Notes to Table 4.1.8.18.:
4.1.8.19. Seismic Isolation (1) For the purposes of this Article and Article 4.1.8.20., the following terms shall have the meaning stated herein: (a) “seismic isolation” is an alternative seismic design concept that consists of installing an isolation system with low horizontal stiffness, thereby substantially increasing the fundamental period of the structure, (b) “isolation system” is a collection of structural elements at the level of the isolation interface that includes all individual isolator units, all structural elements that transfer force between elements of the isolation system, all connections to other structural elements, and may also include a wind-restraint system, energy-dissipation devices, and a displacement restraint system, (c) “seismically isolated structure” includes the upper portion of the structure above the isolation system, the isolation system, and the portion of the structure below the isolation system, (d) “isolator unit” is a structural element of the isolation system that permits large lateral deformations under lateral earthquake design forces and is characterized by vertical-load-carrying capability combined with increased horizontal flexibility and high vertical stiffness, energy dissipation (hysteretic or viscous), self-centering capability, and lateral restraint (sufficient elastic stiffness) under non-seismic service lateral loads, (e) “isolation interface” is the boundary between the isolated upper portion of the structure above the isolation system and the lower portion of the structure below the isolation system, and (f) “wind-restraint system” is the collection of structural elements of the isolation system that provides restraint of the seismically isolated structure for wind loads and is permitted to be either an integral part of the isolator units or a separate device. (2) Every seismically isolated structure and every portion thereof shall be analyzed and designed in accordance with, (a) the loads and requirements prescribed in this Article and Article 4.1.8.20., (b) other applicable requirements of this Subsection, and (c) appropriate engineering principles and current engineering practice. (3) For the analysis and modeling of the seismically isolated structure, the following criteria shall apply: (a) three dimensional Non-linear Dynamic Analysis of the structure shall be performed in accordance with Article 4.1.8.12, (b) unless verified from rational analysis, the inherent equivalent viscous damping — excluding the hysteretic damping provided by the isolation system or supplemental energy dissipation devices — used in the analysis shall not be taken as more than 2.5% of the critical damping at the significant modes of vibration, (c) all individual isolator units shall be modeled with sufficient detail to account for their non-linear force-deformation characteristics, including effects of the relevant loads, and with consideration of variations in material properties over the design life of the structure, and (d) except for elements of the isolation system, other components of the seismically isolated structure shall be modeled using elastic material properties in accordance with Sentence 4.1.8.3.(8). (4) The ground motion histories used in Sentence (3) shall be, (a) appropriately selected and scaled following good engineering practice, (b) compatible with, (i) a response spectrum derived from the design spectral acceleration values, S(T), defined in Sentence 4.1.8.4.(9) for ground conditions of Site Classes A, B and C, and (ii) a 5% damped response spectrum based on a site-specific evaluation for ground conditions of Site Classes D, E and F, and (c) amplitude-scaled in an appropriate manner over the period range of 0.2 T1 to 1.5 T1, where T1 is the period of the isolated structure determined using the post-yield stiffness of the isolation system in the horizontal direction under consideration, or the period specified in Sentence 4.1.8.20.(1) if the post-yield stiffness of the isolation system is not well defined.