Inspections urged for some high-rises
Welds may have led to undetected damage in ’89
Dozens of downtown San Francisco high-rises should be inspected for previously undetected damage they might have sustained during the Loma Prieta earthquake nearly 30 years ago. That’s the conclusion of an unprecedented report on the city’s tallest buildings to be released Thursday.
Beyond their height, the buildings singled out for scrutiny share troubling traits that could leave them especially vulnerable in a major quake.
Between 50 and 65 San Francisco high-rises used specific types of steel welds that were later found to fracture during the 1994 Northridge earthquake in Los Angeles. That weakness was unknown when San Francisco buildings were inspected after Loma Prieta.
The report, compiled by experts hired by the city and called the Tall Buildings Safety Strategy, presents a detailed inventory of the city’s tallest towers and could have a major impact on how the city thinks about making the giants of the skyline more resilient to a major earthquake.
In the midst of a development boom, one seeing increasing numbers of people living in high-rises, it’s become increasingly important to understand how the city’s tallest structures will behave when earthquakes strike and how officials can organize their recovery efforts.
“One thing the city tries to do very well is to be as seismically sound as we can be,” said Supervisor Aaron Peskin, whose district includes a large number of high-rises. The city has, for example, embarked on a mandatory retrofit program for certain buildings seen as susceptible to quake damage and initiatives to get unreinforced masonry buildings shored up.
“To be ignoring this stock of highly concentrated office and increasingly residential developing in our downtown is folly,” Peskin said.
The U.S. Geological Survey estimates that there’s a 72 percent chance of a 6.7 magnitude or greater quake hitting San Francisco before 2043. Loma Prieta was a magnitude 6.9; Northridge was a magnitude 6.7.
According to the report, inspections that took place in San Francisco after the Loma Prieta quake, which was five years before Northridge, might not have detected similar damage. It wasn’t until after the Northridge quake that inspectors knew to look for damage on so-called steel “moment frames.”
“Five years earlier, without the benefit of the lessons later learned in Northridge, San Francisco’s steel buildings were not systematically inspected for weld damage, which they might or might not have sustained in the 1989 Loma Prieta earthquake,” the report says.
San Francisco’s northeastern neighborhoods are home to nearly all of the more than 150 buildings tall enough to be officially distinguished as “high-rises” — at least 240 feet high, or around 22 stories. In all, 156 buildings in San Francisco — either permitted or already built — meet the height requirements.
In all, 56 percent of high-rises in San Francisco are office buildings, 20 percent are residential and the rest are a smattering of hotels and mixed-use spaces.
The report’s authors — experts from the Applied Technology Council, a nonprofit California research organization — recommended conducting intensive inspections necessary to examine the welds on the steel frames in question. Because the potential damage could have occurred decades ago, the report also suggested creating a “special program” that would notify and provide guidance for building owners and tenants.
San Francisco’s building codes trigger seismic upgrades when construction alterations “exceed a certain scope,” the report said. But those sections of the code only apply when two-thirds of a building’s floors are involved in any given project — they almost never apply to tall buildings.
“Therefore, even the most collapse-prone buildings almost never receive the scrutiny intended by the code,” the report states.
Those recommendations are among a number of suggested short-, medium- and long-term actions offered for both new and existing buildings. Some tasks, like creating new training programs for building inspectors and developing bulletins detailing best practices for developers, could be done largely administratively. Others would require more intensive work by city lawmakers.
The report also recommends that the Department of Building Inspection establish design standards for new buildings that include plans for how a building might recover and become habitable again after an earthquake. Current requirements are intended to “provide acceptable safety in extreme earthquakes,” the report said. Under current standards, it could take between two and six months to repair damage from a major earthquake.
“By the city’s tentative recovery goals, even three months of downtime is unacceptably long” for the city and its buildings to begin returning to normal. The report’s authors recommended mandating that new buildings include designs that reflect the city’s earthquake recovery goals, like requiring enhanced design criteria for mechanical and plumbing systems.
A spokesman at the Department of Building Inspection did not respond to a request for comment Wednesday on the report.
With the new report, “we’re able to see and think about both how (a tall building) might perform, but also what we need to get in place so we can move toward recovery as quickly as possible,” said Mary Ellen Carroll, executive director of the Department of Emergency Management.
A major part of recovery following a punishing earthquake is to assess and report the extent of the damage to federal officials — a task, Carroll said, that will be made easier by the new building inventory.
“We know what buildings are in the city through DBI,” she said. “But this is a more organized inventory that will have more details we can pull out post-emergency and have a sense of just how to coordinate the assessment response.”
The report was commissioned by former Mayor Ed Lee after revelations that the Millennium Tower on Mission Street was sinking and tilting to one side. The building’s troubles evoked unsettling fears about what could happen to it and similar structures in a major earthquake. The city had earlier considered compiling a similar report around 2007, but it was eventually scuttled during the Gavin Newsom administration.
“People didn’t want to know the answer, because it might be expensive,” Peskin said.
The report also comes after Gov. Jerry Brown’s decision Friday to veto AB2681, a bill that would have required officials in seismically active regions to submit lists of vulnerable buildings to the state’s Office of Emergency Services.
While he said he agreed with the bill’s goal of mitigating the effects of a large earthquake, Brown was “concerned ... that this bill will not provide the greatest value for the significant investment this enterprise requires.”