© Mike Blake/Reuters/Corbis
In many modern high-rise buildings, engineers use core-wall construction to increase seismic performance at lower cost. In this design, a reinforced concrete core runs through the heart of the structure, surrounding the elevator banks. For extremely tall buildings, the core wall can be quite substantial -- at least 30 feet in each plan direction and 18 to 30 inches thick.
While core-wall construction helps buildings stand up to earthquakes, it's not a perfect technology. Researchers have found that fixed-base buildings with core-walls can still experience significant inelastic deformations, large shear forces and damaging floor accelerations. One solution, as we've already discussed, involves base isolation -- floating the building on lead-rubber bearings. This design reduces floor accelerations and shear forces but doesn't prevent deformation at the base of the core-wall.
A better solution for structures in earthquake zones calls for a rocking-core wall combined with base isolation. A rocking core-wall rocks at the ground level to prevent the concrete in the wall from being permanently deformed. To accomplish this, engineers reinforce the lower two levels of the building with steel and incorporate post-tensioning along the entire height. In post-tensioning systems, steel tendons are threaded through the core wall. The tendons act like rubber bands, which can be tightly stretched by hydraulic jacks to increase the tensile strength of the core-wall.