Tech + Engineering

13
Nov

A Tested, Inexpensive Way to Protect Buildings from Earthquakes

In 1985, construction workers in the southern California city of Rancho Cucamonga put the finishing touches on an unremarkable looking yet revolutionary building. Foothill Communities Law and Justice Center is a four-story courthouse located just 12 miles from the San Andreas fault, the crack in the Earth’s crust responsible for the devastating 7.9 magnitude quake that struck southern California in 1857 and the infamous 1906 earthquake in San Francisco. Geologists think the fault is due for a “big one” with a magnitude that could top 8.0. If and when that comes, the Foothill Communities Law and Justice Center will be prepared, not because it’s anchored in stone, but because sits atop its foundations on pads of rubber.

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The Foothill Communities Law and Justice Center in Rancho Cucamonga, California

For decades, engineers have been using rubber in buildings and structures. Since the 1950s, it has been installed in highway bridges to cope with thermal expansion and in buildings to quell vibrations from trains and highways, though it wasn’t until the early 1980s that the compound was finally used to isolate buildings from earthquakes. Where rubber bearings have been installed, they have been a huge success—and if used more widely, they have the potential to save countless lives and prevent billions of dollars in damage.

Despite that, many buildings still lack the protection that comes with proper seismic engineering. In developing countries, the problem is particularly acute. Take the magnitude 7.0 earthquake which struck Haiti in 2010 and killed more than 100,000 people, many of whom perished when buildings collapsed. Vulnerable structures also plague developed countries like the United States. A recent investigation by the Los Angeles Times discovered that more than 1,000 older concrete buildings in the city could collapse in the next major earthquake.

There are many ways to make a building resilient against earthquakes. In wood structures, extra strappings can be nailed to key parts of the frame. Steel-reinforced concrete is another option, with the strengths and weaknesses of each compound offsetting the other, though even that may not be enough to prevent damage in a large quake. Some buildings use dampers similar to shock-absorbers in a car. The most effective, though, may be base isolation, where the building is separated from the ground using dampers or bearings.

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Buildings that aren't engineered or retrofit to withstand an earthquake can suffer catastrophic failures.

To isolate the base, engineers can turn to a variety of different bearings and dampers. Rubber bearings, like those installed in the Foothill Communities Law and Justice Center, are among the most promising. Like other bearings, they decouple the building from an earthquake’s horizontal motions, literally providing a buffer between the building and the earthquake. The system doesn’t absorb the earthquake’s energy, but rather deflects it through the dynamics of the system. The earthquake’s higher frequencies—and their destructive energy—aren’t transmitted to the rest of the structure. Buildings built on rubber bearings will still shake during an earthquake, but they’ll glide on their base rather than wobble dangerously.

With so many options available, what makes rubber bearings so appealing? For one, they’re effective. The way they’re constructed—thin sheets of natural rubber are bonded to thin steel plates—gives them high stiffness in the vertical direction while being very flexible in the horizontal direction. In other words, they are stiff enough to support a building, but pliant enough to deflect the energy of an earthquake. But perhaps more importantly, they’re affordable, which could bring life-saving seismic safety to poorer, vulnerable countries.

Quelling Vibrations, Large and Small

The idea of reinforcing rubber blocks by thin steel plates was invented by French engineer Eugene Freyssinet. He recognized that the vertical capacity of a rubber pad was inversely proportional to its thickness, but its horizontal flexibility was directly proportional to the thickness. In other words, a thin piece of rubber could be stiff in one direction but flexible in another. Freyssinet is known for another engineering innovation, the development of prestressed concrete, and for the discovery of creep, or deformation, in concrete. Both of these are important parts of bridge engineering today, and it’s possible that he invented the reinforced rubber pad to deal with two challenges of bridge building—supporting the heavy prestressed deck while allowing it to shrink as a result of creep and the prestress load.

Freyssinet’s bearings have been used extensively throughout the world, though most of their use in buildings is to damp unwanted vibrations, like those from an underground train, a practice which originated in the United Kingdom in the 1960s. Since then, many projects in the U.K. have used natural rubber isolators, including low-cost public housing and a number of hospitals, where vibrations can disrupt precision diagnostic equipment.

In that same vein, the Benaroya Concert Hall in Seattle, completed in 1999, and the Walt Disney Concert Hall in Los Angeles, completed in 2003, both use rubber bearings to minimize vibrations. The Benaroya Concert Hall sits atop a train tunnel, while the Disney Concert Hall is built directly above a loading dock for the building next door. Bearings in both buildings work well to isolate vibrations, but they weren’t designed to mitigate earthquakes. Instead, Benaroya and Disney have duplicate systems, though they certainly don’t have to. My colleagues and I used the shake table at the Earthquake Engineering Research Center of the University of California, Berkeley to show that it was possible to design for both.

Engineers test seismic retrofits with a massive shake table.

Old buildings can be retrofit to take advantage of base isolation. In California, Oakland City Hall and San Francisco City Hall, both of which were badly damaged in the 1989 Loma Prieta earthquake, now float above their foundations on dampers. The Los Angeles City Hall, which was damaged in the 1994 Northridge earthquake, was retrofitted with high-damping natural rubber bearings four years later. At 454 feet, is now the tallest seismically isolated building in the country.

Keeping the Structure Separate

Outside of the United States, Base isolation has been very actively pursued in Japan, one of the most earthquake-prone countries in the world. There, the first base-isolated building was completed in 1986, and one of the largest base-isolated buildings in the world is the West Japan Postal Computer Center in Sanda, Kobe Prefecture. This six-story, 500,000-square-foot structure is supported on 120 elastomeric isolators with a number of additional steel and lead dampers. During the infamous 1995 Kobe earthquake, the building—which was just 19 miles from the epicenter—experienced severe ground motion. Fortunately, the postal center wasn’t damaged. The same couldn’t be said of an adjacent fixed-based structure.

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Natural rubber bearings support isolate this building from the ground, protecting the structure during an earthquake.

After the Kobe earthquake, more buildings in Japan were constructed using base isolation, including apartments and condominiums, which are normally constructed using traditional methods. Today, about 100 base isolated buildings are built in Japan each year, not including single family homes.

Now, Japanese engineers are taking the concept of isolation a step further with “ground isolation.” In Sagamihara City outside Tokyo, they have built 21 separate buildings—six to 14 stories tall—on top of a three-acre concrete slab. The slab then sits on 150 isolation devices, including many very large rubber bearings, and all of the buildings move in sync. Ground isolation shows great promise and could bring base isolation to even more high-rise complexes.

Safer Structures for Everyone

Currently, most buildings that are base-isolated are large structures which house sensitive or expensive equipment, but there is increasing interest in using it in public housing, schools, and hospitals, especially in developing countries. A number of base-isolated demonstration projects have already been completed in a variety of countries, from Italy to Chile and Indonesia. Often, these buildings sit right next to identical, fixed-base structures, allowing engineers like myself to compare their behaviors during earthquakes. Time and again, we have seen base-isolated buildings emerge from an earthquake relatively unscathed compared with their neighbors.

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A workers' dormitory in Java, Indonesia that was built atop natural rubber bearings.

My colleagues and I have high hopes that inexpensive base isolation technology, including natural rubber bearings, will be used around the world. One demonstration project is in Indonesia, where workers on a tea plantation in the southern part of West Java now live in a four-story, reinforced-concrete, base-isolated building. It contains eight low-cost apartment units and is supported by 16 high-damping natural rubber bearings.

Apart from base isolation, the building isn’t substantially different from others in Java, which makes it less expensive to construct and more likely that it will be accepted by local building officials. The isolation bearings, which are inexpensive to manufacture, are located at ground level and are connected to the rest of the building using an innovative, cost-effective, and easy-to-install technique.

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A closer look at the bearings that isolate the structure from the ground and the flexible utility hook-ups.

Rubber bearings like these are could dramatically change the way buildings in earthquake zones are designed and built. They allow historic buildings to be easily retrofit without losing their charm. They have the potential to expand earthquake engineering to inexpensive structures like condominiums and single-family homes, which today must rely on less sophisticated measures. And perhaps most significantly, rubber bearings could bring much needed safety and security to make buildings in earthquake-prone developing countries, preventing costly property damage and saving countless lives.

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James Kelly

James M. Kelly

    James M. Kelly is a professor emeritus in the Department of Civil and Environmental Engineering at the University of California, Berkeley, where he has been instrumental in developing low-cost seismic isolation systems for the protection of housing and other structures in earthquake-prone developing countries.