Elastomeric seismic isolation of structures and components

Details Elastomeric seismic isolation of structures and components Elastomeric seismic isolation of structures and components

1995-05-12

2001-02-27

Safavi, Michael (Department: 3635)

Static structures (e.g., buildings)

Processes

Requiring soil work

C052S167700, C052S169900

Reexamination Certificate

active

06192649

ABSTRACT:

TECHNICAL FIELD
This invention relates to a method and structure for mitigation of seismic forces on nuclear power plants, other structures and components, utilizing specially designed elastomers to dissipate the seismic energy.
BACKGROUND
Seismic events create reaction forces in massive bodies and structures that can destroy them, if the seismic energy is not adequately dissipated. It is common practice, and well known in the art of seismic design, to “tune” the structure such that its response to the major portion of the seismic spectrum is minimal. This can be done in a number of ways, such as adding dampers, springs, etc. to the base of the structure in a complicated and expensive array of shock absorbing mechanisms. In addition, the structure design itself is usually made either more rigid, or more flexible, so that its response can avoid the spectral peaks of seismic shock waves. These measures are more or less effective and limited by the frequency “bandwidth”, or response, of the overall design.
DISCLOSURE OF THE INVENTION
With the advent of engineered elastomeric materials, it is now possible to specify means of mitigating seismic response by properly and uniquely applying such materials with frequency-dependent internal damping as a part of the structural design. Thus, energy can be dissipated in the elastomer(s) as the structure attempts to vibrate in response to seismic excitation at a given frequency. In the process, the elastomer experiences a temperature rise, depending on the power density of dissipation internal to the material, and depending on whatever means are provided to dissipate such heat generation. Increasing temperature usually modifies the material properties, so an important aspect of the method here is temperature control by design. This invention specifically addresses both the frequency and the thermal requirements of seismic mitigation as critical parameters in the selection of elastomers for use in damped support structures.
A detailed analysis of this method has been performed to elucidate the trade-offs inherent in it. Using the material properties of elastomers and the time history of typical seismic events, it can be shown that a properly designed base structure provides effective isolation from excessive and destructive seismic forces. It is found that simple structures possess a characteristic frequency (or several frequencies for complicated structures with internal degrees of freedom) that is a function of the structure and elastomer parameters. This frequency is a key factor in designing the system for maximum effectiveness. In addition, the elastomer can be shown to have physical properties that are mathematically similar to electronic filters, thereby filtering the seismic power spectral density in a complicated way that is not obvious to even the most sophisticated practitioners of the art. This is especially true when thermal effects are also considered. The mathematical properties of the system “transfer function” constitute another key factor in the system design and effectiveness.
It so happens that the character of the system response can be described in terms of acceleration, displacement (relative or absolute) or velocity. It is preferred to use net acceleration as the measure of effectiveness. Analysis of the many variables and their inter-relations as described in detail further below, leads to the stated means of mitigating seismic acceleration in structures.
In accordance with an exemplary embodiment of the invention, a damped support structure utilizes one or more layers of an elastomer alternating with reinforced concrete pads. Where multiple elastomer layers are used, the properties of each layer are preferably different, depending on the seismic spectral complexities expected to be encountered. In some cases, several additional layers may be required to obtain the necessary spectral response, and these variants are included within the spirit and scope of this invention. In a two layer elastomer arrangement (again, alternating with concrete reinforcing pads), because of the mass of the upper concrete reinforcing pads, the characteristic frequency is smaller for the lower elastomer pad than for the upper elastomer pad. In addition, the elastomer pads are coupled, and the support structure has multiple degrees of freedom that can be analyzed in accordance with the discussion hereinbelow. The damping factors for the elastomers can be specified by analysis to adequately restrain the motion of the superstructure or building that is to be protected. Clearly, the size and construction of the support structure depends on the properties of the building that is to be isolated from the ground motion, as well as on the anticipated seismic spectrum.
The above described concrete reinforcing pads and elastomer pads are utilized in combination with a plurality of piers or other support members which not only bear a portion of the vertical loads, but also provide some structural rigidity in shear. The piers or support members are also utilized as a major heat transfer path to the bedrock or heat sink. The number, size and placement of the support members are a matter of design, specified by the application. The number and dimensions of the supporting pads are also application specific and dictated by the design. The entire structure is a dynamic system with multiple degrees of freedom, which must be designed to specific criteria. All such designs are included in this disclosure as obvious variants of the basic principles of the invention.
In one aspect of the invention, therefore, there is provided a method of mitigating seismic forces on a structure during a seismic event comprising the steps of:
a) providing a damped support system for the structure comprising a plurality of support piers surrounded by at least two layers of reinforced concrete on either side of at least one layer of an elastomer where the elastomer is formed to include thermal diffusion and damping properties determined as a function of properties of the structure and as a function of an expected seismic event frequency spectrum; and
b) dissipating heat generated in the elastomer during the seismic event.
In another aspect of the invention, there is provided a method of mitigating seismic forces on a structure during a seismic event comprising the steps of:
a) supporting the structure with a plurality of piers; and
b) surrounding the plurality of piers with a seismic damping system including at least one elastomer engineered to include damping properties specified as a function of a predicted seismic frequency spectrum.
In still another aspect, the invention relates to a dumped support system for a structure comprising a plurality of support piers in a predetermined array, supporting the structure; and at least two rigid reinforcing pads sandwiched about an elastomer layer, the plurality of support piers extending through the reinforcing pads and the elastomer layer.
The invention here, as it relates to a methodology and resultant elastomeric isolation structure, provides the following advantages:
1) It provides means of minimizing seismic response of structures and components without the necessity of expensive and cumbersome mechanical springs, dampers, snubbers, etc.;
2) It utilizes internal damping inherent to elastomeric solids to absorb seismically induced energy in structures and components without damaging the elastomer;
3) It provides means of “tuning” the system by judiciously choosing the various parameters and dimensions, thereby enhancing the internal dissipation of energy in the frequency range containing the most excitation of energy;
4) It utilizes a unique “sandwich” configuration that is specific to the minimization of seismic response and conducive to stable structural design;
5) It provides a means of essentially eliminating undesirable frequencies in the excitation spectrum that could otherwise result in undesirable resonance excitation of structures and components; and
6) Elastomeric isolation designs can be retrofitted into existing nuclear p

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