Fri April 20, 2012
Designing A Bridge For Earthquake Country
Originally published on Fri April 20, 2012 1:56 pm
IRA FLATOW, HOST:
This is SCIENCE FRIDAY. I'm Ira Flatow. Where is the safest place to be during an earthquake? Yeah. Here, in San Francisco, everybody is shaking their head.
(SOUNDBITE OF LAUGHTER)
FLATOW: They're all thinking about it. It's always on people's mind, given that 106 years ago this week, two days ago in 1906, a huge earthquake rocked San Francisco and the north coast of California. People are still talking about that historic quake. So the answer to that question may be the safest place to be now or soon to be is on a new bridge they rebuilt after a section of the old one collapsed during the violent Loma Prieta quake in 1989. So 23 years later, engineers say they're putting the finishing touches on the quakeproof new eastern span of the Bay Bridge, set to open next year.
My next guest is the lead design engineer for the new span, which has been called an engineering marvel. Dr. Marwan Nader is also vice president of the structural engineering firm T.Y. Lin International, and he joins us here in San Francisco. Welcome to SCIENCE FRIDAY.
DR. MARWAN NADER: Thank you.
FLATOW: Are you very proud of your new bridge that's going up?
NADER: I am.
FLATOW: Yeah? You should be, right?
NADER: It's certainly been a challenge, but it's certainly coming together.
FLATOW: And you were in San Francisco during the Loma Prieta quake in 1989. How did you get involved with designing this new eastern span of the Bay Bridge?
NADER: Well, I mean, in 1989, I was still a student at U.C. Berkeley. And when the earthquake occurred, I was actually in Davis Hall where the - we study engineering and it - the building shook quite a bit and made it very clear for me how important it is to design structures for seismic loads. And I did my research and dissertation on seismic design of structures. After that - a little bit after that, I joined T.Y. Lin International, and I've been there since then. After 1989, in parallel with that, the California Department of Transportation was looking to retrofit major structures like the Bay Bridge, all the major crossings of the bay. And right around 1997, they put out a RFP, as we call it, for consultants...
FLATOW: Request for proposal.
NADER: A request for proposals for consulting firms to basically be selected to design the new Bay Bridge.
FLATOW: And why did the old one collapse during the quake?
NADER: Well, the old one did very well given the fact that it was built 75 or so many years ago. At the time, we knew as an industry - engineering - very little about earthquakes. In fact, we designed most of our structures for wind. And we basically had a small note that said that the seismic loads will not govern the design. Since then, we've learned a lot more. Mother Nature has a good way of reminding us how to put our structures up. And the bridge itself has two big challenges to it.
Unlike the Golden Gate and unlike the western spans of the bridge - Bay Bridge - where the foundations are sitting on large caissons, and those caissons are supported directly on rock, the eastern spans of the Bay Bridge are sitting in very deep soils.
FLATOW: Mud. They're in mud.
NADER: They're mud - practically, muck...
NADER: ...if you want to call it better than mud. And those soils actually have very little - they get - they amplify the ground motion, and they have very little resistance in terms of - when compared to bedrock. And the piling of the existing structure is about, you know, 30 meters deep. They're actually timber piles. They're 30 meters deep. And there are many, many, many of those. And that particular foundation is not made to resist the seismic motions that we're dealing with.
The other part of it is the fact that the existing eastern spans have - it's a truss structure. And by default, a truss structure has many connections, many members, unlike a suspension bridge. And thereby retrofitting those members to satisfy the new design codes would require a lot of work, overhead traffic and blocking lanes.
FLATOW: But if you look at your design of your new bridge, it's not - it's almost unlike anything we've seen before...
FLATOW: ...around this part of the United States or this part of the world.
NADER: Well, there's a couple of things. Obviously, 90 percent of the bridge itself is a segmental-construction type, and that's rather typical of that. And the part that I believe you're referring to is the signature span...
NADER: ...which is the self-anchored suspension bridge.
FLATOW: Describe that to an audience that can't see it.
NADER: Well, the signature span is like a suspension bridge that you see, where the cables, basically, have a catenary shape to them. There's a couple of big differences about it. The first one is that the cable itself, unlike a suspension bridge where the cable is anchored into an anchorage, and the anchorage is actually anchored in the ground, which is effectively a five-story building that basically anchors in the ground - a self-anchored suspension bridge takes that cable and anchors it in the deck. So it becomes kind of like a hanging basket where the actual loads from the cable are going into the deck, and the deck is subject to compression. And so that's one feature of the self-anchored suspension bridge which makes it very unique.
The other part of it, which is also unique to this bridge, is the fact that the cable itself is in a three-dimensional space. Most cables that we've seen on suspension bridges tend to be in a vertical plain. As you're driving, you see the cables on one side and another cable on another side. And if you look at that cable, it's vertical. The suspenders come - the ropes come vertically from it, and that's how the bridge is suspended.
In this particular bridge, the cable is actually occupying a three-dimensional cable. It starts at the single tower, which is in the middle of the road, and actually splays out and has a - and goes out to the extremities of the roadway, and thereby giving you cathedral rope, if you will, when you drive through it.
FLATOW: So does the cable loop back again?
NADER: It does.
FLATOW: It does? Yeah. You can see that from the roadway?
NADER: No, you can't. You can't. Actually, the looping aspect of it has multiple aspects. One of it, obviously, an engineering design. It was driven to satisfy design equilibrium, and it was driven to make sure that the cable itself is anchored into the deck. The angle at which the cable is coming down, which is close to about 30 degrees, is sharp, and thereby anchoring it into the deck, as you can imagine, would be a little more difficult because you have to turn the cable. And rather than turning it in a vertical plain, we turned it into a - in a horizontal plain, thereby, like the belt around your waist, we looped it around. And this, in many aspects, that aspect of it, is a first of its kind. It's a borrowed industry from pre-stressed concrete bridges where we have a lot of the tendons looped around in a U-form and looped that way, but it's never been done on a suspension bridge like this one.
FLATOW: Well, we're here at the California Academy of Sciences. And the audience, please feel free. You're going to have a chance like you never had to ask about the Bay Bridge if you want to know something about it.
(SOUNDBITE OF LAUGHTER)
FLATOW: So step up to the microphone and - if it concerns your at all. Also the - also, I noticed from the design is that the bridge is made in difference pieces so that parts can move independently of one another, correct?
NADER: That's correct.
FLATOW: Doing so, so that when the Earth shakes, it all just sort of floats.
NADER: Right. The seismic design, the way we understand it, is basically there are effectively two ways to resist the motions. One is to really design a bunker, which effectively is very strong to take the forces...
FLATOW: So you're fighting nature.
NADER: Yes. And what you're doing there, is you're really taking on whatever the motions are. And the earthquake has a very interesting characteristic to it. It's like a musical, effectively. It's got areas where there's a lot of energy, which is at the frequencies that are very, very low or very, you know, very, very high. Excuse me. And then that's where you're getting the most energy. And then as you get the structure to be more flexible, that's where the energy gets smaller. So if you are a little bit careful about it, you can actually design your structure to be in the areas where the earthquake is less damaging. And by making that structure tuned to what Mother Nature's going to apply, you actually avoid that ground of the force.
The other aspect of it is designing components, if you will, that are made to take on the damage. Like when we drive cars. If you think about it, cars - we know we drive cars. We know that we'd like not to get into accident, but we planned for that accident. And the idea...
FLATOW: It's like crumple zones.
NADER: Exactly. And the idea behind it, is you get the damage to occur in areas where you keep the car functional to the extent possible when it's a midsized type of accident that you have. And the idea is that the fenders take all the damage. Very similar to that, is our bridge is designed that way. We actually looked at specific areas which we said that's where it makes sense to have the damage occur. We designed those elements to take on that damage, and thereby protecting the more important elements to it.
FLATOW: So you can replace those damaged pieces later on.
NADER: Exactly. The idea is that, after an event, the bridge is still functional. We would go in - obviously, the engineers at that time would go in and do a, you know, an inspection, evaluate - there will be damage, but it will be in a form where you can actually make it available so that emergency traffic can be - immediately after that, go on it, and shortly after that go through normal traffic.
FLATOW: Mm-hmm. Let's see if we can get some questions from our audience here. Yes, sir.
UNIDENTIFIED MAN #1: Yes, sir. I remember when the Bay Bridge was starting, and it was meared(ph) with a lot of, like, scandal about bad welds. And how's that being addressed? Did they have to start over, or was the cost overruns and things like that?
NADER: Well, I'm not sure I follow the specific event. But in general, this question goes back to quality, if you think about it. And one of the issues that is great of our engineering profession is that we basically have design codes. We have standards and we have qualification for quality and assurance that we follow. And the idea behind it is that every weld, every material that we have, is inspected either during the fabrication or prior to using that material, that it satisfies those qualities. And if it's not satisfying them, it gets rejected, it gets pulled out. There is a sense that when you sometimes find something that wasn't good, that you - we did something wrong - and, in fact, it's the other way around. If you think about it, it's the due diligence that we do that uncovers those situations, so to speak.
FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY from NPR, talking with Doctor Marwan Nader. We talked last week on - a couple of weeks ago on the show about the Brooklyn Bridge. And it turned out that Roebling who had designed the Brooklyn Bridge discovered that it was being made with substandard steel cabling on it, and he had designed so much redundancy or overload into it that it went from 10 times it capacity to only six times what it can hold. I imagine you need to do some, sort of, like redundancy in the bridge.
NADER: And you're so right about that. It's an engineering intent. What you do is you look at, sometimes, the situation you're faced with, and you look at if there's a need to perform what we refer to as, does this particular element satisfy the design intent? And thereby, do you really need to go in there and do work to it, or is it OK the way it is?
FLATOW: And it's been designed to last 150 years, which is a lot longer than most bridges...
FLATOW: ...are designed to last. What are the design features helps in that longevity?
NADER: Well, I mean, if we - if you think about it, what does 150-year mean? Just to give us a sense, most of our buildings that we designed, most of our bridges tend to have a design life which is 50 years to 75 years. And as engineers, what we work with is we work on probability, and we work in recurrence. So the idea is that if you have a longer window than the probability of having a larger earthquake or a larger wind event, becomes - the event itself becomes larger. And you also have to look at the maintenance of the structure and make sure that it satisfies, yeah, the 150-year design life.
What happens in this particular case, obviously, the one driving aspect of it is the seismic loads that we have in the Bay Area, and what it did is it magnified the ground motion that we're designing it for. We wind up designing for a earthquake that has 1,500 year return period, much like the earthquake that we're - we have in - had in 1906 earthquake.
FLATOW: 1906. OK. Let's get a question from the audience here. Yes, sir.
UNIDENTIFIED MAN #2: Yeah. Following up on that comment, how does - what's the process for deciding how strong an earthquake - to design a bridge to expand?
NADER: Well, we obviously have our seismologists and the geologists which look at the pattern and the history. And ultimately, we start with looking at all the faults that are in the Bay Area, the proximity. In the Bay Area, we have two very large faults, frankly. One of them is the San Andreas Fault, which produced the 1906 earthquake, which runs a few miles away from the Golden Gate and runs up and down the coast of California, goes all the way down to Southern California. The other one is the Hayward Fault, which is in the Berkeley Hills, and is also a major earthquake.
So those quakes, we basically look at their seismic hazard, if you will, and from that generates the possible or potential seismic hazard. And then we get intensities that, based on the soils that we have, based on the history of ground motions that we recorded, we come up with those intensities at very different frequencies I was talking about earlier.
NADER: And then from that, we generate a spectrum that ultimately gets used to generate multiple, different simulated ground motions. That we take our structure and in a computer model, we run that structure and we subject this with those displacements. And the computer model is intended to do the equilibrium, statically, at every instant in time, microsecond. And the target would be that we know what the forces are in our structure and what the displacements are in our structure.
FLATOW: (Unintelligible) a quick question in, yes, from...
UNIDENTIFIED MAN #3: Yeah. A quick question is, we have a lot of information, video, interviews and so forth, from the 1989 earthquake on the Bay Bridge, and we know the personal experiences. If a same magnitude earthquake occured with your design, what will the human experience while driving across the bridge?
UNIDENTIFIED MAN #3: What differences when this occured?
FLATOW: Would it be the safest places to be like I mentioned before?
NADER: It would certainly be the safest place to be, and it would actually feel much like a joyride. The difference is that you're not necessarily planning on it.
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NADER: And as you feel it, I would suggest that you slow down and you get to brake your, you know, reach a parking, you know, kind of a situation and just - once the earthquake is done, 50 seconds later, or so, drive on.
FLATOW: To see - well, that doesn't mention about the roads that lead up to it. Is there going to be (unintelligible)...
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FLATOW: Your bridge will be there, but the roads are going to...
NADER: Actually, that's a very good question. And, you know, obviously we looked at these things and the reality of it is that we only have one bridge in the area that crosses the Bay from the east to - East Bay to San Francisco, while we have multiple other...
FLATOW: Yeah. All right. I will - and that's going to be the problem. Thank you very much, Doctor Marwan Nader, for joining us. He's the lead design engineer for the New Bay Bridge Self-Anchored Suspension Bridge. Transcript provided by NPR, Copyright NPR.