Previous posts in the New Narrows Bridge Series:
- The Two Towers I: Introduction
- The Two Towers II: History of the Narrows Bridges
- The Two Towers III: The Caissons
- The Two Towers IV: Anchor Management Class
- The Two Towers V: The Tour Begins
- The Two Towers VI: Struttin’ My Stuff
- The Two Towers VII: To the Top
- The Two Towers VIII: Stairway to Heaven
- The Two Towers IX: Spinning Beginning
- The Two Towers X: Wheels Over Water
- The Two Towers XII: Compacting the Cables
With the completion of the cables, the deck sections have begun transferring by ship to the bridge 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.
No worries: 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).
These gantries are used primarily for the initial deck section lift from 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.
