San Francisco Takes Unprecedented Step to Protect High Rises During Earthquakes

by | October 5, 2018

By Rong-Gong Lin II

For all the tech wealth that has flown into this city in recent years, it's also an unnerving time for city builders.

First, a centerpiece of San Francisco's new downtown, the 58-story Millennium Tower, started sinking and tilting. Then last week, the city's new $2.26 billion transbay bus station was abruptly shut after cracks were found in two steel beams. Both events have fueled anxiety about how San Francisco's growing skyline would hold up in the kind of major quake that destroyed much of the city in 1906.

So on Thursday, the city opened up a new front on California's seismic safety, releasing an unprecedented list of more than 150 of its tallest buildings, which includes data about buildings constructed before modern seismic codes _ a prelude to an assessment of whether they need retrofitting to better withstand an epic temblor.

The inventory comes with a sweeping proposal to more closely examine the city's skyline with an eye of making costly fixes to towers that could collapse amid major shaking. Officials are also calling for rules for new construction, aimed at preventing sinking of buildings as well as making tall buildings even stronger to resist wobbling in an earthquake.

While San Francisco, Los Angeles and other communities have stepped up retrofitting efforts in recent years, none of California's largest cities has gone as far in publicly releasing an inventory of landmark buildings downtown that likely need seismic reviews.

Out of San Francisco's 156 tallest buildings _ those taller than 240 feet _ roughly 100 were built before the era of modern seismic codes for their building type, according to the study by the nonprofit Applied Technology Council.

That doesn't mean all of them are at risk of collapse. Further study would be needed to determine if they are vulnerable and need a retrofit.

The list is a starting point. And the experts who compiled the list for City Hall say officials should take action to get more of these buildings structurally evaluated, and have the ones most prone to collapse retrofitted.

"We must act before the next earthquake strikes to ensure the safety of residents, workers and visitors," said San Francisco city administrator Naomi Kelly.

The tech boom has brought major development to a relatively small section of the city, which is now being crowded with new towers including San Francisco's tallest, the Salesforce Tower. One thing that has been missed in all the focus on new construction is how older buildings in the densely packed downtown area _ built before modern seismic codes _ will fare.

As the collapse of the World Trade Center in New York showed, even the collapse of just one or two could paralyze the city for many blocks for years.

The city has not been tested by a truly cataclysmic earthquake for 112 years. Downtown largely survived the 1989 Loma Prieta quake without major catastrophe. But the 1994 Northridge quake exposed weaknesses in a type of steel high-rise construction used in many California skyscrapers built from the 1970s through the early 1990s.

The recommendation will be controversial. It can be costly to even just evaluate a tall building, and can take millions of dollars to retrofit it. San Francisco is currently requiring owners of about 5,000 wood-frame apartments to be retrofitted, but those buildings are much shorter, and the costs are far smaller, compared to fixing an office tower or high-rise residential building. Kelly, the city administrator, signaled that she backed more than a dozen suggestions she received from the experts to improve safety of tall buildings.

The rhetoric coming out of San Francisco City Hall represents a shift from just a few months ago, when officials emphasized the difficulty in identifying the city's largest vulnerable buildings, such as those made of brittle concrete.

Other cities are now renewing talk of seismic safety. After years of delay, Oakland City Hall has resumed talking about a mandatory retrofit law for nearly 2,000 possibly vulnerable wood-frame apartment buildings at risk of collapse in a seismic event.

The moves come following the publication of a landmark U.S. Geological Survey report in April about the threat of the Hayward fault to the San Francisco Bay Area. The report said 800 people could be killed and 18,000 more injured in a hypothetical magnitude 7 earthquake centered below Oakland. Oct. 21 will mark the 150th anniversary of the last big earthquake on the Hayward fault.

As part of preparations for that anniversary, some seismic experts are urging the public to ask about the earthquake safety of their homes, workplaces, schools and churches.

"Those questions should be more in-depth than just, 'Will this building survive? And will it be damaged to the extent it's going to injure me?' " Keith Knudsen, deputy director of the USGS Earthquake Science Center in Menlo Park, said. "They should follow up with questions (such as) 'Will this building be usable following an earthquake?' "

The city ordered the study of tall buildings amid concern about the safety of Millennium Tower, the skyscraper that has become famous for its sinking and leaning problem. Engineering experts hired by the city say that the earthquake-resisting system of Millennium has not been compromised by the leaning of the building.

The list that San Francisco unveiled Thursday includes details such as the building's structural system, the year it was completed, and a retrofit date, if one was found by the authors of the study.

The report found more than 10 tall buildings that could possibly have a brittle concrete frame. Also known as non-ductile concrete frame, this class of buildings was found to be vulnerable to collapse after the 1971 Sylmar earthquake in the Los Angeles area; 49 died in the collapse of concrete buildings at the Veterans Administration Hospital in San Fernando, and three people died at Olive View Medical Center in Sylmar.

Researchers also found more than 80 steel-framed buildings built before codes were rewritten after the 1994 Northridge earthquake. Engineers discovered that this class of structure could develop troubling cracks in its skeleton. No steel building suffered a catastrophic failure in that quake, but some were so badly damaged they had to be demolished. One _ the Automobile Club of Southern California building in Santa Clarita, open for just 21 months _ came very close to collapse.

Some experts consider the collapse of a steel frame building plausible in an earthquake. The collapse of five high-rise steel buildings in Southern California was seen as possible during a hypothetical magnitude 7.8 earthquake on the San Andreas fault, according to the U.S. Geological Survey's simulation of such a disaster. If the earthquake hit during working hours, about 5,000 people could be inside those five buildings.

In 1995, when a magnitude 6.9 earthquake hit Kobe, Japan, one-third of 630 modern steel buildings in a heavily shaken area were severely damaged, according to a USGS report. One photographed building showed how one story of a steel building collapsed.

In general, concrete buildings built before 1980, and steel buildings constructed prior to 1997, are thought to deserve scrutiny to assess for seismic deficiencies and determine whether an earthquake retrofit is needed.

Just six buildings on the list were found to have undergone modern seismic retrofits, including San Francisco City Hall and the Ferry Building. The most recent retrofit identified by study authors was done in 2015, which strengthened the 25-story Hotel Nikko at 222 Mason St., a steel moment frame building.

Few California cities have required steel buildings to be retrofitted. Santa Monica and West Hollywood recently passed laws requiring vulnerable steel buildings in their cities to be retrofitted. Los Angeles has no such law, although earlier this year, Mayor Eric Garcetti said the city should consider mandatory retrofits to steel-frame buildings.

The San Francisco study authors said it's possible the database is incomplete, and asked those with knowledge of retrofits not mentioned in the database to come forward with their information.

Existing San Francisco laws have generally proved ineffective in requiring tall buildings to be seismically evaluated or retrofitted. Current city law does require a seismic retrofit when two-thirds of a building's floors are renovated. But that almost never happens to tall buildings, when only a small fraction of the building undergoes construction at any given time.

The report suggests finding a way to trigger mandatory seismic evaluations more often during major renovations, but do so in a way that won't discourage developers from modernizing buildings. One possible solution, the authors said, is not to require retrofits except for buildings most likely to collapse.

Other possibilities include triggering seismic evaluations when buildings are purchased or leases are renewed, according to the report. Such changes in city law would require support from the Board of Supervisors and Mayor London Breed.

There are other recommendations in the report. One is to order inspections and, if needed, repairs of steel buildings shaken in the 1989 Loma Prieta earthquake. Among other recommendations:

_ Create rules for new buildings formalizing criteria intended to prevent against settling or sinking after construction;

_ Aim to make new buildings stronger and less wobbly so they can resume operations sooner after an earthquake instead of being shut down for two to six months;

_ Recommend minimum levels of earthquake insurance or another type of collateral to ensure buildings and surrounding neighborhoods can recover;

_ Study whether automatic fire suppression and evacuation systems in tall buildings can function after shaking;

_ Expand the database to include any building taller than 75 feet; and,

_ Create protocols to determine how large a cordon must be around a building at risk of collapse after an earthquake.

Stanford University structural and earthquake engineering professor Greg Deierlein, a co-author, said the report is not meant to be alarming, but "you gotta start somewhere."

Deierlein said he was glad to see city officials were enthusiastic about his recommendations. But now comes the hard part.

"The city needs to take these recommendations _ and see if it's politically viable," Deierlein said.

(c)2018 Los Angeles Times