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Urban seismic risk

Urban seismic risk is special subset of the general term seismic risk. It involves the specific problems of cities when they are subjected to earthquakes. Many problems can be solved with good earthquake construction.

Toronto Old and New

Although the city of Toronto, Ontario, Canada is used as an example because of the author's familiarity, this could apply to almost any city in North America.

Cities are a wonderful mixture of old and new construction as in this picture. Note the old brick building mixed in with the new highrises, and the famous Toronto CN Tower. Similar to methodologies used in nuclear reactors, a seismic walkdown of the city is the best way to identify vulnerabilities and possible places for improvement.

Toronto is located on the shores of Lake Ontario, the site of much microseismicity. [1] (http://www.gp.uwo.ca/) Historic records indicate that there is a strong possibility for a rare, strong earthquake, as with many cities in the world.

Luckily for us, most new construction complies with strict building codes, and buildings designed for loads that go beyond seismic. For example, Toronto's highrises are, for the most part, firmly situated on bedrock (for settlement reasons), and are designed for hurricane wind loads. They are not expected to have any problems structurally during an earthquake, although people may feel quite queasy from the sway.

Toronto elevated expressway

Unfortunately, there are many places where the seismic risk (for damage) is quite high. Old brick buildings on poor soils are highly vulnerable to earthquake damage. Some brick chimneys in Toronto have absolutely no mortar, and you can see through them. This picture shows an elevated expressway on filled land. Although there may not be total collapse, there will be sufficient damage to incapacitate the structure. Problems increase if there is the possibility for soil or earthquake liquefaction.

Finally, there is the problem with interiors. Many items such as suspended ceilings and light fixtures have almost no seismic ruggedness. We have all shopped in warehouse stores where heavy merchandise is stacked in narrow racks up to the ceiling.

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