Engineers have restored one of America’s largest cathedrals in the wake of a devastating earthquake, conferring it with the ability to better weather similar seismic events in future.
On August 23, 2011, a 5.8 magnitude earthquake rocked Washington DC. Huge stone finials, gargoyles and crockets on the 301-foot National Cathedral – the sixth-largest in the world, came crashing down, while the southwest pinnacle of the central tower collapsed onto the cathedral roof.
The earthquake caused millions of dollars in damage to the cathedral. Since the disaster, engineers have been working on a solution to prevent similar havoc from afflicting the structure in future.
According to Joe Alonso, head stonemason for the cathedral, the stones of the old pinnacles had only been held together with small dowels. As works began, the expert repair team soon realised they needed a better system of reinforcement. Steel rods were suggested as a possible solution.
To determine if this was the answer a team led by civil and structural engineers from Columbia University created a simulated scenario, costing around $45,000 and requiring a year to complete.
A 2.5-ton, 10 foot high full scale pinnacle model was tested on a shake table at the university laboratory – the largest object to ever be used with the hi-tech simulation equipment. The goal was to see if a steel reinforcing rod screwed through the middle would hold the limestone sections together.
The table is a custom-designed five foot by five foot piece of equipment that can shake a payload of up to three tonnes with three Gs of acceleration, equivalent to three times the acceleration of gravity in the horizontal direction.
The cathedral was situated almost 100 miles from the quake’s epicentre. Although it was difficult to precisely replicate what the cathedral endured, the Columbia engineers came up with a good estimate before bumping the intensity level up a notch.
Limestone from the original quarry in Indiana which provided stone for the construction of the cathedral was even used to recreate as accurate a scenario as possible. The five pre-cut limestone pieces were assembled to form a new version of the pinnacle, but this time fastened using the steel rod.
A series of shakes of varying intensity was applied. The pinnacle structure remained stable throughout the whole process.
“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,” said Alonso.
After the tests, the engineers used magnifiers to examine the surface of the stone for cracks and concluded that it was in perfect condition.
“The results of these tests are excellent news for all of us,” said George Deodatis, the Santiago and Robertina Calatrava Family Professor of Civil Engineering and chair of the Department of Civil Engineering and Engineering Mechanics. “Even though it didn’t look very exciting, this is just how the earthquake occurred, and it’s a good thing that the new pinnacle design didn’t sustain any damage at all.”
Although the test piece won’t be used again in a new pinnacle, the stone will be recycled.
This wasn’t the only test conducted. Maria Feng, Renwick Professor of Civil Engineering at the Engineering School, also tried out new sensors she has developed, which can take a remote measurement of the displacement response of the pinnacle to the seismic ground motion.
This advanced image-processing algorithm makes use of just a single video camera to track the movements of multiple points along the entire height of the pinnacle.
Feng also tested low-cost accelerometers built into smartphones against high-fidelity accelerometers to measure input and response accelerations of the pinnacle.
“This represents the first effort to use smartphone sensors to measure seismic response of structures,” she said. “We obtained highly comparable measurements, demonstrating the great potential of inexpensive ubiquitous smartphone sensors.”
A $5.5 million construction project for the Phase I earthquake repairs at the Cathedral has already commenced, with an additional $16 million still needed to complete the rest of the repairs.