Bridge 90386 on 7th Street over railroad tracks
Bridge number: 90386
Year built: 1883-1884
Engineer: William A. Truesdell
Contractor: Michael O’Brien; McArthur Brothers, Chicago
Span length: west arch, 41 feet; east arch, 30 feet
Length of arch barrel: 124 feet
Adapted from the National Register of Historic Places nomination form prepared by Jeffrey A. Hess. The Seventh Street Improvement Arches were listed on the National Register of Historic Places in 1989.
The Seventh Street Improvement Arches, carrying East Seventh Street across an abandoned railroad cut on the eastern edge of downtown St. Paul, are a skewed, double-arched, masonry highway bridge constructed according to the helicoidal or spiral method. Buttressed at each end by perpendicular wing walls, the bridge displays two arches surmounted by a well defined coping on top of the parapet. Although the ring stones are cut to simulate the appearance of semicircular right arches, the two vaults are of skewed construction with voussoirs laid in helicoidal or spiral courses. The east arch measures 41 feet in oblique span, with a spring line about 9 feet above grade. The west arch is 11 feet shorter at 30 feet and springs about 12 feet above grade. Originally, the west arch accommodated three railroad tracks, the east arch two tracks. All trackage has been removed. Measuring 124 feet in width, the bridge supports at least 15 feet of earth fill surmounted by a concrete trestle, which, in turn, carries a four-lane black-top roadway and concrete sidewalks bordered by metal railings with concrete balusters. The trestle was constructed when the roadway was widened in 1930. These alterations do not significantly affect the original design.
The bridge features two types of limestone, a locally quarried gray stone that was widely used in 19th century St. Paul for foundation work and a finer-grained, buff-colored stone from Kasota, Minnesota, that was shipped throughout the state for a variety of building purposes. The abutments, pier and wing walls are built of the local material. The voussoirs, ring stones, coping and spandrel walls are of Kasota stone. All stonework is rock-faced, coursed-ashlar masonry with one-half-inch joints. By way of embellishment, intradosal surfaces are bush hammered and ring stones have tooled margins on top and bottom. The center pier also has a rounded, ornamental cutwater.
The Seventh Street Improvement Arches are historically significant for its rarity and the technically demanding nature of its skewed, helicoidal, stone-arch design. At the time of its construction in 1884, the bridge was thought to be one of the few of its type in the United States and today it is the only known example in Minnesota.
In February, 1883, the Minnesota State Legislature authorized St. Paul to issue bonds for the improvement of Seventh Street where it crossed the combined valley of Trout Brook and Phalen Creek, linking the downtown district with Dayton's Bluff to the east. Characterized as the heaviest piece of public work ever attempted in the city, the project called for the construction of four major elements: a roadway embankment measuring 80 feet in height, 640-feet in length and 66 feet in width; a stone-arch sewer, 320 feet in length, for the enclosure of Phalen Creek; a 300-foot-span iron bridge across the tracks of the Northern Pacific Railway; and a double-arch stone bridge at the crossing of the St. Paul and Duluth Railway. In the opinion of the contemporary engineering press, the stone-arch bridge was the most interesting part.
Originally known as the Seventh Street Improvement Arches, the structure was designed in the summer of 1883 by 38-year-old William Albert Truesdell, who had been hired by the City of St. Paul Engineer's Office to supervise the entire Seventh Street Improvement project. Raised on a farm in Wautoma, Wisconsin, Truesdell had studied engineering at the University of Wisconsin, receiving an undergraduate degree in 1867. After enduring uneventful jobs as a school teacher and a surveyor, he joined the engineering staff of the St. Paul, Minneapolis and Manitoba Railway in 1880. Except for occasional special projects, such as his work on the Seventh Street Improvement, he remained with that line and its successor, the Great Northern Railway, for the remainder of his career. During his years with the railroad, Truesdell was involved with all kinds of construction, often serving as an inspecting engineer. The Seventh Street Improvement Arches represent the only documented example of his design work.
The construction of the Seventh Street Improvement Arches presented a number of challenges. Since Seventh Street intersected the St. Paul and Duluth Railway right-of-way at a 63 degree angle, the bridge required a skewed or oblique, design, which, even under ideal conditions, placed extra demands on engineer, stonecutter and mason. As Truedell noted in The Association of Engineering Societies Journal of July, 1886, the ordinary problems of skewed construction were compounded by the fact that "nothing of this kind had ever been built in this western country," so that "very few of our masons in St. Paul had ever seen one, and no one knew anything about the stone-cutting necessary." Truesdell was aware that most American skewed stone-arch bridges were built according to the ribbed-arch method, which utilized "a number of short right arches or ribs in contact with each other, each successive rib being placed a little to one side of its neighbor." Although such a design might have been suitable for a simple highway bridge, the Seventh Street Arches were intended not only to carry pedestrian and vehicular traffic, but also sewer and water pipes, which required a substantial layer of fill. As Truesdell later explained, "The ribbed arch plan was first considered, and then rejected. Such a structure would have been unstable for this locality on account of the great weight of earth the arches would have to sustain. The stones of one rib could not be bonded into those of the next rib." Truesdell next considered the classical French method of skewed construction, where the voussoirs are custom cut in a variety of shapes to fit the configuration of the arch. "The voussoirs of such an arch," he concluded, "could never have been cut in this locality. It would have required a great number of patterns and the cost of such a work would have been beyond all consideration."
One alternative still remained, the helicoidal or spiral method. Introduced by the English architect and mathematician Peter Nicholson in 1828, the method was explained with clarity and precision by the English engineer George Watson Buck, whose Essay on Oblique Bridges, first published in 1839, went through three editions by 1880. In the United States, the helicoidal arch seems to have been little studied and less understood. When John L. Culley, in Van Nostrand's Engineering Magazine, April, 1886, attempted to clear away "the confusion and misunderstanding" surrounding the subject, he noted that "the general opinion has arisen, that helicoidal arches are of the most intricate construction, and too often their consideration has been abandoned with disgust." Truesdell, however, was equal to the challenge of the helicoidal method. According to his obiltuary in the Association of Engineering Societies Journal, June, 1909, he was imbued with "the desire to go the beginning of things" and studied mathematics as a hobby. When he himself discussed his work on the Seventh Street Arches in the Association of Engineering Societies Journal, July, 1886, he passed over the technical difficulties of helicoidal design, remarking only that "very few have ever been built in this country."
In the traditional helicoidal method adopted by Truesdell, the voussoirs are cut with curved surfaces so that they form a series of parallel spiral courses. As Truesdell explained in the journal article, each spiral is "generated by a straight line which intersects the axis of the arch, and is continually at right angles with it, and which moves uniformly along that axis and at the same time revolves uniformly around it." Although the initial calculation and cutting of the curves are difficult, the helicoidal method has one overriding advantage, all the voussoirs (with the exception of the ring stones) are of the same size and shape so "that one set of patterns answers for all . . . and when, the stone-cutters are once taught to cut a stone no further difficulty is encountered." On the Seventh Street Arches, the voussoir stones were quarried and cut at the quarry of W.D. Craig and Company in Mankato, Minnesota. According to Truesdell, "the only difficulty . . . was in making the stone-cutters understand the importance of accurate and careful work with the patterns instead of the ordinary work to which they had been accustomed. This was overcome by placing an intelligent and trustworthy foremen [Mr. Thomas Russell] in charge of all stone-cutters."
Construction of the Seventh Street Arches began in September, 1883, with Michael O'Brien of St. Paul serving as general contractor for excavation, foundations and abutments. After O'Brien's work was completed in June, 1884, the project passed to McArthur Brothers of Chicago as part of a general contract for the Seventh Street Improvement. In late November, 1884, the Chicago firm finished the masonry work and began leading the roadway over the arches, which opened for traffic on December 18, 1884. Although the significance of the Seventh Street Improvement faded from popular consciousness, Truesdell's professional colleagues remembered his achievement. On his death in 1909, the Association of Engineering Societies Journal characterized the Seven Street Improvement Arches as "the most important piece of masonry in the city."