Virtual Validation Aids in the Design of Earthquake-Resilient Buildings

MTS Systems, a manufacturer of mechanical test and simulation equipment, makes many of the shake tables installed around the world today. The company designs and builds shake tables for seismic testing and to test automotive and aerospace components for durability, performance, and vibration transmissibility. 

An inaccurate table design can cost the company millions of dollars. Mechanical engineers at MTS who design shake tables, as well as the company’s customers who use these tables, must be able to predict whether a table can accurately replicate an earthquake’s forces.

Testing the impact of a seismic event on a physical-scale model of a colonial-era church. (Images credit: Universidad Mariano Gálvez de Guatemala, courtesy of MTS Systems)

For this, they rely on a unique tool called the Degree of Freedom System Analysis Tool (DOFSAT). Built by senior MTS staff engineer Bradford Thoen using MATLAB^® and Simulink^®, DOFSAT helps MTS mechanical engineers validate their shake table design. Once the table is built and installed in the customer’s lab, it enables him to design his test to determine key factors such as how big he can make his physical model, whether the earthquake is within the motion and force limits of the table, how long a test can last before pressurized oil runs out, and the fewest number of hydraulic pumps needed. “This improves the accuracy and reduces the operational costs of testing,” says Thoen.

Shaking Up Seismic Simulator Design

MTS shake tables range in size from 1.5-by-1.5 meter, 2-ton tables that move along one axis for testing subscale models all the way up to 16-by-20 meter, 1,000-ton tables with six degrees of freedom for testing full-scale building models.

“For example, a customer will give us some earthquake records and say they want to test for this earthquake on a specimen that weighs 800 tons,” says Thoen. “They want to operate the tables at full capacity to get every last drop of performance out of these very complex and expensive systems by putting the biggest, heaviest possible specimen on the table and operating on it at maximum force levels. Maximizing specimen size mitigates test inaccuracy inherent in scaling down the size of a model structure.”

They can then tweak their design and do multiple iterations quickly until their table design recreates the desired seismic event.

Making DOFSAT More Accessible

Thoen continually updates the DOFSAT app. He recently added the ability to predict swivel joint angles. Both ends of the hydraulic actuators have mechanical swivel joints that allow the actuator to conform to the motion of the table. These swivels have rotation limits. The DOFSAT app predicts the azimuth, elevation, and skew angles that each swivel will experience during an earthquake test, allowing MTS design engineers to verify that their choice of swivel rotation limits is adequate.

“The MathWorks team worked with us to make changes and updates to the executables,” says Thoen. “Working with MathWorks has been invaluable in making DOFSAT accessible around the world.”