Saving lives and businesses: how do you prepare for a massive earthquake?
A massive earthquake is due to hit San Francisco.
If it does hit – and the U.S. Geological Survey estimates there is a 72% chance of a 6.7 magnitude or greater earthquake hitting before 2043 – then tall buildings will be particularly vulnerable. Clearly this puts the safety of the city’s residents, workers, visitors and tourists at risk.
Two towers in the city have construction problems already, even without a seismic event. I wrote about the Millennium Tower last year – The 21st century Leaning Tower of Pisa. This 58-storey luxury tower is still habitable, but it is sinking and tilting, and no solution has yet been found. And the nearby $2.2 billion Salesforce Transit Center was closed in September 2018 after cracks were discovered in two steel supporting beams.
The structural challenges with these buildings, plus the ongoing earthquake risk, has led to a panel of engineers urging the city that building codes need to change. The City of San Francisco’s Tall Buildings Strategy, published towards the end of 2018, suggests that current building codes are inadequate in the face of a large earthquake.
The report calls for existing buildings to be inspected and retrofitted, and for stronger regulations for all new buildings.
Seismic building codes were first created when most of California’s population lived in rural areas with low-lying buildings. As the technology industry has boomed and high-rise construction has increased, San Francisco is now more vulnerable.
On top of this, current building codes are designed to ensure that people survive the earthquake. This will still be the main priority, but there is also a need to make sure that buildings can be used afterwards too – both homes and workplaces.
The report explains that buildings constructed in line with current codes could take two to six months to repair after an earthquake. In a city that is packed full of tech businesses which need an uninterrupted connection to the outside world, a large earthquake could see companies leaving the city if they are not able to get back to work quickly.
Two of the key recommendations of the report are that buildings should be more rigid, and that infrastructure like plumbing, water supply and electricity need to be built to higher standards. One of the report authors – Professor Gregory G. Deierlein – said that the city should consider regulations that ensure buildings are twice as stiff.
Currently, a 300-foot building can sway six feet from side to side in a severe earthquake. This part of the code came from an old design philosophy – the theory being that flexibility allows for dissipation of earthquake energy. But this flex can destroy interior walls and permanently damage elements of the building such as lift shafts.
The San Francisco building code could be adjusted to incorporate these recommendations when the code is next revised in September 2019. There are multiple considerations needed when it comes to the structure of buildings and the priorities of building regulations. These change over time, so staying up to date is vital.
If you need any structural engineering expertise on your next project, please let me know.