All Shook Up
An unusual test took place in Columbia Engineering’s Carleton Laboratory in mid-February: a simulated earthquake to see if a 2-½ ton, 10’ high pinnacle model from Washington National Cathedral could withstand the vibrations similar to the 5.8-magnitude tremor that struck the DC area in August 2011. The pinnacle was trucked up in pieces to the lab in the middle of snowstorms, reassembled by Cathedral stonemasons with a steel reinforcement rod inserted in the center, and then placed on the Lab’s shake table. The table is a custom-designed 5’ x 5’ piece of equipment that can shake up to a 3-ton payload with 3 g acceleration, three times the acceleration of gravity in the horizontal direction.
Each of the four test runs took all of 60 seconds in front of a rapt audience that included Cathedral officials, Columbia Engineering students, lab staff, and the School’s Civil Engineering Professors George Deodatis and Maria Feng, experts in structural engineering, remote sensing, risk and reliability, and hazard analysis. And, to the delight of all, nothing happened. The pinnacle remained intact.
“The results of these tests are excellent news for all of us,” says Deodatis, who is 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.”
During the 2011 quake, the Cathedral, which is the sixth-largest in the world, sustained damage to its structure: several pinnacles twisted out of alignment or collapsed entirely; many of the pinnacles’ finial stones broke off; carvings were damaged; and falling masonry punched holes through the metal-clad roof. Luckily, no one was injured.
Repairs are well underway and the test in the Carleton Lab demonstrated that the addition of a steel rod in the middle of the pinnacle serves as a significant reinforcement. This new pinnacle design was developed by Deodatis and Robert Mark, a long-term colleague who is professor emeritus of civil engineering and architecture at Princeton University and an expert on Gothic construction. One of Deodatis’ students, Antonio Gonzalez BS’13, MS’13, also worked on the design.
The pinnacle is the largest object ever tested on the shake table. “This is an amazing piece of equipment,” says Adrian Brügger, who manages Carleton Lab. “It’s one of several specialized pieces of testing machinery that we have here in Carleton. For more than 50 years, the lab has been testing and monitoring a wide range of important infrastructure around the world, from bridges and railways to unique buildings like Washington National Cathedral. It’s exciting to be able to help solve real-life problems on structures that define our daily lives, and keep them safe for future generations.”
Feng, who is the Renwick Professor of Civil Engineering at the Engineering School, was interested in a test of her own: trying out new sensors she has developed to measure a number of things, including taking a remote measurement of the displacement response of the pinnacle to the seismic ground motion. Her advanced image-processing algorithm allowed her to use a single video camera to track the movements of multiple points along the entire height of the pinnacle. She 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 explains. “We obtained highly comparable measurements, demonstrating the great potential of inexpensive ubiquitous smartphone sensors.”
Construction began on the Cathedral in 1907 and finished in 1990. Decorative work, including carvings and statuary, is ongoing. Its Neo-Gothic design was modeled closely on the English Gothic style of the 14th century. A $5.5 million construction project for the Phase I earthquake repairs at the Cathedral begins this month. There is an additional $16 million that needs to be raised to complete the rest of the repairs.
“This collaborative effort between Washington National Cathedral and the Carleton Lab was an essential component for us to confirm how we should detail repairs for the Cathedral moving forward” said James W. Shepherd, director of preservation and facilities at the Cathedral. “We are trying to improve how the masonry elements might perform if another seismic event should occur.” The Cathedral has a strong Columbia connection: the first bishop of Washington was Henry Yates Satterlee, who was instrumental in realizing the Cathedral vision and a Columbia alumnus (1863).
—by Holly Evarts