The New York Times reports: “The earthquake struck at 12:58 a.m (Thursday morning). Pacific time and caused no major damage. The United States Geological Survey estimated its magnitude at 4.6….The quake’s epicenter was located 3 miles northwest of Chatsworth and 7 miles northwest of Northridge, where a much stronger earthquake of 6.7 magnitude did major damage and killed more than 60 people in 1994. Today’s earthquake was followed by three minor aftershocks, according to the Geological Survey.”
How apropos. Let’s continue our discussion about earthquakes by turning our attention to the buildings they damage. You see, buildings are the problem here in earthquake country. If we could all live and work in yurts, things would be great. If a bunch of grass and felt falls on your head while you’re sleeping, it’s no big deal. But unfortunately yurts are not very practical workplaces, so we must turn to stone and steel.
Prior to 1933, buildings in Los Angeles were mostly made of unreinforced masonry. These buildings are usually made out of brick, adobe, or stone and held together by gravity. Unreinforced masonry doesn’t offer any resistance to shearing forces, so just like a stack of blocks these structures will topple over given sufficient shaking. Lucky for Angelinos is that there aren’t any of these buildings left in our city, or at least none that people live or work in. We don’t have these buildings around anymore because (as we talked about) the Long Beach Earthquake alerted everyone to the dangers of using unreinforced masonry for places like schoolhouses. Between 1933 and 1971 non-ductile reinforced concrete was the popular choice.
Ductility measures a material’s ability to deform without breaking. Something very ductile can be stretched and pulled and won’t snap apart. Think of pulling apart silly putty very slowly, that’s ductile. Most building built between 1933 and 1971 were of the non-ductile reinforced concrete variety, but after the 1971 San Fernando quake, we saw the rise of ductile reinforced buildings. The key difference between the two is the amount of confinement you get in your columns. We’ve all heard the term “rebar”, right? Rebar is stuff that makes up the steel skeleton of a building or structure. Ductile reinforced buildings have rebar vertically through a column but also rebar horizontally around a column to make kind of a cage. By having rebar wrap around a column, you can hopefully avoid column bowing that happens if/when the concrete making up the column crumbles during an earthquake.
Oi, the topic of building response to earthquakes is a broad one. I guess we’ll have to save the discussion of soft 1st floor for another entry because now it’s snack time! I’ve got cookies to eat, folks. I'll see you later.