Outdoor earthquake shake table contributes to greater structural safety

Research on one of the world’s largest earthquake simulators has made buildings, bridges, and other infrastructure more earthquake-safe.

Homes, offices, parking garages, bridges and other structures across the United States are safer during earthquakes thanks to research done at the University of California San Diego’s outdoor earthquake shake table. At the same time, recent tests have also focused on developing new designs that make it more likely that buildings will be safe to use after earthquakes with minimal repairs required.

In short, the United States and the entire world are safer from earthquakes thanks to 20 years of testing life-sized buildings and other large structures at UC San Diego’s large, outdoor earthquake simulator, also known as a “shake table.” It is the only outdoor facility of this kind, and one of the three largest simulators in the world.

“Our UC San Diego earthquake simulator is truly a national treasure that is a resource to structural and geotechnical engineers. It is one of very few sites around the world capable of conducting the types of seismic experiments necessary to validate the design and construction of earthquake-safe structures and to certify the response of critical infrastructure components,” said John McCartney, professor of structural engineering at the UC San Diego Jacobs School of Engineering.

The U.S. National Science Foundation is a key funder of the shake table through support for shake table operations; research projects; a game-changing facility upgrade; student researchers and more. Tests on the shake table are conducted in partnership with many companies and industry groups, which leverage federal investment to perform research that would otherwise be impossible and keeps the U.S. design and construction techniques at the cutting edge, globally.

Research at the shake table is far from complete. Right now, the site is buzzing with activity as engineers get ready to put to the test a 10-story cold-formed steel building this summer.

As the UC San Diego structural engineers and their colleagues prepare for this next big test, we’ve taken a look back at some shake table highlights over the last two decades.

But first, a quick introduction to the shake table.

What is an earthquake shake table?

The simulator is also called a shake table because it comprises a steel plate connected to actuators—similar to pistons—which make the plate move and shake whatever is on top of it. Researchers program the table to move in ways that recreate actual ground motions that have occurred during past earthquakes such as the 6.7 Northridge earthquake which struck Los Angeles in 1994 and the magnitude 6.9 Loma Prieta earthquake which struck Northern California in 1989.

When researchers construct full-size buildings or other large structures and place them on top of the shake table, the buildings get shaken as if they are experiencing a real earthquake.

This allows researchers to study how earthquakes damage—or perhaps don’t damage—buildings, bridges, roads and more. The data can then be used by broader seismic safety communities to design safer structures and to improve construction techniques, while also pushing the envelope on what’s possible.

For the upcoming tests, for example, one of the big goals is to determine if taller buildings made of cold-formed steel can be safely built beyond the current six-floor height limit for this type of construction.

Now, for a look at some of the positive impacts that have come from past tests at the shake table.

Soft-story buildings

In San Francisco, approximately 6,000 “soft-story” wood-frame buildings are being retrofitted to withstand earthquakes. These multi-story buildings typically feature wide open spaces like a garage on the first floor, and are more likely to collapse during a quake as a result.

Full-scale testing of the retrofitting systems to make these soft-story wood-frame buildings safer during an earthquake took place on the UC San Diego shake table in 2013. Professor John Van de Lindt from Colorado State University, who led this project, played a key role in writing the guidelines for the retrofit of these buildings.

Elevators and facades

During a one-of-a-kind test in 2012, a team led by UC San Diego structural engineering professor Tara Hutchinson investigated the safety of a building’s non-structural components—such as elevators, stairs, ceilings, facades and fire suppression systems. These tests were the first in the United States to focus on a broad range of so-called non-structural systems and equipment that can malfunction during an earthquake and make it more difficult to evacuate buildings, which can then lead to more injuries and deaths.

The tests resulted in new building design requirements for the way facades are attached to the structural skeleton of a building, and new construction methods for elevators, to name a few, and they will likely yield more insights in coming years.

Concrete parking garages

In 2008, a test led by structural engineering professors Robert Fleischman of the University of Arizona and Jose Restrepo of UC San Diego led to new recommendations on how precast concrete floors, known as diaphragms, should be built into parking garage structures to improve their behavior during earthquakes.

These new design recommendations are meant to prevent the future failure of buildings, much like what happened during the 6.7 magnitude earthquake in Northridge, Calif. in 1994, when several precast parking structures collapsed.

Older buildings with brick walls

Research by UC San Diego structural engineering professor Benson Shing has led to more accurate methods to assess the seismic safety of older buildings that combine weak reinforced concrete frames and unreinforced masonry walls, and better retrofit methods to improve their performance during earthquakes. In 2011, researchers led by Shing tested a three-story masonry structure on the shake table; this was the first time this type of structure had been tested at this scale on an earthquake simulator.

These are just a few examples of how tests run on the UC San Diego shake table have led to design code changes and retrofits to enhance earthquake safety and resilience. More code updates are expected as results from more recent tests, including the 2023 10-story Tallwood project, are studied and changes implemented.

Looking ahead

This June, researchers led by UC San Diego’s Hutchinson will put a 10-story cold-formed steel building to the test on the shake table. Current cold-formed steel buildings can be no taller than 65 feet, or about six stories— the researchers will study how this 100-foot-tall, 10-story building performs during an earthquake, and assess the safety of taller buildings made of this material. They’ll also be studying the performance of facades, stairs, windows and doors during a temblor.

After the earthquake testing is over and all data has been gathered, the building will be put through a live fire test to study how these components and the structure itself fare in a fire as well.

Large-scale tests like this are often conducted in partnership with various industry collaborators evaluating the earthquake response and safety of their technologies—from connectors to facades, seismic dampers and more.

Since no single company is equipped to fund the full cost of constructing the whole 10-story building that will house these smaller components, support from the National Science Foundation is what enables researchers to initiate a project this large, and then garner industry partnership and participation. The shake table also supports smaller industry projects for critical components such as electrical transformers and nuclear casks.

After this test, the UC San Diego shake table will continue to support academic researchers and industry partners from around the world to test the safety of new building materials and designs.

“This federally funded facility has made us all safer and more resilient in the face of earthquakes and related disasters like fires, and we will continue to work toward gaining critical experimental data to advance our design codes and explore new technologies to retrofit existing buildings to make our infrastructure more resilient,” said McCartney, the UC San Diego structural engineering professor who currently serves as the principal investigator on NSF funding that enables the shake table to operate for the public good.

Under the hood

What makes the shake table shake? Large hydraulic actuators, similar to pistons, that are underneath the table do the shaking. These actuators move the large steel plate which is 40 feet by 25 feet and weighs 330,000 lbs. This system can hold and shake up to 5 million pounds of mass (or 400 metric tons)—more even than an entire 10-story-tall building.

The actuators move the plate in multiple directions—east-west, north-south and up-down—in order to simulate realistic earthquake ground motions. The plate can also rotate about all these axes—pitch, roll and yaw. If you imagine all the subtle directions that airplanes can move in flight, those are the directions that the shake table can move.

The shake table is powered by an ingenious hydraulic system, which pumps 9,500 liters of hydraulic fluid (about 2,500 gallons) into accumulator banks filled with nitrogen. The interaction of the oil and nitrogen pressurizes the oil at 5,000 PSI. The oil is then released at 3,000 PSI to drive the horizontal and vertical actuators, or pistons, that move the table. For reference, a car tire is pressurized at 32 PSI.

A critical upgrade

In 2022, the UC San Diego shake table was upgraded from back-and-forth shaking to today’s capability of shaking buildings in so many directions at once. This critical upgrade was funded thanks to a $16.9 million upgrade funded by the National Science Foundation along with $4 million from UC San Diego.

The shake table went from being able to move in one degree of freedom—east-west—to three translational degrees of freedom—east-west, north-south, up and down, as well three rotational degrees of freedom: roll, pitch and yaw.

The table’s upgraded movement capabilities allow research teams from across the country to simulate how the ground moves during earthquakes much more realistically—and by extension researchers can study, in greater detail, how buildings and other structures fare during earthquakes.

UC San Diego’s shake table also simulates the speed at which the ground can move during earthquakes. In fact, the shake table can simulate ground motion speeds up to six feet per second. It can also simulate accelerations of up to 4gs. For reference, astronauts experience 3gs during a rocket take off. These capabilities allow researchers to create realistic simulations of the most devastating earthquakes on record.

Tests on life-sized buildings at the earthquake shake table have led to important changes in design codes for commercial and residential structures. Research has also led to new insights into the seismic performance of geotechnical systems, such as foundations, tunnels and retaining walls.

Even with all of these advances, there is so much more research that needs to be done in order to make communities all across the country more prepared for earthquakes. This work aims to save lives and also help communities to be prepared to bounce back after earthquakes while reducing the need for hugely expensive and time-consuming repairs to all the built infrastructure in our communities.

The shake table facility is part of the UC San Diego Englekirk Structural Engineering Center within the Department of Structural Engineering at the UC San Diego Jacobs School of Engineering.