The Two Towers:
To the Top
Previous essays on the new Tacoma Narrows Bridge construction:
When viewed from below, the new bridge towers are magnificent and soaring. 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 bird cages, and this is the destination of the 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.
The bird cages are three-story-tall enclosures of corrugated steel which surround the working area of the towers. Seen here 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 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.
The rebar for the tower pours is pre-fashioned as cages in sections offsite, and lifted into position by a 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 water streaks often seen running down the towers.
Between the two tower bird cages there is a narrow catwalk, with an 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 bird cage, 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…
To say it is intimidating to look down from the lofty perch of this catwalk is an understatement — think: Indiana Jones on the rope bridge in Temple of Doom. But again we make it without a newsworthy calamity, and from this bird cage (southern-most in the west tower) we have an altogether-too-close view of the top of the construction crane.
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, which can be seen in the prior picture from the catwalk. 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 facilitate the spinning of the cables for the bridge. Perhaps I’ll skip the next tour after all and keep my fond memories of this one instead.
Next: Tales of cable catwalks and spinning.
