SAN FRANCISCO — Earthquakes are part of the California bargain, a risk that residents accept in exchange for the state’s natural beauty, sunshine and plentiful jobs. This unspoken seismic trade-off came into sharp focus last month when, for the first time, a government earthquake study included a list of potentially vulnerable high-rise buildings in San Francisco. The list includes some of the city’s most prominent hotels and office buildings.
Engineers have known about a major defect in certain steel-frame buildings since 1994, when shaking from the Northridge earthquake in Los Angeles fractured critical joints in more than 60 buildings, bringing at least one very close to collapse. The building code was rewritten to eliminate the flawed technique.
Yet nearly a quarter of a century later, California is still wrestling with what to do with the hundreds of buildings, many of them high rises, that were constructed during the more than three decades when the defective connection system was widely employed. Last year Santa Monica, which has identified more than 70 steel-frame buildings believed to have the problem, became the first city in California to order mandatory retrofits. A similar law in nearby West Hollywood comes into effect in August.
“This is an issue that structural engineers should have been dealing with continuously since the mid-1990s and we just dropped it,” said Keith Porter, an earthquake engineering expert who helped lead the United States Geological Survey study that published the list of San Francisco high rises. “We don’t know how to deal with a problem this big.”
Experts consider these buildings vulnerable to collapse only in extreme shaking caused by rare and powerful earthquakes, similar to the one that struck San Francisco in 1906.
The list, buried among the seismic calculations of an appendix in the U.S.G.S. report, includes around 40 steel-frame high rises clustered in downtown San Francisco and built between 1960 and 1994, the approximate years when the flawed technique was employed. There are more than 200 high rises in the city.
The steel buildings constructed in the years when the technique was used house technology and finance companies, law firms, restaurants, shopping arcades, at least three major hotels, the headquarters of Pacific Gas & Electric, the San Francisco office of Senator Dianne Feinstein, and the former Bank of America building, one of the city’s tallest — and which is partly owned by President Trump.
One cluster of pre-1994 steel-frame buildings known as the Embarcadero Center “welcomes more than 16,000 people each day,” according to its website. A 43-story building known as Salesforce West is on the list and is directly across from Salesforce Tower, the tallest office building west of the Mississippi River. Salesforce Tower was recently completed and is not a steel-frame building.
One address on the list jumped off the page for me: The building that houses the San Francisco bureau of The New York Times.
A number of building owners responded to emailed queries about these buildings. The Hilton hotel near Union Square said it did a retrofit in 2012. And a representative for Columbia Property Trust, the owners of the building where The Times bureau is located, sent a statement that said, “While no seismic retrofits have been made to the building, we believe that the building’s structure compares quite favorably when compared to many other similar buildings constructed before 1994 due to expert assessments that are ongoing.”
Most of us are too busy to think about what lurks behind the walls of our offices or the seemingly far-off possibility that the Big One, generally thought of in California as around an 8-magnitude earthquake, might strike. Researching these buildings took me down a road of uncomfortable questions about risk and probabilities.
So how dangerous are these buildings?
Known in technical lingo as welded steel moment-frame buildings, the columns and beams that make up the skeleton of the building are welded together, an innovation that was adopted to save time and money and a departure from the bolts and rivets used in previous generations of steel buildings. Those constructed before around 1985 were often very flexible — allowing them to sway extensively from side to side. The building code was revised in the mid-1980s to require more stiffness, and again a decade later to correct the defective welding technique.
To live in California is to be forced to draw lines between acceptable and unacceptable seismic risks. And for policymakers, this class of steel-frame building seems to fall somewhere in between.
There are several types of buildings that engineers have identified as vulnerable to even moderate earthquakes, notably wood-frame “soft-story” structures and “non-ductile concrete” buildings, which were typically constructed before the mid-1970s.
Steel-frame buildings, by contrast, are not considered vulnerable to moderate earthquakes.
A 6.9-magnitude earthquake in Kobe, Japan, a year after Northridge caused the collapse of 90 older steel-frame buildings and cracks in newer ones. A prominent Japanese engineer at the time said the Kobe earthquake was a turning point in public perception — it “refuted the popular myth that steel buildings are immune to strong earthquakes,” he said.
“When these fractures occurred it was a very humbling experience — first shocking and then humbling — for the structural and earthquake engineering professions,” said Gregory Deierlein, a Stanford University earthquake engineering expert who is a seismic adviser to the city of San Francisco. “It starkly brought into question one of the fundamental assumptions and building techniques that they had been using.”
The building code today mandates a probability of collapse of no more than 10 percent in extreme shaking similar to what San Francisco experienced in 1906.
Dr. Deierlein was a co-author of a study in 2015 that found that the risk of potential collapse for pre-1994 steel-frame buildings could be five times as much: roughly 50 percent instead of 10 percent. Kenny Buyco, an engineer at the California Institute of Technology, simulated severe ground motions in Los Angeles and found a 20 to 35 percent chance of collapse for a 20-story steel-frame building.
There are many caveats to all the studies. Computer modeling is far from perfect and each building is unique — buildings with tall ground floors or irregular shapes can be even more vulnerable.
San Francisco, which is straddled by the San Andreas and Hayward faults, has spent billions of dollars strengthening government buildings and other structures and has a program of mandatory retrofits for soft-story buildings. The city is still in the assessment phase for pre-1994 steel-frame buildings, according to Naomi Kelly, the San Francisco city administrator.
“It’s not like we are waiting,” Ms. Kelly said. “There are only so many engineers in this city. There’s only so much money.”
Retrofitting a single building can cost tens of millions of dollars and few building owners in California have done it, engineers say. There appears to be relatively little pressure to do anything about the problem because for years it remained a somewhat obscure, technical flaw discussed among engineers.
“I don’t think there’s really any awareness in the general population,” said David Mar, a structural engineer who has retrofitted around half a dozen steel-frame buildings in the San Francisco Bay Area. “Folks need to know — and then you can make decisions about retrofitting or not. More knowledge is better for everybody.”
A bill that passed an initial vote in the California Legislature last month would make it mandatory for cities to produce an inventory of seismically vulnerable buildings, including steel-frame buildings with the construction flaw.
The problem is not confined to high rises; engineers estimate that there are hundreds of defective steel-frame buildings of all sizes in California.
Partly because of the cost, Santa Monica is giving owners of steel-frame buildings 20 years to carry out retrofits. Most of the steel buildings identified as vulnerable are office buildings, but the city also has a number of large steel-frame condominium buildings.
A number of experts described tension between engineers who wanted to raise greater awareness about the vulnerability of the pre-1994 steel-frame buildings and Ron Hamburger, one of the country’s leading seismic engineers, who helped lead federal investigations after Northridge and has been described as a kind of skeptic in chief about the buildings’ potential for collapse.
Mr. Hamburger recently said he had become more convinced of their danger.
“If we have a repeat of 1906 it is probable that some, not all, of the high-rise buildings in downtown San Francisco will experience collapse,” he said.
Mr. Hamburger travels extensively, but on this particular day he was in his office in San Francisco. I mentioned that I was calling him from the 12th floor of a steel-frame building, the likes of which we were discussing.
He paused for a beat.
“I’m sitting in one of them, too.”
On the List
In April, the United States Geological Survey published a report with a list of high rises in downtown San Francisco that included 39 steel-frame buildings constructed between 1960 and 1994, the approximate years when a flawed welding technique was employed. The list was compiled with help from the Structural Engineers Association of Northern California.
1. Hartford Building, 650 California
2. Beal Bank Building, 180 Sansome
3. Bechtel Building, 50 Beale
4. 44 Montgomery
5. 425 California Street
6. 555 California Street
7. McKesson Plaza, One Post
8. Pacific Gas & Electric Building, 77 Beale
9. One Embarcadero Center, 355 Clay
10. Transamerica Pyramid, 600 Montgomery
11. 100 Pine Center, 100 Pine
12. 211 Main Street
13. First Market Tower, 525 Market
14. 425 Market Street
15. Two Embarcadero Center
16. 221 Main Street
17. California Automobile Association Building, 100 Van Ness
18. Chevron Tower (Market Center)
19. Spear Tower (One Market Plaza)
20. Steuart Tower (One Market Plaza)
21. Three Embarcadero Center, 155 Clay
22. Shaklee Terraces, 444 Market
23. 333 Market Street
24. 595 Market Street
25. 201 California Street
26. Two Transamerica Center, 505 Sansome
27. 101 California Street
28. Telesis Tower, One Montgomery
29. 1 Ecker Square, 1 Ecker
30. 100 Spear Street
31. 101 Montgomery
32. Citicorp Center, One Sansome
33. 50 Fremont Center
34. 333 Bush Street
35. 345 California Street
36. 301 Howard Street
37. Hilton San Francisco Hotel, 333 O’Farrell
38. San Francisco Marriott, 55 4th
39. Embarcadero West, 275 Battery