The Two Towers:
The Bridge Cranes
Previous essays on the new Tacoma Narrows Bridge construction:
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 please don’t call your doctor …
There are 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 small problem: the cable bands are in the way.
No worries: when the cranes get to a cable band, they simply hop over them.
The cranes are secured to the cables with four clamshell-like clamps, which can open.
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.
Next: Lifting and positioning the bridge sections.
