Delhi Bridge

Delhi Bridge meets criteria for listing in the National Register of Historic Places under Criterion C (Design/Construction Significance) as one of only a few surviving late nineteenth century Pratt through truss highway bridges of wrought iron construction in Michigan.

A 2007 study by the Delhi Bridge Historic District Committee found that Delhi Bridge is one of only five Pratt through truss bridges built by the Wrought Iron Bridge Company left in the State, and one of only two still in use in their historic locations. Of remaining Pratt truss bridges, the National Research Council considers 19th century examples, such as the Delhi Bridge, to be of greatest significance because of the limited number of extant examples.

The Pratt truss design was patented by Caleb and Thomas Pratt in 1844. It consists of an upper chord, in compression, and a lower chord, in tension, connected by vertical and diagonal members. The loads are applied to the truss at the panel joints, and the reactions are applied at the ends. The principal job of the vertical posts is to keep the chords apart and brace them. The end posts carry only tension, but the others are designed as compression members. The diagonal members resist the shearing forces between the chords that arise when the loads tend to cause the center of the span to sink. In the center panel, there are diagonals in each direction, although only one direction is in tension at any one time, the other being slack. The reason is that a moving load is not applied evenly across the bridge, and, as it moves, one set or the other of the diagonals will find itself in tension. These counters are generally used in one or more of the central panels.

The manufacturer, the Wrought Iron Bridge Company, was founded in 1866 by David Hammond, who later formed another large bridge company, the Canton Bridge Company. Vice President and Chief Engineer of the Company until 1889 was Job Abbott, who designed a number of famous bridges, including the 2,440-foot Canadian Pacific Railroad Bridge over the St. Mary's River at Sault Ste. Marie, the 3,660-foot Lachine Bridge over the St. Lawrence River, and the second cantilever bridge built in North America, a 460-foot bridge over the St. John's River in New Brunswick, Canada. As previously mentioned, the company had erected 3,300 bridges in the U.S. and Canada by 1881. A pamphlet published by the company four years later, in 1885, indicated it had built 4,300 spans in the past 18 years, of an aggregate 50 miles in length. The pamphlet also included a partial list of bridges the company had built. At that time, the company employed 270 men and was the third largest employer in Canton (exceeded only by C. Aultman and Company and the Diebold Safe and Lock Company). It was reported to have built more highway bridges since it started than any other works in the country. It had erected bridges in 26 states as well as in Canada and Mexico.

The Wrought Iron Bridge Company ceased to exist after 1899, when the company was bought by J. P. Morgan's American Bridge Company.

The Wrought Iron Bridge Company, like many other Midwestern companies that built iron truss bridges, marketed their bridge designing and building services through published advertisements and booklets that illustrated examples of the company's work. After the company received an order for a bridge, the engineers prepared detailed plans and instructions for fabrication and erection. Since each bridge was fabricated for a specific site, no two were exactly alike. Fabricators undertook as much assembly as possible before transporting the bridges to the sites where they were erected. The bridge fabricator then prepared each shipment, which was sent by rail. At its site, each bridge was assembled and placed on its abutments.

No documentation for the bridge's date of construction has been found, but the plaque on the portal of the bridge clearly identifies the manufacturer as the Wrought Iron Bridge Company of Canton, Ohio, which dates the bridge before 1899 since the company ceased to exist after this date. Based on records for construction of bridges in the surrounding area, if the current bridge is the original iron truss bridge at this location, the Delhi Bridge was likely built between 1888 and 1890. Dates of construction for the nearby Tubbs Road and Zeeb Road bridges were 1888 and 1890, respectively. Moreover, an eight panel iron truss bridge appears in photographs from the early 1890s. However, if the current bridge replaced the original iron truss bridge after the 1917 tornado, it is only possible to conclude the bridge was built prior to 1899, when the manufacturer ceased to exist. The main members of the Delhi Bridge bear the stampings of the Trenton Iron Company. It was founded by Peter Cooper in 1847 and was absorbed into the American Steel and Wire Company, a subsidiary of U.S. Steel, in 1912. According to the book, Remembering Delhi Mills, by Nicholas Marsh, an iron truss bridge replaced the original 1851 wooden bridge. This wooden bridge appears in an 1874 Atlas of Washtenaw County.

Since 1917, other than periodic decking replacement, only minor changes have been made to the bridge. Vertical members were welded at the hip joints at some point subsequent to 1936 (as they do not appear in historical photographs from this date) and diagonal members in the center panels were reinforced as well. Moreover, abutments have been reinforced with concrete. Also, as previously discussed, engineering design plans for 2008 rehabilitation include some minor structural repairs, such as replacement of some bracing rods and rivets, repair of eyebars, and duplication of the one remaining original structure nameplate so that both ends of the bridge will have matching nameplates. These repairs are being done in a manner consistent with historic preservation. Bridge bearings, abutments, and decking will be replaced as well. These modifications are typical when historic bridges are rehabilitated and are not pertinent to its engineering significance.

The bridge was designated a local historic district by Washtenaw County in 2007 under Michigan's Local Historic Districts Act (Public Act 169 of 1970). It is the only bridge in Michigan with such a designation.

The Delhi Bridge is a single-span, Pratt through truss wrought iron bridge. This bridge spans the Huron River just south of Huron River Drive as an extension of East Delhi Road in Scio Township of Washtenaw County, Michigan. The Delhi Bridge has an overall length of 105' 7" and width of 19' 1". The south bridge portal of the bridge still has a cast-iron plaque containing the legend "WROUGHT IRON BRIDGE CO., CANTON, OHIO." The Wrought Iron Bridge Company fabricated wrought iron and steel bridges that were built across the United States and manufactured thousands of bridges. By 1881 the company had erected 3,300 bridges in the U.S. and Canada. According to an on-site historic inspection conducted as part of an environmental assessment when bridge replacement was being considered, the main members of the Delhi Bridge are wrought iron.

The bridge is single span, with no approaches. The bridge has 8 panels, with no lacing on any members. Truss members are pin-connected. This technology was no longer used after 1910. The vertical brace chords are I-beams riveted to cover plates. Diagonal brace chords consist of a rod with bars on either side. The two middle panels have a second diagonal consisting of a square bar. Upper and lower chords as well as end posts consist of two channels riveted to an upper cover plate with batten plates below. Sway, diagonal, and lateral braces span between the two upper chords. End portals consist of riveted angles connected by lattice bars. Portals also have knee braces. Vertical braces were welded to the end posts at a later date and guardrails added. Abutments and wingwalls are fieldstone. The north abutment was encased in concrete at some point. Wood decking and stringers have also been replaced.

The bridge connects Huron River Drive to the unincorporated village of Delhi Mills, founded in the 1830s, and the site of several mills in the late nineteenth century. The village now consists of 49 houses, a nineteenth-century one-room schoolhouse, and a population of approximately 123 residents. The character of the village remains much as it was when the bridge was put in place and many historic structures remain intact. All land immediately adjacent to the bridge is owned by the Huron Clinton Metropolitan Authority. This property is part of Delhi Metropark, a popular recreation area established in 1947 that includes a canoe livery and a popular kayaking destination, the Delhi Rapids. The bridge is a focal point for many park visitors, who often stroll on and around the bridge to view the rapids.

The current bridge may not be the first iron truss bridge at this location. Instead, it may be a bridge of the same vintage as the original that it replaced in 1918. On June 6, 1917, a tornado swept through the area, damaging the bridge then at the site, lifting it off its abutments, and causing it to land in the river. It is known that the damaged bridge was pulled out of the river by a team of horses, and oral history obtained from a resident living in Delhi Mills at the time indicates it was placed back in its original location. Furthermore, comparisons of the current bridge with archival photographs show no differences in vertical members, horizontal members, and bracing. However, the portals are clearly different from those in archival photographs, and photographs of the bridge after the tornado suggest extensive damage. There is little evidence of such damage on the bridge currently in place, and an evaluation of the bridge by an expert on historic iron truss bridges, Vern Mesler, concluded that the bridge is largely intact and not assembled from pieces of different bridges. Whether or not this is the first or second iron truss bridge at the site, there is no question that it is an intact Pratt through truss wrought iron bridge built by the Wrought Iron Bridge Company.

In 2005, the Delhi Bridge was closed to vehicular traffic. It is scheduled for rehabilitation by the Washtenaw County Road Commission in the summer of 2008. The wrought iron structure of the bridge will be cleaned and painted, with some minor repairs. Abutments and decking will be replaced.