Link-type cable carriers that soar high above the deck of the John T. Alsop Jr. Bridge help to manage its heavy power cables and prevent them from swaying in high winds and becoming worn. The Nylatrac carriers include bolted crossbars that add rigidity to withstand crosswinds, while stainless-steel fittings and guides resist corrosion.
The John T. Alsop Jr. Bridge, built in 1939 to carry U.S. Highway 1 over the St. John’s River, has become a landmark in downtown Jacksonville. Although its builders emphasized functional considerations, the bridge’s strength, its powerful 200-ft-high lift towers and graceful reach across the river also create a stunning aesthetic impact.
Measuring 1,680 ft in length with 14 spans, it includes 11 steel-stringer spans and three Warren through trusses. The 365-ft-long continuous truss vertical lift span at the bridge’s center is the longest such span in Florida. The span-drive system, which places the drive machinery on the movable span, utilizes concrete counterweights enclosed within two large steel frame towers. The bridge has a vertical rack-and-pinion lift mechanism, driven by two 200-hp electric motors located atop the bridge structure. The motors drive the lift mechanism through a single planetary reducer to the main shaft.
A major overhaul of the bridge and its lifting mechanism began in late 2003. Part of the contract with Royal Bridge Inc., Tarpon Springs, Fla., included rewiring the lift motors and related controls.
Engineering for the project was handled by E.C. Driver & Associates Inc., headquartered in Tallahassee, Fla.
The electrical power system for the lift motors was renovated and brought up to current specifications by Quality Electric Contracting Inc., Clewiston, Fla.
Quality Electric replaced all the electrical cables for the bridge’s drive mechanism, the controls and safety equipment. Previously, the power cables hung in a large loop from the top of the bridge and were susceptible to damage and wear as the wind blew them together or into the bridge structure.
Cable carriers were installed that impart a gentle, rolling action and minimize cable flexing that could cause fatigue and cracking. According to Turner, the specific job of the cable carriers is to control the vertical movement of the cables and eliminate the previous wear problems. He said that Hubbel-Kellems wire grips are used to support the weight of the cables themselves.
Two Nylatrac TL-350 Series cable carriers are used to handle the cables that supply power to the gates and vehicle arrestors, as well as the controls and safety guards. One of the carriers also carries a water line that supplies water to the bridge house. The 100-ft-long carriers, designed and manufactured by the Gortrac Division of A & A Manufacturing Co. Inc., New Berlin, Wis., are made of a glass-filled nylon material with a high strength-to-weight ratio. Anodized aluminum crossbars are bolted in place at every one of the more than 200 links in each carrier. Because the larger carriers handle several cables of different sizes, stainless-steel divider bars are positioned at intervals along the carrier length. These include additional vertical separators and horizontal dividers that help manage the cables and keep them in place. One pair of the carriers measures 14.74 in. wide and handles cables for the lift mechanism, controls and safety equipment. These carriers weigh approximately 600 lb each unloaded, while the cables weigh slightly more than 1,000 lb. The other carriers, which carry cables for the tower lighting system, are 7.94 in. wide and weigh approximately 300 lb each unloaded.
The carriers were tested in a wind tunnel to meet side loads in the range of 700 lb. Designed to withstand 40-knot winds, they also incorporate stainless-steel guides that arrest the side loads and maintain the lateral position of the carriers.
Information for this article provided by Bug-O Systems, Pittsburgh.
1. Nylatrac cable carriers on Jacksonville’s John T. Alsop, Jr. Bridge soar upward as they manage the motion of multiple power and control cables. Note guide bracket spacing along height.
2. Carrier location can be seen in this view alongside the bridge structure.
3. Alsop Bridge is the longest continuous truss vertical lift span in Florida, with a 365-foot long vertical lift span at the bridge’s center. 4. Guide brackets like this are spaced along the height of the carriers to keep them aligned vertically during high crosswinds.