Structures can be both earthquake-resilient and sustainable

Several studies show that constructing safe structures in earthquake-prone regions can be accomplished using sustainable materials that range from mass timber to recycled tires.

The building industry is one of the most energy-intensive sectors, accounting for 40% of global CO2 emissions. Steel and cement manufacturing are two of the main culprits. As of 2020, steel manufacturing alone accounted for between 7% and 9% of global anthropogenic CO2 emissions, according to the World Steel Organization. The manufacture of cement, the main ingredient of concrete, accounted for a similar amount of such emissions. What’s more, reinforced concrete — a superstar material that resists collapse and often girds buildings against earthquakes in seismically active regions — is a combination of concrete and steel. Therefore, its manufacturing process also packs a CO2-emitting punch that stems from both its major components. But are there more sustainable alternatives? Ongoing research provides intriguing options that could have great potential.

Mass timber mitigation

Across the globe, mass timber is becoming a popular alternative to concrete and steel. Mass timber products comprise layers of wood joined together to form strong panels or beams. “Basically [you] take smaller dimensional lumber, and glue them up in layers and press them together to make big timber sections,” says Eric McDonnell, a structural engineer with Holmes, an engineering firm. Using timber instead of concrete and steel reduces the carbon footprint of construction; the carbon that trees removed from the atmosphere during their life continues to be stored in timber buildings.

“[Timber] is the only material that we use in our buildings that you could grow with sun and water,” says McDonnell. “I think there’s something very neat and special about that.” And with sustainable forestry practices, new trees are planted to replace those used in the building, mitigating some of the environmental impact of cutting those trees down.

Unlike regular timber, mass timber panels and beams can be used to construct multi-story buildings that are resistant to collapse in earthquakes. Large-scale shake table tests of wooden buildings, like the TallWood project, show that even 10-story mass timber buildings can withstand relatively large ground motions.

There are clear advantages to timber buildings in earthquake-prone regions. “A lot of the earthquake damage is directly proportional to mass,” says McDonnell. Timber buildings tend to weigh less than concrete and steel alternatives. As a result, components designed to prevent collapse — like braced frames and sheer walls — receive less lateral force, he explains. This means that the building may receive less damage.

Smart design features also play an important role in the earthquake resistance of the 10-story building tested in the TallWood project. Four of the strong mass timber panels in the TallWood building have a “rocking wall” design that allows the panels to move with the earthquake, and a metal beam pulls the walls back into place (or back to plumb, which means perfectly vertical) when the shaking stops, McDonell says. Designing seismically resilient structures also prevents them from becoming irreparably damaged, thereby reducing construction waste.

Recycled tires could cushion buildings

According to the U.S. Tire Manufacturers Association, American motorists discard approximately 274 million tires each year. To save these tires from the landfill, they are commonly recycled into shock-absorbent rubber flooring for children’s playgrounds. This led researchers at Edinburgh Napier University in the U.K. to wonder: What are the material properties of recycled tires, and what else could these recycled tires be used for?

When testing the recycled rubber, Juan Bernal-Sanchez, a geotechnical engineer at Edinburgh Napier University in Scotland found that the material can absorb a lot of energy. This observation led him to test a possible application — protecting buildings from earthquakes.

Laboratory experiments show that by placing fine particles of recycled rubber tires below or around a building — for example in the form of trenches — the rubber could act as many tiny airbags, Bernal-Sanchez says. In an earthquake, the tiny rubber particles dissipate energy, thus protecting the building, he explains. But that’s not all. The trenches filled with rubber might also bounce some of the seismic energy back in the direction from which it came, he says.

However, placing rubber within the soil could also have unintended consequences, particularly for water flow and aquatic critters. Recent studies have found that in most cases, those potential impacts should be minimal. Just in case, researchers are exploring how encasing the rubber in recycled bags could mitigate such problems, Bernal-Sanchez says. Their goal is to prevent the rubber from interacting with the soil, while also retaining the air-bag effects.

So far, Bernal-Sanchez and his colleagues have only tested the recycled rubber in the lab. But the project has just received funding to scale up — the recycled tires will be used to test protection of a building in Thessaloniki, Greece, where the real ground will be subjected to vibrations created by a machine that imitates earthquake shaking.

Concrete and cement

“Concrete is the most widely used human-made product in the world,” says Megan Stringer, a structural engineer at Holmes. Buildings that are constructed of reinforced concrete are stiff, strong and ductile and are demonstrably earthquake resilient. And, reinforced concrete structures are cheap to construct.

Conventionally formulated concrete — a mix of sand, gravel, water and cement (made of limestone and clay) — could be responsible for up to 3.3 gigatons of CO2 emissions per year. Given that concrete will remain an important building material in the future, the concrete industry is working to tackle the problem, Stringer says. Because “the cement alone makes up roughly 90% of the concrete impact on the environment,” it makes sense to start there, she says.

(See this image, which shows the process of concrete-making and its contribution to greenhouse gas emissions.)

Swapping some of the cement for materials like fly ash, a byproduct of burning coal, or slag, a byproduct of the iron and steel-making processes, could reduce the overall carbon footprint. However, these potential substitutes both come from energy-intensive processes, so moving toward a more sustainable future may minimize the availability of these byproducts.

Another possible option is to use less cement in the concrete, resulting in a mix that’s higher in concrete’s other components. That could be a simpler avenue for achieving the same goal, provided that the material properties of the concrete mix stay the same. A 10% reduction of emissions is possible just based on using less cement in the concrete mix, Stringer says. Mixes with less cement have been approved for use across the U.S., including in seismically active states like California.

Sustainable construction showcase

Situated in the greater San Francisco Bay Area, Marin County, California is at a high risk for future damaging earthquakes. It is also the first county in the U.S. to develop a low-carbon concrete code, making it a key testing ground for low-carbon concrete. There, all building projects must comply with that code by replacing or reducing the amount of cement used in the concrete.

Mass timber is also showcased in cities across the U.S. For instance, the Carbon12 apartment building in Portland, Oregon is an 85-foot-tall wooden building that also has a braced frame system, making it resistant to earthquakes.

Future steps for decarbonization

These three examples — mass timber, recycled rubber, low-carbon concrete — are not the only ways in which the construction industry can address decarbonization head-on. Other new technologies are being developed. “I’m excited about new biomaterials made of fibers,” McDonnell says. For instance, bamboo and switchgrass can be used for pre-manufactured building components.

Moreover, the future of sustainability is not just about new buildings. The most sustainable building is the one that is already built. The environmental impact of rebuilding or repairing after an earthquake can be significant. Following the 2011 Great Tōhoku earthquake and tsunami in Japan, construction activities generated 26.3 million tons of CO2. Although retrofitting a pre-existing building does have environmental impacts, research shows that the benefits of preventing collapse far outweigh the impacts of the mitigation. Retrofitting an existing building also offers cost benefits, which can be calculated using the CARE tool, a retrofit estimator from the Carbon Leadership Forum.

In societies’ journey toward a greener future, the path to sustainability lies in innovative materials. “There are a lot of new technologies that are really exciting,” Stringer says.

But sustainability also requires thoughtful preservation to increase the resilience of buildings we already have.

“Sustainability comes hand in hand with resilience,” McDonnell says.