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
10. The Two Towers IX: Wheels Over Water
11. The New Bridge at Christmas
12. The Two Towers X: Compacting the Cable
13. The Two Towers XI: Cable Banding
14. The Two Towers XII: The Cranes
15. The Two Towers XIII: Life on the Bridge
16. The Two Towers XIV: Heavy Lifting
17. The Two Towers XV: The Flying Trapeze
18. The Two Towers XVI: Squeeze Play

 
I have been 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 first-hand tour taken of the construction site.

With the deck completed, there are still quite a few tasks to complete before the bridge is completed. The cables, comprised of over 19,000 miles of half-centimeter steel wire, joined end to end and woven back and forth as a single unit, have been spun and compacted, but remain unfinished.

To increase their resistance to corrosion, they are wrapped around their circumference with another layer of galvanized steel wire, leaving a smooth surface ready to be painted. The uncovered cables are first covered to minimize the risk of trapping moisture under the wire wrap,

and an impressive rotating spinner tightly winds the steel around the cable like a monstrous black widow spider preparing her prey for dinner.

The cable is first coated with a corrosion-inhibiting paste. For decades, cables were coated with red lead paste, which worked well, but has not been used since the mid-1990s, when the lead was recognized as an environmental hazard.

Bridge builders now use a urethane-zinc paste, about the consistency of mayonnaise. The idea is to apply it thickly, so that it oozes between the wrapping wires to form a solid anti-corrosion coating. The paste is manufactured in Italy, and 1,710 five-gallon buckets were required, each of which weighed 66 pounds.

The cable is wrapped in “bays” — 40-foot sections of cable between suspension bands, 270 in all. Each bay requires 3.3 miles of wrapping wire, a total of 948 miles. Three coats of rubberized paint then complete the finish, giving the cable a solid appearance.

The deck has been completed, welded together and secured with bolted plates, adjusted to exacting tolerances with the precision of a piano tuner. Or a guitar:

The similarity between tuning a guitar and welding is not something just anybody would pick up on.

But to Bill Madron the connection is obvious.

Madron, who’s an accomplished country and blues musician in addition to being a welding supervisor on the new Tacoma Narrows bridge, says laying down a righteous weld is like making music.

“If you’re tuning the E string against the A string, you know it’s right when you hear it,: he said recently. “A guitar is either in tune or it ain’t.

“Welding is the same way. It’s either on the money or it ain’t.”

Madron, now 66, grew up in the Appalachian Mountains of North Carolina, “a beautiful place to live,” he said, “but you can’t make no money.”

When he left North Carolina, he took his slow, melodious Southern drawl with him. It helps establish an air of calm on the new bridge deck, where he oversees welding crews joining the 46 deck sections into a continuous mile-long sheet of steel.

Madron started welding when he was 20 and has been at it ever since. As a young man in the 1960s, he combined his work with his passion for music, traveling from town to town, welding by day and playing in clubs at night.

Welding now gets more of his energy than music, but he still finds time to play, wherever his work takes him.

“You know how it is,” he said. “Musicians find each other. You start playing with somebody, then somebody else comes along.”

Like music, Madron said, welding is work that takes constant attention and a commitment to quality, and pays off in satisfaction. And, like serious musicians, he said, good welders need to practice constantly to keep their chops.

“Welding is part science, part art,” he said. “It’s not entirely one or the other.”

Normally, it takes young welders at least three years to bring their welding skills to a point high enough to qualify for an exacting industrial job like the bridge, but Madron said career development depends heavily on natural aptitude.

Some people are naturally cut out for welding and take to it immediately, he said. Others never get it.

“You either are a welder or you aren’t,” he said…

The deck itself, whose sections are now joined as a single unit, is still unattached to the anchors. Two giant expansion joints must be placed at either end, to accommodate length changes from changes in temperature, as well as horizontal motion from both traffic, load, and potential earthquake.

These huge joints, manufactured in Minnesota, provided a bit of local drama. The first joint was shipped across 5 states on a monstrous flatbed trailer, happily sailing along until it reached the Washington border — where it ground to a halt, courtesy of the State Patrol.

States have laws governing the maximum vehicle weight allowed on their roads, but vary in how this is determined. Washington determines weight allowances on a per-axle basis: if your load is too heavy, you may transport it legally by increasing the number of axles on the trailer bearing the weight. For huge loads such as this, reconfiguring the axles is no small feat; the original shipping company had to turn the project over to another company, who ultimately delivered the joints safely:

The money quote of this fiasco came from the original trucker: “What I’ve told them is, ‘We’ll do this anyway we can.’ If it’s impossible, then it’s real easy: Y’all can build the bridge in Idaho.”

I love a can-do attitude!

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
10. The Two Towers IX: Wheels Over Water
11. The New Bridge at Christmas
12. The Two Towers X: Compacting the Cable
13. The Two Towers XI: Cable Banding
14. The Two Towers XII: The Cranes
15. The Two Towers XIII: Life on the Bridge
16. The Two Towers XIV: Heavy Lifting
17. The Two Towers XV: The Flying Trapeze

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

As bridge sections continue to be added, either by direct lift or trapezing, the moment of truth must arrive: placing the final sections into position. With all sections in place save the last four, the remaining spaces are only 8 mm. wider than the sections which will bridge them.

Tough problem. Easy answer: make the gap wider.

The 44 sections of bridge are attached to one other in five larger segments. The center span and two short segments between the tower legs are separated from each other by a one-section gap. A second gap is present at each end between the long segment extending from the anchors and the short segment between the tower legs. The segment closest to the anchors is not yet secured directly to them, however, and therefore are able to be moved horizontally, somewhat like a porch swing.

Some porch swing.

The gaps which will hold the final sections lie on either side of the towers. The sections lying between the tower legs are the shortest, and will be moved first to place the 3rd and 4th sections, connecting the short tower segments with center span.

This is accomplished by something of a Rube Goldberg-style pulley system. Winches at the top of each tower (seen on the right) pull cables running down the tower, which then turn toward the center span end of the section to be moved. When the cables are winched, the ends of these short sections are pulled about 6 1/2 feet away from the center span, providing additional space to position the two sections connecting the tower bridge deck section to the center span. They are then eased back, and the inserted sections joined with rivet plates and welding.

Two down, two to go.

The last two sections, on the shore-side of the towers, are a bit more challenging. The long sections between the anchors and the towers are far more massive than the tower sections, and require a different approach. Nine sections are joined — nearly a quarter-mile in length — between the anchors and the gap on the outside of the towers. Engineers fasten metal rods to the last section closest to the anchor, passing them through the concrete of the anchors, behind which are placed hydraulic jacks. These jacks provide the 414,000 pounds of force required to pull the joined sections toward the anchors, increasing the gap sufficiently to place the last section. This move is possible because the final piece joining these sections to the anchors — a large expansion joint — has not yet been placed.

Thus inserted, these last sections complete the 5400-foot deck, leaving only the welding and riveting of the new sections and the wrapping of the cables, before the project is completed. More on these aspects in my next post.

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
10. The Two Towers IX: Wheels Over Water
11. The New Bridge at Christmas
12. The Two Towers X: Compacting the Cable
13. The Two Towers XI: Cable Banding
14. The Two Towers XII: The Cranes
15. The Two Towers XIII: Life on the Bridge
16. The Two Towers XIV: Heavy Lifting

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

Well, I’ve been remiss — delinquent, to be more precise. While you bridge-junkies have been sitting on the edge of your proverbial seat, many months have passed with no updates.

My bad.

Needless to say, the project has not stood still because ol’ Doc Bob hasn’t been on the ball, no siree. In fact, huge progress has been made — and in fact the bridge is due to open within the next few days. So I’ve got my work cut out to bring you up to speed, sweeping seamlessly into the Grand Opening.

In the last post, we covered the construction cranes and their daunting task of lifting bridge sections — 400 to 700 tons each — from their transport ship or barges on the water. The cranes, as you may recall, use the cables themselves as runners, and can move along the cables — albeit very slowly — by lifting and hopping forward. They are easily capable of lifting well over the weight of one bridge section, but there’s one rather large caveat: they cannot move along the cables under a load.

As a substantial portion of the bridge deck lies over land, or other areas not directly over water (between the tower legs, for example), this poses, shall we say, a bit of a dilemma. The sections are far too heavy for freestanding cranes, and the steep banks make any sort of land transport impossible.

But the answer will come with the greatest of ease: the daring bridge deck on a flying trapeze.

Really.

The genius of engineers never ceases to amaze me.

The process involves placing addition temporary cable bands along sections of the suspension cable inaccessible from the water. Sturdy steel cables — called holding lines –are dropped from the these cable bands, initially left hanging in the breeze. The bridge section to be “trapezed” is then lifted straight upwards by the gantries from its barge or ship until it lies slightly above the projected deck level. From the catwalks, the temporary suspension wires are swung toward the new section — in the direction you wish the deck section to move — and attached. So if you are moving the section toward the west moorage, holding lines from cable bands which are west of the new section are attached to the west end of that section.

The lift cranes then slowly add slack to their lines, allowing the deck section to drop slightly, transferring its weight to the temporary holding lines — which swing the deck section forward in the direction you desire.

The lift cranes — no longer bearing the weight of the section — may now move forward, repositioning themselves directly over (or ahead of, as needed) the now-moved section, ready to repeat the process with the next set of temporary lines. And thus the section is moved — very slowly, a few feet at a time, toward its destination.

And I do mean slooowly: slower than a Florida recount with hanging chads. Each move may take one of more days to complete. And it gets even slower near the anchors, where the holding lines by necessity get shorter.

The highest form of trapeze art is seen when positioning the sections which lie between the legs of the two towers. The tower legs have been designed to straddle the deck section rather precisely.

How precisely? Well, how about 5/8 inch clearance between the outside of the section and the inside of the tower legs. Less than a deck of cards on each side. This is parallel parking at its finest.

The process, however, is fundamentally the same. A third gantry crane is brought into play to provide additional control. The sections between the tower legs have brass bearings which much align with bearing plates on the towers, to provide weight support while allowing horizontal movement as the deck expands and contracts with changes in weather or load distribution.

Well, that’s it on this edition. Next time ’round we’ll see how they squeeze the last deck sections in.

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
10. The Two Towers IX: Wheels Over Water
11. The New Bridge at Christmas
12. The Two Towers X: Compacting the Cable
13. The Two Towers XI: Cable Banding
14. The Two Towers XII: The Cranes
15. The Two Towers XIII: Life on the Bridge

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

The towers are completed, the cables strung, the gantry cranes are in place–now it’s time for the big show: building the bridge deck.

Two shipments of bridge deck sections (of three total) have arrived from South Korea, and are waiting on the deck of specialized transport ships. One of these is moored just under the west end of the new bridge; the other waits in the Port of Tacoma until the first has been relieved of its load.

Several weeks passed after the gantry cranes were constructed on the cables. They moved around a bit–and were seen with odd-looking orange bags hanging from their cables:

Just before the first section lift, their purpose became apparent: they were for testing the cranes under load. The bags–12 on each crane, holding 9000 gallons of water–cumulatively weight over 450 tons.

Man, I haven’t seen water bags that big since my plastic surgery rotation…

Since the heaviest deck section weighs about 600 tons–and is lifted by two gantries–this test far exceeded the weight loads required to lift the deck sections.

Having passed the weight test, it was time for the first section lift.

The gantry cranes can move along the cables–albeit very slowly–but cannot move under the load of a deck section. So the cranes can be used to lift the sections vertically, but cannot position them horizontally after they are lifted. The transport ship is far too large and cumbersome to move around under the cables, so the sections must be moved to more mobile barges. Special barges are used which have lateral thrusters to stabilize and fine-tune their position under the cables in the rapid, ever-changing currents of the Narrows. But there’s one more problem: how are you going to get the sections from the transport ship to the barge, since the cranes can’t move the sections horizontally?

The answer is rather ingenious and surprising: move the transport ship–laterally.

The transport ship is equipped with a dozen large winches along its sides and ends, just above the water line, which are attached to fixed anchors. The cranes position themselves over the transport, lift a section vertically–then the winches pull the transport ship to the side, allowing tugs to move the barge under the bridge section. This is quite a treat to watch–take a gander at the time-lapse video below, which recorded the lift of the first section off the ship and onto the barge:

The first sections were then lifted to center span, where their uncompensated weight caused the cables to drop nearly 12 feet–resulting in a shape more like a V than a gentle parabola between the two towers.

The degree of this dip can be appreciated by following the curve formed by the free suspension cables hanging from the main cables. Prior to lifting the center deck sections, they formed a gentle parabola upwards toward mid span; after the deck sections are in place, they gradually slope downwards from the towers to mid-span.
 

The sections are held in place by the gantry cranes as workers prepare them to be connected to the suspension cables and to one another.

One might think that the deck sections would simply be placed end-to-end, starting at the anchors or mid-span. But instead, they are placed in seemingly random fashion–as is this section between the Gig Harbor anchor and the West tower.

This is done to balance the weight load on the cables, preventing excessive stretch or unequal tension which could cause alignment problems later. The engineering math involved in calculating these loads is exceedingly complex. While I know my readers are well-aware of the formulas used to calculate these stresses and the resulting length of the suspension stringers (and have already worked most of these formulas out in their heads), I have provided them here for the less enlightened–and for graduates of the Washington State public school system:

Well, that’s all for now–next time we’ll take a little swing on a trapeze…

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
10. The Two Towers IX: Wheels Over Water
11. The New Bridge at Christmas
12. The Two Towers X: Compacting the Cable
13. The Two Towers XI: Cable Banding
14. The Two Towers XII: The Cranes

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

My fascination with the bridge construction project has led me many times onto the existing Tacoma Narrows Bridge. The existing bridge was designed for another age: completed in 1950, when cars were smaller, traffic much lighter, and average speeds substantially less, there was little thought put into pedestrian traffic. There are two walkways, one on either side, each about 3 1/2 feet in width, with a metal pipe curbing less than 1 foot high separating the pedestrian walkway from adjacent traffic.

Walking on the bridge is an experience which requires some Zen concentration and detachment. The bridge itself moves vertically, especially at the mid-points between the anchor and the tower, and between the tower and mid-span. This vertical motion is several inches or more–especially when heavy trucks or traffic are present–and gives one a decidedly uneasy feeling, recalling for the historically mindful the first bridge which collapsed under admittedly more extreme–but similar–vertical motion.

I have, through repetition, grown rather accustomed to this motion, and no longer even much notice it. I have not yet fully grown used to the other intimidating feature of this pedestrian stroll, however: the experience of having large trucks, double tractor trailers, blow by you at nearly 60 miles an hour, less than 6 feet from your shoulder.The tunnel of light seems not far distant at all at some such moments. The slipstream definitely gets your attention.

The new bridge will have a broad pedestrian lane on one side only, which should make such ambulatory ventures far more pleasant.

In spite of these unpleasant aspects of a walk on the bridge, the rewards are substantial. The views are nothing less than spectacular, particularly on a clear sunny day, when Mount Rainier looms majestic to the North, and a spectacular panorama of the Sound is on display to the South.

The age of the existing Narrows Bridge becomes more evident as well when viewed in close proximity. Despite regular maintenance, the scars of constant exposure to salt air and harsh elements are readily apparent.

Once the new bridge is completed, the existing bridge will be closed for up to one year for a major renovation. Already, structural reinforcements on the tower struts, suspension cables, and deck bracing have been underway to improve resilience to earthquakes.

The construction on the new bridge is no more than one hundred yards to the south, and therefore superb views of this process are unparalleled. With a telephoto lens, you get up close and personal with the engineers and iron workers on the catwalks.

The water below is a constant hive of marine activity. Tugboats, cranes, and barges abound, shuffling equipment about, and stabilizing the large transport ship holding the decking.

And, every now and then, you get a real treat.

Earlier this week, I set out for some photos of the new bridge as sunset approached. Standing near mid-span, I gazed downward to notice a slowly-motoring skiff–a not unusual sight, as recreational boating is a Puget Sound passion. Off the port bow of the boat was an unusual group of eddies–not noteworthy in and of itself, as wild currents are the norm in the Narrows. But these caught my eye: there was motion within them. A dorsal fin–then another, and another, arcing gracefully in a divinely-orchestrated ballet, tossing fine mist upwards on their ascent from now-surfaced blow-holes.

A pod of Orcas was moving through the Narrows–a rare and spectacular sighting.

Their distance–250 feet below, and heading swiftly south in the gathering dusk–made better photographs a challenge–but the moment was captured, nevertheless.

The Orcas–also known as killer whales, although they are not whales, but belong to the dolphin family–are among the most widely-distributed mammalian species on earth, found in waters from the tropics to Antarctica. They have a strong, matriarchal family unit, travelling together in pods numbering between 6 and 18 members. Since female Orcas may live up to 90 years of age, the pod may well contain 4 or 5 generations of males and females.

They are extraordinarily intelligent and resourceful hunters, feeding on a variety of marine life, including salmon, herring, seals, and sea lions. They have been known to toss seals through the air to one another in order to stun and kill them; herring are often caught using carousel feeding, wherein the orcas force the herring into a tight ball by releasing bursts of bubbles or flashing their white undersides. The orcas then slap the ball with their tail flukes, either stunning or killing up to 10-15 herring with a successful slap.

While once plentiful, Orcas have become relatively rare in Puget Sound, and were placed on the endangered species list in 2005. Their habitat has been greatly affected by urban development and pollution around the Sound, and the dirth of salmon due to heavy commercial and tribal fishing. They rarely venture into the South Sound, and so a sighting here is a truly special event.

My day was complete–like the workers on the bridge, climbing the catwalks to head home for food and rest, I had a sense of satisfaction and pleasure at a good day over the Narrows.

Next time around, we’ll take a look at another huge and fascinating stage in the bridge construction: raising the deck sections.