Earthquakes, buildings and the seismic hazard

Whoa, did you feel it? The ground was moving and things were shaking – yes it was an earthquake!

Now where we are based (Toronto, Canada) we don’t get many quakes so our buildings don’t have the rigid seismic code requirements that other jurisdictions have, like Vancouver or San Francisco. In those (and other locations around the globe) the seismic hazard is far more severe.

But have you ever thought what some of the implications of earthquakes are to buildings?

Lets start from the ground up. In severe earthquakes the ground can undergo a phenomenon called liquefaction where what has seemed to be solid soil looses its ability to bear weight and consequently buildings sink in unpredictable ways leading to damage to the structural elements.

Most of the damage sustained by buildings is due to ground shaking and the most vulnerable elements in a house or office building are those that are brittle and can’t resist movement in an up/down or side-to-side manner.

Materials like concrete, brick or block such as those used in foundations have a tendency to crack and could lose their abilities to carry weight loads from above. Structural elements like steel (either columns or posts) and wood are termed “Ductile” and are often able to flex and return to their original positions without failing. This will be important, as we will see in a minute.

The intent of building codes is to prevent collapse and loss of life in such an event, but not necessarily to prevent all damage to a building. Let me explain: ductile elements can flex to lessen the impact of severe shocks on a building’s structure which absorbs the energy of the quake rather than transmitting it to less forgiving elements of the building. So the end result may be some damage to the building rather than complete collapse. As long as the building stays intact so that occupants can get out, the structure has done its job.

Building science now understands far more about the effect of earthquakes on structures and has advanced so that with tools like computer modeling and new materials, we can see how forces will be transmitted through the building and can design accordingly to compensate. The materials have also advanced so that there is more thought given to the loads they have to withstand and the effects of those loads on the connections to other materials.

So even though not all buildings need to be designed to resist the seismic forces of an earthquake there are many things, which engineers and architects have learned in studying them that can be applied to normal structures. And that is definitely something you care about when the ground starts moving.

Did you feel the earth move? Let us know where you were and what you thought in the comments below! And don’t forget to sign up for our newsletter where we give you helpful tips to avoid problems in your projects.

 

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