But, are high-rises really safe? Recent catastrophic events at several high-rise buildings around the world have called that into question, so we're going to take a look at what is causing this raft of failures, and what engineering lessons can be learnt from them.

1. Surfside Condominium – 2021 – Surfside, Florida

Surfside Condominium Source: Mapillary user/Wikimedia Commons

At 1.25am on June 24, 2021, a security camera mounted on a building across the street from the luxury Surfside Condominium building recorded a cascade of water gushing from the ceiling of the building's parking garage. Part of that garage sat beneath the Surfside's swimming pool.

Soon after, an entire wing of the 12-storey building collapsed, with each floor pancaking on top of the one below it. Rescuers who arrived within minutes were only able to pull four people from the wreckage, and over the next two weeks, the bodies of 98 of the building's residents were recovered. 

Surfside Condominium after collapse Source: Miami-Dade FD/Wikimedia Commons

Long before the collapse, residents of the building were aware that the reinforced concrete used to construct their building was eroding, and a $15 million programme to repair it had been scheduled to begin the very next week.

Investigators would come to determine that the cause of the collapse was sea water penetration, which had undermined the steel within the reinforced concrete used to construct the building. Another causative factor cited by investigators was that the land beneath the building was sinking, or subsiding.

Causes of the disaster: Human. According to an article by the New York Times, three years before the collapse an engineer hired to examine the building found "major structural damage" to the concrete slab that sat below the building's pool deck. The engineer also found extensive cracking and crumbling of the columns, beams and walls of the parking garage under the 13-storey building.

Also, an article in the Times of India newspaper, makes allegations – which have yet to be proven – that construction of the tower may have, "involved short cuts and payoffs to navigate through the permit system".

2. Millennium Tower – 2016 – San Francisco, California

Millennium Tower San Francisco, California Source: Michaelgimbel/Wikimedia Commons

Located in a tiny area of San Francisco's downtown, Millennium Tower is San Francisco's tallest residential high-rise. The building opened to residents on April 23, 2009, and for those living in the 58-storey, 196m structure, life was good, with a French restaurant and wine bar located on the ground floor, a private concierge, and an 'Owner's Club' which features its own private lounge, wine cellar, and fitness centre.

That all changed in May 2016 when residents were informed that their building was both sinking and tilting. So far, Millennium Tower has sunk 41cm and it is tilting towards the northwest by 5cm at its base, and by more than 38cm at its top.

The cause of the sinking and tilting is that the building's foundation doesn't reach down to bedrock, but instead sits on a layer of sand, with a layer of clay beneath that. According to an article in the Los Angeles Times, this type of construction was typical for buildings in that area of San Francisco, however, those other buildings were much shorter and lighter than Millennium Tower.

Engineers brought in to assess the problem proposed two fixes: an internal fix where new pilings would be screwed through the foundation of the building, and an external fix whereby pilings would be placed along two sides of the building's outer wall and would take up to half of the building's weight.

The external fix was cheaper, so that's what the building's residents chose, but according to an article by KPIX, a TV station in San Francisco, that couldn't have been a worse decision.

According to the article, when the City of San Francisco asked geotechnical engineer and professor at the University of California Berkeley, Lawrence Karp, to analyse both remediation plans, he raised red flags stating that the pilings used in the external fix were disturbing the already compromised soil.

Even more concerning was that Karp found that the building's lean had increased by 26% over the last year alone. And, Karp determined that in the event of an earthquake, the entire building could rotate.

ESA satellite image of Millennium Tower Source: ESA SEOM INSARAP study

After the building sank an additional 2.54cm in just a month, which was determined by satellite imagery provided by the European Space Agency, the almost $100 million retrofit was halted on August 19, 2021.

Causes of the disaster: Human. Millennium Tower's homeowners are pointing the finger at the building's developers, who in turn, are pointing the finger at construction of the neighbouring Transbay transit centre, which they say is eroding the soil.

3. Knickerbocker Theater – 1922 – Washington DC

Knickerbocker Theater Washington, D.C. Source: Wikimedia Commons

On the night of January 28, 1922, the Knickerbocker Theater in Washington DC was packed with people watching a silent film, despite there being a two-day-long blizzard. The Knickerbocker also housed ballrooms and lecture halls.

The building's managers discussed the possibility of sweeping snow from the structure's roof, but ultimately decided not to. As patrons began watching the silent film, the weight of the snow piling up on the building's flat roof caused the roof to collapse. It came down onto the theatre's balcony, which in turn collapsed onto the orchestra seating area. 

Knickerbocker Theater after collapse Source: Washington Post/Wikimedia Commons

A total of 600 soldiers soon arrived to search the rubble, and the building's architect, Reginald Geare, also arrived, helping to pull people out, and describing to rescuers the layout of the building. In all, 98 people were killed and 133 people were injured.

Investigators concluded that the collapse was due to the use of arch girders rather than stone pillars to support the roof, and in the aftermath of the collapse, both Geare and the building's owner, Harry Crandall, later committed suicide. Crandall left a suicide note stating that he hoped reporters wouldn't be too hard on him.

Causes of the disaster: Human. Had the roof been swept of snow, and had it been built with better support, the disaster could have been averted.

4. Pemberton Mill – 1860 – Lawrence, Massachusetts

Pemberton Mill Lawrence, Massachusetts Source: CSZero/Wikimedia Commons

Originally built by Boston industrialist John Lowell, the cotton textile mill was later sold to new owners, who installed additional machinery on the factory's upper floors.

Just before 5pm on January 10, 1860, the mill was crammed with more than 800 workers, many of whom were women and children, when the building suddenly collapsed, trapping hundreds in the rubble.

Pemberton Mill after collapse Source: open source

Rescuers flocked to the scene, and since darkness came early on that winter's night, they lit oil lamps to help them see. When one of the rescuers accidentally knocked over an oil lamp while climbing through a crack to rescue a young women, the resulting fire spread quickly throughout the cotton waste and oil-soaked wood, burning alive at least 14 of those trapped.

It is estimated that more than 145 people died in the tragedy and another 165 were injured. Investigators determined that the heavy machinery installed on the upper floors, improperly mortared walls, and malformed cast-iron support columns were the cause of the collapse.

Causes of the disaster: Human. The mill's new owners had not determined the effect the new machinery would have on the structure, nor had they properly maintained the building.

5. Hyatt Regency Hotel – 1981 – Kansas City, Missouri

Hyatt Regency Hotel Kansas City Source: Charvex/Wikimedia Commons

When construction began in May 1978 on the hotel, inflation in the US was raging, the unemployment rate was high, and interest rates were in double digits. All this placed additional pressure on the building's developers.

The building's atrium roof had already partially collapsed and been repaired even before the hotel officially opened on July 1, 1980. Like other Hyatts around the world, the building's defining feature was its multi-storey atrium, which was criss-crossed by elevated walkways.

Hyatt's walkways Source: Lee Lowery, Jr./Wikimedia Commons

There were three walkways, one each connecting sections of the hotel's second, third, and fourth floors. Each walkway was constructed of steel, concrete and glass, was about 37m, and weighed about 29,000 kg. The third level walkway was suspended directly from the ceiling of the atrium, as was the walkway on the fourth level, however, the second level walkway support was connected not to the ceiling, but to the fourth-level walkway directly above it.

On the night of July 17, 1981, a tea dance was taking place in the hotel's atrium, with about 1,600 attending. At about 7pm, about 40 people were mingling on the second-storey walkway, and 16 to 20 people were standing on the fourth-storey walkway.

Suddenly, popping noises and a loud crack were heard, before the fourth-level walkway fell onto the second-level walkway, and they both careened down onto the lobby floor where hundreds of people were gathered.

Hyatt's atrium after collapse Source: Lee Lowery, Jr./Wikimedia Commons

Those not killed outright were buried under tons of concrete, steel, and glass, which the equipment brought by the Kansas City Fire Department was powerless to lift. A call went out to Kansas City's building trades and hundreds of people showed up, bringing with them jackhammers, concrete saws, and cranes.

Those trapped in the rubble almost drowned when the building's sprinkler system flooded the atrium with water. In all, 114 people were killed and 216 were injured, making this collapse the deadliest in US history until the collapse of the World Trade Center in 2001.

Investigators attributed the collapse to structural overload caused by design flaws, and the Missouri Board of Architects, Professional Engineers and Land Surveyors found the engineering company behind the building's design to be culpable of gross negligence, misconduct, and unprofessional conduct.

While the engineering company was acquitted of all charges in a criminal trial, it did lose its engineering licences in Missouri, Kansas and Texas, and it lost their membership in the American Society of Civil Engineers.

In the years since the collapse, the Hyatt Regency has became a case study in the world's engineering schools. The chairman of the Missouri Architectural Board said of the disaster, "[The skywalk design] is one of the worst examples of people trying to push off their responsibilities to other parts of the team ..."

The building's chief engineer spent years lecturing at engineering conferences where he said he wanted "to scare the daylights out of" engineers to prevent a repeat of the Hyatt Regency disaster.

Changes made due to the collapse include the American Society of Civil Engineers (ASCE) adopting a new policy making structural engineers responsible for reviewing the shop drawings submitted by fabricators. Kansas City upped its code enforcement department, and for its reporting, the Kansas City Star newspaper won a Pulitzer Prize in 1982.

Cause of the disaster: Human. Investigators cited a lack of proper communication between the building's designer and its steel provider, with the engineering drawings that were submitted to the steel provider being only preliminary sketches that were interpreted as finalised drawings. That designer then approved the steel provider's plan without performing necessary calculations or viewing drawings that would have revealed the design flaw.

Each company assumed that somebody else had performed the necessary calculations, and workers performing the actual construction failed to report that they had noticed the beams holding up the fourth-storey walkway starting to bend, and instead avoided using the walkway.

6. World Trade Center – 2001 – New York City

World Trade Center New York City Source: Jeff Mock/Wikimedia Commons

When the Twin Towers of New York City's World Trade Center were completed in 1973, they were the tallest buildings in the world, following only that of Petronas Towers in Kuala Lumpur, Malaysia and the Willis Tower, formerly known as the Sears Tower, in Chicago.

The morning of September 11, 2001, was particularly beautiful with clear skies and cool temperatures. At 8.46am, an aircraft appeared in the crystal blue sky and improbably sailed right into the North Tower of the World Trade Center.

World Trade Center after second impact Source: Robert J. Fisch/Wikimedia Commons

At 9.03am, as photographers' lenses and all eyes were trained on the North Tower, a second plane appeared in the sky, and it impacted the South Tower. Towering plumes of smoke were coming from both towers before, at 9.59am, the South Tower collapsed to the ground. The North Tower remained standing until 10.28am before it too collapsed, and a third building, 7 World Trade Center collapsed at 5.21pm.

World Trade Center after collapse Source: U.S. Navy/Wikimedia Commons

On the ground, 2,763 people were killed, including 2,192 workers in both buildings, 343 firefighters, and 71 policemen. A total of 147 passengers and crew members on the planes also died along with the 10 hijackers.

An investigation conducted by the National Institute of Standards and Technology (NIST) determined that the fires caused by burning airplane fuel were the main cause of the collapse. The extreme heat caused the building's floors to sag, which in turn pulled the perimeter columns inward. Once those columns buckled, the upper floors of the building came down, pancaking onto the floors beneath them.

Cause of the disaster: Human. Terrorism.

7. Torre dei Moro Building – 2021 – Milan, Italy

Torre dei Moro building Source: Neq00/Wikimedia Commons

A little after 5.30pm on Sunday, August 29, 2021, a resident of the 60m, 20-storey building's 15th floor reported a fire on that floor and alerted fellow residents. 

Despite having to evacuate, residents of the well-heeled building weren't too worried because they had been assured that the panels that clad the building's exterior were fire-resistant. Instead, as a resident told the publication Corriere Della Sera, the panels, "burned quickly like butter".

Within minutes, the entire structure was enveloped in flames, with an Interior Ministry official telling the Daily Mail newspaper that "... it seems that the rapid spread of the flames was due to the thermal covering of the building". 

Torre dei Moro building Source: Vigili del fuoco/Wikimedia Commons

 

As of this writing, all residents of the building have been accounted for and no one has needed hospitalisation, however, firefighters are conducting a door-by-door search. The Torre dei Moro building is now so unstable that it is in danger of collapse.

Cause of the disaster: Human. The Associated Press quoted Milan Mayor Giuseppe Sala who said, "What seems already clear from the start is that the external cladding of the building went up in flames far too quickly ..."

8. Sampoong Department Store – 1995 – Seoul, South Korea

Sampoong Department Store Seoul, South Korea Source: Seoul Met Fire/Wikimedia Commons

Problems began when the developer of the building, Lee Joon, insisted midway through construction that the building be converted from an apartment building to a department store. To instal escalators, several important support columns had to be removed, and when the contractors refused to make those changes, Lee fired them, formed his own construction company, and hired new workers.

Lee next added a skating rink on an unplanned-for fifth floor to the building. This was later changed to restaurants, heated by a system of underfloor hot-water pipes. Air conditioning units placed on the roof had a weight four times what the building was designed for, and even worse, the entire building was constructed with substandard concrete that had only half of the steel reinforcing bars it required.

When tenants of neighbouring buildings complained about the noise from the air conditioning machinery on the roof, management moved the three units to the west by dragging them rather than using cranes to lift them. Cracks opened up during the moving, and they widened every time the air conditioners were turned on.

By April 1995, cracks had started appearing around the building, and on June 29 those cracks became much worse. The building's managers refused to close the building, not wanting to lose a day’s revenue, but instead they fled the building themselves.

Soon, pops and cracking sounds were heard before, over the course of just 20 seconds, the entire building came down. A total of 1,500 people were trapped in the rubble, and 502 people were killed, and another 937 injured.

Cause of the disaster: Human. An investigation determined that criminal negligence, blatant disregard for ethical engineering practices, and shoddy construction were the causes of the largest peacetime disaster in South Korean history.

9. Grenfell Tower – 2017 – London

Grenfell Tower London, England Source: Robin Sones/Wikimedia Commons

On June 14, 2017, a refrigerator-freezer in an apartment on the fourth floor of the 24-story building caught fire. When the fire reached the outer wall of the building, something unexpected and horrible happened.

The cladding of the building, which was thought to be fireproof, sat somewhat away from the exterior wall and this created an air gap which the fire used to leap upwards in what is called a 'stack effect'. This effect is used in the design of chimneys, and it is the upward movement of air due to buoyancy. The greater the heat of the escaping air, and the greater the height of the structure, the greater the upward movement of the air.

Grenfell Tower during fire Source: Natalie Oxford/Wikimedia Commons

The stack effect sucked the fire up the entire height of the building, and so flammable was the structure that it took firefighters more than 60 hours to completely extinguish the blaze.

By that time, 72 people were dead and 70 were injured. Incredibly, 223 people were able to escape from the inferno, despite earlier instructions that in the event of a fire the residents should shelter in place.

In the wake of the disaster, the leader, deputy leader, and chief executive of the council that administered Grenfell Tower all resigned. In May 2018, the British government published a report detailing new building and fire safety regulations, and the Grenfell Tower fire led local governments across England to examine their high-rise buildings for the same cladding that covered Grenfell Tower, and if they had it, to replace it.

Cause of the disaster: Human. An inquiry into the disaster was convened on September 14, 2017, and its results were released in October 2019. The inquiry determined that the building's exterior cladding didn't comply with regulations, and the investigators also felt that the fire services waited too long before ordering residents to evacuate the building.

What you can do

After the Surfside collapse, forensic structural engineer Orlando Ballate told Tampa TV station WTSP, "... high-rise buildings are generally safe. This [the Surfside collapse] is a rare instance but it's a wake-up call if a building is not maintained and inspected and programmes are not in place to maintain life and safety."

Ballate went on to say that inspections should be performed regularly by experienced engineers, and those living in high-rise buildings should request their building's latest inspection reports from the property manager, homeowners association, building maintenance personnel, or their county or city government.

Ballate also said that if you don't feel that your building was inspected recently enough, you can ask your building's management or your county or city government to conduct an inspection.