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”

The Two Towers II:
Concrete Thinking

Previous posts on the new Narrows Bridge:

  1. History of the Tacoma Narrows Bridges
  2. The Two Towers I: Intro

 
Concrete Plant

Our tour starts at the concrete plant. To minimize transport costs, a new concrete plant was assembled on the west side of the bridge. It is a mobile plant, with multiple components trucked in and assembled on site. The components were assembled into a complete plant within a few days, and shortly thereafter it began producing concrete at a rate of 150 to 200 yards a day (the average concrete truck carries 9-10 yards, so 15-20 truckloads a day). Bridge construction requires some serious quantities of concrete: each of the towers alone requires 8000 yards (1000 truckloads, as trucks were filled to 8 yards to match lift capacity), and the caissons and shore anchors required substantially more.

One question which naturally arises: since the existing bridge is a steel structure, why not build the new bridge from steel girder to compliment the appearance of the old? The answer is simple: cost. Steel is extremely expensive. The reasons for its high cost are many: huge increases in Chinese consumption (China’s structural steel utilization went from 100 million tons in 1997 to 260 million tons in 2003, and continues to rise rapidly); the scarcity and expense of raw materials, scrap iron and iron ore; and the high cost of refined coal (coke) to fuel blast furnaces due to environmental restrictions. So concrete wins hands down on a cost basis. It is also far lower in costs of long-term maintenance, not requiring regular painting and rust prevention–a big problem in the salty air of Puget Sound.

Concrete trucks make lousy amphibious vehicles, so how do you get the concrete out the caissons (bridge piers) and the towers? Barges are too slow, and the steep, high banks of the Narrows prevents nearby water access. This problem is solved by pumping it out, through surprisingly small pipes.
Continue reading “The Two Towers II:
Concrete Thinking”

The Two Towers I:
Introduction

Two TowersOpportunities like this don’t come along very often in life.

He was the last patient of the day. I asked him his occupation, and he replied, “construction.” “Contracting? Heavy equipment?” “No, I supervise the concrete work on the new Narrows Bridge.”

Whoa. Cool!

Thirty minutes later–with the staff itching to go home–I finished picking his brain on what is a most amazing engineering feat of the last 30 years: the construction of a new bridge over the Tacoma Narrows. As he started to leave past the checkout desk, he turned and asked: “I’m giving a tour of the new bridge on Sunday. Interested in coming along?”

I considered the pros and cons, weighed the alternatives, and finished the complex decision-making process–nanoseconds later: “What time?” “9 A.M.” “See you there.”

What ensued was an experience you get once in a lifetime. And thanks to the wonder of weblogs, you get to share in it.

There are no public tours of the construction site of the new bridge–the environment is dangerous, and the engineers and work crew are obsessive about safety. Employees face immediate termination if they are on site without proper safety gear, or in riskier areas (such as outside of barriers or railings) without harnesses. As a testament to the rigorousness of the safety precautions, in over 2 years of construction, there have been only 2 injuries (a fractured wrist and a fractured ankle), and no mortalities. No one is allowed on site without hard hat, goggles, and safety vest.

My patient–we’ll call him Mike, although that’s not his real name–was giving a tour for the owner of the site concrete plant and several of his employees–five people initially, but two who were decidedly not interested in going up on the towers–and me, the odd man out. The tour, scheduled to take about 1 1/2 hours, ended up taking three. It flew by like just a few minutes. The weather was typical early May for the Pacific Northwest–mild, overcast, about 50 degrees, but fortunately with almost no wind.

There’s a lot to cover here, so I’m going to break this up into multiple posts, with lots of pictures. For those interested in the photography specifics, the photos were taken using a Panasonic DMC-FZ20, with a remarkable 12x optical zoom Leica Lumix lens and optical image stabilization. Wonderfully light, sturdy camera with one of the easiest user interfaces I’ve used. Images were tweaked a bit in Photoshop, mostly levels and contrast adjustment.

I would recommend my earlier post on the history of the first two bridges over the Narrows (“Galloping Gertie”, which collapsed in 1940, and the current span), which also covers some of the engineering challenges of bridging the Tacoma Narrows.

So put on your hard hat, observe all safety rules, and try to keep you acrophobia in check–we’re off to see the towers of the new Narrows bridge.

Bridge Blogging

I am fortunate to live near an engineering marvel in progress: the new Tacoma Narrows bridge. Most folks have heard of the Tacoma Narrows bridge – at least the first one, “Galloping Gertie”, which catastrophically failed during a windstorm in November 1940.

Built at the cost of $6.6 million dollars, designed by world-famous bridge architect Leon S. Moisseiff (who also designed the Golden Gate bridge), it embraced the light, elegant design principles in vogue at the time – and was designed with complete ignorance of the aerodynamic effects of high winds on bridges. Moisseiff had inadvertently created a mile-wide airplane wing, with its light-weight narrow deck and plate-girder sides. It survived only 4 months after completion. In a strong-but-typical November windstorm, the wave-like undulations were severe enough to unseat a cable from its saddle on the West tower, creating a corkscrew torsional motion which ripped the bridge to shreds. The only casualty, surprisingly, was Tubby the three-legged dog.

May he rest in peace.

The fallen span of the bridge remains at the bottom of the Tacoma Narrows, and has been designated a National Register of Historic Places to prevent salvage. It is one of the world’s largest artificial reefs, and home to a plethora of marine life, as well as the world’s largest octopuses. The remainder was disassembled and sold for scrap during WWII. The caissons and anchors (for the cables, on either bank) were used, largely unmodified, to support the towers and cables of the second Narrows Bridge.
Continue reading “Bridge Blogging”