Building for the Big One
A photograph taken in the aftermath of Alaska's 1964 earthquake illustrates a puzzling paradox. Next to a shattered J. C. Penney store are a glass-fronted HFC office, a beauty parlor, and a toy store, all virtually intact.
At 9.2 on the Richter scale, the Good Friday quake was the most powerful measured in North America and the second highest ever recorded. It released over a hundred times more energy than the quake that flattened Port-au-Prince on January 12. And it begs the question: how could those three little buildings be standing, with nary a broken window?
That photo provides an object lesson in what it takes to shake off a major quake, says William Bulleit, chair of Michigan Tech's Department of Civil and Environmental Engineering and an expert on earthquake-resistant structures. As many things do, it all boils down to physics.
"Force equals mass times acceleration," he states. "Masonry is very heavy. The buildings in Haiti were unreinforced and not designed to withstand the force generated by an earthquake."
Sure, concrete looks sturdy. "But that's because you're thinking only of downward loads," says Bulleit. "In an earthquake, the heavier something is, the more lateral force is exerted."
Using lighter materials like wood can help, but to shake off a quake, a building must be tied together, Bulleit says. "Many can handle the downward load of gravity, but not lateral loads. That's why in quakes you have walls falling down and then the roofs falling down on top of them."
"You need to tie the walls to the foundation, tie the roof to the walls, and make sure no critical piece fails," he explains. Buildings with large spaces held up only by columns are especially prone to collapse. "It's hard to make columns that can handle large lateral forces. Imagine what would happen if you stuck a toothpick in an apple and shook it. When columns are holding up a heavy slab, they can break and the building can pancake. That's one reason why so many people in Haiti were crushed and trapped."
An earthquake of similar magnitude wouldn't do near the damage in the US, which has rigorous building codes. "For most of us, the biggest danger would be having bookshelves fall down on top of us," Bulleit says. Should another Good Friday quake occur, however, the outcome might be different. "We're pretty good at engineering for earthquakes in the US, but there's no guarantee we could handle a big one," Bulleit says. "All you could hope for is that the building wouldn't fall down on you."