The Two Towers XVII: Opening Day

Previous posts in the New Narrows Bridge Series:

  1. The Two Towers I: Introduction
  2. The Two Towers II: History of the Narrows Bridges
  3. The Two Towers III: The Caissons
  4. The Two Towers IV: Anchor Management Class
  5. The Two Towers V: The Tour Begins
  6. The Two Towers VI: Struttin’ My Stuff
  7. The Two Towers VII: To the Top
  8. The Two Towers VIII: Stairway to Heaven
  9. The Two Towers IX: Spinning Beginning
  10. The Two Towers X: Wheels Over Water
  11. The Two Towers XII: Compacting the Cables
  12. The Two Towers XIII: Banding the Cables
  13. The Two Towers XIV: The Bridge Deck Cranes
  14. The Two Towers XV: Life on the Bridge
  15. The Two Towers XVI: Heavy Lifting
  16. The Two Towers XVII: The Flying Trapeze
  17. The Two Towers XVII: Squeeze Play
  18. The Two Towers XVIII: It’s a Wrap

The Two Towers XVII:
It’s a Wrap

Empty cable spools

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.

Wrapping the bridge cables

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,

Bridge cable wire wrapping

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

Plates connecting the sections

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.

Welder on bridge

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…

Anchor joints in transit

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.

Bridge anchor expansion joints in transit

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! Finally, we’ll see the grand opening of the new bridge.

The Two Towers XVIII:
Opening Day

The Two Towers XVI:
The Flying Trapeze

Lifting & positioning bridge deck sections

Since the last segment a great deal of progress has taken place — and now the bridge is due to open within a few days.

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 the flying trapeze.

Really.

The genius of engineers never ceases to amaze me.

Trapezing a bridge deck section

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.

Bridge deck section trapeze

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.

Bridge section in place
bridge deck anchor sections
Bridge section trapeze

And I do mean slooowly: slower than a Florida election 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.

Bridge section between tower legs

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 must 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.

Bridge section between tower legs
adding bridge deck section between tower legs
Bridge deck section between tower legs

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

The Two Towers XV:
Heavy Lifting

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.
First bridge section

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:

Bridge section counterweights
Counterweights

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 bags that big since my facelift…

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 by itself, 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, and follows with subsequent sections to (nearly) complete the deck build. Notice how they “walk” each section with the gantry cranes to move each section into its position:

Lifting a bridge section

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.

First bridge deck section

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 catenary curve upwards toward mid span; after the deck sections are in place, they gradually slope downwards from the towers to mid-span.

2nd bridge section

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 the time lapse video above demonstrates.

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 our Washington State public school system:

Weight balance formulas

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

The Two Towers XIV:
Life on the Bridge

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.
Cable catwalks with gantry cranes

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.

2nd bridge pedestrian walkway

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-minded the first bridge which began its death throes with similar but far more violent 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.

Truck traffic

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.

Mt Rainier from bridge

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.

After the new bridge is completed, the existing bridge will undergo a major renovation. Already, structural reinforcements on the tower struts, suspension cables, and deck bracing have been underway to improve resilience to earthquakes.

Rust on Narrows bridge

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.

Workers on bridge

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.

Tugboat at caisson
Barge on Narrows

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 relatively rare and spectacular sight.

Orca pod
Orca pod

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.

Orcas at dusk
Orca

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 this southern community was placed on the endangered species list in 2005. Their habitat has been greatly affected by urban development, pollution around the Sound, and the dirth of salmon due to heavy commercial and tribal fishing. They infrequently venture into the South Sound, and so a sighting here is a truly special event.

New bridge at sunset

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.

The next episode is back to the construction, and lifting the bridge sections.

The Two Towers XIII:
The Bridge Deck Cranes

Bridge deck ship

With the completion of the cables, the deck sections have begun transferring by ship to the bridge 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.

Bridge crane components

For weeks I pondered the question: What’s blue, and angular, and assists in erection?

The answer came: Viagra! –but I somehow didn’t think this equipment would be of much benefit with that equipment. Seemed kind of … awkward, you know? The quest continued… …until one day, a few weeks later, the pieces were moved, and their purpose became apparent: overhead mobile gantry cranes, using the cables themselves for support. This erection’s definitely gonna last more than four hours–but don’t call your doctor…

Bridge deck crane

There’s eight of these mobile monsters: two between the towers and the anchors at either end, and four on the cables between the two towers.

Bridge deck cranes

The gantries use the cables as tracks and move along them on motorized wheels a few inches at a time — but there’s a glitch: the cable bands are in the way.

Cable bands

No worries: when the cranes get to a cable bands, they simply hop over them.

The cranes are secured to the cables with four clamshell-like clamps, which can open (seen below, upside down, prior to assembly).

Hydraulic lifters

Four hefty hydraulic lifters–two on each cable–lift the entire crane up a few inches, allowing it to ride over the obstacle, then gently light like a butterfly on the other side of the cable band. The clamshell clamps close again, securing the crane on the cables, and the glacial progress onward continues. As you can imagine, this is not a speedy process (think: continental drift)–the cranes can take a day or more to move any substantial distance along the cables (far too slowly, incidentally, to move the deck sections horizontally after they are lifted).

Non moveable gantry cranes

These gantries are used primarily for the initial deck section lift from the transport ship — in a process both surprising and fascinating.

But you’ll have to wait until the next post for those juicy details, because things have been getting really interesting these past few weeks: the deck sections are being lifted. More on that in our next segment.

Bridge deck section gantry crane

The Two Towers XII:
Banding the Cables

Completed cables unbanded

The cables, whose spinning progress was interrupted by problems of premature corrosion of the galvanized steel wire mentioned in my previous post, have 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.

Unbanded Cables

These heavy steel clamps are designed to fit the compacted cable exactly, and their compression of the cable is precisely controlled through the use of precision bolts.

Cable band bolts

These bands will also serve the function of supporting the stringers which will be used to suspend the deck sections. The cable bands are placed precisely 40 feet apart at the level of the deck (the deck sections are 40 feet long) — which seems simple enough, except that due to the variable arc of the cables, the calculations for precise placement are fairly tricky.

Once the bands have been placed, the stringers are simply draped across the top of the cable bands in notches designed for them. The length of the stringers must also take into account the anticipated stretch of the cable which will occur when the massive weight of the deck is attached.