"The Father of Prestressed Concrete": Teaching Engineers, Bridging Rivers and Borders, 1931 to 1999

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The Bank of America Building in Managua: A Seismic Success

What about the Nicaragua earthquake? In Managua? That was in the 1970s.

Lin

Well, I was very well known for that because that was my design.

Which one was that?

Lin

That's the Banco de America, Bank of America building, eighteen stories high in Managua, Nicaragua. Now, about 1960 some, they wanted me to engineer that building. There was an architect; I did the engineering. And it was known that Managua is an earthquake area. So I designed for that. I designed the eighteen stories building to act in two stages. It would resist normal earthquakes with no problem. But in case of large earthquakes, it may have cracks, but would not collapse. The first part was easy, to design against ordinary earthquakes; you put in enough steel. The second part is more difficult. And at that time, our knowledge about earthquake design was not that good. But I had the concept. Putting reinforcing bars--it's not prestressed, just reinforced--at some critical places, so that if they crack, the steel bars would still work. So I call it a second line of defense. There's more details to that thing. So I build in that building a second line of defense.

In 1973, an earthquake of a magnitude of Richter scale 6.5 happened. The whole city was leveled. And that eighteen-story building stood alone, though it cracked.

What was your second line of defense?

Lin

My second line of defense was that after it cracked, the building worked like a tree. A soft tree will move with an earthquake without collapsing.

Oh, it was flexible?

Lin

It becomes flexible.

How did you do that?

Lin

Well, I put steel in the right places. So I did that. Put steel at critical joints, and in fact, it cracked along these joints. But not enough to crack my bars. Concrete was cracked! Almost every story of the eighteen stories, at these junctions, the concrete cracked. But the steel was enough to carry it. So it moved with the earthquake. And then, the building changed from

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the quick movement, when it was stiff, very short period of vibration, to a slow movement, gradually, left to right. And it was resisted by the additional steel I put in. I put in no more than a few thousand dollars worth of steel. Very little. But at the critical points. So that it would be able to take that slow earthquake. You see, when the building first responds to the earthquake, it moves very fast. It cracks. After it cracks, it moves slower, and your added steel was able to stand it. This is fabulous. And it is not only in theory. We put in seismograph machines in the buildings to record any earthquakes. And the recording indicated that's what happened. Before the building cracked, it has a period of about one second [waves arms to demonstrate swaying]. After it cracked, the period changed to about two and a half seconds. Back and forth. And it survived. So actually, the seismograph records proved that to be true. And that became a very famous building. Professor Bertero of Berkeley and many others sustained this and wrote papers on it.

It became very famous.

Lin

Professor Bertero at UC Berkeley, now emeritus, is a world-famous seismic engineer.

I was about to say, how did you learn all this about earthquakes? Were you consulting here with seismologists on the latest--

Lin

Well, not really. Because, when you design a building, you consult a seismologist. And in this case, I did not. This was some thirty years ago; I only knew the general design against earthquakes.

I was in Managua the year after that earthquake--

Lin

In '74. That's interesting!

Yes. I was there, and I saw the damage. Well, a few months after. It was terrible!

Lin

Do you happen to know if it has recovered now?

I understand it has. But at that time, the big hotel was unoccupied, vacant, but it stood. And, of course, the Bank of America building was there--

Lin

Now. Very interesting thing here. After that partial failure, the owner and the architect came back to us. What will we do now to repair these cracks, or repair the whole building? And after

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studying together with Professor Bertero, we figured out the building will stand another earthquake. But we will repair these cracked joints. Now there are two ways to repair it. One is to make it so stiff that the cracks will not occur. That, Professor Bertero and others advised us not to do because it may be so stiff that if an earthquake came the whole building will fall out, somewhat like the one in Alaska.

So we repaired it that it will crack, but the cracks will be controlled. And this is interesting. So we learn from those things. When I first designed that building, the architects wanted to put holes through my beams, every floor. And place these air ducts through my beams and leave big holes. That may weaken the beams and I said, "That's dangerous. Can you move the ducts elsewhere?" "No!" So we put the ducts through. And as I said, I strengthened these joints and designed for the second defense.

Now, after this happened, we discussed with the architects and our consultants. The architects say, "We could put the ducts outside. It goes underneath the beams." But our consultants advised us, "Let the holes be there so that it will again break without breaking the entire building." So we put the weak joints in. See, we completely reversed our positions. At first, I didn't want the holes. The architect wanted the holes. We put it there, and it cracked, didn't fail. Now, architect said, "We don't need those holes." But we engineers figure it's better to have the holes, make it weak. [laughs]

Make it weak in the sense that it's more flexible.

Lin

More flexible--

Doesn't crack?

Lin

It will crack but will not collapse.

It will not collapse.

Lin

Yes. Yes. That's very, very interesting. Very interesting.

So how did you repair the cracks then?

Lin

Essentially, all those cracks were redone. And I think we put more mesh in there. Wire mesh, so that the concrete, if it fail, will not completely broke, sort of held in place by the mesh. It is in a way a partial remedy, but everybody thought that's the best to do.

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So you did go in and just repair the building?

Lin

Yes. Repair the building. Just at these particular joints. These joints were all around the central core. The building itself was okay. So we learned a lot. Even now, I think I have a pretty good idea to design against earthquakes. But I am no perfect engineer or architect. No one is. And I still don't know how the earthquakes are going to act, and how strong the earthquake is going to be. You can't tell. Same thing with the Bay Bridge here. We can do better, but not necessarily full earthquake-proof.

About this text
Courtesy of University Archives, The Bancroft Library, University of California at Berkeley, Berkeley, CA 94720-6000; http://bancroft.berkeley.edu/info
http://content.cdlib.org/view?docId=kt4w1003s9&brand=oac4
Title: The Father of Prestressed Concrete: Teaching Engineers, Bridging Rivers and Borders, 1931 to 1999
By: T. Y. Lin, Creator, Eleanor Swent, Interviewer
Date: 1999
Contributing Institution: University Archives, The Bancroft Library, University of California at Berkeley, Berkeley, CA 94720-6000; http://bancroft.berkeley.edu/info
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