Saturday, July 30, 2016

Galloping into oblivion





The Tacoma Narrows Bridge Disaster

Puget Sound, near the city of Tacoma, Washington, USA

November 7, 1940


Background:

The original Tacoma Narrows Bridge was known as "Galloping Gertie" because of its rolling, undulating behavior. It had a length of 5,939 feet (1,980 metres approx) and was opened to traffic on July 1, 1940 linking Tacoma and Gig Harbor by road.




The bridge was an unusually light design, and, as engineers discovered, peculiarly sensitive to high winds. Rather than resist them, as most modern bridges do, the Tacoma Narrows tended to sway and vibrate. This progressively worsened due to harmonic phenomena.

Four months after the opening of this bridge, there was a 42-mile-per-hour (70+km/h) wind storm around the bridge area on November 7, 1940. The wind caused the bridge to sway violently from side to side, and it finally tore the bridge apart. This incident happened because of the structure of the bridge itself which caught the wind instead of let the wind pass through. The combined force of the winds and internal stress was too great for the bridge, and it self-destructed.





No one was killed, as the bridge had been closed because of previous swaying. This is one of the best-known and most closely studied engineering failures, thanks in large part to the film and photographs that were taken to record the collapse.





The Tacoma Narrows has to rate as my all-time-favorite bridge collapse. Certainly, none has been more spectacular. That sucker just snapped like a twig, went down like a pile of kindling. It's possible to find immensely long and boring web sites devoted strictly to the reasons why this happened. We won't go there, because I have ideas of my own. It's my blog, and I'll surmise if I want to.

The original design for a standard, four-square, virtually-indestructible bridge was just too expensive. America had not yet entered the war, but it was getting close, and things like steel and whatever-else-they-make-bridges-out-of were expensive as they rode out the end of the Depression. Then someone stepped forward (Moisseiff, I think his name was - anyway, his name was mud after this) who claimed to be able to build it at a fraction of the cost.

Bad idea.




It maddens me that this is never mentioned as a reason for the bridge's collapse. It's always some sort of fancy laws-of-physics thing that goes on for pages and pages. Often one web site contradicts another, humiliating the person who wrote the competing theory as an idiot and a know-nothing. All part of the fun of the bridge-building world, I guess. Or perhaps it's the mean-spiritedness of physicists everywhere. 

But this bridge was only 39 feet wide! By bridge standards, it was a skipping rope. While it was being built, the men working on it were laying bets as to how long it would last. Most said less than six months.

Those guys were right.

Even as they worked on it, the span heaved and bucked, a feature which made it extremely popular after it opened. People came from all over the country just to ride the rollercoaster, which had become known as Galloping Gertie. And still, in a profound state of denial, the city kept the thing open, perhaps realizing what a boon it was for touristry.




It was sheer dumb luck that no one was killed (except for poor Tubby, a black cocker spaniel trapped in a car).  A few people had to run for their lives however, and barely made it. So why did it collapse? This thing was put together with spit. They should have known better! Never mind torsion and flutter and resonant frequencies. This thing was a piece of shit! They built it on the cheap, and look what happened. Though I can't find the account, I remember reading that after the disaster there was a huge war of words in the newspaper about "whose fault it was". The designer of the bridge never lived it down, but the worst abuse was saved for an engineer who tried to warn everyone during the building of the bridge, insisting and insisting it would never hold. Why was he abused? Because he didn't stop it? Because he tried to stop it? I think it was because he embarrassed everyone by being right.




Something similar happened with Challenger. The guy who had sounded the warning about the O-rings was a pariah for the rest of his career. Everyone hated him for being right. The nerve of that guy! It was just a random accident, wasn't it? It couldn't be helped. So what if he was right! He should have either stopped the disaster or (better yet) kept his mouth shut. Either way, didn't he cause it just by thinking it was possible?

Post-notes. My favorite gif is the guy running for his life as the bridge literally collapses behind him. It's one of the more nightmarish things I've ever seen. Reminds me of something out of a Popeye cartoon, where Popeye gallops along as a flimsy bridge falls away behind his footsteps. The "newsreel" isn't really a newsreel, though I am not sure who put it together. The music on the soundtrack sounds as if it was salvaged from an old Western. As devastating as the collapse was, this guy attempts to whip us into a frenzy over it. He talks about people running for their lives when they're merely walking away. One wonders why they aren't just standing there gawking, but this was a different era; people didn't feel the need to capture everything on video (or anything, for that matter - no one had movie cameras except professionals, and who could take a still picture of this thing?). One account spoke of the sickening noise this thing made as it twisted back and forth, grinding and screeching. If you've ever taken a wire and bent it back and forth a lot of times, first it gets hot, then it breaks in two. To me, that encompasses all the physics I can handle in one day.

An excerpt from Catastrophe to Triumph: Bridges of the Tacoma Narrows by Richard  S. Hobbs (who also composed an identical web site meant for student use, so I can quote whatever I like, see).




By 11:00 a.m. the extreme twisting waves of the roadway, magnified by the aerodynamic effect of wind on the sides of the bridge, began to rip the span. Huge chunks of concrete broke off "like popcorn" (in the words of one witness) and fell into the chilly waters far below. Massive steel girders twisted like rubber. Bolts sheered and flew into the wind. Six light poles on the east end broke off like matchsticks. Steel suspender cables snapped with a sound like gun shots, flying into the air "like fishing lines," as Farquharson said.

The strange sounds of the bridge's writhing filled the air. When the tie-down cables failed, the side spans began to work the main cables back and forth. The movement shifted the steel covers where the cables entered the anchorage, producing a metallic shrieking wail. By now, several hundred bystanders stood on the eastern shore of the Narrows. From the bluff, a workman on a pile driver repeatedly tooted his whistle to try to warn the approaching Coast Guard cutter, Atlanta, which passed under the bridge. The shrill whistle blasts mixed with the howl of gusting winds and the grinding and screeching of metal and concrete. The wild noises gave onlookers a sense of dread and impending calamity.




At 11:02, a 600-foot long section of roadway in the eastern half of the center span (the "Gig Harbor quarter point") of the heaving bridge broke free. With a thunderous roar, the massive section wrenched from its cables in a cloud of concrete dust, flipped over, and plummeted 195 feet into Puget Sound. A mighty geyser of foam and spray shot upward over 100 feet. Great sparks from shorting electric wires flew into the air.

Farquharson ran from the East Tower toward the Toll Plaza, covering the 1,100 feet of the side span length as fast as his legs could carry him. He followed the centerline, where he knew there was least motion. Twice, the roadway dropped 60 feet, faster than gravity, then bounced upward, finally settling into a 30-foot deep sag. Just in front of him Howard Clifford ran, fell, and scrambled up the roadway.

Successive deck sections rapidly fell out toward each tower.Coatsworth's car and Tubby followed the plunging roadway into the wind-swept Narrows.

By 11:10 a.m. it was over. The cold waters churned, eddied, and swirled. The heart of Galloping Gertie sank beneath whitecaps, coming to rest on the bottom of Puget Sound.

By this time, hundreds of cars bumper-to-bumper were driving to the bridge, making their way west on 6th Avenue from Tacoma and clogging side streets.

The most spectacular failure in bridge engineering history was over. The world's third largest suspension bridge, the latest and most advanced in its sleek design, was a twisted tangle of steel and broken concrete.

 




And this, which I'd call "why didn't they close the sucker before it opened?":

Even in a light breeze, the Narrows Bridge moved. Suspension bridges are supposed to move. But, this was different.

The roadway sometimes "bounced" or "rippled" in a wind of 3 or 4 miles per hour. Often, several waves of 2 to 3 feet (and on a few occasions up to 5 feet) would move from one end of the center span the other. There seemed to be no correlation between the wind speed and the size of the waves. Sometimes the span would "bounce" for a few moments then stop. Other times, the waves lasted for 6 or even 8 hours.




Thrill-seekers drove to the Narrows from miles around when the ripples started. Some motorists became "seasick" and avoided using the bridge. But, for adventurous spirits the bridge became an amusement ride. Drivers crossing the span at times saw a car in front of them suddenly disappear into the trough of a wave. Moments later it reappeared as the roadway rose. According to one report, a couple of times drivers experienced waves 10 feet high.

What was that motion? In the first weeks after the bridge opened, the newspapers referred to its movement as "the bounce" or "the ripple." Here are other terms used to describe the movement, used by a variety of locals, engineers, and other observers:
gallop
wave
undulation
up and down
crests and troughs
peaks and valleys
rising and falling
like a roller coaster
vertical oscillation
vertical flexibility

How did she get her name? Only later, in the autumn of 1940 about the time of the collapse, did the nickname "Galloping Gertie" make it into the newspapers.




I am never linear, don't know how to be, so things leap into my head after the fact. This jumped into my head when I thought of the sea-serpentine heaves of the Tacoma bridge. Ribbon candy. It was disgusting stuff, but we had to pretend to like it, or at least tolerate it. We went to the Kiwanis Club Christmas party every year, we had no choice in the matter, and had an insipid turkey dinner (identical round grey slices of processed turkey with grey mush underneath them, purported to be dressing) which always began with a little glass dish of fruit cocktail. Then there would be a magician, a very bad one. Then - Christmas carols? I think not. THEN, finally, what we had come for: Santa. I don't think we had to sit on his knee - and really, can you think of anything worse for a small child than sitting on the knee of a complete stranger, not just any stranger but an old man who looks like a goon with a huge beard and a frightening booming way of talking? Or that muttering whiskery "tell me what you'd like, little girl" way of talking, which is even worse. Anyway, for enduring Santa without a shrieking fit we'd get a Christmas stocking that was made out of some kind  of netting, like the type you buy for your dog. In it were a few things, candy canes for sure, but I also remember a bag of nuts in the shell (nuts were always in the shell back then - shelled nuts were considered obscene). And pink or white almonds encased in a sort of creamy candy, "sugared almonds" they were called. And - hard candy, a big cellophane bag of hard candy which I hated. Humbugs. Square brown things all indented, like pillows with buttons in them. And the dreaded "horehound" (hound from hell!). And worst of all, ribbon candy.






Ribbon candy looked like - to me, it looked like a train wreck. It looked like the Loch Ness Monster. It looked like Cecil the Seasick Sea Serpent.  It looked like the Tacoma Narrows Bridge before it collapsed, when it was "sound" and merely heaved up and down to a height/depth of ten feet! Did anyone eat this stuff? I tried to eat it. The ribbon candy was either pepperminty (I never liked peppermint candy and nearly fell over the first time I tasted a cinnamon candy cane), or horehoundy, that awful medicinal taste, worse than cough drops (which I sort of liked but which gave you a sore throat if you ate too many). The only thing I can compare it to is Pepsin Life Savers, which I don't think are made any more. Pepsin comes out of the stomach of a cow or something, and tastes like it. It has a sort of cigar ash/cemetary/Grandma's-old-trunk-that-used-to-store-dead-bodies taste.

So why all these tales from the crypt? I don't know, I'm tired, it's sort of late and things seem a bit pointless right now.




Ribbon candy. Do NOT DIY.

Applications (from Wikipedia)

Commercial pepsin is extracted from the glandular layer of hog stomachs. It is a component of rennet used to curdle milk during the manufacture of cheese. Pepsin is used for a variety of applications in food manufacturing: to modify and provide whipping qualities to soy protein and gelatin, to modify vegetable proteins for use in nondairy snack items, to make precooked cereals into instant hot cereals,and to prepare animal and vegetable protein hydrolysates for use in flavoring foods and beverages. It is used in the leather industry to remove hair and residual tissue from hides and in the recovery of silver from discarded photographic films by digesting the gelatin layer that holds the silver. Pepsin was historically an additive of Beemans gum brand chewing gum by Dr. Edward E. Beeman. It also gave name to Pepsi-Cola, originally formulated with pepsin and kola nuts.

No comments:

Post a Comment