The rubber in the isolator acts as a spring. It is very soft laterally but very stiff
vertically. The high vertical stiffness is achieved by having thin layers of rubber
reinforced by steel shims. These two characteristics allow the isolator to move
laterally with relatively low stiffness yet carry significant axial load due to their high
vertical stiffness. The lead core provides damping by deforming plastically when the
isolator moves laterally in an earthquake.