<img class="styles__noscript__2rw2y" src="https://dsx.weather.com/util/image/w/sanfrantallbuildings.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com/util/image/w/sanfrantallbuildings.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com/util/image/w/sanfrantallbuildings.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > Downtown San Francisco (Public Domain) (Public Domain)

At a Glance A new report commissioned by the city of San Francisco is calling for stricter seismic safety building codes.

The Tall Buildings Safety Strategy suggests inspection and retrofitting of existing buildings and stronger regulations for new ones that tower 240 feet and higher.

Of the 156 buildings, nearly 100 were constructed before the era of modern seismic codes were enforced for their respective building types.

The city of San Francisco has called attention to its building codes, which a group of experts says are inadequate to handle the aftermath of a large earthquake in a recent report commissioned by the city's former mayor. The Tall Buildings Safety Strategy calls for both the inspection and retrofitting of existing buildings and stronger regulations for new ones .

The report addressing the seismic vulnerability of the city's tall buildings comes in the wake of officials' struggle to find answers in the construction flaws of two of San Francisco's premier new structures: The Millennium Tower and the Salesforce Transit Center.

The former, a 58-story skyscraper, has sunk about 17 inches and tilted 14 inches to the west and six inches to the north since its creation in 2009. The Millennium Tower has had reports of windows cracking as recently as September. The latter, the Salesforce Transit Center, is a $2 billion transbay bus center built in 2016 that was closed in late September after cracks were found in massive support beams.



The Tall Buildings Safety Strategy, which classifies tall buildings as those that exceed 240 feet, suggests that buildings should be built to be more rigid and that important infrastructure - such as electricity, plumbing and water supply - be constructed to meet higher standards. In addition to the report, the city created a collection of the 156 buildings that exceed the set height and categorizes details, such as the height, date built, number of stories, structure material and type of occupancy.

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com/util/image/w/tallbuildingsf.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com/util/image/w/tallbuildingsf.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com/util/image/w/tallbuildingsf.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > The tall buildings (240 feet and higher) that span downtown San Francisco. (Applied Technology Council) (Applied Technology Council)

(MORE: Central California Residents May Hear Booms, See Bright Lights on Sunday; Air Force Says Don't Worry )

Of those 156 buildings, nearly 100 were constructed before the era of modern seismic codes for their respective building types. Just six of have undergone modern seismic retrofits.

“What you are seeing here is the city’s recognition that it cannot protect their citizens from the biggest earthquake without dealing with these issues,” Lucy Jones, a prominent California earthquake specialist who's been urging stronger building codes, told the New York Times.

San Francisco, once a low-rise city, has seen a rapid growth in high-rise construction with the influx of money from the technology industry. The city's current philosophy behind seismic building codes was set when most of the state's population lived in low-lying structures in rural areas. The codes are focused on protecting lives and not on a building's structural integrity following an earthquake, which becomes an issue with tech companies that require a constant connection to the outside world.

The Tall Building Safety Strategy calculates that buildings that meet the current code could take two to six months to repair. It's feared that this could cause tech companies, which can't afford that downtime, to pick up and leave.

“The building code is usually looking to make sure you survive the earthquake,” said Naomi Kelly, the San Francisco city administrator. “We want to go a step further — and make sure you can go back into the building.”

Both the San Andreas and Hayward faults run closely by the San Francisco region - and both of them are capable of producing very powerful earthquakes. The infamous San Andreas fault has a possible maximum earthquake magnitude listed as an 8.0 in a US Geological Survey report.

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com/util/image/w/haywardsanandreas.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com/util/image/w/haywardsanandreas.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com/util/image/w/haywardsanandreas.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > The earthquake faults, notably the San Andreas and Hayward, that surround the San Francisco area. (American Geosciences Institute) (American Geosciences Institute)

When the USGS ran a simulation of a 7.8 earthquake along the San Andreas fault, five steel-framed high rises completely collapsed in Southern California. If a quake was to hit during work hours, some 5,000 people could be in those buildings.

Engineers have known about a major flaw in certain steel-frame buildings in San Francisco since 1994 when Los Angeles' Northridge earthquake caused a fracture to joints in more than 60 buildings. At least one came close to collapsing , the New York Times reported in a separate article.

“There’s never been a systematic reassessment of those buildings,” said study co-author and Stanford University professor Gregory G. Deierlein. Deierlein added that some buildings could have undergone structural damage when the Loma Prieta earthquake struck in 1989, but they've yet to be inspected.

“There is no doubt that those older steel frame buildings are far more likely to collapse than their designers anticipated,” said Keith Porter, a structural engineering professor at the University of Colorado, Boulder. “A collapse of one of these buildings could not only potentially kill many of its own occupants, but also people in nearby buildings.

“This is a problem we have known about for decades,” said Porter.

(MORE: Michigan Man's 1930s Doorstop Turns Out to Be 22-Pound Meteorite Worth $100,000 )

Porter helped lead a study conducted by the USGS that lists some 40 steel-frame and potentially vulnerable high rises sprinkled throughout downtown San Francisco and built between 1960 and 1994."Even the most collapse-prone tall buildings almost never receive the scrutiny intended by the code," the report reads.

Of the most notable suggestions made by the report, San Francisco has required buildings to reduce the amount of sway in them, becoming significantly stiffer. Right now, a building reaching 300 feet skyward is allowed to sway a maximum of six feet during a powerful earthquake. While the idea is to allow sway to let the energy from the earthquake dissipate, the flexibility could severely damage interior walls and elevator systems.

San Francisco should implement regulations making buildings twice as rigid, Deierlein said.

Though no legislation has been formed, recommendations from the report will be presented to the city's board of supervisors in the coming weeks. The report's recommendations could be put into effect as soon as September 2019, when the city's building code undergoes its next revision.