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
- The Two Towers IX: Wheels Over Water
- The New Bridge at Christmas
- The Two Towers X: Compacting the Cable
- The Two Towers XI: Cable Banding
For those who may be new to this series, I have been 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 first-hand tour taken of the construction site.
With the arrival of some of the deck sections by ship last month, one was struck by the task at hand: here’s these enormous deck sections (between 450 and 700 tons apiece), and there’s the graceful cables arcing gracefully over the water, with their attached-but-empty suspension cables.
How ya gonna get those bad boys up there?
Good question. They’re far too heavy for construction cranes to lift, much less anything smaller.
As the cables were being spun, some unusual-looking equipment began to appear in the staging areas behind the anchors. Light-blue in color, they appeared at first to be part of the bridge structure itself.
For weeks I pondered the question: What’s blue, and angular, and assists in erection?
The answer came: Viagra! –but I somehow didn’t think this equipment would be of much benefit with that equipment. Seemed kind of … awkward, you know? The quest continued…
…until one day, a few weeks later, the pieces were moved, and their purpose became apparent: overhead mobile gantry cranes, using the cables themselves for support. This erection’s definitely gonna last more than four hours–but don’t call your doctor…
There’s eight of these mobile monsters: two between the towers and the anchors at either end, and four on the cables between the two towers.
The gantries use the cables as tracks, and move along them on motorized wheels a few inches at a time–but there’s a glitch: the cable bands are in the way.
Not to worry–when the cranes get to a cable bands, they simply hop over them.
The cranes are secured to the cables with four clamshell-like clamps, which can open (seen below, upside down, prior to assembly).
Four hefty hydraulic lifters–two on each cable–lift the entire crane up a few inches, allowing it to ride over the obstacle, then gently light like a butterfly on the other side of the cable band. The clamshell clamps close again, securing the crane on the cables, and the glacial progress onward continues. As you can imagine, this is not a speedy process (think:continental drift)–the cranes can take a day or more to move any substantial distance along the cables (far too slowly, incidentally, to move the deck sections horizontally after they are lifted).
At two other locations, between the towers and the anchors, smaller, non-moveable gantries have been placed attached to temporary cable bands.
These gantries are used primarily for the initial deck section lift off the transport ship–in a process both surprising and fascinating. But you’ll have to wait until the next post for those juicy details, because things have been getting really interesting these past few weeks: the deck sections are being lifted. More on that in our next segment, after a word from our sponsors…