The Two Towers:
Wheels Over Water
Previous essays on the new Tacoma Narrows Bridge construction:
The process of laying the wires begins by securing one end to the anchorage. This wire is then looped up and over the spinning wheel, and the top wire returns to the spool which feeds it. As the spinning wheel moves outward from the anchorage along the tram, the lower wire called the dead line — remains fixed, while the upper wire, the live line — is drawn out from the spools onshore, much like a fishing line after a strike.
As the wheel moves outward from the anchor, it lays both lines in their selected position — guided by the positioning forks and the saddles on both towers — until the opposite anchorage is reached.
The complexity is greater than it sounds, however: the spinning wheel can carry up to four simultaneous loops — effectively laying down up to 8 wires in one pass. The process is automated but under manual control and supervision; when working at optimal efficiency, the spinning wheel can achieve speeds up to 40 miles an hour. Workers are positioned on the catwalks at intervals to monitor the process and address any problems or glitches which may arise.
Once the wheel reaches the opposite anchor, the loop (or loops) of wire are removed from the spinning wheel and placed around the strand shoe (see the previous post), a semicircular casting with grooves for the wires, secured by anchor bolts to the opposite anchorage. When 454 wires have made this journey, they are bundled into a single strand or tendon. A total of 19 strands comprise the final cable, which is then compacted, wrapped, and sealed with epoxy to form the final cable.
Next: Compacting the Cables.
The complexity is greater than it sounds, however: the spinning wheel can carry up to four simultaneous loops — effectively laying down up to 8 wires in one pass. The process is automated but under manual control and supervision; when working at optimal efficiency, the spinning wheel can achieve speeds up to 40 miles an hour. Workers are positioned on the catwalks at intervals to monitor the process and address any problems or glitches which may arise.
Once the wheel reaches the opposite anchor, the loop (or loops) of wire are removed from the spinning wheel and placed around the strand shoe (see the previous post), a semicircular casting with grooves for the wires, secured by anchor bolts to the opposite anchorage. When 454 wires have made this journey, they are bundled into a single strand or tendon. A total of 19 strands comprise the final cable, which is then compacted, wrapped, and sealed with epoxy to form the final cable.
Next: Compacting the Cables.
