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CLIENT: MACTEC, Inc.
June 10, 2010 : Public Works.com
By Dr. Marshall Lew, PE, MACTEC Engineering and Consulting, Inc.
Data gathered by a team of renowned structural and geotechnical engineers from the magnitude 8.8 earthquake that struck Chile on Feb. 27 may help to better understand how we can mitigate damage in quake-prone areas in the United States.
A six-member team of private sector quake experts was dispatched by the non-profit Los Angeles Tall Buildings Structural Design Council (LATBSDC), specifically to investigate the performance of tall buildings. Chile has similar design and construction practices to the U.S., where there's also an abundance of skyscrapers in major population areas.
Chile, population 16.6 million, is almost twice the size of California, with 85 percent of the population living in urban areas. Nearly half live in three metro areas – Santiago, Valparaiso-Vina del Mar, and Concepcion-Talcahuano.
Chile is one of the most seismically active areas worldwide; in fact, the University of Chile's Servicio Sismologico reports that between 1570 and 2007, there have been 107 destructive quakes with a magnitude of 7.0 or greater. And since 1960, two of the world's five most powerful quakes (1960 and 2010) occurred in Chile (others were Kamchatka, 1952; Alaska, 1964; Sumatra, 2004).
The Chilean quake occurred at the convergence of the Nazca (oceanic) plate with the South American continental plate along a subduction zone where the Nazca plate moves under the South American plate. Confirmed fatalities to date are just under 500; more than 800,000 people were injured or displaced and damage is expected to top $30B(U.S.).
Chile's building code, issued by the National Institute of Normalization in 1996, is similar to building codes used in the U.S. Concrete design, for example, is largely based on the American Concrete Institute (ACI) 318 Building Code Requirements for Structural Concrete. Chile's building code did allow for some exceptions to ACI requirements that may have caused damage in some buildings. These exceptions were that boundary elements should be provided at boundaries and edges around openings of structural walls when there is high extreme fiber stress under loading (including earthquakes), and that boundary elements should have transverse reinforcement with proper anchorage within the core of the boundary element.
A peer review system by recognized experts in seismic design was established in Chile in 2003; this process is somewhat similar to plan check review in the United States.
There is one noticeable difference – the U.S. practice of independent construction inspections and testing doesn't exist in Chile. Construction observation and testing are performed as part of the building contractor's services.
Chile's steady economic growth has transformed Santiago into one of Latin America's most modern metropolitan areas, with extensive suburban development, dozens of shopping centers, and impressive high-rise architecture. Santiago has a modern transport infrastructure, including the Santiago Metro, public bus transport and a toll-based ring road and inner city highway system, part of which is tunneled underneath a large section of the Mapocho River.
Santiago is also the regional headquarters to many multinational companies, and a regional financial center. SkyscraperPage.com lists 65 commercial buildings 15 stories and higher constructed or under construction since 1981 in Santiago. The 55-story Torre Titanium La Portada was just completed this year and is currently the tallest building in South America; the building also has seven subterranean levels. The nearby Gran Torre Costanera is currently under construction and will be 70 stories high.
The Valparaiso-Viña del Mar metropolitan area is on the coast of the Pacific Ocean, about 120 km northwest of Santiago. Valparaiso is the home of Chile's National Congress and is an important seaport and commercial center. Viña del Mar is also a popular tourist and beach destination and has numerous tall residential buildings.
Concepción has the second largest metropolitan area in Chile. Although economic development is not at the same level as Santiago, the city does have a significant number of tall buildings, but most are residential.
The Chilean quake has significance to the United States as Chile's tectonic setting is similar to tectonic settings of the Pacific Northwest and Southern Alaska.
A tectonic plate boundary condition, for instance, similar to that on the west coast of South America also exists on the west coast of North America in what is known as the Cascadia Subduction Zone. The Cascadia Subduction Zone is off the coast of British Columbia, Washington, Oregon, and the northern part of California. A large earthquake is believed to have occurred on this zone in 1700 resulting in a tsunami that hit Japan.
In addition, the U.S. is also vulnerable to the Aleutian Subduction Zone where the Pacific Plate is being subducted under the North American Plate to the south of Alaska and the Bering Sea. The Great Alaska earthquake (9.2 magnitude) of March 27, 1964 occurred in this subduction zone in Prince William Sound, the second largest earthquake ever recorded.
Practically everyone in Chile has experienced a significant earthquake event at some point during their life. But structural engineers in Chile have not been able to effectively educate the public about what to expect about structural performance from a massive quake – this perception could also be developing in the United States.
The Chilean earthquake is providing us with invaluable data of how tall buildings might behave if there were a Cascadia or Aleutian subduction event, especially in cities like Portland, Seattle, Vancouver and Anchorage, which could experience similar earthquake ground motions and durations. Even in non-subduction zone areas, the effects of a large distant earthquake with strong long-period ground motions and long duration can be instructive.
As an example, a few catastrophic failures of tall buildings occurred in the Chilean earthquake. In Concepción, two notable failures occurred -- the 15-story Alto Rio residential building and the 21-story Torre O'Higgins office building.
The Alto Rio building suffered damage most likely attributed to excessive compression on lower walls, coupled with insufficient tension capacity. In the Torre O'Higgins building, failure occurred in the upper part of the building, above the tenth floor. Probable cause may have been due to structural modifications of the building by a tenant up to the 10th floor.
Despite these few catastrophic failures, most of Chile's tall buildings performed well.
Since 1985, buildings in Chile, like buildings all over the world, have increased in height. But the walls in these buildings have either stayed the same or decreased in thickness primarily due to architectural and parking needs. Walls in basements are generally reduced in length from walls above in the superstructure and vertical loads can be very high. With the exception granted to omit boundary elements on these walls, localized failures occurred resulting in some buildings tilting and being evacuated.
The recent and past experience in Chilean earthquakes provided an excellent opportunity to evaluate building structures subjected to major seismic events with strong ground shaking and long duration. Improvements in these four areas may help mitigate damages in subsequent quakes, not only in Chile, but in the U.S.:
As tall buildings get even bigger, it's paramount to consider the consequences of pushing the limits of our existing knowledge. The structural loads and seismic loads will increase requiring greater demands on the structural materials to carry these loads. We must be certain that yesterday's solutions will work for today's and tomorrow's challenges.
Peer review for major structures is an important element of the overall design and permitting process. Also critical is the need for quality independent inspections during the construction phase.
The bottom line? Putting some of the lessons learned in Chile into practice in the U.S. may save lives and mitigate property damage.
Dr. Marshall Lew is senior vice president at MACTEC, Inc., an environmental and engineering firm. Based in the firm's Los Angeles office, he's an internationally recognized seismologist and has also served as secretary-treasurer of the Oakland-based Earthquake Engineering Research Institute. Contact Dr. Lew at 323/889-5300, email@example.com.
Return to: 2010 Feature Stories