Previous posts on the new Narrows Bridge:
- History of the Tacoma Narrows Bridges
- The Two Towers I: Intro
- The Two Towers II: Concrete Thinking
- The Two Towers III: Anchor Management Classes
- The Two Towers IV: Out & Down
- The Two Towers V: The Struts
- The Two Towers VI: To the Top
- The Two Towers VII: Stairway to Heaven
- The Two Towers VIII: Spinning Beginning
For those who may be new to this series, I am blogging the construction of the new Tacoma Narrows Bridge. See the above posts for more information on the Narrows Bridges, the engineering challenges, and a recent first-hand tour taken of the construction site.
In the previous post, I discussed the history of cable spinning technology, and looked at the wire spools and spool feeding mechanism on the East end of the bridge. Now we’ll examine the tram itself, and the “traveler” wheel or sheave.
The tram is essentially a ski lift: a continuous loop cable, powered by winches, which moves the traveling sheave back and forth on cables hung from overhead gantries (called tram frames) on the anchorages, the towers, and evenly spaced 200 feet apart along the catwalks.
The tram frames are secured to the catwalks, and below them lie positioning guides–like upturned forks–which help position the individual wires in the proper location as the tendons are formed.
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 feed 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, 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 acheive 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 oppositie 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.
As the strands enter the anchorage, they are disbursed by a specialized saddle called a cable sorter (seen on the left, from the Tsing Ma bridge construction in Japan), and enter the splay chamber (seen below from the Jiangyin Bridge across the Yangtze River in China). Splaying the individual tendons as they enter the anchor plate greatly increases the cross-sectional area and distributes the pull on the anchorage over a wider area.
Watching the cable spinning in action is fascinating–and has been the precipitant cause of more than a few fender-bender accidents on the existing bridge, as gawking rubber-neckers on their cell phones have a tendency to back-end their SUVs into the Mini Cooper they’re following–not pretty. It’s far more pleasurable–and safer–to watch from the walkway–although passing drivers are wont to wonder if you’re planning a deep-six dive from one of the favorite suicide spots in the South Sound. So I carry a camera and smile a lot to avoid such untoward suspicions.
I’ve provided a couple of Quicktime movies taken from this vantage with my Panasonic digital camera. The resolution and exposure aren’t ideal–but you can get a pretty good idea of how the process looks up close and personal. These are pretty hefty downloads (16 to 25 MB), so they’re not for the dial-up crowd (sorry!), but if you have broadband mojo, take a gander. The first movie shows the spinning wheel just after leaving the East anchorage, as it moves through the tram frame on its way up the ascent to the East tower. The second (longer) movie shows the descent from the West tower toward the Gig Harbor anchorage.
Hope you enjoy these–more updates to come in the future as the construction proceeds.
Update: Here’s several YouTube videos for the cable spinning: