Previous posts on the new Narrows Bridge:
- History of the Tacoma Narrows Bridges
- The Two Towers I: Intro
- The Two Towers II: Concrete Thinking
- The Two Towers III: Anchor Management Classes
Topical and temporal exigencies have distracted me from the ongoing series about the construction of the new Tacoma Narrows Bridge, but it is time at last to continue our journey. In brief, several months ago I was given an extraordinary opportunity to tour the construction of the new suspension bridge, up close and personal–a once in a lifetime experience. If you are new to this series, take a glance at the links above for the background on the history of the Narrows bridges, the engineering challenges, and the beginning of the tour.
Having toured the new anchorage, Mike now leads us to the access catwalk which will take us to the caissons and out to the tower. But first we make a brief stop at the anchor and suspension cable for the existing bridge, shown above. While the cable appears to be fixed at first glance, it is actually capable of motion–a great deal of motion, in fact. The steel ring surrounding the cable where it enters the anchorage is part of a rocker mechanism, which allows the cable to stretch as weight on the bridge varies. The cable actually moves back and forth through the ring–sometimes as much as 8-12 inches during heavy load periods, as the cable stretches and relaxes. Watching this motion is more than a little unnerving–especially if you have ever strung a guitar, and waited for the high E string to snap as you bring it to pitch. Not that this would ever happen to the bridge cable–at least one would hope.
At this point, several members of our party gracefully bow out–this is an interesting tour, to be sure, but walking out on a narrow catwalk 200 feet above the water–much less heading up the tower–has made several of the folks a bit squeamish. The catwalk is built underneath the existing roadway, on the bottom of the structural frame which supports it. It has a rather solid feel–as it rests on steel girders–but there is a 36 inch railing made of construction webbing and 2 by 4’s separating you from a horrible death, screaming in terror and clawing the air, as you fall for seemingly endless seconds to a crushing impact in the frigid waters and wild currents of the Sound, where octopuses dine on your bloated flesh and rescue boats circle hopelessly above … oops! Get a grip! I’ve let my imagination run away from me again. Focus … breathe …. Aaah, there, that’s better. Needless to say, the first looks over that railing are a bit–how you say?–anxiety-provoking.
And actually, walking out under the bridge is in some ways better than walking on the pedestrian walkway above. The pedestrian sidewalk is at road level, 3 1/2 feet wide, separated from the outside traffic lane by a steel pipe curb 12 inches off the road. Cars and tractor trailers roar by at 50+ MPH, about six feet from your shoulder, with deafening noise. Add to that the constant up-and-down motion of the bridge from the weight of passing vehicles, and your scenic bridge stroll becomes somewhat less than relaxing. Things are much quieter down on the catwalk, and even the bridge motion seems less pronounced.
It is 1/4 mile from the shore to the first tower. Evident on this walk is a lot of work unseen from the shore, replacing girders in the existing bridge deck support to bring the structure up to modern earthquake standards. One-quarter mile doesn’t seem like much–until you stand out at the tower and look back at the catwalk you’ve just navigated:
The yellow pipe seen on the right above is the slick line, which transports concrete out to the tower.
Of course, now that you’re at the West tower of the existing bridge, there is one small problem: you need to get down to the caisson, at water level. At this point you are over 200 feet above the water, so it’s time to hike down the stairs.
Yup, the stairs – about 250 of them. Open steel grate footing. Open tubular steel frame, single steel handrail. A one inch steel pipe between you and the fishies below. This makes the catwalk look downright secure.
Of course, construction guys (and gals) have a certain mojo, a macho not found in ordinary mortals: they have races up and down these stairs. Mike tells us the record time coming up is 2 minutes 12 seconds. The best time down: 5 seconds–“but that was a jumper”–referring to a favorite Tacoma Narrows Bridge pastime: suicide. Black humor is not found solely among physicians, it appears.
Near the top of the stairs–out of view, unfortunately–there is a peregrine falcon nest. This magnificent raptor–long endangered, but making a comeback in many parts of the country–is the fastest bird on earth, and has been clocked at nearly 200 MPH in vertical dives, feasting on pigeons, gulls, and other birds, which it hits in mid-air. Mike relates that the “pop” of a falcon hitting its prey is very impressive, and the mother often swoops workers near the stairs to protect the young in her nest.
No falcons are visible today.
The stairs are surprisingly stable, and before long we are at the water level, on the caisson of the current bridge. Above is a view looking up at the tower, with the roadway overhead and the stairway seen on the left. The caisson is the original one, built for the first Narrows Bridge (“Galloping Gerty”), and although showing its age is still very sturdy in appearance. And it turns out, we’ve also arrived at the offices of the construction crew.
To provide a local workplace out on the bridge piers, container offices were brought out by barge and located between the legs of the existing bridge towers. Far from the bustle of the roadway above, they would appear to be quiet and safe–except for one problem: falling objects. Objects falling or thrown from the bridge above are moving at over 100 MPH when they reach the water. In one incident, a short piece of PVC pipe was knocked off the catwalk above, and pierced the roof of the offices, the seat of a chair, and impaled itself in the floor. Fortunately, no one was in the chair at the time; the roof was reinforced with steel plating shortly afterwards.
The pier is loaded with equipment: electrical generators, air compressors (used for the flotation tanks when the new caissons were being constructed), and lots of pipes and wires. Yet everything is orderly: there is an obsessiveness about orderliness and cleanup to minimize the risk of injury.
The view from the caisson, under the bridge, is rather spectacular. It gives one an appreciation of the extraordinary feat of engineering involved in building a modern suspension bridge. Even the current bridge–built half a century ago–inspires a kind of awe when seen from this perspective.
So, that’s all for this chapter of the bridge contruction series. Next time, we’ll be putting your acrophobia to the acid test: riding the elevator to the top of the towers.