The Two Towers VI:
To the Top

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

 
For those who may be new to this series, I am blogging a tour of the new Tacoma Narrows Bridge construction taken recently. See the above posts for more information on the Narrows Bridges, the engineering challenges, and the first parts of the tour.

Tower

When viewed from below, the new bridge towers are impressively tall. Unlike a tall building, there is nothing surrounding the towers to detract from their majestic height: one’s insignificance in the face of an engineering miracle is readily apparent. Seen at the top are two square enclosures, connected by a narrow walkway. These are called birdcages, and this is the destination of this final part of the tour. I suspect that vultures live therein, to feast like mythical harpies on their prey–those foolish enough to ascend to such absurd heights.

Elevator view-1

If it was not apparent already, this final leg of the elevator ride brings you really high above the water–550 feet high, to be exact. Viewed from within the elevator cage, you can see the west bank of the Narrows, with the new road construction and the Key Peninsula stretching beyond. The elevator cage–once a terrifying prison–now seems a rather cozy and secure refuge as we reach the top–and step out.

Elevator view-2

Upon leaving the elevator, like Lot’s wife, I make the mistake of looking back–only to gaze at the yawning gap between the elevator and its dock. Little toy cars move below–blithely unaware of the impending tragedy above, where a foolish curiosity seeker is about to fall 550 feet into the swirling black waters of Puget Sound, screaming “Safety First!” as he tumbles like a rag doll from the yawning chasm between elevator and landing. Becoming a pillar of salt seems like a pure joy at this moment. But somehow this tragedy is averted, and I step onto the top floor of the birdcage–at the very top of the West tower of the New Narrows Bridge.

North view

Once I have recovered my composure, the view from up here is nothing short of spectacular. You are immediately adjacent to the top of the tower of the existing bridge, just to the North.

West view

Looking West, once again the Key Peninsula, Gig Harbor area, and West end of the existing Narrows bridge is seen.

East view

To the East lies the other bridge tower and remainder of the Narrows Bridge span arching gracefully across the Narrows toward Tacoma.

South view

To the South lies Fox Island, and about 30 miles to the South, Olympia, the State capital.

Birdcages

The birdcages are three-story-tall enclosures of corrugated steel which surround the working area of the towers. Seen above shortly after the start of tower construction, they provide protection from the wind and the elements which become increasingly hostile as the tower height grows. They also serve–ahem!–to allay the anxiety some workers have about working at ridiculous heights. Those of us who are not such girlie-men of course would not require such measures, being entirely comfortable dangling from rebar hundreds of feet above the water in howling winds and driving rain. But such is life–we must accommodate our weaker brothers.

Within the birdcages the rebar for the towers is positioned and tied, the concrete forms placed, and concrete poured to increase the tower height. Once a section of the tower is completed, the birdcages simply lift themselves to a higher position. The birdcages have four hydraulic jacks–one at each corner–secured to temporary steel anchors attached to the tower, which work in unison to lift the cage to its new elevation, where it is secured and the hydraulics repositioned for the next lift several weeks later.

Rebar

The rebar is pre-fashioned as cages in sections offsite, and lifted into position by the construction crane. The main rebar is about 1 inch in diameter, and tied and secured using lighter weight rods. Once the concrete is poured over the rebar, inside the forms, it is tamped using vibrating plates to eliminate air pockets, then left to set. A sprinkler system irrigates the setting concrete to control moisture and temperature–which results in the dark water streaks often seen running down the towers.

catwalk

Between the two tower birdcages there is a narrow catwalk, with open steel grate flooring and a fairly skimpy railing. Mike–our tour guide–informs us that we are going to cross over to the other birdcage, and I marvel at his quirky sense of humor–until I realize he is serious. That wet spot on my trousers may be from the sprinklers, but it seems awfully warm…

catwalk view

To say it is intimidating to look down from this lofty perch is an understatement–think: Indiana Jones on the rope bridge in Temple of Doom. But again we make it intact, and from this birdcage have an altogether-too-close view of the construction crane.

crane cab

Construction cranes (also called tower cranes) themselves are a more-than-fascinating work of engineering. Carefully balanced to handle large loads without tipping over, they perform the most amazing feat of gymnastics in engineering: they can grow themselves taller. The rotating gearbox at the top (called a slewing unit) is detached from its supporting section, and is lifted 20 feet by hydraulics, contained in a three-sided mobile cage surrounding the tower sections. The crane then lifts a 20 foot section of tower, and positions it under the slewing unit, between the four hydraulic jacks. The new section is then attached below and to the slewing unit above–and all is well until a new section is required. This process strikes me as akin to performing your own hemorrhoidectomy–it’s a job best left to the experts, so don’t try this on your stepladder at home.

At this point, we take a few more photos, and Mike informs us it’s time to head back down. The fear has long since passed, and the amazing tour is drawing to a close–with far more sadness than relief. It has been the opportunity of a lifetime, and despite my acrophobia–substantially diminished having conquered this demon–I would do it again in a heartbeat. Mike informs us that in a few weeks, the only way to reach the top of the towers will be by catwalks strung between the shore anchors and the towers–which will be used to spin the cables for the bridge. Perhaps I’ll skip the next tour after all, and keep my fond memories of this tour instead.

3 thoughts on “The Two Towers VI:
To the Top

  1. Excellent description of bridge engineering for gearheads and non-gearheads alike! Great photographs of that really show the vertiginous heights of the tower bridge!

  2. I’m very impressed by your photos – I live in Tacoma, and enjoy seeing this bridge being built. Your photos really give a vantage point that is outstanding, remarkable. Thanks.

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