Preparing Infrastructure For Earthquakes

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ROBERT SIEGEL, host:

As we just heard, Japan works very hard at preventing havoc in the event of a natural disaster. It has invested heavily in infrastructure, putting up buildings that can withstand strong quakes and save lives.

Stephen Mahin is a professor of structural engineering at the University of California Berkeley and director of the Pacific Earthquake Engineering Research Center. Professor Mahin, welcome to the program.

Professor STEPHEN MAHIN (Structural Engineering, University of California Berkeley): Thank you, Robert. It's very nice to be here.

SIEGEL: When we hear of a magnitude 8.9 earthquake, kind of stunning to me having been in the Szechwan 7.9 earthquake. This is tens times bigger than that. To what magnitude are buildings designed to withstand an earthquake?

Prof. MAHIN: Generally speaking, buildings are designed to be serviceable following an earthquake for an event that might occur once or twice during the life of the structure. So this might be an earthquake that might only happen every hundred or maybe even fewer years. When you get up to these big earthquakes, it's more like having a collision in your automobile when the airbags and the bumpers that collapse, are all brought into play.

So it's normally expected that the buildings aren't going to perform without damage in a really, really big earthquake like this. But we're aiming to try to prevent collapse ultimately to save lives.

SIEGEL: But is the assumption that you avoid collapse, but in a quake this big, assume structural damage will be so great that ultimately you'll have to knock structures down and replace them?

Prof. MAHIN: Exactly. I think that's something that many people don't realize, that we're doing our best to protect life safety and to allow people to, you know, recover their lives but in a truly big earthquake, a magnitude 8 earthquake or similar in California or an 8.9 earthquake in Japan, if one designed to the minimum requirements of the building codes, you basically can walk away from the building, but you may be not able to go back into it.

There's a big concern now exactly for that of what would happen to a big city like Tokyo or San Francisco or Los Angeles or any other large city, of what would happen if you had a large number of buildings that were damaged and could not be occupied following an earthquake. So there's a big push now for resiliency, so that they can be repaired following an earthquake.

SIEGEL: Professor Mahin, when you say the Japanese have done a great deal to make buildings resilient, so that they could simply withstand a big earthquake, what have they done, actually? Is there any especially novel engineering and construction that they've used?

Prof. MAHIN: There's a lot of new technology, high performance materials, concretes or steels that are more strong or have improved material properties that we can exploit. But I think there's a big push now toward things like seismic isolation where you put the buildings on top of devices that will simply isolate the building from the movement of the ground, reduce the forces that go up there.

There are devices that are like shock absorbers that you would find in your car's suspension, so that when you go over a bump, your car doesn't bounce too much. And so, the Japanese have gone over and have some actively controlled buildings. If you go down and buy a new car now, you have many options for automatic braking systems to prevent your car from spinning on ice or that you can put them on soft ride mode. And the same thing is now being done in Japan for buildings.

SIEGEL: For buildings, you're saying?

Prof. MAHIN: Yeah.

SIEGEL: Of course these would be features of new construction.

Prof. MAHIN: Right.

SIEGEL: I mean, you couldn't retrofit seismic isolation for an existing building, I would think.

Prof. MAHIN: Well, many of the buildings in California now that are seismically isolated are older buildings simply because they are historic buildings, in these cases, that are so fragile that it's pretty hard to strengthen them. You basically have to destroy the building in order to strengthen it using conventional methods.

So, the San Francisco City Hall, the Oakland City Hall, the Los Angeles City Hall are all seismically isolated as retrofits.

SIEGEL: And seismic isolation in this case means actually, in effect, retrofitting the foundation of the building, so that...

Prof. MAHIN: Yeah, in an existing building what is done is to put props under the building, shore it, so that you can cut some of the columns out in the basement or the bottom floor or in some cases, go down and dig under the building and put new foundations in it.

They're really spectacular, historic structures and you can't really do too much to the upper part and still keep their historic and cultural fabric in place. So, it turns out to be cost-effective to do these what seem to be fairly drastic measures in these kind of structures.

SIEGEL: Professor Mahin, thank you very much for talking with us.

Prof. MAHIN: Thank you.

SIEGEL: That's Professor Stephen Mahin of University of California Berkeley, where he is a professor of structural engineering and director of the Pacific Earthquake Engineering Research Center.

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