Design of Structures with Seismic Isolation

This chapter surveys the principles, benefits, and the feasibility of seismic isolation. The basic principles of seismic isolation are introduced first. Contrary to a perception held by many engineers, neither the concept of seismic isolation is new nor its application is necessarily complex. What is new is the availability of relatively new materials and devices worked to perfection over the last two decades and advances in computational techniques now commonly in use by practicing engineers. Force-deflection characteristics ot commonly used isolation devices are introduced next followed by guidelines for evaluation of the feasibility of seismic isolation as an alternative for a given project. The differences in approach to new construction and rehabilitation of existing structures are highlighted. The building code provisions for seismic isolation are covered next. The very recently released year 2000 edition of the International Building Code (IBC-2000) takes a much more simple approach to seismic isolation than did its direct predecessor, the 1997 edition of the Uniform Building Code (UBC-97). This is true even though the theory and objectives implemented in both of these codes are the same. The simplification is largely due to incorporation of spectral hazard maps in IBC-2000. A very practical side-effect of this incorporation is elimination of near-fault factors from the design process simply because now they are explicitly contained in the map. In many cases, design according to the new IBC-200 requirements will result in smaller displacement and force demands on the isolation system and the structure above the isolation plane. This in terms mean that seismic isolation can be implemented much more economically than it was possible under UBC-97. The IBC-2000 design provisions for seismic isolation are discussed in detail. A simple preliminary design procedure is provided to aid engineers in initial sizing of the isolation devices. Several examples are provided to illustrate the practical application of the material covered in this chapter.