June 16, 2014

Watch buildings move during earthquakes

Posted by Austin Elliott

Through literal eons of Earth’s history, earthquakes have heaved the ground, shuddered the trees, and sent fauna scurrying. Yet aside from the occasional tsunami and the localized sloughing of rock faces and hillsides, they’d never really been directly injurious to the animal kingdom. …Until the animals started “sheltering” themselves under ponderous weights of precarious things. As the adage goes, “earthquakes don’t kill people; buildings do.” That edifice that so importantly protects you from wind, rain, cold, vermin, and attack–and so literally structures modern civilization–can become an incredible threat when its normally sturdy foundation is shaken by the Earth. Fortunately, we’re learning–quickly–how to design our structures so that even the uncommon but extreme threat of seismic shaking poses little danger to a building’s human occupants.

Getty Images

re-creation of the 1906 quake. source: http://www.gettyimages.com/detail/video/buildings-shaking-falling-apart-in-earthquake-stock-video-footage/562-38

Major quake-prone cities around the world have begun to take sweeping action to upgrade building codes and bring their huge stocks of older buildings into compliance. The most notable of these modern resiliency efforts come from Istanbul, San Francisco, and most recently Los Angeles. Each of these cities was recently dealt a near-miss with a violent earthquake, but that luck won’t last forever. It takes great collective investment to tackle the expensive problem of seismic retrofitting, especially when the violent shaking of earthquakes is usually unimaginably remote from anyone’s experience. To help make the hazard of shaking buildings clear, here is a collection of simulations, data animations, and YouTube video footage of buildings responding to strong ground motions. Some of these are jaw-dropping, and ought to give you a little shiver when you consider whether it’s worth risking it in an unfit building.

The legions of video cameras running during the March 11, 2011 Tohoku earthquake provided ample documentation of tall skyscrapers swaying the way they’re designed to. I’ve posted about that before, here:
http://blogs.agu.org/tremblingearth/2011/05/05/swaying-high-rises-and-resonance-frequencies/

In the following videos we see mid-rise to low, 2-story buildings wobbling, responding to higher frequency ground motion.

Heavy roof tiles are flung to the ground from these heavy-topped Japanese buildings, whipping wildly back and forth (a reminder to get away from the sides of structures):

In this long and terrifying video of the Tohoku quake we see plenty of two-story buildings whipping around and losing their window frames, among other non-structural elements. The building frames flex enough to pop concrete and plaster off their walls:

Another clip from Japan shows a base-isolated building sloshing independently from the street around it, buckling the pavement along the joints. You can clearly see how much the base isolation dampens the shaking, because the building next door (complete with dozens of chandeliers!) flails wildly:

Here’s a motion-stabilized version of a clip from an office showing two narrow buildings in Tokyo shaking in 2011:

A similar video from a less violent quake in Mexico city in 2014 shows two narrow buildings responding differently and swaying towards each other:

A more vast structure, the domed and open Monument to the Revolution, wobbles alarmingly during that same Mexico City earthquake:

This Getty stock film shows a scale-model recreation of San Francisco’s brick and wood apartment buildings clattering together as they might have appeared in the 1906 earthquake–and will appear during a violent quake in the city’s future:
http://www.gettyimages.com/detail/video/buildings-shaking-falling-apart-in-earthquake-stock-video-footage/562-38
(I always wonder, when I’m in San Francisco, what it must sound like to have your neighbors’ houses clapping against your own during even a modest earthquake…)

Of course in all of these examples, the buildings survived. One doesn’t need to imagine, however, that a little more shaking or a little less structural integrity can tip these buildings over the edge. We see it on display in the last scene of this footage from the 2010 Haiti earthquake. Starting at 3:19 the camera shows violent shaking and the gradual and tragic destabilization of the neighboring building.

Fortunately the motions of these buildings are predictable, or at least model-able. Simulations of seismic response are a routine component of engineering a building. The models people make (see, for example, the animations at Computers & Structures, Inc.’s YouTube channel) are currently being improved by real data from heavily instrumented buildings. Caltech’s Millikan library has accelerometers installed throughout to measure its tweaking and torquing during even tiny seismic events in Pasadena. Its motion during a 4.2 is shown here, along with a great narrative explanation.

Similarly, the Factor Health Sciences building at UCLA has multiple accelerometers on every floor, giving an amazingly precise view of the building’s motion during earthquakes. It’s been through plenty, and each one gets an animation. Have a look at any of them here–these are actual motions recorded during quakes in L.A…. exaggerated a bit:
http://factor.gps.caltech.edu/animations

Data recorded within these well instrumented buildings help scientists and engineers understand the spectrum of motion possible during earthquakes, and calibrate models of structural response, two key ingredients in designing earthquake-safe structures for the seismic safety needs of a towering city.