Bridgeport Swing Span Railroad Bridge, Bridgeport Alabama
Bridgeport, in the northeast corner of the State of Alabama, is the location where the Louisville and Nashville Railroad Company's (L&N) main line track between Nashville, Tennessee and Atlanta, Georgia crosses the Tennessee River on a single-track railroad swing span bridge structure. The crossing is also used jointly by the Southern Railway.
The river was first crossed in 1854, as a result of construction by the Nashville and Chattanooga Railroad's (N&C) rail line between Nashville, Tennessee and Chattanooga, Tennessee, 1850-1853. The railroad bridge crossing was apparently built with timber.
The existing 364'-0" long swing span, steel, pin-connected bridge was built in 1890-1892. This bridge replaced an earlier bridge, or bridges, and was constructed on the same alignment. The replacement bridge was constructed by the Nashville, Chattanooga and St. Louis Railway (N.C.& St. L.), a successor company of the N&C, and consisted of a 114'-6" approach truss span on the Chattanooga (East) end, a 364'-0" swing-span truss span over the river, and a 145'-0" truss span on the Nashville (West) end with a short timber approach trestle. The total length of the bridge was 690'-0".
The 1890-1892 construction provided new stone masonry piers and abutments and the 364'-0" steel thru truss pin-connected swing span. The adjacent approach truss spans were apparently re-used spans from the earlier bridge, which was in use at the time of this major rebuilding and upgrading construction. The 114'-6" truss span on the Chattanooga end was replaced in 1910 with a 114'_8 1/4" riveted thru truss span. The 145'-0" truss span on the Nashville end was replaced in 1912 with a second-hand 143'-3 3/4" riveted thru truss span which was fabricated in 1905.
The swing bridge was designed by the Louisville Bridge and Iron Company, Louisville, Kentucky, in 1890 for the owner, N.C.& St.L, Railroad. The span is 364'-0" long and comprised of two swinging arms, each 174'-0" long. The steel trusses are of the sub-divided Warren type, partially continuous over four supports, with the thru spans being 174'-0", 16'-0", and 174'-0" long. The two middle supports, being only 16'-0" apart, rest on the center/pivot round pier. These middle supports transfer their load down through a series of distributing steel girders to sixteen points atop a steel circular drum with a 25'-5" diameter.
The steel circular drum is fixed to and underneath the distributing steel girders, thus, rotating as the span is opened and closed. This circular drum sits atop of forty 20-inch diameter tapered wheels that are equally spaced around the circumference of the 25'-5' diameter circular drum. The wheels are held in position by forty steel rod spokes which radiate out from a capstand which is centered at the center of rotation.
These steel wheels run in a raceway on a circular steel rim bearing casting with tread plate placed atop the round center/pivot pier. The rim bearing casting was fabricated in segments, anchored to the top of the pier, and contains gear teeth around its outside perimeter. The pitch diameter of this gearing is 26'-7 9/32".
The-swing span bridge, 364'-0" long, weighing 600 tons, is turned with two small pinion gears, with power being supplied by electric motors, gear reducers, shafts, etc. The operating controls for operation of all machinery for opening and closing of the swing span bridge is in the operator's house positioned above the track within the steel superstructure at its center of rotation.
The truss members of the 364'-0" long swing span bridge are pin-connected. The chords of the trusses and main diagonals (web members) are fabricated from Bessemer Steel, with the subdiagonals and compression verticals being of wrought iron. The swing span bridge has no floor system of floorbeams and rail stringers, thus, the track is supported by 8" x 16" x 18'-0" long timber ties spaced at 16-inch centers. These ties are placed transverse (90 degrees) to the track and trusses and span from bottom chord to bottom chord. The two steel thru trusses are spaced at 16'-0" centers. The ties, which are supported atop the bottom chords of the trusses, not only produce direct loads to the chord members but produce bending moments as the chord is supported at ten panel points (joints) that span 17'-4, 13/16" on each 174'-0" arm.
The 364'-0" long swing bridge was designed in 1890 for a fixed dead load of 2,000 pounds per lineal foot of bridge, plus a "rolling load" (live load) of two consolidated type (131,500-pound) steam locomotives followed by a uniform live load of 3,000 pounds per lineal foot. To this was added impact, wind and other longitudinal and transverse loads.
The 1890 design loading is equivalent to somewhat less than a Railroad Cooper E30 loading. Present-day railroad design specifications specify a Railroad Cooper E80 live load when designing new railroad bridge structures.
In 1943, the owner, N.C. & St.L. Railroad, engaged the consulting engineering firm of Howard, Needles, Tammer and Bergendoff to make a study of the bridge on the basis of increasing the weight limit by strengthening. The swing bridge was showing signs of weakness and by design was carrying as much weight as could safely be permitted to cross the bridge. The owner desired to operate both heavier steam locomotives of the Mikado type (313,000-pound) and heavier rail cars in series weighing 210,000 pounds per 4-axle cars.
The conclusion reached by the consulting engineering firm as a result of
this study and evaluation along with their conclusions is summarized as follows:
1. All principal parts of the structure, except the swing span, are adequate for all loadings contemplated, within certain speed regulations.
2. The swing span, originally designed for loads not much more than half the present loading, is in part seriously overstressed, even under the liberal provisions of the stipulated rating specifications, and but for the alert attention and maintenance it has received during the past year, a serious accident might have occurred. The most pronounced overstresses are found in the bottom chords, arising largely from improper application of track loads to them. With so many members of the span rating as acceptable as an old bridge under the stipulated rating specifications, we conclude that with suitable corrective work to other members, this span can be made adequate for the engine and train loadings specified, according to A.R.E.A. rating for OLD bridges, for operation at speeds which will produce impacts not to exceed 15 percent. For this we recommend a speed limitation of ten miles per hour, strictly observed.
3. Continued use under present conditions prior to corrective repairs involves hazards. Until reconditioning shall have been completed, suitable precautionary measures must be observed. After reconditioning, the span can be kept in service as long as desired under the speed limitations stipulated, for live loads not greater than herein considered, provided it receives careful and thorough maintenance and is given frequent close and intelligent inspection so that incipient failures may be detected and promptly remedied.
4. We desire to make clear that this is an old bridge, and while usable as an old bridge under the conditions set forth, it cannot be regarded as in any way equal to a new bridge designed for the present engine loadings and in accordance with modern design specifications and modern methods of manufacture. Such a new span would have a wide factor of safety for the actual loadings. The stipulated rating specifications in effect use up most of the margin of safety provided in the original design.
5. We estimate the cost of the contemplated corrective repairs of the swing span will be approximately $90,000.00.
The above conclusions and summarization were made in letter dated January 15, 1944 from Ernest E. Howard of Howard, Needles, Tammer and Bergendoff, Kansas City, Missouri, to R. L. Schmid, Chief Engineer, N.C.& St.L. Railroad, Nashville, Tennessee, the then owner of the Bridgeport swing bridge.
As a result of this study and in order to operate heavier rail locomotives and equipment across the Bridgeport swing bridge, the N.C.& St.L. Railroad made arrangements with the Nashville Bridge Company to make an engineering design analysis of the 364'-0" swing bridge, formulate a method to strengthen the weak
members, prepare shop detailed plans, furnish materials and strengthen the span in the field to support a railroad live load equivalent to a Cooper E50. This strengthening work was done in 1944-1945 by adding plates, bars, yokes, Ushaped rods, etc., both by welding and bolting. An improved method of supporting the ends of the track ties, supported atop the bottom chords, was incorporated into this work. This improvement was to eliminate twisting of the bottom chord as the live load crossed and the ties deflected down, causing their load to be on the edges of the chords.
Prior to the strengthening work, the swing bridge span had on various occasions been repaired, including, principally, replacement of a considerable number of loose rivets, attempts dating back to 1928 to correct pin wear, tightening of elongated eyebars, renewal of rim bearing tread wheel plates, and replacement of various parts of the lateral bracing system. The machinery was kept repaired with replacement parts, upgrading, etc., as the need arose.
Subsequent to the strengthening work in 1944-1945 and the operation of heavier rail loadings crossing the swing span bridge, the span continued to need repairs, including re-welding of patches on diagonals, re-welding of pin and member wear, and repairs to cracks in the inside of the lower/bottom chords at the West end of the north and south trusses.
Again, between the years 1970 and 1974, extensive repairs had to be made. In 1971, the swing bridge span was jacked up with eight 100-ton jacks, thus permitting the replacement of the forty 20-inch diameter tapered wheels and the rim bearing raceway. The tread plate and raceway were made heavier and the bridge raised nine inches, thus replacing the worn-out and light parts. In addition, all of the floorbeam hangers (although there is no actual floorbeam system in the bridge) had to be strengthened and carefully inspected routinely due to several hanger failures by breaking of these eyebar members. Also, the tops of the stone masonry rest piers had to be grouted and encased with concrete collars to hold the individual stones from further separation and ultimate failure of these piers. This further augmented previous repairs to these piers where they were banded with steel rails and loop rods to hold the stones together.
During the years 1970 to 1974, it became very evident to its present owner, Louisville and Nashville Railroad Company (L&N), successor to the N.C.& St.L. Railroad, that the Bridgeport 364'-0" long swing bridge span could not continue to function and meet the needs of modern railroad clearance requirements and increased weights of cars and equipment. To safeguard against serious failure, a load limit of 254,000 pounds, 4-axle cars, with a speed limit of 10 M.P.H., was permanently placed on the Bridgeport bridge.
This prompted serious consideration to seek a way to construct a heavier replacement bridge of sufficient load carrying capacity and clearance to meet the Railroad's needs as well as the navigational users of the Tennessee River. This swing bridge not only restricted the L&N Railroad's operations and service to its users but also affected the Southern Railway, which has trackage rights over L&N at Bridgeport. The navigational users of the Tennessee River needed more horizontal clearance than the 145 feet provided by the 1890-1892 swing bridge.
Working with the Tennessee Valley Authority, the United States Coast Guard (Second Coast Guard District) and the Department of Army, Nashville District, Corps of Engineers, the L&N obtained Bridge Permit No. 38-80, dated May 6, 1980, to construct a replacement lift span bridge 93 feet downriver from the 69O'-0" long bridge crossing. Upon completion of the new bridge and diversion of rail traffic, the old bridge was removed.