Lab- created quake creates shake tests for building tech
NEW YORK — The “earthquake” was seconds away. The floodlights were on. The cameras and the sensorswere ready.
All eyes were on the eight- foot, two- ton section of a Washington National Cathedral pinnacle that stood like a missile in the Columbia University engineering laboratory.
It was the biggest object ever tested on the lab’s shake table. ( The table can hold three tons, or 6,000 pounds.) The plan was to see whether a steel reinforcing rod screwed through the middlewould hold the limestone sections together.
Shortly before noon, a warning buzzer sounded. A blue emergency light began flashing, and the crowd of student bystanders put on their hard hats.
Lab director Adrian Brugger announced that the quake was about to begin. “And, start,” he said. The metal table on which the pinnacle was bolted began to jerk back and forth, squeaking as it did. There was no rumbling. No falling debris. No one fleeing into the streets. It lasted 30 seconds. Brugger turned from a bank of computer screens and said, “That’s it.” The room was silent. That was it? Yes, he said, thatwas it. Applause broke out.
Steel rodsworkwell
Lastweek, an earthquake similar to the 5.8- magnitude quake that struck the Washington area in August 2011 was simulated in New York City. But itwas in a laboratory. There were no injuries, and the whole thing was sort of underwhelming.
The simulation was a success, though. The steel rod held the pieces of the test pinnacle firmly together. There was nary a crack in the stone or the mortar.
“Well, there you have it,” said Joe Alonso, the cathedral’s head stonemason, whohad helped put the test together and was watching. “I’m glad what we didn’t see. We saw it on August 23, 2011. That was enough for me.”
“It’s not very interesting,” said George Deodatis, chairman of Columbia’s Department of Civil Engineering and Engineering Mechanics. “But this is howthe earthquake occurred.”
He said the engineers thought about adding some low- frequency noise to the test for effect. “But we’re not Hollywood,” he said. “We’re scientists.”
The real earthquake, on a Tuesday afternoon, rattled the 301- foot- tall Episcopal cathedral in Northwest Washington to the tips of its majestic towers. Gargoyles,
“Without that rod, if that was doweled together with those little bronze dowels ... there’s no doubt this thing would have come apart just like they did in the quake.”
Joe Alonso, Washington National Cathedral’s head stonemason
crockets and huge stone finials shook loose and came crashing down.
The quake knocked off the top of the southwest pinnacle on the cathedral’s central tower. The pinnacle fell on the cathedral roof, and the shaking nearly knocked over the tops of three other pinnacles, which were within seconds of falling.
All loose or dislodged stone was subsequently removed from the structure, which suffered millions of dollars in damage. Full repairs are expected to take years.
The stones of the old pinnacles had been held together with small bronze dowels, Alonso said. As earthquake repairs proceeded on the English Gothicstyle cathedral, experts realized they needed a better system of reinforcement.
The steel rod seemed to be the answer.
Alonso, who has been immersed in the cathedral’s repairs since the quake, said he thought that it worked.
“That rod that’s in there, theway we [ screwed] it down, I would imagine that did what it was supposed to do,” he said.“Without that rod, if that was doweled together with those little bronze dowels ... there’s no doubt this thing would havecomeapart just like they did in the quake.”
As for the simulation, Deodatis said it was difficult to replicate exactly what was experienced at the cathedral, almost 100 miles from the quake’s epicenter near Mineral, Va.
The Columbia engineers came up with a good estimate and then bumped the intensity up a notch. “It’s very close to what happened that day,” Deodatis said.
The test, which cost about $ 45,000, was about a year in the making, Alonso said.
It was largely arranged by longtime cathedral consultant Robert Mark, a professor emeritus of architecture and civil engineering at Princeton University. Mark and Deodatiswere colleagues at Princeton.
Mark, who was present at the test, said an early plan had been to place a model of the entire cathedral on the shake table. But he realized that a model would not generate the data theywanted.
“We decided it’s better to do it at full scale because we wanted to check how the stone and the mortar stood up,” he said.
Tosetupthe test, Alonsoordered five pre- cut limestone pieces from the same quarry in Indiana that provided stone for the construction of the cathedral. He and other cathedral stonemasons then assembled the pieces into a rough copy of the pinnacle section that fell.
Reassembling history
Last month, the disassembled pieces were trucked to Columbia, andthis month, Alonsoandhismen reassembled them in the Columbia laboratory and fastened them together with the rod.
“The intent was ... to simulate as close as possible the finished and fully re- created pinnacle that’s going to go back up on the central tower,” said James W. Shepherd, the cathedral’s director of preservation and facilities.
The test piecewon’t be used in a new pinnacle, Alonso said, but the stone will be recycled.
The test included two other simulated earthquakes, a lesser shake and a more severe shake. The pinnacle structure was sound throughout.
After the tests, Alonso and Deodatis used magnifiers to examine the surface of the stone for cracks.
“It’s in perfect condition,” Deodatis said atonepoint. “You’re absolutely safe.”
He added later: “I would stand next to this during an earthquake.”
“You can’t get a better guarantee than that,” Mark said.