The convergence of an earthquake and a pandemic shuttered the gorgeous, historic Salt Lake City & County Building in March, 2020. A handful of broken windows during BLM protests in June contributed to its turn inward.
Mayor Mendenhall has no plans to open the building to the public until Covid transmission levels are at the Low level. Most employees who normally populate City Hall have been encouraged to work from home.
Yet the City & County Building survived the area’s biggest earthquake since 1934 with rave reviews.
It’s an occasion to celebrate. Thanks to its original quality and major seismic retrofitting from the 1990s, the seat of Salt Lake City municipal government is still intact after the 5.7 Magna earthquake of March 18, 2020.
The Utah State Capitol building copied and improved on the city’s seismic retrofit in a renovation a decade later, completed in 2008. Its stately columns and dome, too, had no problem dealing with the 2020 Magna temblor.
The Utah State Capitol reopens to the public today. Due to threats of right-wing civil unrest – not Covid nor earthquake repairs – the Capitol building was closed to the public for the first week of the Legislative session in 2021, which ends March 5.
The City & County Building
The grand dame of public architecture in Downtown Salt Lake City, the City & County Building (completed 1894) was the first structure in the world to be retrofitted with base-isolation technology to safeguard from earthquake damage.
Part of a comprehensive $30M restoration project completed in 1999, the historic building owes its survival to resourceful and forward-thinking engineers, preservationists, and city political leaders.
By the 1990s, base-isolation technology had been used in new construction projects, but never to underlie an existing structure. It’s proven to be an extremely cost-effective way to not only avoid the collapse of an historic structure, but keep it intact. The isolators allow the building to move horizontally during a seismic event – isolating the earthquake’s energy away from the building.
Since that city-defining moment in the mid-1990s, base-isolation technology has been incorporated into several other high-profile historical preservation projects in Utah, including the State Capitol building (completed in 2008) and, currently under construction, the LDS Church’s Salt Lake City Temple.
What happened to the building on March 18, 2020?
According to city engineers, the City & County building performed wonderfully in the March 2020 Magna earthquake.
Jerod Johnson, whose team at Reaveley Engineering led the seismic retrofit projects at City Hall and the State Capitol, told us “It could have suffered severe damage. The outcome would have been much worse without the isolation system to stop the vibrations from getting up into the building.”
City officials say the only damage suffered was some fallen plaster on upper floors and cracks opening between the structure and the east and west stairs.
Which is exactly what was supposed to happen. The building moved, all together, horizontally with the seismic waves on top of its 443 base-isolator 18-inch platforms. The stress on the structure, including its 256 ft-high clock tower, was minimal.
When asked what he learned from the quake, he replied, “It works, therefore you rarely hear about it.”
If given the chance to change the technology underlying City Hall, Johnson says he would have installed larger isolators, as they did a decade later at the State Capitol, to be able to handle a larger seismic event.
To mitigate the potential damage of larger earthquakes on the City & County Building, Johnson’s team was charged with improving the system in 2015. Instead of replacing the 443 18-inch isolators, which would have run into the tens of millions of dollars, they supplemented the existing system in case of the big one – currently estimated to be 7.3 for the Salt Lake Valley.
Engineers designed a new set of piers and columns, so that the building has something to land on in case it is displaced off its isolators during a major event. Finished in 2018, the backup system was not needed during the Magna quake.