High-rises shouldn’t give L.A. the shakes
ALTHOUGH Mayor Antonio Villaraigosa vetoed an ordinance that could have paved the way for a high-density, vertical Los Angeles, the idea of a taller city remains much in play. But is it a good one in an earthquake-prone region such as Southern California? Graphic images of piles of rubble and bodies where tall buildings once stood before big earthquakes in Taiwan, Pakistan, Turkey and Iran certainly should raise questions about the wisdom of building more high-rises in L.A. The collapse of the World Trade Center’s twin towers after the 9/11 attacks makes for more high anxiety. Are tall buildings safe during an earthquake?
Large (and even not so large) earthquakes release enormous energy that moves the ground and everything attached to it. What happens next tells you a lot about where you want to be. In the simplest terms, brittle buildings don’t respond well to shaking because they cannot move much without coming apart. Think about a house of cards: Move it just a little and it collapses.
Now think about attaching each card to its neighbors with tape. If you poke the house, the connected cards will move as a group but not fall down because the structure is ductile.
The large number of fatalities associated with big earthquakes is generally caused by the collapse of multistory, non-ductile buildings. In many instances, poor building codes and shoddy construction contributed to the loss of life.
Since the Santa Barbara temblor in 1925 and passage of the Field Act after the Long Beach quake in 1933, California has been a leader in requiring better-designed buildings that resist earthquakes. The building codes enacted to implement earthquake-resistant design were a major factor in the relatively few deaths caused by the 1971 San Fernando, 1989 Loma Prieta and the 1994 Northridge earthquakes. Researchers, architects and builders from around the world have studied these events and used the knowledge to make tall buildings even safer.
Aside from what the Hollywood special-effects folks would have us believe, the safety of well-engineered and well-constructed high-rises during earthquakes is indisputable.
If buildings of all heights receive the same level of attention as to design and workmanship, studies indicate that tall buildings are substantially safer than smaller ones when subjected to strong ground motions caused by an earthquake. During the Northridge quake, office skyscrapers of 50-plus stories in downtown Los Angeles were generally unscathed and fared far better than many smaller structures elsewhere in the city.
In Mexico City, the 44-story Latin American Tower survived earthquakes in 1957 and 1985 despite devastation across the city. In fact, the only documented case of tall buildings collapsing was at the World Trade Center site.
Tall-building technology continues to advance. The 55-story One Rincon project in San Francisco uses cutting-edge design in the structure and external outriggers to resist shaking on a hilly site. Because of the upsurge in high-rise construction in Los Angeles and other West Coast cities, demand will continue to grow for new framing and bracing systems and ways to isolate buildings from ground motion or to dampen the effects of shaking. The Pacific Earthquake Engineering Research Center, headquartered at UC Berkeley, is leading the Tall Buildings Initiative, a collaborative of universities, government agencies and the engineering community that develops design criteria to ensure safe and usable high-rises following future earthquakes.
ALTHOUGH THERE may be many reasons to question the vision of Los Angeles as a vertical city, the seismic safety of tall buildings is not among them. Engineers can generally do a much more effective job of designing seismic resistance into larger, more expensive structures than for small commercial buildings or multifamily apartments -- decades of statistics bear this out. If we look at fatalities caused by building failures in earthquakes worldwide, low- to medium-rise structures with often dubious pedigrees are the killers -- not tall buildings. I know where I’d want my family to be during an earthquake.
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