Happy New Year

Narrows Bridge at nightHere’s wishing all my readers, and your families, a happy and prosperous New Year. May you all be blessed with good health, optimism, peace and prosperity this year.

The new Tacoma Narrows Bridge has been decked out for the holidays, with colored lights on the catwalks, Christmas trees on the towers, and even colored construction cranes. Here’s a shot I grabbed last night — one of the few nights without rain in the past few weeks.

Back soon — God bless and stay well.

The Two Towers IX:
Wheels Over the Water

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

 
For those who may be new to this series, I am 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 recent first-hand tour taken of the construction site.

Spinning wheel

In the previous post, I discussed the history of cable spinning technology, and looked at the wire spools and spool feeding mechanism on the East end of the bridge. Now we’ll examine the tram itself, and the “traveler” wheel or sheave.
Continue reading “The Two Towers IX:
Wheels Over the Water”

The Two Towers VIII:
Spinning Beginning

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

 
For those who may be new to this series, I am 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 recent first-hand tour taken of the construction site.

Bridges at dusk

The engineering beauty of modern suspension bridges lies in the graceful catenary curves of their cables. Starting out as near-gossamer threads of steel wire–no larger than that which you could purchase at your local hardware store–their massive girth and strength support almost unthinkable weight across impossibly wide chasms. Taken for granted as we drive across such majestic spans–perhaps more drawn to the views of water or wilderness they afford–one rarely ponders how such muscular steel sinews are created. It’s a fascinating process–and it’s just begun on the new Narrows Bridge.
Continue reading “The Two Towers VIII:
Spinning Beginning”

The Two Towers VII:
Stairway to Heaven

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

 
For those who may be new to this series, I am 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 recent first-hand tour taken of the construction site.

Birdcages

Quite a bit of work has transpired since my tour of the construction site in April 2005. The birdcages–enclosed structures used for work on the towers themselves–have been removed (see above), the top struts completed, and the saddles for the forthcoming suspension cables have been placed at the tops of both the east and west towers. The concrete pours are complete, and the concrete plant has been disassembled. It’s time to start spinning the cables from shore to shore.

So, how will they string the suspension cables from both anchorages on shore and across the tops of the towers?
Continue reading “The Two Towers VII:
Stairway to Heaven”

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.
Continue reading “The Two Towers VI:
To the Top”

The Two Towers V:
Struttin’ My Stuff

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

 
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.

Elevator

Having survived the journey over the catwalk, and the descent into Hell–sorry, the 250 steps down to the old bridge caisson–the time has come to explore the west tower of the new Narrows Bridge. A narrow catwalk connects the new and old caissons, over which course air pressure hoses, electrical wiring, the slick line (for pumping concrete-the yellow pipe above)–leaving a fairly narrow walkway over the water to the new caisson. Seen above, the orange elevator by which we will ascend into the heavenlies, leaving behind family, friends, hope, and all that we cherish, lies waiting to escort its terrified captors to certain death. But like a proverbial last meal of Chicken McNuggets, we get to savor some of the mechanics of work on the new caisson first–specifically, the area where the slick line transfers its concrete to the yawning bucket for transport by construction crane to the top of the towers. Large droplets of water fall on our heads–although it is not raining: the irrigation systems used to keep the setting concrete wet drip down from high on the towers.

But before long, like some ancient prophet, Mike tells us “The Time Is At Hand:” it’s into the elevator. It’s a tight steel cage, designed to hold maybe 6 or 8 occupants, max. On the right sits a lever which controls its ascent, and Mike moves beside it as the cage door clanks closed behind us, sealing our doom. He turns to me, instructing me on how it works–“Sometimes the elevator jams, so in case I need to get out to fix it, you can operate it.”

We are most decidedly not amused. But evidently this does happen on occasion. For my next exciting adventure, I think I’ll try the petting zoo…
Continue reading “The Two Towers V:
Struttin’ My Stuff”

The Two Towers IV:
Out & Down

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

 
West Cable Anchor
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.

Catwalk
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:
Catwalk
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.

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.
Peregrine falcon
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.

Stairs & tower
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.
Offices
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.
Bridge from pier
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.

The Two Towers III:
Anchor Management Class

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

 
West Cable Anchor

Suspension bridges are an ingenious application of engineering. They are surprisingly old–the oldest suspension bridge still operational in the world, the Wheeling Suspension Bridge across the Ohio River in West Virginia–was built in 1849.

Suspension bridges come in two different designs; the elongated “M” shape, and the “A” shaped design called a cable-stayed bridge. The engineering is quite different between them. The weight of the bridge deck on a suspension bridge is borne by two different types of forces: compressive–the downward pressure on the bridge towers, and tension–the transfer of weight horizontally along ropes, chains or cables. In the cable-stayed bridge, the weight of the deck is borne primarily by the tower (or towers), as each radiating cable bears the tension of supporting only a small section of the bridge platform. The recently-completed Millau Viaduct in southern France is a stunning example of such a design.
Continue reading “The Two Towers III:
Anchor Management Class”