‘Earthquakes don’t kill people – poorly built buildings do’. This is the message posted on the home page of the Build Change website.
Build Change is an international not-for-profit social enterprise that designs earthquake-resistant houses in developing countries and trains builders, homeowners, engineers, and government officials to build them. By developing local skills in countries like China and Indonesia, the aim is to create permanent change in construction practice.
With an estimated 500,000 detectable earthquakes in the world each year and 100 of them causing damage of some kind, it’s important that those who live in high risk areas acquire the skills and knowledge to protect themselves and their buildings.
INDONESIA AND ITALY
Self-built residential buildings were worst hit in the Indonesian earthquake last year (and in Haiti last month) as they were not covered by earthquake codes. Buildings in Indonesia are often built from unreinforced masonry which does not perform well in earthquakes. In fact, as there was no requirement before the earthquake for seismic design for low rise structures, less than two percent of building stock in the region was designed in line with current Indonesian seismic codes for buildings of two or more stories.
Lessons are also being learned in the West. According to a report from the Natural Hazards Center in the US, following the earthquake in L’Aquila, Italy last April, 38 percent of residents living in reinforced concrete structures experienced moderate or heavy damage to their homes.
The lighter modern buildings, erected prior to the detailed site investigation introduction of seismic building standards in 1980, were most badly affected. L’Aquila’s medieval, Renaissance and Baroque buildings have survived several earthquakes over the centuries.
Scientists agree that one day new buildings will be able to withstand earthquakes of the magnitude that destroyed San Francisco in 1906 – 7.8 on the Richter Scale. Work is under way using extra strong materials that can bend, stretch and compress without breaking. At Lehigh University in Bethlehem, PA, which has the largest structural testing facility in the US, scientists tested a ‘self-centering system’ using gigantic rope-like steel bands to hold building columns and beams in place during earthquakes.
The bands are encased in plastic and designed to prevent buildings from buckling by allowing the beams and columns to separate, rock and twist independently of one another. Friction plates are also used to dissipate the earthquake’s energy. After the quake the steel bands pull the beams and columns back to their original positions. Engineers believe this system could be used commercially within the next 10 years.
PRINCIPLES OF SEISMIC DESIGN
Successful seismic design requires an understanding of geological hazards. Structures straddling geological faults can be at risk during earthquakes depending on the amount of movement and the type of ground they are built on. Loose saturated soils, for example, can liquefy as sand particles separate from water. This leads to uneven building settlement. The solution is either to strengthen the ground or to build deeper foundations.
Liquefaction caused the settlement of a number of buildings during earthquakes such as in Niigata, Japan in 1964 and more recently in Mexico City in 1985 and Adapazari, Turkey in 1999. As a result it is now common practice to investigate ground conditions and design accordingly.
Similarly, if buildings are very close to a slope they can become unstable due to rainfall and ground movement. Following an earthquake in El Salvador in 2001, 585 of the 844 people killed by the consequences of the earthquake died in one major landslide. In Pakistan during the 2005 earthquake many buildings were damaged because they were hit by rocks falling from the hills.
“The key principle of seismic design is to build structures as flexible as possible,” explains Ziggy Lubkowski, Associate Director Arup Geotechnics – Seismic Engineering Group. “Some damage will inevitably occur and there needs to be sacrificial elements in the design, but as long as damage is not excessive buildings won’t collapse.
“Plastic hinges and seismic isolation bearings can be used to absorb energy shock waves and allow some degree of movement. There are of course cost implications associated with this level of specialist design which is usually only used in structures such as hospitals, schools, police stations and airports; operational buildings that need to be in use following an earthquake.”
Other Arup projects include Istanbul airport which is in an area with a 60 per cent chance of a major earthquake in the next 20-30 years. It includes seismic isolation design and is the world’s largest earthquake-resilient building.
Beijing is also in one of the world’s most active seismic zones. Advanced computer analysis and modelling ensured that the Bird’s Nest Olympic stadium was designed to withstand major earthquakes. And the city’s new China Central TV building is a 234m high ‘three-dimensional cranked loop’ braced tube structure with the required robustness to withstand any likely seismic activity.
The building’s primary support is achieved through the irregular grid on its surface, a visible expression of the forces traveling through the tube structure; the smaller the diagonal pattern, the stronger the load and the greater the support.
Arup adopted an innovative approach to the seismic design of the sub-structure of a new cable-stayed bridge connecting the city of Incheon, near Seoul, and the west coastal highway to Incheon International Airport, one of the longest fixed-link sea crossings in the world.
The bridge is located in a seismically active region and the design ensured that the crossing was structurally sound. It also introduced cost savings as a result of the reduction in construction complexity.
“Seismic design is constantly changing; it’s a growing science and with each earthquake we increase our knowledge of how buildings perform and how the ground reacts to forces,” says Lubkowski.
“However, high tech design and construction is expensive and is not always the highest priority in a developing country struggling to feed a large population. From this perspective, development in the application of seismic design still has a long way to go.”