied arch resists spreading (drift) at its bearings by using the deck as a tie piece. The few historic suspension type bridges in Iowa consist of a pedestrian bridge in Floyd County and Iowa-Illinois Memorial Bridge in Scott County.

Warren truss

The Warren truss was patented in 1848 by two British engineers, James Warren and Willoughby Monzoni. The original form of a Warren truss was a series of equilateral triangles, and as such, represents one of the earliest, simplest truss types. Later modifications included subdivision by verticals or addition of alternate diagonals. The Warren truss was widely built throughout most of the United States from about 1860 to the 20th century.

Whipple Pratt truss

The Whipple Pratt truss, also termed double intersection Pratt truss, added additional diagonals to the basic Pratt truss, which extended across two panels, but kept the parallel top and bottom chords of the simple Pratt profile. Squire Whipple's double intersection truss was patented in 1847. In 1863 John W. Murphy, chief engineer of the Lehigh Valley Railroad, slightly modified the Whipple Pratt truss by adding crossing diagonals. The Whipple Pratt truss was widely used for long span railroad bridges.