This article comes from the WeChat public account: SME Technology Story (ID:> MEA> ID:> span class = “text-remarks”>, author: SME, the original title: “multi-million bridge, opened to traffic only four months hit The wind blows off, but it has become a milestone in the history of construction “, header image source: IC photo
Recently, the video of Humen Bridge shaking in the wind spread to WeChat. Some people asked curiously: Is there really a modern bridge blown down by the wind?
Do n’t say it, it really is. A Channel Bridge in the United States ended its life in this way more than half a century ago.
But what is incredible is that the wind blowing down the bridge is no more than level 8.
After being in traffic for 4 months, it was easily blown down by the strong winds commonly seen by the sea, and the bridge seemed to become a laughing stock. However, its sacrifice has become a classic in the history of bridges, frequently included in various engineering textbooks.
The Tacoma Strait Bridge is a strange name to many people. But its strange twisting pictures in the wind are widely circulated. If you haven’t known it beforehand, you will not believe your eyes.
This bridge was the third largest suspension bridge in the United States at that time, and was once called “Pearl Harbor in the engineering world.”
The designer is an elite in the industry and has participated in the construction of famous bridges. Construction workers are also conscientious, and there is no violation of regulations and fraud.
But none of this can stop it. Just 4 months after the bridge was opened, it fell into the strait in frightened gaze. Fortunately, as one of the most serious engineering design errors of the 20th century, it collapsed without causing any casualties.
This is a classic case that cannot be avoided in all engineering disciplines.
As early as 1889, people proposed to build a trestle for the North Pacific Railway on the Tacoma Strait.
Because the completion of the bridge will greatly facilitate the transportation of the Navy ’s shipyard in Bremerton and the Army ’s military base in Tacoma, the construction of the bridge has also received strong support from the US military.
However, when it comes to funding issues, the Tacoma Channel Bridge construction plan was not on track until 1937.
According to the preliminary plan, the Federal Government Public Works Administration (PWA) needs to allocate $ 11 million for the construction of the bridge.
But Leon Moisef, an engineer from New York, (Leon Moisseiff) thought he had a better way.
Leon Moisev (first right)
Moisev graduated from Columbia University in 1885. He joined the New York City Bridge Department only three years later and participated in the design of almost all large suspension bridges from the 1920s to the 1930s.
He once published an article about the construction of the Manhattan Bridge. This article quickly won him an international reputation and was very well known in the engineering world.
Manhattan Bridge under construction
Moisev believed that he could build the suspension bridge to be lighter, thinner and longer than before .
This idea is fully reflected in his design plan for the Tacoma Strait Bridge.
Construction drawings of the Tacoma Strait Bridge
Moisev intends to use ordinary steel beams of 2.4 meters instead of the 7.6-meter truss beams originally planned. This not only greatly reduced the construction cost to $ 6.4 million, but also made the bridge more slender and elegant.
But Moisev did not expect that the plan was officially approved, which also meant that his career was about to come to an end.
héng frame—a spatial structure formed by connecting rods
The wind resistance of the original bridge design reached 120 mph.
But after the bridge is hoisted and closed, as long as there is a relatively mild wind of 4 mph, the main span of the bridge will rise and fall slightly.
Bridge Cable
However, the ups and downs did not cause too much concern. Construction on the bridgeWorkers also invented a method of chewing lemon to resist the dizziness caused by the ups and downs.
In their view, the Tacoma Bridge that meets the design requirements is safe and reliable.
On July 1, 1940, the Tacoma Strait Bridge was completed and opened to traffic as scheduled.
Before opening to traffic, the bridge has been shaken by the wind, which has attracted many people to drive to find out.
And people soon discovered that the amplitude of the bridge fluctuations was a bit unusual.
Even when people are driving on the bridge, you can see the cars in the distance disappearing for a while and then appear again as the bridge rises and falls.
Engineers also noticed this problem, and some professionals were sent to the site for on-site monitoring.
During the following months, the volatility of the bridge deck continued to increase. The bridge management department tried to reduce the fluctuations and reduce the impact on the traffic by binding cables and installing hydraulic buffers, but it was not successful.
On the morning of November 7, 1940, Fenger seemed to be more noisy than ever.
Technicians measured a wind speed of 38 mph at 7:30 and reached 42 mph in two hours. The wave-like undulation of the bridge actually reached more than 1 meter.
At this time, reporter Leonard Cotsworth (Leonard Coatsworth) is driving across the Tacoma Bridge.
In the back seat of the car is his pet dog, a three-legged black cub, Tubby. Both are unaware of what will happen next.
Cotsworth-the last person on the bridge
Without any warning, one side of the bridge pavement was suddenly lifted by the wind. This caused intense lateral twisting of the bridge, which was very different from the previous ups and downs. Under the excessive turbulence of the bridge deck, Cotsworth was forced to park the car 137 meters away from the East Tower.
And his parking place is 578 meters from the toll booth.
Cotsworth stumbled down the car, the bridge began to twist like a twist, and his ears were filled with the sound of tearing concrete.
Crazy twisting caused the side of the road to tilt up to 8.5 meters and tilt to 45 degrees.
Most of the time Cotsworth can only crawl on his hands and knees, but the wavy roads make it very difficult to escape. When he tried his best to reach the safety zone, he turned around and witnessed the unforgettable sight of his life.
The slings that bear the weight of the bridge break one after another, and the bridge deck that lost its tension is like an angry python struggling in the air.
Only four months after it was opened to traffic, the main body of the 120-meter-tall bridge crashed into the Tacoma Strait, which caused a huge splash of water.
This incident caused no casualties.
Although Cotsworth and his friends tried to rescue many times, the carbinese in the car still became the sole victim of the accident.
After the bridge collapsed, the United States formed an accident investigation committee. Among them is the aerodynamicist von Carmen (Theodore von Kármán) .
Carmen is a Hungarian Jew. After moving to the United States in 1930, he was responsible for guiding the design and construction of the Guggenheim Aerodynamics Laboratory and the first wind tunnel at California Institute of Technology. NASA’s famous Jet Propulsion Laboratory (JPL) was also created by him.
Wind Tunnel Lab
Carmen in life is a humorous and talkative person. When Chinese scientist Qian Xuesen went to study in the United States, Carmen served as his mentor, and the two had a deep relationship.
It was also because of Carmen ’s investigation that the cause of the Tacoma Strait Bridge accident fell.
Von Carmen
After preliminary research, the committee found that there are defects in the design of the bridge that cannot be ignored.
First of all, the main span of the Tacoma Bridge is 853.4 meters long, but the width of the bridge is only 11.9 meters, which is very rare on the suspension bridge of the same period. Not only is the bridge deck too narrow, but a steel girder with a height of only 2.4 meters cannot produce sufficient rigidity.
Rigidity-the ability of objects to resist deformation
Second in the original plan, Wind can pass freely between the truss beams. But after changing to ordinary steel beams, the wind can only pass through the upper and lower sides of the bridge.
In addition, the wall skirts on both sides of the bridge are made of solid steel plates, and the cross-section forms an H-shaped structure, which will make the wind blocking effect more obvious.
However, experts did not agree on the exact theoretical reason for the collapse of the Tacoma Bridge. Some engineers believe that the Tacoma BridgeThe vibration is similar to the flutter of the wing.
Another expert, represented by Carmen, believes that the Tacoma Bridge has an H-shaped cross-section, which is different from the streamlined wing.
After testing the model in the California Institute of Technology ’s wind tunnel, Carmen guessed that the disaster stems from a phenomenon- Carmen Vortex .
This is a widespread phenomenon in nature.
For example, if a wooden pile is inserted into the water flow, under certain conditions, two vortices will be generated on the two sides downstream of the wooden pile. The two rows of vortices rotate in opposite directions and are staggered with each other, just like street lights on both sides of the street.
From space overlooking Carmen Vortex on the coast of Chile
In this accident, the steel plates on both sides of the bridge were like wooden piles in the water. When the high-speed vortices formed by the wind continuously detach from both sides of the bridge body, an alternating lateral force will be generated on the bridge body.
Carmen vortex street is a regular periodic phenomenon, that is to say, the formation of vortex and the effect of lateral force have a certain frequency.
The Tacoma Bridge itself also has its own frequency, and resonance will occur when the two frequencies are close. Everyone knows the consequences of resonance.
After some calculations and experiments, Carmen’s analysis was confirmed.
The collapse of the bridge was (Barney Elliott) recorded by the local photo studio owner Barney Elliott.
This precious video material was selected by the U.S. Library of Congress to be kept in the National Film Registry, and is said to be:
“Important in culture, history and aesthetics” .
At the same time, it has become one of the best classroom education films for architectural engineering students.
The wreckage of the bridge now sleeping at the bottom of the Tacoma Strait has become the largest artificial coral reef in the world. A large number of giant Pacific octopuses choose to settle here.
At the top of the site are two new suspension bridges, which are responsible for the continuous flow of traffic between Tacoma and Gig Harbor. They will continue to complete the work of the first Tacoma Strait Bridge.
Of course, we should not blame the engineers who built the bridge too much.
People in that era knew very little about the aerodynamic characteristics of suspension bridges, so this disaster was basically unpredictable at the time.
And it is the collapse of the Tacoma Strait Bridge that triggered the research of wind vibration by scientists all over the world, and led to the establishment of various new disciplines such as bridge wind engineering.
Bridge Cross Section
Now when we build large bridges, we can improve the air flow around the cross-section by modifying the shape of the cross-section of the structure or installing air stabilizers. The vibration of the bridge can also be reduced by installing dampers, etc., making the bridge safer and more durable.
Research on the wind vibration of bridges has enabled humans to continuously break through the span records of bridges in recent decades.
The collapse of the Tacoma Strait Bridge has become the cornerstone of future bridge construction. Guided generations of engineers to continue to move forward in the lessons.
References
Yuan Jiajun. Skywalk Collapse, the worst structural disaster in American history: the collapse of the flyover of the Hyatt Hotel in Kansas City. The accident returned to the scene. 2010.2.
Peng Xiaowu. [On the ruins] The collapse of the Hyatt Hotel in Kansas City. Jianshu. 2017.04.10.
This article comes from the WeChat public account: SME Technology story (ID: SMELab) , author: SME