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

 
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.
Continue reading “The Two Towers XIII:
Life on the Bridge” →

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

 
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.

With the arrival of some of the deck sections by ship last month, one was struck by the task at hand: here’s these enormous deck sections (between 450 and 700 tons apiece), and there’s the graceful cables arcing gracefully over the water, with their attached-but-empty suspension cables.

How ya gonna get those bad boys up there?

Good question. They’re far too heavy for construction cranes to lift, much less anything smaller.

As the cables were being spun, some unusual-looking equipment began to appear in the staging areas behind the anchors. Light-blue in color, they appeared at first to be part of the bridge structure itself.

For weeks I pondered the question: What’s blue, and angular, and assists in erection?
Continue reading “The Two Towers XII:
The Cranes” →

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

 
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.

I’ve been a bit remiss on bringing you up to date on the new Narrows Bridge construction. Over the past few months, there’s been a lot of progress, but much of it in ways and places without much, if any, visible change. The cables, whose spinning progress was interrupted by problems of premature corrosion of the galvanized steel wire, mentioned in my previous post, has now been completed. The North cable has been compacted and banded, and the infrastructure supporting cable spinning has been dismantled. The catwalks remain in place, but the overhead tram system used for spinning has been removed.

After the cable is compacted, it is necessary to maintain its compressed configuration, particularly since it will be placed under enormous stress shortly by the bridge decking. Temporary metal bands are used initially, but permanent cable bands are the long-term solution.
Continue reading “The Two Towers XI:
Banding the Cables” →