US engineers study Chile earthquake to bolster California building codes
Since the Chile earthquake, dozens of US engineers have visited Santiago and other affected cities to study the failures and successes of building codes here. They say it provides valuable insights for California.
Mr. Celebi had come here from the office of the US Geological Survey in Menlo Park, Calif., to study concrete structures damaged by the massive 8.8-magnitude earthquake that rattled Chile on Feb. 27. Reluctantly, and a bit incredulous, a guard lifted a band of yellow tape to allow Celebi to pass. The building, as it leaned precariously, had become one of the now-iconic images of havoc caused by the quake.
Celebi pored over the building, looking for the design failures that caused walls at the base of the building to crumble.
"This is what you call 'soft story,' " Celebi said, using the term to describe buildings that are weaker at the lower level than up top. When the quake hit, the largest load came down on the weakest part of the structure and the walls buckled.
"I think we have observed this is crushing," Celebi concluded, pointing to twisted rebar spilling out from the wall's interior. "This is a prize from an engineering point of view."
In the wake of the fifth-largest earthquake ever registered, dozens of scientists such as Celebi are flocking to Chile to better understand how structures perform when pushed to the limit. The Earthquake Engineering Research Institute in Oakland, Calif., sent more than 30 researchers to record data from Chile's homes, hospitals, bridges, and apartment buildings.
A unique opportunity in Chile
Structural engineers typically work as consultants to the architect. The architect designs the look, function, and spaces in the building, while the structural engineer designs the skeleton that ensures the building will withstand wind, earthquakes, and other forces. This is done according to codes often based on computer simulations. But when an earthquake occurs, it provides a unique opportunity to study a 'living laboratory.'
"It's fascinating to see the kinds of things that we do calculations on every day do occasionally get put to their ultimate test," says Joe Maffei, a structural engineer and principal with Rutherford & Chekene in San Francisco, who made a recent trip to Chile to study the behavior of concrete structures in Santiago, two coastal cities, and ConcepciĆ³n, the city nearest the epicenter. "This is an opportunity to see if we are applying the science right in building codes."
Chile is of particular interest to American engineers. It employs similar building codes to those in California and also has widespread use of reinforced concrete.
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," Mr. 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. Mr. 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 US 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?"
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 US 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 US 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 didn't allow for sufficient support. Celebi shifted his attention to the building in perfect form.
"We have to appreciate the successes too," Celebi said.