Friday, April 24, 2009

Earthquakes in Italy part 3

Other than un-reinforced masonry buildings the most potentially dangerous are those built up on posts with little or nothing between the posts. Try standing a pencil on end, what happens, it falls down. Now if we take our little model, even the one with the tape on it and put some posts under it we would have a hard time making it stand up. If we taped or glued the posts to the model it would probably stand up under normal conditions, but if we again moved the cardboard base back and forth some of the connections of the top of the posts to the model would start to give way and the model would eventually fall down. If it were a real building everything and everyone below it would be crushed.

By trying to understand how we could make the posts below the model strong enough to resist the horizontal forces breaking the connection between them and the bottom of the model we get into the real meat of the problem of making buildings earthquake resistent. In an earthquake the earth is moving horizontally under the building. The weight of the building or it's mass makes the building want to stay where it is. Since the earth is much stronger than the opposing mass of the building, we know that the earth will move. The bottom of the building is connected to the earth so it moves too. What we would like is for the rest of the building to move at the same time as the part connected to the earth.

In our model example that isn't happening. So how can we make it happen? One way is to make the connection between the top and bottom of the posts strong enough to not come apart when the earth moves. These are called moment resisting connections, remember moments from physics class, they would involve steel frames, steel rienforcing in concrete columns and such things as steel plates in wood columns. Another way is to add some shear walls to the posts. Picture this, take a pencil, cut two pieces of cardbord the same height as the pencil about 5cm wide. tape them to the pencil and now the pencil stands up. But if you push on it at the top it will still fall down. However, if you tape the cardbord ''shear walls'' to the carboard base it will take much more force to push it over.

The same thing that causes the failure of the connection between the posts and the floors they support happens between walls and floors. And thus we get into all the technical apects, hold downs, shear walls, moment resisting connections, floor and roof diaphrams, etc. To make it all more complicated we have old buildings designed before much was know about earthquake forces. Retrofitting these old buildings is too much to get into here. The main point being that it's a costly business, but not impossible. Many Italian engineers are quite capable of designing reftrofits if given the chance and they are capable of designing new buildings to withstand earthquakes. It will be interesting to see if the failure of some relatively new structures in the recent earthquake was due to poor design or poor construction.

Next time I'll talk about what to do in an earthquake, and what you can do to minimize your risk.

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