Earthquake-resistant Foundations and Materials
If a building's foundation sits on soft or filled-in soil, the whole building may fail in an earthquake regardless of the advanced engineering techniques employed. Assuming, however, that the soil beneath a structure is firm and solid, engineers can greatly improve how the building-foundation system will respond to seismic waves. For example, earthquakes often knock buildings from their foundations. One solution involves tying the foundation to the building so the whole structure moves as a unit.
Another solution -- known as base isolation -- involves floating a building above its foundation on a system of bearings, springs or padded cylinders. Engineers use a variety of bearing pad designs, but they often choose lead-rubber bearings, which contain a solid lead core wrapped in alternating layers of rubber and steel. The lead core makes the bearing stiff and strong in the vertical direction, while the rubber and steel bands make the bearing flexible in the horizontal direction. Bearings attach to the building and foundation via steel plates and then, when an earthquake hits, allow the foundation to move without moving the structure above it. As a result, the building's horizontal acceleration is reduced and suffers far less deformation and damage.
Even with a base-isolation system in place, a building still receives a certain amount of vibrational energy during an earthquake. The building itself can dissipate, or damp, this energy to some extent, although its capacity to do this is directly related to the ductility of the material used in the construction. Ductility refers to the ability of the material to undergo large plastic deformations. Brick and concrete buildings have low ductility and therefore absorb very little energy. This makes them especially vulnerable in even minor earthquakes. Buildings constructed of steel-reinforced concrete, on the other hand, perform much better because the embedded steel increases the ductility of the material. And buildings made of structural steel -- steel components that come in a variety of preformed shapes, such as beams, angles and plates -- offer the highest ductility, allowing buildings to bend considerably without breaking.
Ideally, engineers don't have to rely solely on a structure's inherent ability to dissipate energy. In increasingly more earthquake-resistant buildings, designers are installing damping systems. Active mass damping, for example, relies on a heavy mass mounted to the top of a building and connected to viscous dampers that act like shock absorbers. When the building begins to oscillate, the mass moves in the opposite direction, which reduces the amplitude of mechanical vibrations. It's also possible to use smaller damping devices in a building's brace system.
Even with extensive testing on laboratory shake tables, any seismic engineering design concept remains a prototype until it experiences an actual earthquake. Only then can the larger scientific community evaluate its performance and use what it learns to drive innovation. In the next section, we'll examine some of those innovations, as well as what the future may hold for seismic engineering.