The Two Towers XII:
The Cranes

Previous posts on the new Narrows Bridge:

  1. History of the Tacoma Narrows Bridges
  2. The Two Towers I: Intro
  3. The Two Towers II: Concrete Thinking
  4. The Two Towers III: Anchor Management Classes
  5. The Two Towers IV: Out & Down
  6. The Two Towers V: The Struts
  7. The Two Towers VI: To the Top
  8. The Two Towers VII: Stairway to Heaven
  9. The Two Towers VIII: Spinning Beginning
  10. The Two Towers IX: Wheels Over Water
  11. The New Bridge at Christmas
  12. The Two Towers X: Compacting the Cable
  13. 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.

deck sections
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.

gantry assemblies

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…

gantry cranes

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.

gantry cranes

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.

cable bands

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).

crane cable clamps

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.

non-mobile gantry

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…

cable gantry crane

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5 thoughts on “The Two Towers XII:
The Cranes

  1. Having lived in Gig Harbor until two years ago, I’ve certainly enjoyed your series on the new bridge. Fascinating and amazing stuff! I look forward to your future posts.

  2. Thanks so much for continuing this series. Much as I know it’s important to get our minds around the craziness of the medical billing field, it sure is a lot more enjoyable to see how the engineers have put their minds to constructing something so massive, concrete and practical.

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  4. Hey, have been reading about the construction from werribee australia. looks intersting! wish i could be there to see it! thanks for the posts!!!!!!!

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