Base Isolation by a Soft First Story with Inclined Columns

Abstract

An idea of passive base isolation of buildings is explored, using inclined rubber base isolators or inclined “soft” first-story columns. Such a system behaves as a physical pendulum, “pivoted” above center of mass, and is more stable than the standard system. Another advantage of the inclination is that the inertia forces of the structure due to rotation about the pivot point cancel to some degree the inertia forces due to the base translation. This is expected to result in smaller relative deformations of the building and smaller internal forces. In this paper, the consequences of this concept are illustrated on a simple model (base isolated single-degree-of-freedom oscillator), but also considering the soil flexibility and the wave nature of strong earthquake shaking (these are associated with additional rotations of the system and also affect the system period). A frequency domain solution is presented for small deformations, rigid foundations embedded in a homogeneous elastic half-space, and horizontal in-plane and Rayleigh wave foundation driving motion.