The lessons of Chile earthquake to California building code

concrete (see “Chile’s Concrete Code for Buildings Called into Question,” 2 April 2010 HSNW).

One of Chile’s more successful earthquake-resistant designs was a “base-isolated” building. The building is placed on rubber or sliding supports designed to dissipate energy the earthquake imposes. The disruption to these buildings was markedly less than for non-isolated buildings. Despite widespread damage, a remarkably small number of large structures collapsed. “This earthquake overall can be considered a success in terms of the engineering and construction practices in Chile,” Maffei says.

Preparing for tsunamis

In fact, more damage was caused because of inadequate storm preparedness. The greatest concentration of destruction and loss of life in Chile was the result of successive tsunami waves that denuded the coastline.

Eli Robertson, a professor of civil and environmental engineering at the University of Hawaii at Manoa, led a team of researchers here to study the performance of buildings hit by the tsunami. Robertson has been working on a National Science Foundation grant for the past four years trying to develop design building guidelines to protect from tsunamis. Hawaii is one of the few places in the United States to have seen a significant number of deaths from tsunami impact.

“We need to know what to do with the folks in the coastal areas,” Robertson says. “Do we tell them to go up in buildings and wait it out? Do we need to reevaluate buildings to include tsunami design?”

Nelsen notes that here again, Chile provides invaluable lessons. Light-framed and masonry built homes stand little chance of surviving, but reinforced concrete frames performed quite well, Robertson found. Chile uses U.S. codes for concrete design. By and large, these structures, though not specifically built for tsunamis, proved resilient to a 30-foot surge. “We’ll take some of the observations in Chile and compare them with what our design guidelines would say,” Robertson says. “In other words, we’ll try to design one of the structures we looked at and decide if it would have survived and compare that with what actually happened in the tsunami.”

A forewarning for California

One immediate observation from Chile’s earthquake that could find its way into U.S. building code is on confining reinforcement, says Maffei, who is on the committee that decides building code in California. Confining reinforcement is meant to keep vertical bars from bucking, but the design proved insufficient in Chile. Maffei says he would consider requiring confining reinforcement along a greater length of the wall.

In preparing an earthquake-proof building code, engineers must also take into account the land the building sits on. Chile provides a case study in this, too. While the center of Santiago withstood the quake relatively well, on the outskirts of the city the metropolitan area of Maipu fared much worse because it sits atop soft volcanic ash, which amplifies the motions of the earthquake. Several apartment complexes in Maipu will have to be demolished.

In assessing these cases, according to Celebi, engineers try to correlate a building’s design with the level of shaking. If the building was designed to withstand the shaking and damage still occurs, then there are two possible explanations: either the design is bad or the construction is bad.

Standing under Chile’s mid-afternoon sun recently, Celebi considered two buildings positioned 90 feet apart. One was broken and leaning, the other was impeccable. The slumping building lacked sufficient shear walls, which restrict the relative displacement between floors. The design is widely used in Chile, and according to Celebi, is probably what saved many buildings from collapse. This particular building, however, failed because the design did not allow for sufficient support. Celebi shifted his attention to the building in perfect form. “We have to appreciate the successes too,” Celebi said.