This hybrid material is so strong and cheap that it's widely used around the world for offices, apartments and bridges. But problems can arise. In Turkey, "Reinforced concrete was the biggest problem," says Natali Sigaher, a doctoral candidate in earthquake engineering at the State University of New York at Buffalo. "All residences are reinforced concrete; we don't have single-family houses." Generally, she adds, the apartments are three to six stories high, and the floors weigh enough to crush those caught in a collapse.

Sigaher, a native of Turkey who was home in Istanbul during the August quake, says the biggest problem was not design but construction. Concrete that was mixed on site was a major problem. She says one contractor admitted using unwashed sea sand in the mix. Sea sand contains chloride ions, which are detrimental to concrete and rusts the steel reinforcement rods.

Doing the post-mortem
A key way to keep reinforced concrete buildings on their feet after a large earthquake is to study the effects of past earthquakes. In California, a hotbed of earthquake engineering, the 1971 San Fernando quake led to a serious tightening of standards. "There was a drastic change" in building codes, says Jose Pincheira, an assistant professor of civil engineering at the University of Wisconsin-Madison who studies the earthquake performance of reinforced concrete buildings built to the prior standards.

Engineers got other lessons in the power of earthquakes from the 1994 Northridge quake in California, and the 1995 Kobe disaster in Japan, both of which caused more damage than anticipated. These quakes have set the stage for a further increase in earthquake design standards, Pincheira indicates. "There will likely be changes in the level of force for which one will have to design, especially for structures located very close to the fault."

After most major earthquakes, engineers do postmortems to compare collapsed structures to surviving buildings. Photos of almost every earthquake show collapsed buildings side by side with intact or lightly damaged ones. What's the difference?

One obvious key is columns, the vertical elements that hold up the floors. "We look at the integrity of columns," says Pincheira. "They need to be standing after the end of the earthquake. Beams, walls and floors are also important, but in general, it's the vertical members that are going to prevent collapse, although structural walls are also key elements."

Oddly enough, earthquake designers do not want most buildings to be in perfect condition after a large quake, Pincheira says. Rather, the goal is to make sure they are standing, so occupants can leave safely. "The forces from an earthquake are so large, in most cases it would be too expensive to design a building that would remain intact." In an area that gets a large quake every 50 years, he points out, a building may never experience a big shock.

The no-damage philosophy, he says, has been traditionally reserved for critical structures like nuclear power plants. For most buildings, "The idea is to be able to go back and repair them."

In recent years, engineers have tried to institute "performance-based design," which would allow owners to specify the acceptable level of damage, giving the engineer guidance on how much strength would be required.

Repairs and retrofits
Damaged concrete buildings, as revealed by cracks in beams and columns, can often be repaired after a quake. Sigaher, who has participated in post-quake postmortems, says the degree of damage determines what happens next. Small cracks can be repaired with injections of epoxy compounds. Damaged columns can be "jacketed" with steel cladding to prevent further deterioration, or a larger column can be cast around them. Some buildings, however, must be demolished even though they are standing afterwards.

Other structural changes aim to reduce the movement between floors of a building. "Shear walls," built between adjacent columns, are one approach to stiffening the building. An alternative is dampers -- giant shock absorbers -- installed between columns and beams.

Unlike a shear wall, dampers can attenuate some of the earthquake energy. Dampers "limit the displacement and the forces," says Sigaher, who studies the technique. The kinetic energy that originated as heat in the Earth's mantle moves the Earth and the building's foundation, and some of it becomes heat again as it exerts a force on the fluid in the damper.