Historic Bridge Project

The Historic Bridges of Iowa links to information and photographs of a wide range of historic bridges such as the metal truss bridges, concrete arch and beam bridges, steel bridges, and the magnificent covered bridges of Madison County, built in the early 1870s and 1880s.

Bridge Types

• 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. 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.

Project Background

In 1994 the Iowa Department of Transportation (DOT), in cooperation with the Federal Highway Administration and State Historic Preservation Office, hired FraserDesign to complete an inventory of Iowa’s state, county and city historic roadway bridges constructed prior to 1942. The Iowa DOT has hired the Louis Berger Group Inc. to expand the 1994 inventory to include bridges constructed between 1942 and 1970. The inventory is intended to be used as a planning tool for state and local agency officials as they perform bridge maintenance, rehabilitation and replacement projects. However, many members of the public are also interested in Iowa’s historic heritage and would like to learn more about our historic bridges.

Iowa currently has a total of 25,075 bridges, placing it fifth among the states in the nation. If Iowa’s roadway bridges were laid end to end, they would stretch across the state and half way back. In 1994, approximately 9,000 bridges were looked at in 1994 for the historic inventory and about 10,000 are being considered in the present inventory.

The inventories include:

• A historical overview of bridge development and construction providing the context in which the bridges are evaluated.
• An inventory of several types of vehicular bridges including a description of their structural configuration.
• An evaluation of each structure’s National Register of Historic Places significance.
• Results from the 2003 research by the Louis Berger Group Inc. They set out to find each of the historic bridges recorded in 1994, photograph them, record their locations with global positioning system equipment and add information on their current status into a database.
• Since the 1994 inventory, some of the bridges had been moved, burned or destroyed by floods or fire. The historic bridges that still stand in the state today are listed in this Web site.

The reader should be aware that the bridges listed represent the results of data collected in 2003. For a number of reasons, the status of any listed bridge may change. While the database is periodically updated, the information shown may not always be current.

Selection Criteria

The bridges listed in this page have been determined eligible for or are listed on the National Register of Historic Places (NRHP). The criterion used to evaluate the bridges is based on the National Park Service’s NRHP historic evaluations.

• Criterion A. Criterion A is associated with having made a significant contribution to the broad patterns of history. In a liberal interpretation, most bridges older than 50 years could qualify under this criterion as an element of Iowa’s transportation system. To exercise a degree of discrimination under Criterion A, bridges need to have been important in the settlement and development of a geographically definable area.
• Criterion B. Structures important under Criterion B have an association with lives of significant people in history. Important bridge designers have created a few of Iowa’s significant bridges.
• Criterion C. Criterion C embodies the distinctive characteristics of a type, period or method of construction. Most bridges that qualify for the NRHP fit in this category. To winnow the group to a meaningful list, the bridges evaluated under Criterion C paid particular attention to identifying aspects of the bridges that make them stand out among their group (i.e., earliest/oldest example of type, longest span, longest total length, unusual structural or architectural detailing, etc.).

Historic Surveys

First Survey

In 1995 the Iowa Department of Transportation (DOT), in cooperation with the Federal Highway Administration and State Historic Preservation Office, completed an inventory of Iowa’s state, county and city historic roadway bridges constructed prior to 1942.

This first survey work was completed by FraserDesign.

Second Survey

In 2011, the Iowa DOT contracted with Louis Berger Group Inc. to expand the 1995 inventory to include bridges constructed between 1942 and 1970. These inventories and evaluations have been intended to be used as planning tools for state and local agency officials as they perform bridge maintenance, rehabilitation and replacement projects.

Many members of the public are also interested in Iowa’s historic heritage and would like to learn more about our historic bridges.

• Intensive Architectural Survey - 1942 and 1970

Third Survey

Starting in 2023, the Iowa DOT, with support from the Iowa Highway Research Board, began collaborating with a team of consultants on another bridge survey.

The consultants include:

• Bear Creek Archeology
• Historical Link
• Bentonsport Preservation

This third generation bridge survey will review bridges constructed up through 1985 and reevaluate determinations from the earlier two bridge surveys. We are hoping to conclude the third bridge survey in late 2025.

Looking Ahead

If you have historical information about a historic bridge that you would like to share, we will enjoy hearing from you. Consider using DOT.HistoricBridges@iowadot.us to share your information.

Since the 1995 inventory, some bridges had been moved, burned or destroyed by floods or fire. The historic bridges that still stand in the state today are listed on this page.

The bridges listed on the website represent the best information we have about a particular bridge. For a number of reasons, the status of any listed bridge may change. While the database is periodically updated, the information shown may not always be current.