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.

Monday, April 20, 2009

More Venice & the Beach at Jesolo

Earthquakes in Italy part 2

Last time I give an idea of how earthquakes can cause problems with old buildings. Now I want to say a little about how we prevent the damage. It is a very complicated subject, but I just want to give you a feeling for it. Lets say we take our little building of blocks and this time put a vertical row of transparent tape down both sides of each row of blocks. We should also put a couple horizontal rows. Now if we shake the cardboard base the blocks will stay together until the force applied becomes really strong. The tape seems much weaker than the blocks, but it is much stronger in tension than the weak joints between them (in this case only weight holds them together without the tape). The thing is that masonry and concrete are very weak in tension, even though strong in compression and the joints between masonry units are even weaker. So we see in modern buildings steel is used to provide the tensile strength masonry and concrete lack. We have to also apply the same logic when attaching the floors and roof. We can just sit the floor on top of the walls which is fine for the vertical forces but as soon as we move the building horizontally as in an earthquake the floor might just slide right off.

You probably noticed if you built the model that even though the model did not fall down it slid around a bit. In a way this is good as the building is still intact, in fact some high rise buildings are designed to sit on a kind of rubber cushion that allows for a small amount of horizontal movement. But in most cases we don't want our buildings sliding around. So steel is again used to tie the building to the concrete foundation.

After the Northridge, California earthquake a few years ago I did some inspections for FEMA of damaged homes. It was interesting to see how some had slid sideways on their foundations, as they had not been properly tied down to the foundation.

Next time, buildings with lots of windows, or built up on posts over open spaces, shear walls and moment resisting connections.

Friday, April 17, 2009

Earthquakes in Italy

Italy is a country where earthquakes are a constant threat. The main problem is that the old buildings have very little resistance to them. If you want to understand what happens in an earthquake try this. Take a piece of cardboard and build a little house of blocks just staked one on top of the other on the cardboard. Now grab an edge of the cardboard and pull it back and forth horizontally. As you move the cardboard the blocks will start to separate and eventually fall down. This is basically the way the old Italian buildings, and buildings in many other countries are built. They are made of stones or masonry blocks/bricks just stacked up and held lightly together with cement mortar.

Earthquakes generally move the earth horizontally, with some vertical movement as well. If you imagine the house of blocks model several floors tall with heavy concrete slabs for floors and a heavy roof on top you can imagine what happens. The inertia of the mass of the floors and roofs make them want to stay where they are. The earth moves under them and the walls start to separate and or tilt. Then they can't support the mass above and the whole thing comes falling down.

More on this to follow.