How can California’s skyscrapers withstand a major earthquake? LA County is trying to find out.

Faced with the prospect of extensive and expensive seismic retrofitting of its downtown headquarters built in the 1960s, Los Angeles County decided to scrutinize alternatives. It’s a much newer building just a few blocks away. This could be because not only were they manufactured to stricter standards, but they were also available at a significant discount compared to pre-pandemic prices.

However, the possibility of the gas company’s relocation to the tower is far from official. And before officials put any agreement to paper, they want to assess how seismically safe the skyscraper actually is.

The scrutiny highlights a growing understanding of the structural weaknesses of many buildings across Los Angeles, leading to steady progress in regulations requiring the strengthening or demolition of potentially vulnerable structures.

Many of these rules, and much of the policymaking attention, have focused on older buildings and buildings made of materials known to be susceptible to shaking, such as unreinforced brick and non-ductile concrete.

However, gas company towers are different. It is relatively modern, completed in 1991 and made of steel.

However, as the 1994 Northridge earthquake demonstrated, common types of steel buildings can still be seriously damaged in stronger earthquakes. The magnitude 6.7 earthquake caused significant damage to 25 steel moment frame buildings, including nearly destroying the Automobile Club of Southern California building in Santa Clarita.

Even more worrying is the fact that the earthquakes of the past half century have not really tested the kind of shaking that can occur where California’s tallest buildings stand. Based on the U.S. Geological Survey’s definition of shaking intensity, neither downtown Los Angeles nor San Francisco has experienced “severe” shaking from an earthquake since the era of steel skyscrapers began in the 1970s.

Other cities in Los Angeles County (Torrance, Santa Monica, West Hollywood) typically require buildings such as gas company towers to be seismically evaluated and retrofitted if necessary. The city of Los Angeles is not like that.

But a Los Angeles County spokesperson said seismic engineers are conducting a “detailed assessment” of the gas utility tower, the fifth tallest in the downtown skyline, to determine if there are any vulnerabilities that need to be addressed. That’s what it means.

“These are exactly the issues we are investigating through our due diligence,” the county said in a statement. “Without pre-empting the work currently underway, one of the factors is to evaluate how this building will perform compared to the performance of the administration hall and the respective costs of each approach.”

The county has submitted a non-binding letter of interest for the 52-story, 749-foot-tall building, but the Board of Supervisors must approve the deal.

With little demand for office space downtown, the county plans to pay $215 million, a steep discount from its pre-pandemic valuation of more than $600 million. However, this price does not include any possible renovation costs.

The tower, at 555 West Fifth Street, was completed to stronger seismic standards than one of the buildings it would likely replace, the nearby Kenneth Hahn Administration Hall, completed in 1960.

The Executive Building is the headquarters of the nation’s most populous county and home to the five-member Board of Supervisors, its top elected officials. This expansive building is located in the Civic Center about three-fifths of a mile northeast of the skyscraper.

But like its decades-old neighborhood, the gas company’s tower may still need seismic upgrades.

This is one of many LA skyscrapers that incorporate steel moment frames as part of the structural system. Such a frame consists of horizontal beams and vertical columns and is characterized by an approximately rectangular skeleton. Steel moment frame buildings rely on connections between horizontal beams and vertical columns to maintain the building’s skeleton without damage during earthquake shaking.

(Steel moment frames are different from steel braced frames, which add diagonal braces to create triangles. Triangles are stiffer than rectangles and strengthen the overall skeleton. Steel moment frames are (It bends less during an earthquake than a frame.)

Steel moment frames were found to have potential problems after the Northridge earthquake, which killed at least 57 people. No buildings of this architectural style collapsed or caused any loss of life, but some were damaged so badly that they had to be demolished.

Deficiencies in steel moment buildings prior to Northridge-Tembler included problems with welding techniques and inspection, the filler metal used in welding, and the basic configuration of connections between vertical columns and horizontal beams. After the earthquake, construction methods were changed to solve these problems.

David Koch, former director of the Earthquake Engineering Research Institute and co-founder of the Gardena-based structural engineering firm Structural Focus, said that when it comes to steel moment frames, Northridge says, “I showed that I was receptive to it,” he said.

In the worst-case scenario, steel moment-frame buildings would collapse during a strong earthquake, something that has not happened in the United States.

It remains unclear how vulnerable the gas company’s towers are, and experts say more research is needed to find out. Generally speaking, concrete buildings that require renovation, such as administrative halls, are much more vulnerable than steel buildings that require seismic reinforcement.

The gas company tower doesn’t rely solely on its steel moment frame to withstand earthquakes, said Prabodh V. Banawalkar, the building’s structural engineer. Rather, it uses a dual-wall system that incorporates a brace core. This design reduces the stress on the connections between horizontal beams and vertical columns that are tested by earthquake shaking.

Banavakar said the U.S. Bank Tower and the Figueroa at Wilshire skyscraper (he was the structural engineer for both) also use dual-layer systems.

Steel buildings have suffered significant damage in the United States and abroad. In 1995, a magnitude 6.9 earthquake occurred in Kobe, Japan, causing a one-story steel building to collapse. A third of the 630 modern steel buildings in the violently shaken area suffered severe damage, according to a U.S. Geological Survey report.

Its 2008 report, titled “Shakeout Scenario,” quoted a structural engineer as saying that “the collapse of some pre-1994 welded steel moment-frame buildings is a credible scenario.” Therefore, five times the height of the iron moment frame may be reasonable. High-rise buildings in Southern California could collapse in a hypothetical magnitude 7.8 earthquake on the San Andreas Fault.

In such an earthquake, many buildings would be shaken for nearly two minutes, much longer than the seven to 15 seconds felt in 1994, according to the USGS.

The Northridge earthquake and the 1989 magnitude 6.9 Loma Prieta earthquake were scary enough for many people, but their epicenters were far away from the city centers of San Francisco and Los Angeles, and the shaking they brought to their downtowns was considered “strong” or “strong.” It was just very strong.

This type of movement, as defined by the modified Mercalli intensity scale, is more severe than the “severe” shaking felt in parts of Santa Cruz County in 1989 or the “severe” shaking felt in parts of the suburban San Fernando Valley. It is also calm and causes little damage. In 1994.

Non-reinforced brick buildings, non-ductile concrete buildings, and unretrofitted tilt-up buildings are considered much more dangerous than steel moment frame buildings, and in California these types of buildings are There are many examples of collapse. “But that doesn’t mean they (steel moment frame buildings) shouldn’t be evaluated or retrofitted,” Koch says.

If even one steel moment frame building collapses due to an earthquake, the consequences can be catastrophic. According to the USGS, 5,000 people could be trapped inside five such buildings that could completely collapse in the event of a powerful earthquake.

The question is whether gas company towers need seismic retrofits to cope with strong earthquakes. The fact that it has a braced core helps. Generally speaking, it provides stiffness and strength that helps buildings cope with side-to-side shaking, Koch said.

Since many changes have been made to seismic assumptions since design, an up-to-date seismic analysis of the building will be helpful. Scientists say the ground shaking from the current earthquake could be much more severe than previously thought, with buildings in downtown Los Angeles and San Francisco at risk of the kind of violent shaking we now know is possible. states that it has not been tested against.

“These older buildings were designed for much lower (earthquake) loads in the ’80s,” said Daniel Zepeda, a structural engineer with Degenkolb Engineers.

Other advances since skyscrapers were built include the discovery of new earthquake faults.

Scientists discovered the Puente Hills thrust system in 1999. This fault system underlies a belt of downtown Los Angeles, southeastern Los Angeles County, the San Gabriel Valley, and northern Orange County. This fault system can produce earthquakes of magnitude 7.5, and one simulation estimates that such an earthquake could kill between 3,000 and 18,000 people.

It’s unclear how much it would cost if the skyscraper needed to be retrofitted, but the gas company’s tower has a dual-frame system that incorporates a braced core, so it could end up with only a steel moment frame. Koch said it could be cheaper. .

While some applaud the county’s move to buy the skyscraper at a “fire sale” price and argue that such a sale would revitalize downtown, most members of the Board of Supervisors He has remained silent about the proposal negotiated by the chief. Secretariat.

Supervisor Janice Hahn was the only exception, arguing that the county didn’t need to move away from Los Angeles’ Civic Center, where many of its government offices are located.

The Management Hall of Fame is named after Hahn’s father, the late Kenneth Hahn, who retired after serving a record 10 terms as superintendent. He represented South Los Angeles from 1952 to 1992, and died in 1997 at the age of 77.

The administrator said the county has some steel buildings but has not compiled a comprehensive list like non-ductile concrete buildings. Officials say there are no immediate plans to make steel buildings more earthquake resistant as the focus is on retrofitting high-risk structures.

Relocating the county office to a gas company tower will not solve the problem of earthquake-vulnerable non-ductile concrete buildings in the county, and will require the building to be renovated, demolished, sold, or left vacant. Questions remain as to whether it should be kept. The Board of Supervisors voted last year to ask officials to create new rules requiring renovations of county-owned non-ductile concrete structures and buildings in unincorporated areas.

Although the Government Building is the most famous, the county also owns other non-ductile concrete facilities. One of the most important is the coroner’s office, where employees perform autopsies. Headquarters of the Department of Public Health and Health Services.

The collapse of just one building, even if unoccupied, can injure or kill nearby people and motorists, and potentially damage nearby buildings, such as the Civic Center, for years. It may also have a negative impact.

The Los Angeles Times in 2024. Distributed by Tribune Content Agency, LLC.