- For decades, engineers have been using a technology called base isolation that can protect buildings from earthquakes.
- The system isn’t common in the US, but it has been installed in the city halls of Los Angeles, San Francisco, and Pasadena, as well as Apple’s new headquarters in the Silicon Valley.
- While many structures in the US are built directly on the ground, base-isolated structures sit on bearings that help absorb the shock of an earthquake.
- The system could help mitigate a major disaster, but property owners must be willing to pay an additional upfront cost.
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Fourth of July weekend ushered in two powerful earthquakes in Southern California. The first, a 6.4-magnitude quake, produced gas leaks and power outages in Ridgecrest, a small city near the Mojave Desert. The second, a 7.1 magnitude quake, rocked the same area on July 5, causing fireplaces to collapse and buildings to shift from their foundations.
At around 4:30 a.m. on Jan. 17, 1994, residents of the Northridge Meadows apartment complex saw their windows shatter, their walls crumble, and the ground below them begin to crack. From there, the building’s upper levels collapsed onto the first floor, killing 16 people inside.
Twenty five years later, the complex represents the largest number of concentrated deaths from the Northridge quake, one of the costliest natural disasters the world has ever seen. It also symbolises the dangers of conventional construction elements such as wood frames and concrete cores, which can be particularly vulnerable during a quake.
In the years since the 6.7-magnitude earthquake, nations like Japan, China, Italy, and Turkey have turned to a sturdier method of construction, known as base isolation, to save their buildings from collapse.
Many buildings in the US currently sit on fixed bases, which means they’re built directly on the ground. When an earthquake hits and the ground begins to shake, the building will sway along with it. A heavy quake can cause vital parts of the building’s infrastructure, such as walls and columns, to topple, and the whole building to eventually come tumbling down.
A base isolation system, on the other hand, places bearings between the floor of the building and the foundation. These bearings, known as “base isolators,” help absorb the shock of an earthquake and minimise the impact of shaking on the building. The technology is similar to a car’s suspension system, which prevents you from getting jostled when you drive over a bumpy road.
Take a look at the difference:
Base isolation isn’t ideal for every location, said Ronald Hamburger, a senior principal at the engineering firm Simpson Gumpertz & Heger.
Hamburger said the technology works best on stiffer soil and becomes less effective in buildings located extremely close (about a few kilometers away) to a large fault, since the system can become overwhelmed by the “large, impulsive motions” of earthquakes. In these cases, he said, buildings with concrete walls can offer better protection.
But base isolation has long been considered a superior form of construction when it comes to earthquake resistance.
“Believe it or not, the concept of base isolation can be dated back to the 1920s,” said Hamburger. “At that time, some engineers thought it was beneficial to design structures with a weak first story, that would ‘absorb’ all the earthquake damage and protect the stories above. This was not a good approach.”
Base isolation, he said, was “rediscovered” in New Zealand in the early 1980s, before quickly travelling to the US, where it was installed in buildings like the San Bernardino County Law and Justice Center and the Salt Lake City and County Building. The system has also been implemented in the city halls of Los Angeles, San Francisco, and Pasadena, and Apple’s new headquarters in the Silicon Valley.
But the method hasn’t spread widely across the US, as outlined recently by the New York Times.
For one thing, base isolation is more expensive than conventional engineering methods. Hamburger estimated that a base isolation system may cost roughly $US30-50 per square foot, or between $US600,000 and $US1,000,000 for a five-story building. That adds about 5% to 10% to a building’s total construction costs, Hamburger said, though he estimated that a developer could could save 20% in repair costs if an earthquake were to strike.
In the event of an earthquake, base-isolated buildings won’t necessarily be the only ones that save lives.
“Whether we isolate buildings or not, we believe that we design them to be very safe in earthquakes,” Hamburger told Business Insider. “The risk of serious injury in a modern building designed for earthquake resistance is very low.”
But the cost of structural damage – multiple buildings out of operation for months after an earthquake – could constitute a major disaster for a city, Hamburger said.
In 2015, the US Geological Survey put the likelihood of “The Big One,” a magnitude 8.0 earthquake located along the San Andreas Fault, occurring in the next 30 years at 7%. The agency estimated that there was a 72% chance that the San Francisco Bay Area would see at least one earthquake of a magnitude of 6.7 or greater by 2043, and a 60% chance that Los Angeles would witness the same scenario. That prediction almost came true on July 5, with the 7.1 magnitude quake striking about 125 miles northeast of LA.
In California, Hamburger said, an earthquake may affect a building site once every 500 years.
“The issue comes down to judging the economic welfare of the individual property owner against that of society as a whole,” he said. “Most developers believe it is a low enough risk to warrant taking a chance.”
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