The East Coast earthquake rattled buildings, nerves and our historic national monuments, all in just a few seconds.
In Washington, D.C., about 90 miles from the magnitude-5.8 earthquake's epicenter in Mineral, Va., the National Cathedral sustained millions of dollars in damage to the building's tallest limestone spires (thankfully, the Darth Vader gargoyle remains unharmed).
The 154-year-old Gothic-style Smithsonian Castle located on the National Mall had some minor cracks and broken glass.
Then there's the city's tallest peak, the 555-foot-tall Washington Monument, which National Park Service officials closed indefinitely after engineers discovered cracks near the top of the memorial.
Park Service spokesman Bill Line told Discovery News that one of the cracks is 4 feet long and 1 inch wide and located entirely in one stone.
Line said a team of structural engineers is currently analyzing the damage, after which it will recommend options for repairing the monument.
"At that time is when the Park Service would step in to conduct those repairs, and only once the repairs have been made, and the structural engineers have taken a look at them, will the memorial be reopened to the public," he said.
Construction of the marble and granite obelisk began in 1848 and was finished in 1884 (with a 20-year stoppage in between -– thus the memorial’s two-tone look, since the marble was quarried from two different places).
At the time of its completion, the Washington Monument was the tallest building in the world, until it was eclipsed by the Eiffel Tower in 1889; to this day, the monument remains the world’s tallest freestanding stone structure. That means that besides an independent iron staircase inside the structure, the rest of the obelisk is all stone and mortar. So how did it not fall down?
"This is a truly significant and shallow earthquake and so it generated a decent-size acceleration for the region," said Matt Tobolski, president and CEO of Tobolski Watkins Engineering, which specializes in structural and earthquake engineering. "But D.C. happens to be far enough away that what it sees is not as substantial in comparison."
The monument has survived several other strong eastern earthquakes, according to Line, including a magnitude-5.9 quake in 1897 that was also centered in Virginia, as well as a 1944 tremor in New York that was the last quake of equal power to hit the region. One in 1884 centered near Columbus, Ohio, is said to have been felt by workmen on top of the then-unfinished monument.
Turns out the monument, like the man it was named for, is no pushover.
"But," Tobolski added, "the earthquake definitely showed the vulnerability of unreinforced masonry systems. Masonry is really, really good in compression, so I can push on it all day long. But as soon as you overcome its cracking capacity, if it's not reinforced, cracks just propagate."
Tobolski's San Diego-based firm has retrofitted numerous historic buildings. "If you wanted to somehow strengthen the Washington Monument, it’s one heck of a challenge just when you think about trying to hide any sort of upgrade."
One technique could be to use steel reinforcing bars, something he describes as a kind of rubber band system that holds the structure in place.
"When you were a kid, did you ever have one of those toys that had those legs that when you pushed a button they always spring back?" That's the kind of mentality. You basically put this long rubber band inside of it that holds the building together. So as you push it, it always wants to come back together."
Tobolski also wondered if the cracks found on the monument were due solely to the earthquake.
"This is just speaking aloud; I'm not really sure from the pictures I saw how much of this really might have been the earthquake. The accelerations it would have seen, it most likely would have seen larger forces from wind loads over its lifetime."
Winds, huh? Good thing we don't get too much of that around … ohhh wait.
Photos: U.S. Park Police Aviation Unit, Amy Enchelmeyer
Tags: Earthquakes, Engineering, Geology, Safety
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