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Understanding earthquakes

As part of the earthquake engineering test, Lehigh fabricated replicas of the columns of the off-ramp bridge on I-10 in Santa Monica, California that were affected by the 1994 earthquake.

Some experiments are too big to be confined to one laboratory or one testing method—even when researchers have access to the world’s finest labs and most powerful supercomputers.

Such was the case last spring when Lehigh University, the University of Illinois and Rensselaer Polytechnic Institute (RPI) conducted one Internet-linked experiment at three separate laboratories.

In five hours, the three institutions re-enacted the impact of the Northridge (Los Angeles) Earthquake of 1994 on Collector-Distributor 36, an off-ramp of the I-10 Freeway in Santa Monica, which was severely damaged by the event. Overall, the earthquake, which was the costliest in U.S. history, left 51 people dead and caused $40 billion in damage.

The goal of the three-way experiment was to improve the methods of testing new structural systems that are expected one day to enable bridges, buildings and other structures to survive earthquakes with little or no damage.

The three schools synchronized and coordinated lab tests on replicas of the various parts of the Santa Monica Bridge. The three test sites were linked by Internet2, which enabled researchers to share data and interact as the test progressed.

The experiment was coordinated by the University of Illinois’ National Center for Supercomputer Applications (NCSA).

Researchers call the unprecedented experiment a “distributed hybrid test” because it was conducted at three sites and consisted of physical tests as well as numerical simulations on a computer. In a distributed hybrid test, selected parts of a structural system are modeled as physical test structures in different labs, while the remaining part of the systems are modeled analytically as a numerical substructure.

A test proceeds in a synchronized manner by having the experimental coordinator generate command displacements for the test structures and numerical substructure for a given “time step” during the earthquake.

The Lehigh-Illinois-RPI researchers say the results of their experiment, which were presented recently to the National Science Foundation (NSF), will help engineers in their quest to design and build earthquake-resistant structures.

“A distributed hybrid test obtains an integrated picture of what happens systemically to a bridge and its columns and soil foundation during an earthquake,” says Jim Ricles, director of the RTMD (Real-Time Multi-Directional) Laboratory in Lehigh’s Advanced Technology for Large Structural Systems (ATLSS) Research Center.

Until now, said Ricles, earthquake engineering experiments have focused on the performance of individual structural members, such as a column or a connection, as opposed to the performance of entire structures.

“Before, one isolated specimen was tested without considering the overall bridge structure,” Ricles said. “If you test only one isolated member, you can obtain the wrong answer by neglecting the effects of that member’s interaction with the remaining parts of the structure that were excluded from the test setup.

“Now that we have proven the worth and accuracy of distributed hybrid testing, we can use it to verify alternative types of constructions and renovations. It can be a tool to demonstrate as realistically and accurately as possible the real-time performance of a structure during an earthquake.”

A national effort

The Lehigh-Illinois-RPI test, called the Multi-Site Soil-Structure-Foundation Interaction Test, or MISST, was funded by NSF through its George E. Brown Jr. Network for Earthquake Engineering Simulation. NEES is a consortium of 15 universities, including Lehigh, Illinois and RPI, that conduct earthquake engineering research.

The three schools fabricated half-scale replicas of components of the 420-foot-long off-ramp bridge on I-10 in Santa Monica and then portioned out their experiment.

One bridge column was tested in Lehigh’s RTMD Lab and a second at the University of Illinois. The soil foundation of a third column was tested at RPI. The remaining structural members of the bridge, including the bridge deck, were simulated, or tested numerically (using finite element analysis models), on the NCSA supercomputer.

Lehigh’s RTMD Lab, one of the largest facilities of its kind, can subject structures to loads (forces) and loading rates similar to what they would sustain during the most powerful earthquakes. Likewise, the NCSA at the University of Illinois is one of the world’s largest supercomputing facilities.

Nevertheless, said Ricles, “the entire bridge structure, with its multiple components and spans, was too large for us to conduct one test in one lab. Distributed hybrid testing allowed us to break the project down into manageable pieces.”

The 1994 earthquake lasted about 25 seconds, said Ricles. Researchers were able to conduct the recent test over a much longer period of time without skewing the results because the bridge material—reinforced concrete—is not sensitive to the rate of loading, or earthquake forces.

Remarkably, said Ricles, the columns in the distributed hybrid test sustained almost the exact behavior and degree of damage as that sustained by the columns of the actual off-ramp bridge. Shear cracks formed in the column, expanded into diagonal cracks and then opened, allowing one part of the column to slide over the other part.

During the 1994 earthquake, the columns of the off-ramp bridge failed (requiring the bridge to be replaced), said Ricles, but the bridge did not collapse and no one was killed.

In the distributed hybrid test, the Lehigh column failed a fraction of a second later than the University of Illinois column, in part because the collapse of the Illinois column forced a redistribution of the seismic loading onto the Lehigh column.

The distributed hybrid test will help engineers verify the accuracy and effectiveness of numerically simulated testing methods, said Sougata Roy, ATLSS research scientist, who developed the numerical simulation model for the test.

“We do physical lab tests to verify the simulation model,” said Roy, “and to develop and refine new models that can predict the performance of complex phenomena in the real world.

“The distributed hybrid test has enabled us to calibrate and verify our numerical simulation model.”

“Our conclusion,” said Ricles, “is that the method of distributed hybrid testing works and can help us design and build a structure that will survive earthquakes essentially damage-free and that will also be more reliable, safe and economical to monitor and maintain.”

Systems that are expected to make future structures more earthquake-resistant include self-centering columns and braced frames in buildings and “smart” dampers in bridge abutments. These systems are being tested at Lehigh.

“When the Santa Monica Freeway off-ramp was built,” said Ricles, “the design philosophy was to accept damage during an earthquake as long as people did not get killed. Today, we are working on next-generation structures that will seek to avoid seismic damage altogether.”

--Kurt Pfitzer

Posted on Wednesday, May 02, 2007

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