In the first four months of 2010, 16 additional earthquakes with magnitudes ranging from 6.0 to 8.8 hammered the planet, and many more quakes of lesser force struck remote areas worldwide. We didn’t hear about those because there was little -- and in several cases zero -- loss of life.
Why did so many more people die in the Haitian quake? Haiti, a poor country, could afford to build only inelastic, frail structures, whereas structures in Chile, a more developed nation, were strong and somewhat flexible and, for the most part, withstood the 8.8 quake.
How can we prevent similar large losses of life in future seismic events? We can't control plate tectonics, but we can engineer earthquake-resistant structures.
The motions of an earthquake alone cause very little loss of life. Instead, falling objects often are the instruments of death and injury. Fires break out from broken gas or power lines. Hazardous chemicals escape when holding tanks rupture. Sewage lines break, releasing effluents that contaminate water supplies and, in turn, laying the groundwork for cholera, typhoid, dysentery and other serious diseases. Power outages, communications breakdowns and transportation interruptions after an earthquake impede rescue and recovery efforts. Records and supplies go missing, crippling businesses and government, slowing recovery even more. But amid all of that chaos, perhaps the most lethal immediate threat to human life is the collapse of buildings, bridges and other structures. Thus the old engineering adage: "Earthquakes don't kill people; buildings do."
In general, engineers erect buildings to withstand the static forces of gravity and those inherent in the structures' designs. But earthquakes generate an entirely different set of forces -- a quake's motion, particularly the side-to-side and rolling movement, subjects buildings to tremendous dynamic forces along vectors that bypass load-absorbing areas and, in the process, eliminate structural integrity and shred the structures like paper. Unless buildings are flexible enough to bend with those dynamic forces or sturdy enough to withstand them, a strong earthquake will take them down.
Knowing Where and How to Build
|Image by Doug DeWitt |
Engineers approach earthquake-resistant structures in a number of ways. For small- to medium-sized buildings, they use simple reinforcement techniques such as bolting buildings to their foundations and providing shear walls, which strengthen the structure and help resist rocking forces. Shear walls in the center of a building, often around an elevator shaft or stairwell, form a shear core. Engineers might also reinforce walls may with cross-bracing.
Researchers in the Civil and Environmental Engineering Department at the University of Michigan (U-M) simulated the effects of a large earthquake in the Structural Engineering Laboratory to test their new technique for constructing high-rise reinforced concrete buildings. Their proposed design procedure for coupling beams in a core-wall structural system passed the test, withstanding more lateral deformation than an earthquake would typically demand. The engineers used steel fiber-reinforced concrete to develop a better kind of coupling beam that requires less reinforcement and is easier to construct. Coupling beams connect the shear walls of high rises around openings such as those for doorways, windows, and elevator shafts.
U-M New Building Design Withstands Earthquake Simulation
Engineering earthquake-resistant skyscrapers is much more problematic than building small- to medium sized buildings, and engineers approach them with a couple of techniques. One practice is to tie the foundation to the superstructure -- the upper part of the building -- as solidly as possible to the foundation so that, when the ground shakes, the building and the foundation move as a unit, which helps prevent the collapse of upper floors. Another method, seemingly at complete odds with the first line of thought, is to design the superstructure to move on the foundation. Base isolators between the building and its foundation act like shock absorbers, damping some of the lateral motion that would otherwise cause damage. This configuration helps the building stay in one place while the foundation shakes beneath it. Base isolators are effective but, unfortunately, prohibitively expensive for construction in poor countries such as Haiti.
Using yet another approach, engineers erect a building to be as stiff and strong as possible in the hope that it'll retain its integrity and stay intact through a quake. Once again, there's a second method that's completely unlike the first. In this second approach, engineers design segments of the frame to move so that, during a quake, the entire frame deforms slightly and absorbs the quakes energy, dissipating it throughout the entire structure and in that way maintaining the structure's integrity.
The approached might be different but the objective is the same: Improve the construction of buildings to withstand one of Earth's awesome forces and subsequently save lives.
Wired Science 8: High Tech Quake Survival