On a unique outdoor earthquake simulator, engineers recently subjected to testing the tallest steel-frame building ever tested in this way. The ten-story structure, approximately 30 meters high, swayed, twisted, and shook as powerful hydraulic actuators replicated the devastating forces of some of the most infamous earthquakes, including the 6.9 magnitude Loma Prieta earthquake that struck California in 1989. The primary goal of this ambitious project was to scientifically determine whether existing height limitations for buildings constructed from cold-formed steel could be significantly increased, opening the door to a new era in construction.

This series of tests took place on an earthquake platform, better known as a "shake table," at the University of California, San Diego (UC San Diego), a facility funded by the U.S. National Science Foundation (NSF). It is one of the three largest earthquake simulators in the world and the only one located outdoors. This characteristic is crucial for tests that push the boundaries, such as this one, as it allows for the testing of structures taller than 27 meters, a feat that cannot be performed anywhere else in the world. Just two years ago, a 35-meter tall mass timber building was tested on the same platform, at the time the tallest building ever subjected to seismic simulation.

Focus on the Material of the Future: Cold-Formed Steel

This year's research is focused on a building whose load-bearing structure is made of cold-formed steel (CFS). This material stands out as lightweight, sustainable, and, extremely importantly, non-combustible. Its environmental component is also significant, considering it is produced from 60 to 70 percent recycled metal. Despite these advantages, current building codes in many regions, including seismically active areas, limit the height of CFS structures to 20 meters, or six stories. The researchers posed a key question: can this limit be safely raised to 10 stories, or 30 meters? The test results so far strongly suggest an affirmative answer.

“The building performed remarkably well,” stated Tara Hutchinson, the project leader and a professor in the Department of Structural Engineering at UC San Diego. “Despite a series of 18 earthquake simulations of increasing intensity – including three extremely strong earthquakes that were at or even above what designers must consider in their designs – the load-bearing structural system maintained its integrity.”

Although some damage to the non-structural elements of the building was expected, key safety aspects remained functional. The staircases, which are vital for the safe evacuation of occupants and designed to move with the building, remained fully passable and usable even after the strongest earthquakes. “Inside this building, we installed nearly a thousand sensors to measure its response in terms of acceleration, displacement, and local stresses. We have an outstanding set of data to analyze that will ultimately help us improve building codes and support the design community's desire to use this excellent material in the construction of taller, lighter, and more resilient buildings,” Hutchinson added.

One of the key advantages of cold-formed steel is its low mass, which allows for the assembly of modular units in factory conditions. These units are then joined on the construction site to form a complete building, reminiscent of assembling giant Lego bricks. This technique dramatically shortens construction time compared to traditional on-site frame construction from scratch.

The Technological Marvel Behind the Scenes: The Upgraded Earthquake Simulator

These tests also highlighted the importance of the recent, $17 million upgrade to the earthquake platform, also funded by the NSF. The project, completed in April 2022, gave the simulator the ability to move in six degrees of freedom (6-DOF). Before the upgrade, the platform could only move in one direction, east-west. Now, it can also move up-down (vertical), north-south (lateral), and perform rotational movements known as pitch, roll, and yaw.

Footage of real earthquakes shows that the ground does not just shake in one direction. It moves back-and-forth, up-and-down, from side-to-side, and even oscillates. "Here we are able to simulate what we call near-real earthquake conditions," explained Joel Conte, one of the lead researchers at the simulator and a professor at UC San Diego. During one of the tests, the researchers observed a significant amount of torsional, or twisting, motion in the building's movements – something that could not have been observed while the platform was moving only one-dimensionally. "The motions we saw today demonstrated why the platform upgrade was critical for the science we are doing here," added Ben Schafer, co-leader of the CFS10 project and an engineering professor at Johns Hopkins University.

What's Next? Post-Earthquake Fire Resistance Testing

The series of tests, however, is not yet over. In addition to a detailed inspection of the building's physical condition after the seismic tests, the research team is preparing for the final phase: a live fire test, which will take place during July 2025. These tests, led by Professor Richard Emberley of Cal Poly-San Luis Obispo, aim to understand the spread of temperature, smoke, and particles through parts of the building previously damaged by the earthquake. This is a realistic scenario known as a "fire-following earthquake," which can be triggered by gas leaks or other hazardous materials that serve as an ignition source.

"Cold-formed steel is a non-combustible material, unlike wood and some other building materials, which is an important beneficial characteristic if fires are a cause for concern," Hutchinson emphasized. Understanding how a damaged structure behaves in fire conditions is crucial for developing comprehensive safety standards.

Broad Support for the Future of Construction

The CFS10 project would not have been possible without broad support from academic, government, and industry circles. In addition to key funding from the NSF, the tests were also sponsored by the U.S. Department of Housing and Urban Development, the California Seismic Safety Commission, the California Governor's Office of Emergency Services, and the National Institute of Standards and Technology.

Significant support was also provided by numerous industry organizations, such as the American Iron and Steel Institute, the Steel Framing Industry Association, and the American Institute of Steel Construction. Many companies made significant contributions to the design and construction efforts, including ClarkDietrich, Clark Construction, Standard Drywall Inc., Mid-Rise Modular, Bapko Metal, Grabber Fastening, and others. This strong collaboration demonstrates a shared interest in innovations that will enable the construction of safer and more resilient communities in the future.

Source: University of California