Earthquake resistant design of structures using the variable frequency pendulum isolator

The essential proper-ties of sliding isolators used for earthquake resistant design are period shift, energy dissipation, and the restoring mechanism. Isolation systems using a curved surface incorporate all of these in a single unit. The systems currently available have limitations due to their period and restoring force characteristics. The writers have recently proposed a new isolator called the variable frequency pendulum isolator (VFPI), which overcomes these limitations while retaining the advantages. The oscillation frequency of the VFPI continuously decreases with increase in sliding displacement, and the restoring force has an upper bound so that the force transmitted to the structure is bounded. The mathematical formulation for the response of multi-degree-of-freedom (MDOF) structures isolated using the VFPI has been discussed in this paper. Parametric studies have been carried out to examine the behavior of MDOF structures and structure-equipment systems isolated with the VFPI, friction pendulum system, and pure friction isolator. The VFPI performance is found to be effective and stable during low- and medium-intensity excitations, and fail-safe during high-intensity excitations. The VFPI is also found to exhibit robust performance for a wide range of isolator and ground motion characteristics, clearly demonstrating its advantages for vibration control of MDOF structures

Behavior of torsionally coupled structures with variable frequency pendulum isolator

The essential characteristics of an effective base isolation system are isolation, energy dissipation, and restoring mechanism. Some friction type base isolation systems proposed earlier have been found to be very effective in reducing the structural response and incorporate simple restoring mechanisms through gravity. However these isolators are effective only under certain excitation and structural characteristics. To overcome these limitations the authors have recently developed a new isolation system called the variable frequency pendulum isolator (VFPI) that has been found to be very effective under a variety of structure and excitation characteristics. In this paper, the mathematical formulation for the three dimensional behavior of an asymmetric structure isolated by a sliding type isolator, with particular reference to VFPI has been presented. The behavior of torsionally coupled Structure isolated by VFPI has been examined. The factors affecting the torsional coupling of isolated structure have been identified. The behavior of structure isolated by VFPI has been compared with the structure isolated by other type of friction isolators such as friction pendulum system and pure-friction system