After the February 2011 earthquake in Christchurch, New Zealand, seismologists discovered new fault lines in areas previously assumed as seismically non-active. Surprised residents had felt stronger ground motion for a 6.2 quake in comparison to other seismic events of the same magnitude. The new fault line runs through the city’s high density urban center where the community experienced a long sequence of aftershocks. Local structural engineers reminded the public that even though the country’s buildings and homes are built to the latest codes, the nature of improving a building’s structural performance is a constantly evolving practice. Today’s recognized seismic mitigation strategies and technologies for buildings can quickly become outdated tomorrow - especially when the scientific community is witnessing new tectonic behaviors and finding unmapped ruptured faults.
The ever-changing science of earthquake engineering is one of the reasons engineers cannot design a magic solution for all buildings, according to DCI Engineers’ Director of Seismic Design Tom Xia.
“There are many reasons why a building performs well or poorly during an earthquake,” Xia said. “It really depends on the regional practice and the unique seismic circumstance. For structural engineers to design and build to higher seismic standards, professionals in the industry need to regularly attend engineering conferences and post-earthquake investigations, plus participate in discussions about lessons learned from real seismic events. You start by asking what were the lessons learned from catastrophic events, and then you start asking more questions about what can be applied to our local engineering practice.”
DCI’s Director of Seismic Design Tom Xia was part of the Structural Engineers Association of Washington Reconnaissance Team who inspected damage after Japan’s 2011 Tohoku earthquake. In the background is the remainder of the village’s emergency response tower. When the tsunami overpowered the area, half of the building’s occupants held onto the building structure to survive the flooding.
In the past few years, several destructive seismic events have occurred in various locations. By investigating the earthquakes in Christchurch, New Zealand, Santiago, Chile, the Van earthquake in Turkey, and the Napa Valley earthquake in California, engineers were able to reassess their knowledge of the regional areas, note observations from local and international authorities, and consider the financial availability of seismic upgrades.
What did the global community learn about the region?
- The ground accelerations during Christchurch’s February 2011 quake were the largest ever recorded in New Zealand . Most of the city’s buildings that met current standards performed well during the seismic occurrence. The buildings that collapsed had damaged sprinkler systems. Local officials determined they need to make more provisions ensuring that fire protection systems will remain operational even if the building collapses.
- When Chile experienced an 8.8 earthquake in 2010, international observers learned most of the deaths were tsunami-related, not building related. Chilean officials explained that most of the mid-rise and high-rise buildings are built to modern seismic codes. The international community discovered Chilean residents foster a “seismic culture” of being prepared for recurring earthquakes in the country. Therefore, emergency and medical response teams are routinely trained for crisis mode.
- After Turkey’s 2011 7.2 earthquake, investigators and the media discovered the country’s latest building codes were not enforced in all provinces and the codes didn’t address buildings built before 2001. The New York Times reported 2 out of 3 buildings in Turkey were illegally constructed and more than 600 deaths could have been avoided.
- California’s August 2014 South Napa 6.0 quake was the largest seismic event in the Bay Area since 1989. The activity ruptured a 12-kilometer stretch of the West Napa Fault. Emergency response agencies were able to react rapidly with real-time information from monitoring stations and social media posts from the general public . Older brick and mortar buildings located in the historic downtown area received the most damage. The post-earthquake outcome emphasized the importance of modern engineering, building design and quality construction practices to protect residents in the area and professionals in the wine industry.
Why did some buildings perform better?
- Christchurch’s 27-story Hotel Grand Chancellor avoided collapse because there was sufficient resiliency in the overall structure. The building’s framing systems were designed with alternative load paths to redistribute loads during earthquakes.
- During the 2010 Chile quake, most of the occupants of a toppled building survived because the framing system was built to modern codes and preserved the integrity of the individual apartment units.
- Napa Valley’s newer buildings exhibited less damage from their most recent earthquake because the venues complied with northern California’s earthquake-resistant construction standards. Well-anchored homes survived the quake’s prolonged ground shaking. One home had a massive concrete foundation with piers 15 to 25 feet deep into the ground. Homeowners who retrofitted cripple walls with bracing in Victorian-era homes reported less home damage.
Partially intact building in Santiago, Chile. (Claudio Nunez)
Why did some buildings perform poorly in comparison?
- In all catastrophic events, buildings with non-retrofitted, unreinforced masonry received the most damage during seismic activity.
- Most of Christchurch’s central business district sits on soft sediment or loose silt, therefore the area received widespread liquefaction-induced damage.
- In Chile, the buildings which received the most damage from the earthquake were improperly sited on poor soils, according to Chilean officials.
- The city of Napa’s late 1800s-era buildings and homes took the most damage since many were not reinforced with internal steel or retrofitted with braced water lines.
Post-earthquake examiners measure how much the ground shifted for a previously level tile floor in Tohoku Village.
Does this new information mean citizens should change their practice or do structural engineers need to change building design?
- Christchurch officials increased the seismic hazard factor from 0.22 to 0.30 based on the new knowledge acquired from the 2011 quake. New guidelines were published by their Department of Building and Housing, encouraging builders, planners, designers and engineers to consider integrating concrete foundation slabs reinforced with steel.
- After the 2010 earthquake, Chileans developed new building codes to address new standards for major interior elements (e.g., tie-downs for equipment, etc.).
- Napa Valley and Sonoma wineries were encouraged to arrange sturdier, shorter stacks of wine barrels (four barrels maximum) in their industrial-sized storage facilities. Some winery owners were advised to install steel cages or barrel gates to address life safety issues for employees in the event of another earthquake.
When Tom returns from industry conventions, seismic committee meetings or reconnaissance trips, he leads lunch-and-learn meetings with DCI staff to share more about new and alternative design practices that address safety and life preservation. “The best way to stay sharp with your structural engineering abilities is to learn through work and field visits, or from mentors,” Xia says. “We do both at DCI.”
About the Author
Rose Bechtold, Communications Specialist | Rose comes from a journalism and technical writing background. She is in her element while in research mode and naturally immerses herself in expert knowledge by interviewing staff members about new subjects. In her spare time, Rose practices plein-air sketching of buildings and random scenes around town.
Tom Xia, Ph.D., PE, SE, LEED AP, Principal | With more than 25 years of civil and structural engineering experience in various sectors, Tom Xia brings technical expertise and the “big picture” concept to any project. Actively involved with numerous committees, he contributes to setting new industry standards with the Building Seismic Safety Council, American Society of Civil Engineers seismic committees, the National Council of Structural Engineers Association Seismic Committee, and the American Concrete Institute for Performance Based Seismic Design and for Performance Based Wind Design. Always resourceful, he regularly presents new scientific research to the firm and motivates colleagues and new engineers to pursue creative structural concepts.