Seismic-Isolator

1. Field of the Invention

The present invention relates to earthquake base isolation of buildings and other structures. More particularly, the invention relates to antifriction base isolators.

2. Description of the Prior Art

It is considered to be the ultimate in seismic isolation to place a superstructure on roller or ball bearings. In this case, almost no horizontal force will be transmitted in a superstructure. However, extreme yieldingness under wind load and inability to restore the initial position prevented such systems from practical implementation.

The invention entitled "Earthquake Shelter" (Ser. No 07/363,592, U.S. Patent Allowed) incorporates ball bearing base isolator where the ball bearing is supported on a pedestal plate having a concave upper surface; the ball bearing permits the superstructure to remain horizontally undisturbed during an earthquake and retains its initial position with respect to the footing. But this system is not intended for resistance against wind.

The invention entitled "Earthquake Stable Support" (SU-666-266) has a sphere contained between two belts with conical recesses, each side having additional recess in its center to remain the sphere in position under wind pressure. But the conical shape of the recesses makes the system auto-tuning with the earth excitation, because the growth of the ground period is accompanied by the increase in ground displacement which in its turn gives rise to the increase of the isolated system period, and the system has to perform under periresonant conditions. Besides, functioning of a support of this kind is associated with travelling the sphere horizontally regarding the supported superstructure which generates alternating eccentrically applied vertical base reactions that might result in excitation of damaging flextural stress waves.

To minimize the transmission of destructive ground motion into a superstructure, to prevent permanent horizontal post-earthquake offsets and at the same time to keep the system's ability to withstand wind pressure, as well as minor earthquakes, a seismic isolator is offered which consists of a ball transfer unit raggedly constructed and rigidly connected to the supported superstructure. The ball rests on a depression which is shaped in compliance with the configuration of the contacting surface of the ball and is centered at the lowest point of the pedestal plate having a concave upper surface and resting on a foundation.

In the description of the invention herein presented, references are made to the accompanying drawings, in which:

FIG. 1 is a schematic perspective of a seismic isolator with associated superstructure and foundation.

FIG. 2 is an enlarged vertical elevation of the ball, depression and adjoining part of the pedestal plate with a pattern of acting forces.

The present invention will be described with reference to the accompanying drawings. As illustrated at FIG. 1 the seismic isolator according to the invention has a ball transfer unit (1) which consists of a large ball that supports the superstructure (2) and permits the foundation to move horizontally not involving the superstructure in this movement. The large ball is positioned in a massive steel housing which is located above the ball and is coupled rigidly to the superstructure in order to ensure irreversible foot reactions and to prevent an induction of secondary waves of flexure in bearing elements of the superstructure. The large ball is separated from the housing by several smaller balls running in a hemispheric shell (conventional practice, The large ball rests on a depression (3) of a pedestal plate (4). The depression is shaped in compliance with the configuration of the contacting surface of the ball and is centered at the lowest point of the pedestal plate having a concave upper surface and resting on a foundation (5) to which it is firmly attached. The depth of the depression d at given radius of the ball r is governed mainly by weight of the structure F.sub. g and by design wind load F.sub.w (FIG. 2). For most of structures with the exception of slender ones such as high-rises, tall chimneys and open-frame towers, the maximum wind load averages a relatively small fraction of a strong earthquake base shear. Therefore, the force of gravity will keep the structure in a steady position on the pedestal plate both at any wind and at slight earthquakes. When magnitude of the earth movement exceeds a certain threshold the ball gets out of the depression, any transfer of horizontal movement to the superstructure practically gets ceased, and a hazardous shaking of the foundation does not influence the superstructure.

The upper surface of the pedestal plate (4) is shaped as a spherical segment to ensure an independence of the natural period of the isolated structure from the foundation amplitude and thus to prevent the structure against auto-tuning to periresonant frequencies. The chord (horizontal diameter of the cavity) is equal or more than the double maximum amplitude of any possible vibration of the ground during a strong earthquake. The radius of vertical curvature of the upper surface of the pedestal plate is designed as big as to provide a proper tuning-out the natural frequencies of base-isolated from fixed-base structure.