Tappan Zee Constructors, LLC (TZC) has begun creating the new bridge’s iconic 419-foot main span towers by utilizing a unique type of self-climbing concrete “jump forms.” These forms, along with a series of climbing tower cranes and catwalks, will allow crews to assemble steel reinforcement (rebar cages) and pour concrete for the towers in segments, starting from the bridge’s foundations and slowly working up to the towers’ full height.
Four towers will be constructed on each of the project’s two football-field-length main span pile caps (eight towers total), with the individual jump forms encapsulating an approximately 25-by-26-foot base (650 square feet).
As crews complete segments of the towers, the jump forms are raised through a screw jack system to proceed higher into the air, so that the following segment can be built on top of the preceding segment. Each of the bridge’s eight towers will require a total of 26 lifts, ranging from 12 to 18 feet in height. The forms will follow the slight outward angle of the towers, as well as their decreasing area as they reach higher and higher levels. Anchorages for the new bridge’s cables will be installed after the towers are more than two-thirds complete.
Temporary catwalks will accompany the jump forms as they rise, extending both vertically and horizontally to connect the towers for ease of access. Staircases and temporary elevators will also be mounted to the sides of the towers for crew members to access the jump forms.
The forms are composed of two different frame systems, one internal, the other, external. The external frame moves along with the crew as they install the steel reinforcements for the towers, providing a safe work area. The internal frame remains below, with the previous segment of the tower, until the reinforcing steel is installed and the new segment is ready to be filled with concrete. The internal frame acts similar to a casting mold, forming the shape of the towers as the concrete cures. Once this process is complete, a series of rails are raised to the next level along with the external frame and the process begins once again.
Fall protection lines are installed around the external frame to provide continuous safe access for crew members as construction continues high above the Hudson River.
TZC personnel trained for the elaborate procedure by creating a full-sized mock-up of a tower base off-site. The success of the mockup in July 2015 has properly prepared the crew for the real operation, which is scheduled to be completed in 2016.
Tower Construction Facts and Stats:
- Completed Tower Height – 419 feet
- Number of Towers on the new bridge – 8, with four on each of the two spans
- Number of Jump Form lifts for each Tower – 26
- Total number of Jump Form lifts on the project – 208
- Angle of the towers – 5 degrees
- Size of towers at their base – 25 feet by 26 feet
- Size of towers at their maximum height – 15 feet by 17 feet
- Tower crane maximum load – approximately 25 tons
- Maximum height of tower crane – 490 feet
- Length of tower crane boom – 164 feet
- Maximum height tower crane can lift to – 460 feet
