Historic Structures

Bridge Design and History Bridgeport Swing Span Railroad Bridge, Bridgeport Alabama

The 364'-0" long swing span bridge is of the thru sub-divided Warren type steel truss, partially continuous over four supports, and of the rim bearing swing bridge type for balancing and turning atop the center, or pivot, round pier. The span lengths for the entire span are 174'-0" (arm), 16'-0" and 174'-0" (arm), with both arms of the same length. This swing drawbridge opened by revolving about a vertical axis on a cylindrical drum, supported by steel wheels (rollers) running on a rim bearing circular raceway on the center pier; the truss loads being delivered to the drum by a system of distributing girders. In the Bridgeport swing bridge, the circular drum and wheel rim bearing raceway have a 25'-5" diameter and forty 20-inch diameter steel tapered wheels. The tapered wheels serve to keep the span properly placed on its imaginary pivot point when opening and closing.

An alternate method used during this period (1890-1892) to accomplish the same results was a simpler arrangement where the entire weight of the swing bridge, when turning, is carried on a central pivot. In this case, the trusses are two spans and are continuous over three supports when the span is closed and the ends lifted.

As previously stated, the Bridgeport swing bridge has three spans having four supports, when the bridge is closed and the ends are lifted by wedges. When the web bracing is continuous across the middle span/panel, above the circular drum, the trusses are continuous over the four supports. As the span/panel above the drum is much shorter (16 feet) than the two end spans (174 feet), heavy negative reactions would be caused at one of the two middle supports> thus lifting the drum up off its wheel roller supports, when only one arm of the span was loaded.

In order to avoid the difficulty just mentioned, the Bridgeport drum bearing swing bridge was designed and built as two simple spans with light web members and top chord above the middle supports, making the span only partially continuous. It is partially continuous across the middle span/panel for bending moments and discontinous for shears.

When the end wedges/supports are removed and the entire swing bridge is balanced on its two middle supports, resting atop the center/pivot pier, the top chord is in tension and was designed to carry the direct tensional load delivered to its members by the dead load of the cantilevered swinging structure.

The Bridgeport swing bridge was designed and built without the usual floor system of rail stringers and floorbeams. To support the track within the thru trusses, 8" x 16" x 18'-0" long timber ties, spaced at 16-inch spacing, were placed transverse (90 degrees) to the track and trusses. These ties spanned from center of truss to center of truss, a distance of 16'-0", and were placed atop of the bottom chords and supported thereon. As a train crossed the swing bridge, the ties supported the load and delivered it to the steel trusses, which then bore this load and transferred it to the four supports on the stone piers, thence down the piers to bed rock.

The truss members were designed and built of metal, wrought iron and steel, in relative light pieces both in length and weight. Rivets were used in the making and fabrication of the members which were built up by the use of plates, channels, angles and bars; however, certain tensional members were made of bars with enlarged ends. These members were called eyebars. Machined holes were provided in the ends of these members, or spaced where needed along a long continuous member, to provide a method of connecting the various individual pieces to form the desired truss bridge.

The erection in the field, at the site of the bridge crossing, was accomplished by progressively assembling the individual pieces and connecting their ends with steel pins, placing the pin through all the necessary holes that had been previously lined up in their desired location and position. Falsework, usually timber supports, was used to support the dead load of the members during erection and was removed when the trusses were completely assembled atop their pier supports.

During the lifetime of the Bridgeport swing bridge, pin wear and hole elongations developed, necessitating corrective measures to prevent the members from excessive movement and sawing. Where schemes could not be formulated to hold the members in place at their joint, it became necessary to remove the old pin, enlarge the hole, and insert a larger replacement pin-secured with pin nuts each end. During this work, rail traffic was halted and could resume only after the corrective work was completed. A milling machine, mounted at the joint being worked on, enlarged the original hole diameter to the desired size to correct the hole enlargement, thus providing the size needed to accommodate the larger replacement pin.

Highlights of happenings to the Bridgeport swing bridge include - air dump car with clam shovel dumped on span and damaged end vertical post on February 15, 1946; diesel boat "Elisha Woods" handling two empty oil barges struck and damaged upstream end of open draw span on January 24, 1947; lightning struck operator's house, gasoline exploded, blowing house away and damaging machinery, the four center steel vertical members above machinery floor and interrupted rail operations on May 14, 1957; towboat "Elisha Woods" with four empty barges enroute downriver struct west rest pier and damaged rails and rods banding on stone pier on October 7, 1957; and five runaway rail cars, set loose by vandals in Bridgeport, Alabama, rolled to the swing span and plunged into the west channel of the Tennessee River, while the swing span was in the opened position, on November 4, 1980.

Much can be written about the history of the Bridgeport swing bridge. The bridge served its intended purpose as a railroad structure, allowing the crossing of the Tennessee River by the Railroads it served since its construction in 1890-1892 to 1981. This is a span of 90 years. The old structure gave way to a much needed heavier, wider and safer railroad bridge crossing, and it was be removed upon completion of the replacement bridge.

In 1981 an average of 32 trains (L&N-20 and Southern Railway) crossed the bridge daily, and an average of three boat openings were made daily to permit river navigational users to pass through as they ply the Tennessee River. 42 million gross ton miles of freight traffic crossed the bridge annually.