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CU Boulder’s 400 g-ton Centrifuge Applies Vibration Research Control Software

Vibration Control

Dynamic centrifuge modeling replicates the gravitational field of a geotechnical event for studying soil deformation. However, accepted centrifuge scaling laws are demanding on the shake table system. To maximize the control system’s capabilities, researchers at the University of Colorado Boulder introduced a new servo-hydraulic actuator for its 400 g-ton centrifuge. The research team presented their findings at the 10th International Conference on Physical Modelling in Geotechnics in Daejeon, Korea.

Earthquakes and Soil Mechanisms

Engineers must build structures like buildings and bridges to withstand the region’s potential natural events, including earthquakes. The properties of the soil structure, such as strength and stiffness, change with varying stress conditions. Consideration of the soil’s behavior during a seismic event is as important as the construction of the physical structure.

A seismic shock deforms the nearby soil and can trigger liquefaction, where loosely packed, saturated soil takes the form of a convenience store-like slushy, no longer able to support the surrounding structures. With the release of pressure from the solid material, the groundwater pressure rises and forces the liquified material up and out to the open ground, cracking the foundations in its way.

Historically, liquefaction and similar phenomena have been the source of much damage during an earthquake. In seismic applications, geotechnical engineering largely works to understand soil mechanisms following excitation and devise improvements to prevent a disaster.

It is not reasonable for an engineering team to perform a field test of the earth’s stability during a complex loading event because of cost, time, and safety. Instead, engineers create a scaled model of the project and simulate the gravitational effects of an earthquake by spinning the model in a centrifuge.

Centrifuge Modelling

Researchers have developed scaling laws for the model development of many physical phenomena, including earthquakes. Centrifuge modeling is a scaling technique that replicates soil deformation during events such as natural disasters.

Unlike the tabletop device that shares the same name, a geotechnical centrifuge does not function to separate liquids. Instead, engineers use its uniform circular motion to emulate the stress conditions of soil-related geotechnical problems. More specifically, its circular, or centripetal, acceleration increases the sample’s exposure to gravitational acceleration. If gravity were the sole influence, the acceleration of a moving object would be its gravitational acceleration.

The shift in the earth’s mass distribution during an earthquake temporarily disturbs the surrounding gravitational field. A scaled model may pass a test at the earth’s gravitational field (1g or about 9.80 m/s^2), but the gravitational field during a geotechnical event may exceed that value. Under-testing can have dire consequences for the full-scale structure.

The centrifuge technique allows engineers to adjust the gravitational field, making the response of the scaled model closer to what would be expected during a seismic event. First implemented in the 1930s, centrifuge modeling is now considered an “essential tool for geotechnical research” [2].

A New Shaker System

The 400 g-ton centrifuge at the University of Colorado Boulder is the second largest in the United States. An asymmetrical rotor arm is balanced at its center, with a fixed counterweight compartment at one end and a platform for the payload at the other. When the arm is fully extended, the platform can operate at a radius of 18ft, and it can accommodate a payload of up to 4x4x3ft and 4,000lbs.

A research team at CU Boulder sought to upgrade the centrifuge from a mechanical to a servo-hydraulic actuator. In a servo-hydraulic system, a large release of energy from pressurized hydraulic fluid drives the actuator. The team’s goal was to maximize the payload capacity and increase the range and repeatability of earthquake motions. They also incorporated Vibration Research’s control software to optimize the drive signal.

New centrifuge modeling designs can be challenging. Centrifuge scaling laws require that an increase between the full-scale and applied model accelerations and the shaking frequencies follows a model scale factor, which can be demanding on the control system. Additionally, the enhanced gravitational field and increased payload capacity can affect the structure or individual components.

The CU Boulder team outlines their hydraulic design, including the shake table control strategy, in the paper “Development of a new servo-hydraulic earthquake actuator for the 400 g-ton Centrifuge at the University of Colorado Boulder.” The paper also presents the preliminary results in terms of the shaker’s ability to replicate a given acceleration time history. If you are interested in learning more about seismic vibration control, please visit our Seismic Testing page.

Date

January 5, 2023

Author

Cherie Stoll

Category

Vibration Control

References

[1] Westcott, Joelle, Srikanth S. C. Madabhushi, Brad Wham, et. al. “Development of a new servo-hydraulic earthquake actuator for the 400 g-ton Centrifuge at the University of Colorado Boulder,” In Proceedings of the 10th International Conference on Physical Modelling in Geotechnics, Daejeon, Korea, September 2022, 166-169. Seoul, Korea: Korean Geotechnical Society.

[2] Boulanger, Ross W., Daniel W. Wilson, Bruce L. Kutter, et. al. “NHERI Centrifuge Facility: Large-Scale Centrifuge Modeling in Geotechnical Research,” Frontiers in Built Environment 6 (Article 121): 2020. https://doi.org/10.3389/fbuil.2020.00121.

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