Tall buildings must be braced throughout their height with cross-bracing, rigid connections, or stiff structural walls called shear walls. Most tall buildings are constructed of steel or reinforced concrete. These materials are flexible—that is, they can bend slightly without breaking. When strong winds blow, the steel and concrete buildings give way to the force of the wind by bending a little. For example, the Sears Tower in Chicago was designed to sway as much as 10 inches (25 centimeters). If tall buildings did not sway, they could be snapped in two by violent wind. The buildings are not pulled out of the ground because they are firmly anchored in strong foundations.
