Engineering the Impossible

Are earthquake-proof buildings possible? Sinan Acikgoz, PhD Student in the structures group at Cambridge University Department of Engineering is working on new ways to make buildings resistant to earthquakes. He showed Ginny Smith how to better protect buidings against damage...

Sinan -  Well, there's a very important saying which I think belongs to Nicholas Ambraseys who was a very big important man in our field.  He said, "Earthquakes don't kill people.  Buildings do."  I think this lies at heart of the issue that we all live in buildings.  We all need civil structures to actually sustain our society and we need to protect them from earthquakes.  Earthquakes happen over hundreds of years.  They don't stop and our Earth is going to keep on giving us shakes here and there.  So, we need to be prepared for these important events.  They're very costly when they happen even when we actually manage to stop buildings from collapsing, stop that occurring during earthquakes.  They can be very costly and they can be - if that happens on a regular basis, that can be very damaging economically.

Ginny - Are there some buildings that are more susceptible to earthquakes than others?  I mean, I imagine the sort of really tall skyscrapers you see are probably more dangerous than old fashioned, sort of lower houses.

Sinan - I'm not so sure because it's a complex issue in the sense that when you make a structure which is small, compact, stiff, that structure will end up attracting more load.  Actually, the structures you mentioned, the skyscrapers, they're all engineering structures.  It's actually quite rare that you would find them in places where there's recurrent danger of earthquakes.  So, there will be building regulations which would actually limit the number of stories or suggest that there should be technologies in place to actually prevent these buildings from moving a lot.  

So actually, certain buildings are certainly more susceptible to earthquakes.  For instance, masonry buildings would be one example and a recent earthquake that happened in Christchurch for example was a good example of this where the earthquake hit at a city centre and city centre was full of beautiful old masonry buildings.  You must've all seen the Christchurch Cathedral after the earthquake and it was in shambles.  So certainly, there are structures which are more susceptible and these are generally structures which are not engineered to withstand earthquakes with modern science.

Ginny - So, what can you do to make a building more resistant to earthquakes?  Do you build one that won't move when the earthquake happens, it'll just stay nice and still so everyone inside is safe?

Sinan - That ideally would be great because if we had something that's rigid, if the ground moves, if you've got something that's sitting on a ground and it's completely rigid, the accelerations in that building will not exceed that of the ground.  Whereas if you've got a flexible structure, I imagine a very tall skyscraper as you've given an example, that building actually will amplify its motion at the top stories, you will get a lot of sway.  And the strategies that we can do is actually, we can design our buildings to be completely elastic during an earthquake which is, the earthquake will happen just like you move a rubber band, you pull it back, you leave it and then it comes back to its own original state.  So, we would ideally like to do that.  We would ideally like to have a building which doesn't get damaged, but that's just far too expensive.  So, what we do instead is we try to decouple the structure from the motion of the ground during an earthquake.  So, that would be called base isolation.  

That's one strategy and the second strategy is what is really commonly adopted today.  You let the building get damaged during an earthquake, which is quite counterintuitive to people because one would hope that an earthquake-resistant building is damage proof.  But what happens is this damage that occurs in a building is actually what saves the building.  So, this is another strategy, but what earthquake engineers want right now, what is our biggest challenge is, actually making structures which are safe during an earthquake and which do not get damaged.  It's actually a big challenge.

Ginny - Now, I've never actually been in an earthquake, so I'm finding it quite difficult to kind of imagine what happens to buildings during one.  Dave, you've got something over there that's going to show us a bit more about how it would actually look if you were in an earthquake - a very, very small earthquake.

Kate - Yeah, Dave has been building something all week.  What have we got here, Dave?

Dave - Basically, what I've got here is a platform which acts like the ground which if I turn it on, wobbles.  It's basically like an earthquake.  I can change the speed of that wobble.  And so, at the moment, it's wobbling quite slowly and the building is behaving fairly stiffly and it's just moving with the ground, and you're probably happy in that building.  But if I change the speed a bit faster...

Kate - So, this is the ground shaking more quickly.

Dave - As you see now Kate, the building is starting to - it's bending.

Kate - It's buckling under - the thing is, it's no longer shaking just in line with the Earth moving.  It's sort of bending in the sections in the towers.

Dave - So basically, buildings can have a natural speed at which they want to vibrate.  And if it's a very strong speed which is resonance of the building, that the building wants to vibrate at, and if the earthquake happens to hit that, the vibrations get bigger and bigger, and bigger, and you get into real trouble.  So, I imagine designing buildings to avoid the resonance being anywhere near an earthquake speed is very, very important to them.

Sinan - This is something that's quite important that we try to actually take care of in our research.  So, the concept of base isolation that we just discussed is essentially actually just this.  So, there's a frequency range during an earthquake which the ground will move and you want to move away from that frequency.  You want your structure to be away from that frequency so that the motion doesn't build up.  So, the typical example of a resonance is, if you're pushing somebody when they're in a swing, so the motion will just build up, build up, build up, and that will be too dangerous.  So, what we try to do in a base isolated building is actually, put something very soft underneath the structure so that its frequency will just go very low, and actually, this will decrease the loads that is acting on a structure.

Kate - I remember the wibbly wobbly bridge in London.  When it first opened the Millennium Bridge, it was meant to be this huge thing and as soon as people marched over it, it started shaking.  That was because people were marching in the same resonance.  Do all earthquakes have the same resonance though?  Can you predict at what sort of frequency it's going to be shaking about?

Sinan - Well, this is actually a field where there's a lot of research as well.  We try to characterise earthquakes, but you cannot really specifically guess what specific earthquake is going to be like.  So, we need to actually design it for a specific range of frequencies, specific range of large range of frequencies and be very conservative about how we do it.  So, what happened with the Millennium Bridge was a very interesting example because the lateral frequency of the bridge actually really matched what the frequency of the people would be walking which is roughly about 1 hertz.  So, this is a very specific example. With buildings, interestingly, the frequency actually falls in the range where the earthquakes would be most damaging.  So especially, for short stocky structures, this is a big, big danger that we have.  My personal research about earthquake engineering actually works on the structures which are rocking and these rocking structures are slightly different because we let them separate from the ground during an earthquake. So, the common strategy is, you would just tie down the structure to the ground and you would not let it separate so it would eat up all the energy of the earthquake.  Whereas if you let a structure separate from the ground, it will actually take the earthquake energy and there's actually a setup there which we're going to be demonstrating with...

Kate - So, Dave's model is about the height of my hand, but this one's a little bit larger.  So, what are we going to be showing with this model?

Sinan - So, this is a 3-storey shear frame which is a normal building frame.  It's bare.  It doesn't have all the other non-structural components.  But what would happen is, let's say there's an earthquake and if the building is fixed to the ground, it will start to shake and the building will actually start to take all the load.  It will start to vibrate.  But what we propose is, instead of - and this idea is not actually our proposal.  It has been around for a long, long time.  Actually, if you let the building separate from the ground during an earthquake, it will just... If you push a structure like this and hold it from the base, you can see that the columns are all taking all the load and that's actually quite heavy on the columns because we have to design them for bigger forces. But if the structure is rocking and if you push it, what will happen is, it will take the load, but the load will be actually translated into movements. This movement is actually causing the structure to have less load because it's more flexible and it's easy, ready to move and the concept of resonance that Dave mentioned.  So, if you move something really far away, this has got a very big motion.

Kate - I'm sorry Sinan. So, when you pushed it and held it down to the ground, I could see that the columns were bending. But to me, literally walking the building, tipping half an edge off the ground and sort of letting it tilt backwards, if I was in this building, I'd rather have the columns bend.  Is that the wrong way of thinking about it, just looking at this tilting at a 45-degree angle here?

Sinan - Well personally, I've never been to one of these things.  I would like to be and it's so much easier working on paper.  But the interesting thing about these is normally, in practice, what would happen is, instead of this fiddly little bits that we have here which restrain the structure, which restrain the shear frame down, we would have big steel members, steel tendons, which will run through the structure and hold it down. So actually, we would be allowing only very small motions. In terms of percentage, it would be only 1% of the height of the total building. So very, very small movements, we would allow and you would probably not be able to tell the difference in between a structure which is rocking and which is not. The interesting thing probably is that when you're in a structure which is rocking, yes, you would feel the low frequency motion of the structure going from the left to the right. You would feel this, but there would be actually less acceleration of the stories doing this fiddly motion. So, you would actually decrease loads on the structure and that's sort of the interesting thing because that causes the loads to be less. We can design the structures to be more economical and more resilient actually during an earthquake because they don't get damaged.

Kate - So, by allowing it to tip from one side to another, imperceptively if I was inside this building, you're putting it under less stress and that means in a big earthquake, it might get less damaged.

Sinan -  Exactly, that's the case because as I said, the current methodology that we have, we allow buildings to get damaged. This is not good for the long term because after an earthquake, there are millions and millions of dollars of damage. This is very bad for the economy and we want our structures to be safe.

Kate - So basically, you're saying, I'm wrong.  I want to be in a rocking building rather than sort of the wibbly wobbly one. How can you be sure if you're allowing it to sort of tilt to one side and then come back down again that it will come down in exactly the same place?

Sinan - So, that's a very important part of the research that we do. What we want to do is we want to restrain these structures. So, we don't let them fiddle about. As you can see from this model, when it actually rocks, it starts to walk off from its foundations which is quite dangerous. So, we want to keep it in place and the way we do it is, as I said, we would have large tendons which run through the structure. We would have other restrainers, other energy "dissipaters" which would dissipate the earthquake energy coming from the ground and these would actually restrain the movements. So, that's one thing that we would engineer.

Ginny - Right. Who's got any questions for Sinan?

George - I'm George. I'm 10, and I'm from Ely. Do you really have to make the buildings earthquake proof because an earthquake is caused when two plates crush together or they overlap? Couldn't you just prevent that somehow and then you wouldn't have to have earthquake proof buildings?

Ginny - Good question. Why can't we just stop earthquakes? Come on guys.

Sinan - That's an excellent question. I think that would actually be a solution to all the problems. But the motions of the Earth are very big. So, these forces of plates moving are massive. When they create these earthquakes, comparing an earthquake force to let's say, a Hiroshima bomb, you can see actually, I don't really have the numbers with me, but you can see actually how big an earthquake force is. It's an unstoppable force. It would be great if we can actually stop it from the beginning, but these earthquakes do tend to happen and they create these as you have said, they're plate motions and they create these big mountains, so you can imagine what sort of big forces we're talking about.

Kate - The Earth is still more powerful than a scientist at the moment.

Venud - Hi. I'm Venod and I'm from Cambridge, but I'm originally from India. While growing up, I did experience a couple of earthquakes and the major one was the 2001 in West India. That was 7.7 magnitude I think and there was a lot of destruction and loss of life and property, but the main thing was, the capital near my city, there wasn't much destruction, but there were 50 multi-storey buildings collapsed because there was absolutely no earthquake planning there. One of the major local rumours was that few buildings collapsed because they had a big swimming pool on the top. Was there any truth to that or does that affect if you really have a big water body on the top of a building?

Sinan - Well, this is very interesting question. I would imagine, this might not exactly be true because it's actually at the top. So probably, what happened was, if there was collapse especially was probably related to lower storeys. In most of these cases, in recent earthquakes in Haiti for instance where there was massive damage, one of the things that earthquake engineers realise is, during these earthquakes, we are not currently learning sometimes many things new because we actually have the technology to withstand the earthquake, but it sometimes boils down to actually having the regulation, doing it properly.

This is a very big problem and especially in the developing world where an earthquake can still actually cause very big damage whereas in developed nations for instance when the earthquake happened in New Zealand, life safety was actually reduced to a minimum. This is quite an achievement in terms of what we've done, but in that case, the economical damage was just beyond control. So, we have to manage both of these ends and that's a very big task I think which extends beyond earthquake engineers, goes to policy makers, and beyond.

Justin - Hi. I'm Justin. I'm from Cambridge. I have a question for both the first speakers.  Would it be possible to use superconducting technology to act as maybe dampers or motion restrictors for earthquakes?

Ginny - Could we levitate our buildings so that the Earth just moves underneath them? That sounds like a brilliant idea.

David - It is a great idea and it has been suggested, but it's completely impractical. The best way is, I think there's a guy called Robinson, designed the building isolators, specifically like the Christchurch earthquakes, but it's much better to have a rather viscous damper rather than some of it levitated.

Sinan - Taking the fantasy further I think, we would actually want to lift the plates up so that they don't crush into each other, so that there's no earthquakes. But...

Ginny - What, levitate the Earth's whole plate? I think that might need a very strong magnet. Okay, I've got another question from the front row.

Jess - My name is Jess from St. Yves. You spoke about partially destructive structures. Did you give an example of that because I didn't appreciate that?

Sinan - So, when I say partially destructive structures, imagine just ordinary reinforced concrete frame which is currently designed, again, going back to this Christchurch example, many buildings just after the earthquake which was conventionally designed, which is just structures which are fixed to their ground which don't uplift as in the example that I've illustrated which don't have a base isolation at the bottom. So, these buildings just get damaged and when I say damaged, this is all control damage.

So, what would happen is, first, the beams would get damaged and then slowly, they would migrate to the columns because we designed beams to be weak so that they can dissipate the earthquake energy because they're not as essential as the columns. So, they get damaged first and then the damage would migrate to the columns. In the meanwhile, the building is supposed to stay intact. But in terms of damage when I mean damage, you would see cracks on the building, you would see big cracks on the column and beam interfaces. This is all stuff you would expect. On top of it, after an earthquake, if it has gotten to this level of damage that it cannot be repaired which is very common because if you imagine a reinforced concrete building again, it's got steel bars running through it. So, how are you going to be actually removing those steels which have yielded completely during the earthquake without taking the column out and without replacing the whole structure. So, it becomes a very difficult issue. It is actually, as I said, very common way of designing structures and it will just happen to any ordinary structure that you can imagine.