text/Phoenix News Client Honorary Chief Editor Tang Puihu
Core tips:
. At 1:30 am on June 24, the Champlainta Apartment Building in Miami, USA partially collapsed. So far, 12 people have died and at least 149 have disappeared. Rescuers moved 35 people away from the uncollapsed part that day, and rescued two people from the collapsed part on Friday, and overall progress was slow. White House announced that Biden and his wife Jill will go to the scene on July 1 and meet with the families of the victims.
. Champlainta is a modern high-end apartment in the sea. The residents are mainly Hinayana. The rich are particularly fond of this place. Therefore, the missing list includes the cousin of the former president of Chile and the sister-in-law of the president of Paraguay. Before the apartment building collapsed, it was undergoing 40 years of building safety certification, but before it even started, the building collapsed.
. Years of sea water soaking and years of lack of maintenance are the main causes of the collapse of the building structure. It is said that Champlainta is a beamless floor structure with low cost and low reinforced concrete savings, but has weak resistance to deformation. It was also designed according to early American specifications and did not meet modern requirements. Therefore, local structural damage quickly triggers continuous collapse.
. Scientists have long warned that houses should not be built on the quicksand of miami beach , etc. Because seawater can easily erode the sea sand substrate here, corroding the reinforced concrete structure. According to statistics, Americans have built about $3 trillion worth of real estate on the islands of the Weir and coastal plains. Now, they are all facing underground seawater erosion.
On Thursday, June 24, at around 1:30 a.m. local time, a 12-story apartment building in Miami, Dyde County, Florida, southeastern United States collapsed.
As of the 29th, 12 people have been confirmed to die and at least 149 people have disappeared. The White House announced that Biden and his wife Jill will go to the scene on the "first seven" this Thursday (July 1) and meet with the families of the victims.
What happened to this 40-year-old building? Everyone knows that the US infrastructure has been in disrepair.
But no one can explain the direct reason for the collapse of the building so far.
Basic information
This group of apartments named Champlain Towers (Champlain Towers is a French name) is located at 8777 Collins Avenue, Surfside neighborhood. The collapsed
The south and 111 units of the unit were built in 1981 and 1982, and the east of the unit of 99 units was completed in 1991.
all have 12-storey high, just two blocks away from the Arte Marina Apartment building rented by Ivanka Trump and Jared Kushner.
The developer of "Champlanda" is Nattel Construction held by Nathan Reiber. The company's main performance is these three apartments.
After Laiber's death, Natel Construction no longer had any business.
Even by current standards, Champlainta is still a modern apartment. Because of its superior seaside location, each apartment has both selling and renting prices. It is a premium apartment and has a high occupancy rate.
Some recent listing transaction records show:
On June 17, a three-bedroom and two-bathroom unit with a 9th floor and a 162-square-meter unit transaction price was US$710,000 (4.58 million yuan, a square meter unit price of RMB 28,000);
On May 11, a four-bedroom and four-bathroom suite with a top floor, 418-square-meter unit transaction price was US$2.88 million (18.6 million yuan, a square meter unit price of RMB 44,500).
The North Tower of this apartment still has several two-bedroom apartments for sale on the market, with a price of between $600,000 and $700,000.
Some people live here all year round, and some people use it as a vacation. The median monthly rent here is US$2,000 (13,000 yuan).
When the incident occurred, the apartment was almost full of people, and 55 of the 136 households were destroyed.
Before the apartment building collapsed, it was undergoing 40 years of building safety certification and had begun to prepare for large-scale repairs for rusted steel bars and damaged concrete, but before it started, the building collapsed.
Rescue workers moved 35 people from the uncollapsed part that day, and rescued two people from the collapsed part on Friday.
residents who live here are mainly Latino , followed by Russian and Jewish .
Therefore, the list of missing persons includes the cousin of the former president of Chile and the family of the president of Paraguay (the first lady's sister, brother-in-law and three children).
After all, due to distance position, Miami is regarded as the "capital" of the entire Latin American region.
Rich people in Latin America like to own real estate here and even settle down.
Various surface questions
Is it a building defect or an engineering defect?
Now, the American media focuses on some trivial building damage.
For example, the beams, columns and walls of buildings have cracked and peeled to varying degrees, and the steel bars have aged and exposed. There were also a large number of cracks and peelings in the underground parking lot of
. In addition, cracks and water stains appeared on the exterior wall, which must be fully repaired.
However, these are not the key load-bearing structures for construction.
It was also because the roof of the apartment building was undergoing renovation and construction at that time.
Some people speculate that the apartment may not be able to bear it because the roof construction has increased weight.
But common sense also tells us that this is unlikely to cause the entire building to collapse.
Some residents also said they had lived in the apartment building for decades and had never had any major problems with the structure of their building before construction started in a new building next to it.
But just last year, some residents complained that when the construction site next door was excavating and blasting, their building would also shake and vibrate, which may aggravate the cracks and some kind of damage.
This is basically a blind association for ordinary people who don’t understand architecture.
In addition, many media have reported that Shimon Wdowinski, a professor in the Department of Earth and Environment at Florida International University, found that the building has been sinking since the 1990s. The sinking rate is about 2 mm per year.
But in fact, anyone with a little knowledge of architecture knows that settlement is an inevitable phenomenon. All I fear is uneven settlement.
To understand why the local collapses so thoroughly? And the rest of the things are fine?
You also need to understand the load-bearing structure of this building first.
Building structure
It is understood that "Champlanda" is a beamless floor structure, and a floor slab directly supports only 4 corners.
The conventional beam-column frame structure is what is commonly known as the "four beams and eight columns". The beam is a cross beam and the column is a vertical column.
, and the beamless structure means that the floor slab is directly mounted on the column. Since there are no beams, the slabs are often thicker than the slabs in common concrete structures, and prestressed is required.
Considering the difficulty of being near the column, it will sometimes be partially thickened, just like a hat, commonly known as a column hat.
Compared with conventional beam-column frame structures, beamless floor slab structure generally saves more formwork, steel bars, and concrete, and the construction cost will be much lower, and the actual structural layer height can be compressed.
However, compared with reinforced concrete frame structures, the column heads of beamless floor slab structures have weak resistance to deformation.
For example, this is the traditional wood building block, which is a weak connection and can be knocked down with a slight push.
, and the new Lego building blocks , due to the structural junction, is a strong connection, and it cannot be shaken.
Generally, high-rise offices and residential buildings in the West have beamless floors, and few are made of beams.
First, they don’t like beams (pipeline layout is inconvenient), and second, they save labor costs, save the effort of building formwork and binding steel bars, save the construction period of beam formwork, and speed up the construction progress.
Beamless structure is also allowed by specifications, and steel bar structure connections must be made between the slabs across floors.
, but it is understood that "Champlanda" was designed in accordance with early American standards, with too few integrated steel bars and the steel bar connections do not meet the current specifications.
Therefore, local structure damage quickly triggers continuous collapse.
The current engineering structural specifications in various countries require more steel bar structural connections and stronger bearing and deformation capabilities.
In this way, after the local structure is damaged, the system can still be connected through steel bars, withstand tensile loads, and resist continuous collapse without serious consequences.
If it is a modern beam and column frame structure, the consequences are likely to be better.
▎Photo/Visual simulation of the collapse process Tsinghua University Professor Lu Xinzheng's research group
Many media use colloquial "collapse" when describing events. In fact, this is a catastrophic "vertical collapse".
As shown in the surveillance video and simulation, the center of the building first began to collapse, just like a huge part falling vertically;
This part of the building was razed to the ground almost instantly. About 6 seconds later, the east wing of the building also collapsed. Like "a nightmare avalanche".
At the same time, due to the lack of beams in the structure, most of the building collapsed in an instant and turned into ruins like a thousand-layer cake.
There is almost no gap in the ruins, so there is almost no living space for survivors. The old beamless floor structure used by
"Champlanda" has aggravated the damage consequences.
But what is the starting point of destruction? This has to first break out of the field of architecture and come to the field of geographical geology.
Weizhou Island that is not familiar to Chinese people
Some media reported that this building was built on the "reclaimed wetlands" back then, but this is actually an inaccurate common name.
Carefully observe the high-resolution map of the United States and you will find an interesting phenomenon - there are slender "dikes" distributed on the east coast of the Atlantic Ocean.
"dike" is generally only a few hundred meters wide, but it can be as long as hundreds of kilometers. Many dikes extend thousands of kilometers together.
It is located between the mainland and the ocean, and in the middle is the invading shallow bay and lagoon, which surrounds the mainland like a necklace.
This is the "long dike" that Chinese people are not familiar with, but Americans love very much.
Are these slender "dikes" artificially built in the United States?
of course not. This is the magical art of nature.
This narrow island distributed along the coast is called Barrier Islands, which is called Weizhou Island in Chinese.
How did the peculiar dam form? There are two main ways to form it.
First, the sea level rises first, the sea water overflows the coastal highlands and accumulates in low-lying areas offshore.
Then the sea water recedes, and the coastal highlands are exposed to the sea, forming a narrow and long island.
This process mainly occurred at the end of the last ice age, more than 10,000 years ago.
Another more common method is related to the erosion of rivers and oceans and the handling effect of .
Coastal rivers transport sand and gravel to offshore seabed to deposit. At the same time, under the guidance of wind and ocean currents, seawater pushes the deposited sand and gravel in the direction of land.
After long-term accumulation of waves, sand ridges rise along the coastline, and eventually form sand bananas.
From the formation process of the Weir Island, it can be inferred that the large-scale development of Weir Island needs to be in offshore areas with stable geology and gentle terrain.
Therefore, the harbor islands of North America are basically distributed on East Coast , and rarely exist on the west coast.
This is because the west coast is close to the boundary between the American plate and the Pacific plate, with frequent geological activities and a steep seabed topography, which is not conducive to the long-term accumulation of sand and gravel.
and the east coast is far away from the plate junction (the plate boundary is located in the middle of the Atlantic Ocean and the ridge at the bottom of the ocean), and the geology is stable; the offshore seabed topography is gentle, which facilitates sand and gravel deposition.
In addition, the river originating from the Appalachian mountainous areas in the eastern carries a large amount of sand and gravel into the Atlantic Ocean, providing rich raw materials for the formation of the weir island.
This strange long embankment is not only available on the Atlantic coast, but also in large quantities along the southern coast of Gulf of Mexico.
The eastern coastline of mainland China has a wide continental shelf and shallow water level. Moreover, because the seabed sedimentation is mainly muddy, no similar "dike" has been formed.
However, on the west coast of the Jianan Plain in the western part of Taiwan Island, there are also some dams and islands, forming new Haipu, such as Waishandingzhou, Zhuyouzhou, Qingshangangshan, and Wangziliaoshan.
Weir Island is composed of sea sand and is greatly affected by nature and human activities.
For example, the sea breeze will sweep away soft sand and gravel, thereby pushing the island of the weir to move or disappear toward the land.
Also, in recent years, climate change has increased the erosion of seawater on the island, and has also forced the island of the weir to move towards land or sink into the sea.
When encountering strong sea breezes (such as typhoons and hurricanes), the island of the weirs will even be cut off directly. By planting windbreaker , or building artificial projects (such as building seawalls or strengthening pavement) can slow down the loss of sand and gravel, stabilize the island, and make island development possible.
Coastal resort in the United States
Weir Island is connected to the mainland, with water on both sides and excellent sea views. It happened to be developed into a tourist destination that integrates beaches, vacations and sightseeing.
For example, Far Island, south of Long Island, New York, , but the most famous is Florida , the Miami area in the south of .
Because it is the farthest distance from China, it is not as familiar to Chinese people as Los Angeles, San Francisco in , Seattle in China, and New York and New England in the northeast.
In fact, this place is a grand human wonder - a 180-kilometer coastal viewing residential area.
This area spreads along the Atlantic Ocean, with a depth of about 30 kilometers and a total area of about 5,000 square kilometers, of which there are at least 7 airports embedded!
This is a super vacation and retirement area for Americans. It is also unique on the entire earth.
By the way, many people say that Hainan is the Florida of China. This is really far from that. Hainan's coastal vacation and vacation development area is only a few dozen square kilometers at most.
But scientists have long warned that houses should not be built on the quicksand of weir islands such as Miami Beach, which is composed of a mixture of fine-grained sandstone and loose soil.
Because seawater can easily erode the sea sand substrate here, causing corrosion to the reinforced concrete structure.
In Miami, there will be a "king tide" from October to November every year, which means that when the tide is high, the sea water will invade the basement and parking lots, causing more serious corrosion.
A former maintenance worker William Espinosa, who worked in Champlainta, pointed out that seawater had penetrated into the foundations of the apartments about 20 years ago, especially at high tides. This leads to water leakage and water accumulation in the underground parking lot of the apartment.
He was the repairman of the building from 1995 to 2000, when the staff had to pull the excess sea water out of the garage. "There is one foot (about 30 cm) at the bottom of the parking lot, sometimes two feet of sea water."
staff said they need to pump water frequently in the underground garage, so that they have to replace the water pump motor every two years.
Direct cause of the collapse of the building structure
And just 36 hours before the collapse of "Champlanda", a contractor went to the underground garage and found that the ground was full of water, the concrete was cracked, and the steel bars had been severely corroded.
He visited the apartment building on the morning of June 22, originally to restore the appearance of the pool and price the new pool equipment. He found no abnormalities in the apartment pool and lobby area.
But when he went to the underground garage, he found that "the parking lot was full of water", and in the underground area below swimming pool , the concrete and steel bars had cracked and corroded.
The contractor took photos of the places and sent the photos to his supervisor, pointing out that the repair work might be more complicated than expected. But before the bid was completed, the apartment collapsed.
contractor said he has worked in the industry for decades and has been to some bad places, but is still shocked by the lack of maintenance of the apartments that occurred.
So, the problem is already very clear.
The perennial sea water immersion penetrates the pores of the building's base columns. It weakens the bond between metal and concrete, creating cracks and breakages in fragile places. The main hydration products of
silicate cement are high calcium hydrated calcium silicate C-S-H(II), calcium hydroxide Ca(OH)2 and calcium aluminate hydrate C3AH6.
sulfate in seawater undergoes chemical reaction with calcium hydroxide in cement, and produces calcium sulfate and magnesium hydroxide and precipitate.
Calcium sulfate reacts with calcium hydroxide and calcium aluminate in cement to form calcium sulfaluminate hydrate, and volume expansion occurs at the same time.
The leaching effect and volume expansion of the newly formed substance cause corrosion of cement by seawater. The corrosion of cement opens the door to the invasion of chloride ion in seawater.
chlorine ions penetrate into the deep part of the concrete along the cracks formed by corrosion. When steel bars encounter them, they rust, causing them to rust, causing the concrete components to further crack or even crack.
In order to improve the durability of marine cement concrete, researchers have developed a series of technical measures, mainly starting from two aspects:
First, to enhance the resistance of steel bars to chloride ion corrosion, such as using epoxy coated steel bars, stainless steel cladding steel bars, adding steel bar corrosion inhibitors and setting up cathode protection systems;
Second, to improve the resistance of cement concrete to corrosion. Add volcanic ash mixtures such as blast furnace slag, fly ash and silica fume to cement; add water reducing agents, gas induction agents and other organic admixtures when making concrete; apply epoxy resin protective layer on the surface of concrete components, etc.
Of course, it also includes taking waterproof package measures where conditions permit. In short, additional protection is required for reinforced concrete structures.
Now, the direct cause of the collapse of the building structure is very clear. As long as one pillar has concrete cracks, when it is destroyed, the entire building can collapse like the domino effect .
In Miami Beach, a large area of buildings are built on the Imperial Island. The large number of corrosive media in seawater damages concrete.
The concrete materials used in accordance with the construction standards 40 years ago did not fully consider the impact of seawater corrosion, so it creates real risks.
Due to the unknown maintenance situation, for safety reasons, Chinese people in the United States are also advised not to stay in high-rise buildings located on the Weizhou Island or very close to the coastline.
It is obvious that similar buildings in Miami, Florida and around the world will also encounter the same dangers.
The coastal area cannot be built, but it requires good waterproofing to resist the corrosion effects of seawater.
At the same time, close monitoring and maintenance are required. The corrosion and protection of reinforced concrete structures in seawater and their protection is a special science.
In the long run, coastal buildings should use the best quality water-resistant and salt-resistant concrete in the future.
According to statistics, Americans have built real estate worth about $3 trillion on the island of Weir Island and coastal plains. Now, they all face the risks posed to the foundation by eroding underground seawater.
This incident is an ominous sign, a turning point in the history of American infrastructure. It is also a wake-up call for coastal towns across the United States.
