In the aftermath of the earthquakes that hit Venezuela last week, FIU civil engineering professor Atorod Azizinamini explains how engineers design buildings to survive earthquakes, why some buildings collapse and more.
How do engineers design buildings capable of withstanding earthquakes?
It depends on the height of the buildings. It's not the just the strength. You have to make sure that the building is able to sway because during the earthquake, basically the ground is shaking. It has to have enough ductility [the physical property that allows it to survive stress such as stretching or pulling without fracturing]. In order to make that happen, you have to design the columns, beams, joints a little bit differently. You have to put more steel reinforcement in it, and all that is spelled out in the seismic design.
There are many older buildings that were not designed this way. There are a couple of different ways to make them earthquake proof. One way is that you can provide additional confinement [structural reinforcement]. Or if you have an old building that really cannot do much, you can isolate it from the ground. So you can basically, pick the building up, put it under devices that will isolate the ground. The ground can move, but the building is not going to move.
Can you explain some ways that buildings collapse during earthquakes?
If a column is not designed adequately when one floor collapses...for example, in the 1989 earthquake in San Francisco, we saw examples of the cases where the first floor didn't have enough walls around it. It was like a parking garage and so on. So that floor will collapse and then the second floor will fall right on top of it. So that's a phenomenon that's called the soft story in the earthquake engineering terms. It's called the soft story phenomenon that happens.
Now, in the case of the Kobe earthquake [in 1995 Japan], I saw police stations and several other buildings, not the first floor, but the floors like fifth floor or seventh floor had collapsed. And the reason for that is a dynamic behavior of the building and the ground motion that happens. The ground motion can create, based on the dynamic characteristics of the building, a situation where the critical columns are in the third, fourth or fifth floor, and that's why the floors above the first floor collapse sometimes, and then pancakes on the floor below.
Why is protecting hospitals, essential facilities and roads so important after an earthquake?
Some of the important buildings, such as hospitals, have to remain functional. You have to make sure that the electricity, the water, all that is not interrupted. You have to make sure that the roads leading to the hospital are functional. You have to make sure that the contents of the equipment in the hospital continue to be functional.
So in order to do that, you have to make sure that the roads and the bridges there are earthquake proof. You have to make sure that the emergency vehicles, the ambulances and so on can get the people, for example, the injured from a collapsed building, to the hospital. So there are multiple things that you have to worry about in order to, to say that I have done everything possible in order for my society to survive the earthquake.
Once an earthquake is over, what concerns engineers the most?
The major concern is that when the earthquake comes, it is possible that it inflicted damage to the building, but it doesn't collapse. After the major earthquakes, usually you have aftershocks. Those aftershocks for a building that is already damaged could cause a collapse.
One of the challenges that, for example, structural engineers usually have after the earthquake is that they go inspect the buildings. It's just visual. The number of the buildings and structures that you have to inspect is very large and you cannot really assess completely what happened. The concern is that you take a look at it... usually, if you look at it visually, if you see the major elements of the building are really badly damaged, you abandon it right away. But if there is just a cracking around the corners of the windows or in the columns, usually you say, 'Okay, there is a damage, but it's not [too much].'
But if you see one story, of course, is leaning, you just have to leave it. So usually the visual inspection should be in general able to say, 'Okay, should I abandon this building or should I not abandon this building?'