Monday, April 20, 2009

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.


  1. Interesting stuff John. I felt the earth shake at 3:32 on the
    6th. I was sleeping on the sofa in my living room. There were guests sleeping in my bedroom and they slept right through it. I'm told that since I live in an open field, in a wood house, that I am in a much safer place should anything ever happen in my area of Molise. How true is this, and why?

  2. Probably fairly true. 1. You live in an open field so nothing can fall on your house. 2. Your wood structure is much lighter than concrete and masonry. And therefore less likely to fall down, and if it does your chances of survival are much higher. 3. A wood structure is somewhat flexible so it can deform and move to a point without catastrophic failure.

    Since I haven't seen your house and in spite of the above comments you aren't necessarily home free. But I would be much more comfortable in you home than one of the old structures in an ancient area. You could have it checked by a structural engineer, but I doubt you will find one familiar with seismic design or retrofitting a wood structure.

    Fire is an added danger in a wood structure in an earthquake. It can be started when a gas appliance like a water heater falls over breaking the gas line. If you have a tank type water heater make sure it is tied well to a wall with metal straps and that the stove etc. is connected with flexible hoses.

  3. Actually, the house was designed by my cousin who is structural engineer. He is well versed in seismic design and he worked closely with others in his field after the earthquake at S. Giuliana. The only problem with him is that he speaks very technically and only in Italian, so I'm usually lost.

    I say you and the family take a trip this summer to my part of the country. You can even camp out at my place along your travels...We'll make it a get together.

    Thanks for the information and I hope you keep writing more on earthquakes and's much easier to understand in English.

  4. Giovanna,
    Sounds like your house should be in great shape. I have to ask why is it built with wood as opposed to the usual? I am trying to get some interest in my experience with wood construction in the US but there is a lot of opposition.

    We may take you up on the camping idea.

  5. Please do come...I would love it! As for the choice of wood was simply because my cousin wanted to build a protype of sorts. He knew that at the time there were no known wooden homes in Molise. He wanted to build a model and then he showcased it. Originally, it was also completely powered by the sun and natural gas. It was actually also in some magazines and in a book about this village.

    Now there are little wooden houses popping up everywhere.


  6. Could you post or send some pictures, I would like to see it.