Wabash Bridge (St. Charles)

By the late 1920s, the 1880s bridge had become obsolete for modern traffic, and the Wabash sought to replace the structure. The bridge had become extremely costly from an operations standpoint, and numerous attempts to reinforce the structure had not provided the capacity required by the railroad. Surveys for a new bridge were begun in late 1929. Due to the height and irregular alignment of the old line, it became advisable to construct a new bridge on a new alignment. A suitable location for a new bridge was found approximately one half mile downstream of the existing bridge. The new bridge required a line change of over 7 miles between Robertson and St. Charles. The requirements imposed by the War Department necessitated that the bridge have a 600-foot main span, a 426-foot auxiliary span and cross 46 feet above the Missouri River. Modjeski, Masters and Case was contracted to serve as consulting engineer for the new bridge, with Wabash Bridge Engineer C.S. Johnson overseeing the project. The chosen design involved constructing a large cantilevered truss bridge, with lengthy deck plate girder viaducts on either side. Due to the size of the structure, several contractors were hired to complete the work. Missouri Valley Bridge & Iron Company was contracted to construct the river piers, the Inland Construction Company was contracted to construct the concrete substructures for the east approach, and Bates & Rogers received the contract for constructing the west approach substructures. The main truss spans would be fabricated and erected by Mt. Vernon Bridge Company, with American Bridge Company fabricating and erecting the approach viaducts.

During the winter of 1930-1931, the five main piers of the bridge were constructed. Due to the varying depth of bedrock beneath the river, the piers were constructed using different methods. The easternmost main pier, or pier #1 was constructed on concrete piles. Piers #2 and #3 were constructed using pneumatic caissons, while piers #4 and #5 were constructed using a cofferdam, and were constructed directly on bedrock. The piers for the bridge were mainly constructed of concrete, but lower portions of the piers were faced with limestone quarried near Bedford, Indiana. In May 1931, the five main piers and significant grading had been completed, but work was halted due to the Great Depression.

Work would resume in 1934, after a Federal loan helped finance the bridge. Erection of the main portion of the bridge was completed from the east end, starting with the Warren truss span. This work was completed by means of a traveler inside the truss, which continuously worked west. The approach viaducts of the bridge were delivered the ground, with spans erected by a crane located on the ground. The bridge required 8,800 tons of steel, and the new line required 1,650,000 cubic yards of earthwork and grading. The new bridge would run at a 0.50% grade for a portion of the structure, significantly lower than the 1% grade of the old bridge. The new structure was constructed at a cost of $1,720,000. The bridge was completed in the current configuration in October 1936, and demolition soon commenced on the old bridge, which was dropped to the ground and cut up for scrap.

The central span of the bridge consists of a cantilevered 624-foot, 20-panel riveted Warren through truss. This span is anchored by a 431-foot, 12-panel span of the same design on the east, and a 274-foot, 8-panel span of the same design on the west. The suspended portion of the central span is 312 feet. In addition, a 316-foot, 12-panel riveted Warren through truss span approaches the cantilevered spans on the east end. The east approach viaduct of the bridge consists of, from east to west, a 61-foot deck plate girder, thirteen 60-foot deck girder spans, seven 30-foot deck girder spans, a 72-foot deck plate girder span over Missouri Bottom Road, thirty-six 60-foot spans, nineteen 30-foot spans and a 92-foot deck plate girder span. The west approach viaduct consists of, from east to west, a 107-foot deck plate girder span, three 60-foot and two 30-foot deck plate girder spans, a 70-foot span over River Road, a 100-foot deck plate girder span over the MKT Railroad (now Katy Trail), twenty 60-foot and eleven 30-foot deck plate girder spans, a 104-foot deck plate girder span over 3rd Street, a 30-foot and a 106-foot deck plate girder. The 60-foot and 30-foot deck plate girder spans were arranged in an alternating pattern, consisting of two 60-foot spans and a 30-foot tower span. The 30-foot tower spans were designed to be as deep as the 60-foot spans due to the economics and to improve appearance. The substructures of the approaches utilize steel towers and bents, set onto concrete pedestals and abutments.

Cantilevered spans are relatively unusual for railroad use, requiring precise calculations and additional engineering. These types of spans were typically used in the 20th Century, particularly to cross navigable waterways requiring large channels and greater clearance. The cantilevered span of this bridge is made up of three components, including a central span and two anchor spans. The anchor spans are differing lengths, another unusual feature of the bridge. The cantilevered spans are connected by towers, located at pier #3 and #4. The truss spans of this bridge utilize solid members, some laced members, heavy riveted connections and an X-frame portal bracing, designed to help stabilize the structure. The Warren design was arguably the most popular railroad truss design in the 20th Century, as the design provided durability and strength, while being cost effective. The use of deck plate girder viaducts was common for large crossings such as this, as the design provided the most cost effective solution for long approaches or viaducts.

Since the initial construction, the bridge has seen few changes. The bridge was dedicated as the Norman Pitfcairn bridge in the 1950s, in honor of a former Wabash president. Missouri Route 370 was constructed immediately downstream of the bridge in the early 1990s, and the Earth City Expressway was constructed under two 60-foot spans on the east end of the bridge at the same time. Overall, the bridge appears to be well maintained, and no signs of significant deterioration were found during a 2016 visit. The author has ranked this bridge as being nationally significant, due to the unusual design, large size and unique history of this crossing.