Previous posts on the new Narrows Bridge:
- History of the Tacoma Narrows Bridges
- The Two Towers I: Intro
- The Two Towers II: Concrete Thinking
- The Two Towers III: Anchor Management Classes
- The Two Towers IV: Out & Down
- The Two Towers V: The Struts
- The Two Towers VI: To the Top
- The Two Towers VII: Stairway to Heaven
- The Two Towers VIII: Spinning Beginning
- The Two Towers IX: Wheels Over Water
- The New Bridge at Christmas
- The Two Towers X: Compacting the Cable
- The Two Towers XI: Cable Banding
- The Two Towers XII: The Cranes
- The Two Towers XIII: Life on the Bridge
- The Two Towers XIV: Heavy Lifting
- The Two Towers XV: The Flying Trapeze
- 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.â€
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.â€
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.â€
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.â€
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!
As soon as the story hit my first reaction was “What will Dr. Bob say?” You are the most informed observer of bridge construction I know. I find it ironic that components of the Tacoma Narrows bridge came from Minnesota.
I understand about risk-reward metrics. Statistics always justify cutting it close. But that is no comfort to those who have lost family members in this tragedy. Like intersections that only get attention after enough traffic fatalities, or earthen dams that finally get looked at after one or two fail…now a category of old bridges is about to get overdue scrutiny.
Yes, it’s a horrible tragedy. Of course there will be the obligate finger-pointing, and hordes of salivating lawyers wait in the wings, their Gucci loafers tapping in impatience to suck the evil perpetrators — real or perceived — bloodless.
But it seems very likely that there was some unrecognized design flaw in the Twin Cities bridge which led to catastrophic failure. You shouldn’t have to perform a major rebuild or replacement on a bridge after only thirty years.
I heard a very reasonable explanation this morning on the radio by a fellow with a British accent. Don’t recall who or where, but he was explaining “metal fatigue.” The phenomenon can be seen by anyone bending a clotheshanger. Flex and straighen a few times and the wire “breaks.” That, he says, illustrates metal fatigue in its most basic form. Small vibrations repeated over a period of time can have the same effect at the microscopic level resulting in weakening the material. Apparently aircraft engineers know about this and checks for metal fatigue routinely. The phenomenon was not well understood, however, in the sixties and before. Just one man’s opinion.
I would be interested if any of your bridge experts said anything about “metal fatigue.”
Arches and bridges made of stone have been in use since Roman antiquity, but they were over-engineered because they didn’t know any better. We have become so smart we know how to cut every margin to the tightest tolerance. I’m not sure we are on the right track in this regard.
My Dad was a car mechanic. I mentioned to him once that every car could be made with a safe and effective built-in jack for emergencies by putting lift rods in the door casings midway on the car body, one on the driver’s side, the other on the passenger side. Either could be deployed if needed to lift the vehicle clean off the ground, resting on two wheels and a jackpost. He said no automaker would ever do such a thing because it was cheaper to keep using the cheesy little jacks that have been tossed in for years. I think he was right. And I still think that built-in jacks make a lot more sense.
Kinda like over-building bridges so they don’t drop a line of bumper-to-bumper slow-moving traffic. And maybe cheaper in the long run…
But part of the issue of over-engineering is the law of diminishing returns. Sure, you can make a bridge dtronger, but there’s always a trade-off. Sometimes you end up making the bridge heavier, and the extra weight negates any value from the added strength. Or maybe you make it less flexible, which is a critical thing on the Ring of Fire.
On one of the anniversaries of the Golden Gate Bridge, they shut down the traffic and let people walk across it. They were about halfway across when some engineer looked at what was happening and started swearing— there were so many people walking shoulder-to-shoulder that they weighed more than even rush hour. A postcard of the event shows something rather frightening if you know what you’re looking at— the section of bridge between the towers was level, not gently curved up to meet the suspension cables.
From what I heard, they asked the local constabulary to discreetly start cutting the flow of people, and prayed that there wasn’t enough stress to do something terrible. As you know, everything worked out all right. But they just didn’t know at the time. We always think we know what’s going on but there’s no real way to tell.
As a former engineering student who used to calculate strength of materials, I can only say that somewhere, somehow some small piece of the load carrying body reached its limit, then exceeded it.
The rest is history.
As much as we’d all like it to be different, we’re never going to create a perfect world, this side of heaven, though we do learn from our experiences.