The earth shakes millions of times every year. Often, these earthquakes strike in familiar places, such as the recent, deadly quakes in Ecuador and Japan. At other times, a quake may hit in a spot less familiar with the temblors, such as the magnitude-5.8 earthquake that struck Virginia in 2011 and damaged the Washington Monument.
Historic structures are often vulnerable during a quake. Several World Heritage Sites in Nepal were destroyed or badly damaged in 2015 during a magnitude-7.8 earthquake and aftershocks as strong as magnitude 7.3. Older building practices and aging construction materials make most historic structures less able to withstand the vibrations that occur during an earthquake or from high winds. Modern building techniques can be used to update these structures to mitigate some of the potential damage, but even then they are more vulnerable than their modern counterparts.
Now engineers at NASA’s Marshall Space Flight Center in Huntsville, Ala., say they can help historic structures survive these devastating events. They have developed a way to change how buildings respond to motion caused by movements in the earth’s crust. And it all started with a rocket.
The technology comes from work on the Ares rocket, a launch vehicle designed for the Constellation program that, before it was cancelled in 2010, was expected to replace the Space Shuttle for taking astronauts into space. The rocket vibrated so badly it would have injured anyone onboard, so NASA engineers had to find a way to make the vehicle safe. However, the usual way to control shaking, by adding more weight, wasn’t an option because the rocket would have been too heavy to lift itself out of Earth’s atmosphere.
The team figured out a way to use the rocket’s fuel to solve the problem. And the same solution can work for vibrating buildings, including those built hundreds of years ago, says Rob Berry, a NASA project manager at Marshall.
Historic structures can lack the kinds of connections, such as steel reinforcement, that transform the individual pieces of a building into a more durable, cohesive system. Engineers, however, can retrofit those buildings with external ties that hold the building together. “On [some] of these buildings, you’ll see plates on the exterior with a bolt coming through them and a big old nut on the end,” says Michael Kreger, director of the Large-Scale Structures Laboratory at the University of Alabama. “They’ll usually paint these things black so they kind of look like they’ve been there forever.”
Another option is to remove interior finishes, such as paneling and trim moldings, and build new, steel-reinforced walls around the originals. Those walls are then covered up, so the modifications can’t be seen.
These efforts are costly, though, and don’t bring the entire structure up to current building codes, says Kreger. And some historic structures don’t have the space necessary to add walls or hide steel beams for earthquake mitigation.
New buildings incorporate many of these technologies during construction. The most common method for decreasing a building’s movement has been a device called a tuned mass damper (TMD). An example of this would be a very heavy object, the mass, added to a building on top of springs set to a specific frequency. When a quake happens, or wind blows by, the mass is set in motion by movement of the building. This added weight moves in the opposite direction and reduces the overall motion of the building. Such a device isn’t perfect, though. The building has to move before the TMD will work, and those first few seconds of an earthquake can be incredibly destructive.
Berry’s team found a new way to use the building itself or small amount of added mass to bring about a more dramatic drop in motion. Most TMD use an object equal to about 1 to 2 percent of the building weight to achieve a reduction in movement of about 50 percent. In a skyscraper, that object can weigh as much as 2 million pounds. To solve the rocket problem, the NASA engineers used the rocket fuel to mitigate the vibrations and achieved a 95 percent reduction in motion for their 650,000-pound rocket. That was possible with a simple balloon-like device called a Fluid Structure Coupler, says Berry.
“Think of a balloon. Put air inside the balloon, it gets larger; take air out and it gets smaller,” he says. “If I put [the balloon] down into a swimming pool, the water is going to react. When that balloon contracts, the water follows the contraction of the balloon. It if expands, the fluid moves away from it.”
Because the water responds to the motion of the balloon, it is possible to change the natural frequency of the liquid by adjusting the pressure inside the balloon. With a building, an engineer can use that concept to adjust how the structure will move.
First the engineers determine the natural frequency of the building to learn when it will start to move. Then they set the coupler (balloon) to a different frequency. By placing the coupler into a body of water, such as in a swimming pool, or adding pipes filled with water attached to the roof, the water alters the building’s natural vibration. The liquid acts like an anchor for a swing—the swing will still move, but it will be much harder to push. The building, likewise, moves less during a quake or high winds.
NASA successfully tested this concept on a historic structure of its own, the Dynamic Structural Test Facility in 2013. But Berry and his team recognized that not all building designs would have the space to add this kind of fluid-based system. So they applied what they learned to develop a mechanical device that would take up less space but provide the same kind of anchor.
Now, the team has come up with a new version of the technology, called a disruptive tuned mass (DTM), which uses a hunk of metal, instead of water, to mitigate a building’s movement. It is much smaller than a conventional TMD and costs a lot less to produce—but is just as effective.
Earlier this month, Kreger and his colleagues, who were skeptical of NASA’s claims, put the device through its first test in a simulated earthquake at the University of Alabama Center for Sustainable Infrastructure. It was a success.
“The test clearly showed the disruptive tuned mass outperformed tuned mass damper, and it clearly showed that it’s useful for earthquake mitigation,” says Berry. This new approach, he says, “is another great example of where technology derived for the space program can provide new capabilities to industry.”
Kreger agrees and hopes to partner with NASA on testing and developing future DTM systems.
These technologies are prototypes, but NASA is working with private companies to develop commercial products that can be used for earthquake mitigation in public and private buildings, including historic structures.
This new technology might even help the Washington Monument withstand the vibrations of earthquakes and wind, Berry says. “I’ll bet they’ve looked at the various ways to mitigate,” he says. “But if that same earthquake went through there with a disruptive tuned mass installed, the response would have been totally different. We could have muted the response.”
He continues, “I’d love to have the Washington Monument people call. This technology was developed with taxpayer money, so it belongs to them.”