under the hooves of a squadron of guards cavalry
The Egyptian bridge across the Fontanka River in St. Petersburg collapses.
Imagine that you are standing on a swinging wooden slatted bridge. It is clear that if you start swaying in time with the swaying of the bridge, the bridge will begin to sway even more.
Real modern bridges also, in fact, oscillate imperceptibly to the naked eye. Architects know that the phenomenon of resonance (that is, the coincidence of the natural frequency with the frequency of external influence) can lead to catastrophic consequences.
Egyptian chain bridge over the Fontanka
So, on February 2, 1905, the Egyptian Bridge in the city of St. Petersburg collapsed when a horse squadron was passing across it. It is believed that the cause of the incident was that the riders, while prancing on their horses, came into resonance with the bridge’s own vibrations.
In school physics lessons, when studying the phenomenon of resonance, they often give an example of this destruction, when a squadron of the Horse Guards Regiment passed “in step” across the bridge in one direction, and 11 sleighs with drivers in the opposite direction.
Typically, a military squad takes 120 steps per minute, and this frequency (2 Hz) coincided with the natural frequency of the structure. With each step, the range of vibrations of the span increased, and finally the bridge could not stand it. The bridge resonated and collapsed. It was one of five suspension bridges in the city.
The entire deck of the bridge, along with the railings and fastenings, broke the chains and broke part of the cast-iron support, broke through the ice and ended up at the bottom of the river.
Fortunately, there were no casualties and everyone managed to get ashore. According to official information, there were no serious injuries.
Subsequently, the military was forbidden to walk across the bridges in lockstep. There was even a special command: “Step at random!”
Egyptian bridge over the Fontanka River. The bridge got its name because of its unique design.
Currently, the sphinxes are all that remains of the first bridge. Now this bridge is neither chain nor suspension.
And in 1940, the Tacoma Bridge in the USA collapsed due to resonant vibrations. The photo shows how it was “twisted”.
The Tacoma Narrows Bridge (Tacoma Bridge) belongs to the category of hanging bridge structures. Located in Washington State, United States of America. It is laid through the Tacoma Narrows, which, in turn, is part of Puget South Sound.
History of creation
Initially it was built according to the design of Leon-Solomon Moiseev, a native of Russia. He is known as a design engineer, bridge builder, and an active participant in public life. The Tacoma Bridge opened to traffic in July 1940. Already during its construction, the builders noticed vibrations and swaying of the bridge road surface when the wind increased. This was due to the insufficiently high stiffening beam. In everyday life, the bridge began to be called “Galloping Gertie”.
Bridge characteristics
At the time the Tacoma Bridge was built, it was a remarkable structure. It was a hanging (cable-stayed) three-span structure. Its total length was 1810 meters. And the length of the central suspended span is 854 meters. The bridge was about 12 meters wide. The main supporting cables had a diameter of 438 millimeters. The stiffening beam reached a height of 2.44 meters, which was later recognized as a miscalculation. The bridge structure was supported by steel pylons standing on concrete supports (bulls).
Crash
On November 7, 1940, when the operation period was only four months, the destruction of the Tacoma Bridge occurred. On this day, the wind speed reached 65 km/h. Given that there was minimal traffic on the bridge that day, this made it possible to avoid casualties.
The very fact of destruction in dynamics was captured on film. This made it possible to subsequently carefully study and investigate this process. The newsreels and photographs of the Tacoma Narrows Bridge in the process of its destruction are indeed very impressive.
The film was used to create the internationally acclaimed documentary The Tacoma Narrows Bridge Collapse.
Causes of destruction
Based on the results of research and the study of documentary materials, it was established that the main factor leading to the accident was extreme dynamic torsional vibrations caused by strong winds. It was found that the Tacoma Bridge project was calculated and designed taking into account only statistical and wind loads. However, the possible impact of aerodynamic factors on its design has not been studied.
The vibration of the bridge deck arose due to It began to intensify due to the vertical vibration of the cables. The weakening of the cable on one side of the bridge and the tension on the other gave rise to torsional phenomena, led to the tilt of the pylons and, as a consequence, to the breakage of the suspensions of the central span. The bridge turned out to be structurally too flexible, with little resistance to absorbing dynamic forces.
Filming recorded that the bridge began to sway when the wind speed was about 19 meters per second. Although in the project its resistance to winds was calculated based on 50 meters per second.
conclusions
The destruction of the Tacoma Bridge forced bridge designers (and others) to begin research in the field of aerodynamics, aerodynamic stability of structures and structures. This has led to a change in thinking about the design of long span bridges.
In theory, the cause began to be designated as the phenomenon of forced mechanical resonance. However, in practice it is believed that the so-called led to it. aeroelastic flutter (torsional vibrations) due to insufficient calculations of wind loads at the design stage.
New bridge
The dismantling of the collapsed structure began immediately after the accident. The pylons and side spans were dismantled. This process lasted until 1943, when construction of a new bridge began. The bases of the pylons, anchor abutments, and some other parts from the old structure were used. The recreated bridge was put into operation in October 1950. At that time it became the third suspension bridge in the world (based on its length of 1822 meters).
In order to impart and reduce loads of an aerodynamic nature, open-type trusses were introduced into its elements. Installed additional stiffeners. It is equipped with expansion joints and vibration damping systems. The bridge could carry up to 60 thousand cars per day.
In 2007, another bridge was built parallel to the existing one. The purpose of the construction is to increase the highway's capacity. Its length is 1645.9 m, and its width is 853.4 m. The height of the pylons is 155.4 meters.
On August 14 of this year, a road bridge in Genoa collapsed; according to the latest data, 42 people became victims of the disaster. While engineers and investigators are looking into why and how this happened, Around the World decided to recall and list the main possible causes of bridge collapses and notable examples of such collapses from the past.
Humanity began to build bridges more than three thousand years ago, which allows the bridge to claim the honorary title of itself. Moreover, many bridges built thousands of years ago - especially by the Romans, who achieved amazing heights in the field of bridge construction - are still standing and even performing their functions.
But, like any engineering structure, the bridge can collapse, which has often happened over the past three thousand years. And it’s also good if it’s right in the process of construction. It’s worse if this happens after the work is completed.
Why are bridges being destroyed? Often there can be several reasons at the same time, and they, successfully complementing each other, lead to disaster. For example, the engineer made the calculations incorrectly, the builders skimped on materials or violated construction technologies, then the bridge was not operated properly and, in the end, collapsed when a too heavily loaded train or a large number of cars or people passed through in bad weather. However, in most cases one of the reasons acts as the main one.
Design and operation errors and excessive wear
Perhaps errors in design can be called the primary reason for the destruction of all engineering structures - bell towers, fortress walls or bridges. Moreover, the problem may appear immediately, or under certain conditions after construction is completed. This is what happened, for example, with a railway bridge over the Firth of Tay in Scotland in 1879. Engineer Thomas Bautsch, the author of the project and knighted for him, did not take into account the wind load when creating the project and planned the supports that supported the bridge trusses to be too thin. Added to this is the poor quality of materials and work. As a result, during a severe storm (10 out of 12 on the Beaufort scale) on the evening of December 28, 1879 (two years after construction was completed), a train with 75 people drove onto the bridge and soon found itself in the water: the spans of the longest bridge in the world at that time ( about 3000 meters) collapsed into the river along with the carriages and the locomotive.
This is what the bridge looked like a few weeks after the collapse. Today its structures have been dismantled, but the remains of the supports are still visible
But users of the suspension road bridge across the Tacoma Narrows between the city of Tacoma in Washington State (USA) and the Kitsup Peninsula were luckier. Problems with this long and rather elegant structure became known already at the construction stage: workers erecting the bridge noticed that when a side wind rose in the strait, the road surface began to vibrate and bend. For this they even nicknamed the bridge “Galloping Gertie” (Gallping Gertie). This, however, did not prevent the construction from being completed and the bridge being inaugurated on July 1, 1940. Moreover, although the vibrations of the road surface in the wind were noticeable to the naked eye and immediately began to cause concern among engineers, inspectors of regulatory authorities and drivers, the bridge was considered completely safe. Simultaneously with its operation, solutions to the problem were developed. What was the problem? The fact is that during the construction, advanced at that time solid carbon steel beams were used, on top of which the road surface was laid. If more conventional through beams were used, the wind blowing across the bridge would pass through them, and the solid beams would deflect air flows above and below and thus set the roadway in motion. Projects to correct the deficiency did not even have time to be fully thought through: on November 7 of the same year, 1940, the wind in the strait rose to strong, but not catastrophic, 18 m/s (about 64 km/h; 8 points on the Beaufort scale), and the bridge was at the end could not stand it at the end: the cables burst and the road surface, along with the car of the driver who miraculously escaped, fell into the strait; One dog died when it accidentally ran onto the bridge. And we received unique footage - they were taken by a local resident who happened to be at the bridge that day with a camera.
ResonanceOne of the most well-known causes of bridge destruction, although not the most common, is resonance, that is, the phenomenon of a sharp increase in the amplitude of vibrations of a system (in our case, the bridge structure) under periodic external influence. At school, this phenomenon is even explained in physics lessons, citing as an example the story of how a detachment of soldiers, walking in step, can cause a bridge to collapse. In fact, two reasons converge here: errors in design and improper operation; Sometimes bad weather can also be involved. This is exactly what happened with the Tacoma Narrows Bridge mentioned above. Resonance is often cited as the cause of the collapse of the Egyptian chain bridge in St. Petersburg on February 2, 1905, when the Life Guards Cavalry Grenadier Regiment was following, although the commission that investigated the causes of the incident indicated that the low quality of the iron of the chain was to blame Unfortunately, not all disasters of this kind occur without loss of life. The record for the death toll was the destruction due to the resonance of the suspension bridge over the Maine River in the city of Angers in central France on April 16, 1850, when more than 200 soldiers died while walking across the bridge in a thunderstorm and strong winds. And one of the first recorded cases of this kind was the collapse of the Broughton Bridge in England near Manchester 19 years earlier. Then no one died, although two dozen of the 74 soldiers were injured when falling into the water, and a team appeared in the army break step(“go out of step”), used when crossing bridges, especially suspension bridges, which are more susceptible to resonance. The soldiers in Angers, by the way, carried out such a command, but this did not save them from trouble. |
Exceeding the permissible load
Strictly speaking, exceeding the permissible load is also a violation of operating rules, although, as a rule, it is not a consequence of neglect of such rules and common sense motives as untimely repairs or carrying out repairs in violation of regulations (which destroyed the 710-meter bridge across the river in 2011 Mahakam in the Indonesian part of the island of Borneo), but by coincidence. This is exactly how we can evaluate, for example, what happened at 17:00 local time on Friday, December 15, 1967 with the Silver Bridge (Silver Bridge) across the Ohio River, connecting the states of Ohio and West Virginia. The bridge, built in 1928, was part of the highway U.S. Route 35 and enjoyed great popularity, which was reflected in the fact that a dense traffic flow regularly passed through it. In the weeks leading up to the holidays, traffic increased even more than usual, and the tragedy occurred on a Friday evening ten days before Christmas. The bridge collapsed due to the destruction of one of the suspension rods with which the road surface was attached to the cables, and behind it the rest of the bridge structures began to collapse - the entire destruction took about a minute. As a result, 46 people died.
The most accurate list of those killed in the bridge collapse in Dixon, Illinois, includes 46 names, and 37 of them were women, that is, 80%. Moreover, 19 of the dead were under 21 years of age. The reason for this disproportion is that women and children were allowed to go ahead so that they could better see the baptism ceremony in the waters of the river - precisely on that side walkway where the largest mass was concentrated. Heavy dresses, people falling from above and the structures of the ill-fated bridge completed the job.
Another example is also from America - from the city of Dixon, Illinois. The beginning of May 1874 was warm and sunny, so the pastor of the local Baptist church decided to hold a baptism ceremony in the waters of the Rock River for six new members of the community on the first Sunday of the month, the 4th. The convenient location was near the bridge, and such ceremonies usually attracted the attention of the townspeople (there were few alternative entertainment options in a provincial town with a population of just over 4,000 people in 1874). The bridge was built five years earlier and had a popular lattice design that was new for those years, which made it possible to assemble long crossings from short metal parts and, therefore, spend less money and build bridges in hard-to-reach areas.
On Sunday morning, between 150 and 200 people gathered on the bridge, all dressed for Sunday, with the largest number of people concentrated at one end of the bridge and within one span. The pastor took a theatrical pause before immersing the baptized person in the waters of the river. Suddenly, in the ensuing silence, a loud creaking sound was heard, and the bridge span began to fall along with the people gathered on it (men, women in heavy dresses with crinolines and petticoats, children, including small ones), who flew into the water from a height of more than five meters. About 50 people died. Officially, the cause of the incident was called the design of the bridge, but the tragedy would not have occurred if it had not been overloaded, and unevenly.
Warfare and terrorism
In all the cases described above, bridges were destroyed due to unintentional actions of people. But this does not always happen; often people destroy crossings built by other people. Most often in human history this has happened during wars, and the largest number of bridges were destroyed in the 20th century during World War II by airstrikes or shelling - either to stop the advance of troops or to disrupt the economic activities of the enemy. Thus, the Hohenzollern Bridge, built in 1907–1911 in the center of Cologne, allowed road, rail and pedestrian traffic to cross the Rhine and was therefore considered the most important element of the infrastructure of the Third Reich - during the war it was the busiest railway bridge in Germany. It is not surprising that since 1942 the Allies have been trying to destroy it with air raids. However, they were never able to completely disable it from the air - the bridge collapsed into the waters of the Rhine only on March 6, 1945, when it was blown up by American sappers.
The Hohenzollern Bridge, destroyed two months before the end of the war (pictured in the center) began to be restored soon after the end of hostilities in Germany. And in 1948, railway traffic along it was already launched. The car line was put on a different route, and to the left and right of the tracks there are now pedestrian and bicycle paths, which offer magnificent views of the city in general and Cologne Cathedral in particular.
However, even after the end of World War II, bridges continued to be destroyed by air bombing and explosions - this fate befell, for example, the very beautiful cable-stayed Freedom Bridge in the Serbian city of Novi Sad in 1999 during the NATO military operation against Yugoslavia (the bridge, however, was restored in 2005).
Bridge collapses in literatureThe bridge often became the hero of literary works, and some of them described the destruction of the crossing. Thus, the Scottish poet of the second half of the 19th century, William McGonagall, wrote the poem “The Wreck of the Bridge over the River Tay,” which we discussed above. The poem is famous for being considered one of the worst poems in the history of British literature. The writer Archibald Cronin in his novel “Castle Brodie” describes this event, although in prose, but much better. However, writers do not necessarily have to describe things that actually happened. For example, the main character of one of the best and most popular novels by Ernest Hemingway “For Whom the Bell Tolls” (eighth place in the list of the hundred best novels of the 20th century, according to the French edition Le Monde) Robert Jordan joins a detachment of Spanish partisans just to blow up a strategically important bridge (spoiler: he blows it up and dies), moreover, the author claimed that all the events in the novel are fictitious. However, the greatest attention to the collapse of the bridge is perhaps given in the novel by the American writer Thornton Wilder, “The Bridge of Saint Louis,” written in 1927. The story centers on the collapse of a century-old suspension bridge built by the Incas in Peru on the road between Lima and Cuzco in 1714 just as five strangers were passing across it; they all died. The witness to the misfortune, the Franciscan monk Juniper, on whose behalf the story is told, is investigating why exactly these people ended up on the bridge at that unfortunate moment.
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Disaster
This category of causes includes floods and sudden sharp rises of water that simply wash away a bridge or destroy its supports and the soil underneath them, and earthquakes, as well as landslides. It was the latter that caused the collapse of the bridge over the Pfeiffer Canyon (98 meters deep) on Highway 1 in California in March 2017. Over the course of a month, more than 1,500 mm of rain fell in the area of the bridge, which caused the displacement of a thick layer of soil on the slope of the canyon along with the bridge support dug into this slope. Fortunately, there was no one on the bridge at that moment.
The 92-meter-high bridge over the Kinza River partially collapsed after encountering a tornado in 2003. Before its collapse, it was 625 meters long and was the 4th tallest bridge in the United States. In 1977, the structure was included in the US National Register of Historic Places, and in 1982 - in the List of US Historic Civil Engineering Landmarks
Another, albeit rather exotic, scenario is a tornado. It was he who destroyed the famous railway bridge over the Cilantro River in Pennsylvania (USA) - a monument of engineering, built in 1883 and served until 1963, and then became the main attraction of the park Kinzua Bridge State Park. And on July 21, 2003, a tornado hit the park, hit the bridge and knocked down 11 of its 20 supports - the 120-year-old structures could not withstand wind speeds above 150 km/h.
Collision
A great way to bring down a bridge is to crash into it, and for the greatest success of this undertaking, it is worth aiming at the support. Although you can, if you wish, try to demolish the span, for example, by rushing under the bridge in a vehicle of greater height than the span itself. It must be said that in most cases the bridge wins (see the so-called “Bridge of Fools” in St. Petersburg), but not always, as happened with the Almö bridge, which connected the Swedish island of Cörn with the mainland. This beautiful arched structure (at the time of construction, the longest bridge of its type in the world) spanned a busy waterway and stood for 20 years without incident until it encountered a bulk carrier on a dark, foggy night from January 17 to 18, 1980 MS Star Clipper. He, following in difficult navigation conditions, passed not in the center of the arched span, touched the arch and demolished it. The road surface and bridge structures fell onto the ship's bridge and destroyed it. It is noteworthy that no one was injured on the ship. But, unfortunately, there were no casualties at all: in the fog, several cars drove at full speed onto the bridge from the direction of Chern and, not noticing that there was no bridge, fell from it into the icy waters of the strait - eight people died. There could have been more casualties if the truck driver coming from the continent had not noticed that the barriers had suddenly disappeared and did not have time to brake a meter from the cliff, blocking the road.
When a barge collides with a highway bridge I-40 in 2002, in the United States, no one was directly injured by the impact, but eight cars and three trucks managed to fall into the water - 14 people were killed, 11 were injured
And yet, a more reliable way to demolish a bridge is to crash into a support and preferably at full speed, as the loaded barge did Robert Y. Love in the Kerr Reservoir on the Arkansas River in Oklahoma, USA. Her helmsman collapsed at the helm, and the out-of-control vessel crashed into one of the road bridge's supports and carried it off, causing the collapse of a 177-meter section of the span. As in the case of the Almö bridge, the victims of the crash were car drivers who did not have time to brake on the edge (this happened on a May morning).
Photo: Wikimedia Commons, Stephen Lux/Getty Images, Posnov/Getty Images
Before you begin to get acquainted with the phenomena of resonance, you should study the physical terms associated with it. There are not many of them, so it will not be difficult to remember and understand their meaning. So, first things first.
What is the amplitude and frequency of movement?
Imagine an ordinary yard where a child sits on a swing and waves his legs to swing. At the moment when he manages to swing the swing and it reaches from one side to the other, the amplitude and frequency of the movement can be calculated.
Amplitude is the greatest length of deviation from the point where the body was in the equilibrium position. If we take our example of a swing, then the amplitude can be considered the highest point to which the child swings.
And frequency is the number of oscillations or oscillatory movements per unit time. Frequency is measured in Hertz (1 Hz = 1 cycle per second). Let's return to our swing: if a child passes only half the entire length of the swing in 1 second, then its frequency will be equal to 0.5 Hz.
How is frequency related to the phenomenon of resonance?
We have already found out that frequency characterizes the number of vibrations of an object in one second. Imagine now that an adult helps a weakly swinging child to swing, pushing the swing over and over again. Moreover, these shocks also have their own frequency, which will increase or decrease the swing amplitude of the “swing-child” system.
Let's say an adult pushes a swing while it is moving towards him, in this case the frequency will not increase the amplitude of the movement. That is, an external force (in this case, pushes) will not increase the oscillation of the system.
If the frequency with which an adult swings a child is numerically equal to the swing frequency itself, resonance may occur. In other words, an example of resonance is the coincidence of the frequency of the system itself with the frequency of forced oscillations. It is logical to imagine that frequency and resonance are interrelated.
Where can you see an example of resonance?
It is important to understand that examples of resonance are found in almost all areas of physics, from sound waves to electricity. The meaning of resonance is that when the frequency of the driving force is equal to the natural frequency of the system, then at that moment it reaches its highest value.
The following example of resonance will give insight. Let's say you are walking on a thin board thrown across a river. When the frequency of your steps coincides with the frequency or period of the entire system (board-person), the board begins to oscillate strongly (bend up and down). If you continue to move in the same steps, the resonance will cause a strong vibration amplitude of the board, which goes beyond the permissible value of the system and this will ultimately lead to inevitable failure of the bridge.
There are also areas of physics where it is possible to use such a phenomenon as useful resonance. The examples may surprise you, because we usually use it intuitively, without even realizing the scientific side of the issue. So, for example, we use resonance when we try to pull a car out of a hole. Remember, it is easiest to achieve results only when you push the car as it moves forward. This example of resonance increases the range of motion, thereby helping to pull the car.
Examples of harmful resonance
It is difficult to say which resonance is more common in our lives: good or harmful to us. History knows a considerable number of terrifying consequences of the resonance phenomenon. Here are the most famous events where an example of resonance can be observed.
- In France, in the city of Angers, in 1750, a detachment of soldiers walked in step across a chain bridge. When the frequency of their steps coincided with the frequency of the bridge, the range of vibrations (amplitude) increased sharply. There was a resonance, and the chains broke, and the bridge collapsed into the river.
- There have been cases when in villages a house was destroyed due to a truck driving along the main road.
As you can see, resonance can have very dangerous consequences, which is why engineers should carefully study the properties of construction objects and correctly calculate their vibration frequencies.
Beneficial Resonance
The resonance is not limited to dire consequences. By carefully studying the world around us, one can observe many good and beneficial results of resonance for humans. Here is one striking example of resonance that allows people to receive aesthetic pleasure.
The design of many musical instruments operates on the principle of resonance. Let's take a violin: the body and the string form a single oscillatory system, inside of which there is a pin. It is through it that vibration frequencies are transmitted from the upper deck to the lower one. When the luthier moves the bow along the string, the latter, like an arrow, overcomes the friction of the rosin surface and flies in the opposite direction (begins to move in the opposite area). A resonance occurs, which is transmitted to the housing. And inside it there are special holes - f-holes, through which the resonance is brought out. This is how it is controlled in many stringed instruments (guitar, harp, cello, etc.).
