Black Warrior River Railway Bridge Near Demopolis
The new single-track bridge of the St. Louis & San Francisco Railway over teh Black Warrior River near Demopolis, Ala., on its new line from Aberdeen, Miss., to Pensacola, Fla., was erected without the use of any falsework in the river channel. The bridge consists of trestle approaches and three steel spans - two through-truss side spans of 100 ft., and a vertical lift center span 184 ft. long with a 22 ft. lift. This center span weighs 294 tons and was the part of the bridge for which no streambed falsework was required.
Two conditions at the bridge site pointed directly to the advisavility of using no falsework. Black Warrior River is navigable to Birmingham, one to six or seven boats daily passing the bridge site, so that it was necessary to keep a channelopen during construction. In addition, the river at this point ran in a rock trough, making piledriving difficult if not impossible, and ran at such a speed that any falsework erection possible would have been exceedingly difficult.
The side spans were erected with ordinary falsework. The first plan for the erection of the lift span was to drive falsework bents as far out into the stream as possible, leaving a narrow ship channel open. Because of the rock this method was abandoned and a scheme of using cantilever or bracket falsework was devised. As shown in the picture above, a bracket arm projected outward from each main pier. This arm was 52 ft. long and its timber anchor bent was approximately 30 ft. high, made up of eight 12 x 12 in. posts. The horizontal member of the bracket consisted of two 12-in. 25-lb. channels, while the main inclined member was made up of two 15-in. 40-lb. channels. Angle-iron web members were used.
Steel was brough in on barges and unloaded into a storage yard. Derricks set up on the ground erected the side spans. On teh north side a mast derrick with a 115-ft. mast and a 100-ft. boom was placed on the top of the timber bracket bent, where it erected half of the center span and the north counterweight supports. On the south side an A-frame derrick was placed on teh approach span, from which position it erected the remaining half of the center span. It was then moved shore-ward on teh side span until it was in position to place the south counterweight supports. The lift span was made of material from an old bridge and both the bottom and top chords were in three sections. The two splices occurred at the ends of the bracket arm so that the center secion fo the bottom chord could span between the brackets, the north side to the right.
The center span was erected
slightly above its usual lifted position. After erecvtion was complete and
were added, the span was lifted from the timber bents and teh brackets removed.
This information was taken from:
March 14, 1929