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by Michelle Walter
In a state where the worst earthquakes may not even rattle windows or shake a flowerpot off the table, powerful tremors will soon be wrenching and twisting massive steel beams and small buildings. Fortunately, these quakes will be powered by precisely controlled hydraulics at the new Multi-Axial Subassemblage Testing (MAST) facility, part of the National Science Foundation's new Network for Earthquake Engineering Simulation (NEES).
Along with 15 other facilities around the world, the MAST facility will advance earthquake engineering research by combining precise experimentation, computation, theory, databases, and model-based simulation to increase our understanding of how earthquakes and other strong forces affect various structures.
The $6.5 million facility is housed in a new building at the edge of campus, between the University transitway and University Village. The 6,000-square-foot testing bay contains an assembly floor and crane to move test structures into position.
The equipment itself consists of a number of unique parts. The largest pieces are two perpendicular "strong walls"--35 feet square and seven feet deep--and a "strong floor" platform that connects them. Together they form three sides of a huge, concrete cube that provides a sturdy foundation for the moving parts of the testing equipment, the crosshead and the actuators.
The crosshead--a 30-foot square cross made of steel four feet thick--applies forces to test structures. It is attached to the strong walls with four horizontal actuators (two on each wall) and to the strong floor with four vertical actuators. The actuators are hydraulically powered and can move the crosshead in six degrees of freedom--allowing researchers to slide and twist the structure along the three axes.
The strong walls and floors have a system of "through holes," which are simply inch-wide pipes that span the entire seven feet of the walls and floors. The actuators can be attached at any height with this system, although the maximum clearance between the crosshead and the floor is 25 feet.
To undergo testing, a structure is placed under the crosshead and secured to the strong floor to prevent it from moving.
Surrounding the strong floor is a shallow trench containing hoses and power cables of the hydraulic system that runs the actuators. The pump room near the lab floor provides the power for the hydraulics, a total of 180 gallons per minute.
The actuators can apply up to 1.8 million pounds of force vertically and 880,000 pounds in each direction horizontally--the equivalent of 65 and 44 full-grown elephants, respectively, jumping up and down on the test structure.
Shaking things up
According to civil engineering professor Cathy French, director of the MAST facility, MAST's ability to move in six degrees of freedom offers researchers a great advantage. Performing controlled experiments that accurately simulated real situations was nearly impossible using older, one-actuator systems, she says.
"In the past, usually you have a single actuator or a couple of actuators applied to different stories of a structure, and you just push in one direction, maybe back and forth to simulate wind or earthquake loading," she explains. "The trouble is, an earthquake doesn't hit just to excite the structure on one axis. It gets deformed in all different directions."
In addition to the eight main actuators, secondary actuators can be attached to the walls to apply lateral forces to simulate wind.
MAST will also use a mixed-mode technique, which allows deformations to be applied in some directions while weight is applied in others.
Structures such as walls or bridges can be built in the assembly area or brought in from an off-site location. In addition to evaluating new materials, the equipment can analyze the condition of existing structures. For example, a piece of a building that underwent an earthquake could be removed and brought to the lab.
After determining the structure's physical condition after the quake, researchers can apply additional deformations and measure their effects. Computer simulations based on this data can help determine how an even stronger quake would affect this type of structure.
The data from this type of experiment can be used to build structures better suited for seismic areas or develop ways of retrofitting existing structures so they will better withstand earthquakes.
The NEES network
The MAST lab is only one part of the new NEES system--the first system with integrated experiments that tests for different criteria and combines all the results in a comprehensive report of a new building's material or construction design.
To facilitate collaboration, the lab will be connected electronically to other NEES facilities, allowing researchers around the country to control and view experiments remotely.
Results from all experiments will be preserved in an archive that won't deteriorate (as previous research archives have) and is open to anyone interested in earthquake engineering.
Steve Olson, a research associate for the MAST lab, says the ultimate goal is to condense the length of time it takes to test a new method and build safer and more earthquake-resistant structures.
Minnesotans may not see the physical results of this research, but homes, businesses, and people around the world will be safer because of this groundbreaking new facility.
