Shiang-Jung Wang

Periodic materials-based vibration attenuation in layered foundations: experimental validation

Guided by the recent advances in solid-state research in periodic materials, a new type of layered periodic foundation consisting of concrete and rubber layers is experimentally investigated in this paper. The distinct feature of this new foundation is its frequency band gaps. When the frequency contents of a wave fall within the range of the frequency band gaps, the wave, and hence its energy, will be weakened or cannot propagate through the foundation, so the foundation itself can serve as a vibration isolator. Using the theory of elastodynamics and the Bloch–Floquet theorem, the mechanism of band gaps in periodic composites is presented, and a finite element model is built to show the isolation characteristic of a finite dimensional periodic foundation. Based on these analytical results, moreover, a scaled model frame and a periodic foundation were fabricated and shake table tests of the frame on the periodic foundation were performed. Ambient, strong and harmonic vibration attenuations are found when the exciting frequencies fall into the band gaps.

DESIGN FORCE TRANSMITTED BY ISOLATION SYSTEM COMPOSED OF LEAD-RUBBER BEARINGS AND VISCOUS DAMPERS

In seismic isolation design of structures located at soft soil sites or near field areas, viscous dampers (VD) are often included as part of the isolation system to minimize its maximum displacement. Due to the 90° phase angle existing between the force and displacement of the VD, the maximum force transmitted by the isolation system cannot be calculated by simply combining the forces of the isolation bearings, such as lead-rubber bearings (LRB) or high damping rubber bearings (HDRB), and VD in association with the design displacement. Conforming to the code-specified equivalent lateral response procedure for isolation design, this paper presents a formula for determining the seismic design force of the combined LRB and VD isolation system, taking into account the phase angle between the combined force of the LRB and VD and the displacement of the isolation system. The numerical results have shown that the maximum responses of the isolation system predicted by the proposed formula are conservative and comparable with those from the inelastic dynamic response history analysis.

Analytical and Experimental Studies on Seismic Behavior of Buildings with Mid-Story Isolation

The mid-story isolation design method is recently gaining popularity for the seismic protective design of buildings particularly located at highly populated areas. In a mid-story isolated building, the isolation system is incorporated into the midstory rather than the base of the building. In this paper, the dynamic characteristics and seismic responses of mid-story isolated buildings are investigated using a simplified three-lumped-mass structural model for which equivalent linear properties are formulated. It is found that the nominal frequencies of the superstructure and the substructure respectively above and below the isolation system have significant influences on the isolation frequency and equivalent damping ratio of a mid-story isolated building. The mass and stiffness of the substructure are of greater significance than the superstructure in affecting the dynamic characteristics of the isolated building. Moreover, based on the response spectrum analysis, it is noted that the higher mode responses may contribute significantly to the story shear force of the substructure. The adverse effect arising from the coupling of higher modes on the acceleration responses of the superstructure is presented numerically and experimentally. A simple method to guarantee the mid-story isolation design against the modal coupling effect attributed to the improper design of the substructure and superstructure is proposed. Consequently, the equivalent lateral force procedure of design codes should carefully include the effects of higher modes.

Seismic isolation of small modular reactors using metamaterials

Adaptation of metamaterials at micro- to nanometer scales to metastructures at much larger scales offers a new alternative for seismic isolation systems. These new isolation systems, known as periodic foundations, function both as a structural foundation to support gravitational weight of the superstructure and also as a seismic isolator to isolate the superstructure from incoming seismic waves. Here we describe the application of periodic foundations for the seismic protection of nuclear power plants, in particular small modular reactors (SMR). For this purpose, a large-scale shake table test on a one-dimensional (1D) periodic foundation supporting an SMR building model was conducted. The 1D periodic foundation was designed and fabricated using reinforced concrete and synthetic rubber (polyurethane) materials. The 1D periodic foundation structural system was tested under various input waves, which include white noise, stepped sine and seismic waves in the horizontal and vertical directions as well as in the torsional mode. The shake table test results show that the 1D periodic foundation can reduce the acceleration response (transmissibility) of the SMR building up to 90%. In addition, the periodic foundation-isolated structure also exhibited smaller displacement than the non-isolated SMR building. This study indicates that the challenge faced in developing metastructures can be overcome and the periodic foundations can be applied to isolating vibration response of engineering structures.

Experimental Study on Seismic Performance of Mechanical/Electrical Equipment with Vibration Isolation Systems

A prototype diesel generator equipped with a vibration isolation system consisting of restrained isolators (denoted as I/system) is quasi-statically and dynamically tested. Sequentially, the seismic simulation tests are conducted to further investigate the effectiveness of additional snubbers incorporated into the vibration isolation system (denoted as I/R system). Comparing the test results to the static design demands specified in ASCE 7-10, the recommended component amplification factor could represent the horizontal acceleration amplification phenomenon of the generator equipped with I/R system; however, the seismic force demands for static design of I/R system might not be appropriate and conservative enough.

Seismic response prediction of base-isolated structures with high damping rubber bearings

To characterize the highly nonlinear hysteretic behavior of high damping rubber (HDR) bearings, a previously developed mathematical hysteresis model, rather than a simplified bilinear hysteresis model, is adopted in this study. Unilateral and bilateral seismic simulation tests of a scaled-down multistory structure isolated with HDR bearings subjected to three recorded earthquakes were conducted. Based on the unilateral test results, different sets of model parameters can be identified. The fidelity of the adopted hysteresis model is investigated by comparing the analytical predictions using all sets of identified parameters with results measured from the tests. The significant influence ascribed to different ground motion characteristics such as the effects of near-field and far-field earthquakes on modeling the hysteretic behavior of HDR bearings is observed. Besides, a feasibility study on applying the identified parameters of the adopted hysteresis model to predict the bilateral seismic responses is performed. Even though the prediction accuracy of the seismic responses is satisfactory in two orthogonally horizontal directions, the bilateral hysteretic modeling of HDR bearings requires further intensive studies considering the coupling effect under bilateral excitations.

Periodic Material Based Seismic Isolation for Small Modular Reactors

The concept of periodic materials, based on the theory of solid-state physics, is applied to earthquake engineering. The periodic material is a material that possesses distinct characteristics that prevent waves with certain frequencies from being transmitted through it; therefore, this material can be used in structural foundations to block unwanted seismic waves with certain frequencies. In this paper, small modular reactor with one dimensional and three dimensional periodic foundations were studied. The theoretical band gaps of one-dimensional (1D) and three-dimensional (3D) periodic foundations were obtained using transfer matrix methods and finite element method, respectively. The dynamic response of the small modular reactor supported by a periodic foundation to that of conventional concrete foundation was compared. It is showed that the periodic foundations are an effective way to reduce the seismic response of small modular reactors (SMR) under seismic excitations.Copyright

Analytical Study of 1D Periodic Foundations for Structural Vibration Isolation

The application of periodic foundations for structural vibration isolation has been intensively studied in the past decade. The ability of the periodic foundation to block the seismic wave on certain frequency ranges has put this kind of foundations as a prosperous next generation of seismic isolators. This paper first describes general idea of basic theory of one dimensional (1D) periodic foundation. The parametric studies based on the material and the geometric properties conducted in the past are discussed. The experimental tests on 1D periodic foundation are reported as well. Based on the promising results of the past research, 1D periodic foundations have been regarded as one of the simplest, yet effective foundation to block the seismic wave in all directions.In addition to the review of the existing studies, enhanced analyses of 1D periodic foundations have been performed and are presented in this paper. Finite element analysis model of 1D periodic foundations with a three story steel frame as the superstructure was utilized for the parametric study. The study was conducted under both Shear Wave (S-Wave) and Primary Wave (P-Wave). The independent variables include the number of unit cells in the periodic foundation, the foundation and superstructure interaction, and the main frequency content of earthquake ground motion. It is found that the main frequency content of the earthquake and the natural frequency of the super structure-periodic foundation system play the major role in the superstructure response. While the number of unit cell affect the superstructure response, the effect is not as significant as the aforementioned.Copyright

Health Monitoring of RC Frame Structure to Shake Table Excitations

Structural health monitoring of RC structures under seismic loads has recently attracted attention in the earthquake engineering research. In this paper, a piezoceramic-based device called “smart aggregate” was used for the health monitoring of RC frame structures under earthquake excitations. A two-story one-bay RC moment frame instrumented with smart aggregates was tested using a shake table. The distributed piezoceramic-based smart aggregates embedded in the RC structures were used to monitor the health condition of the structures during the tests. The sensitiveness and effectiveness of the proposed piezoceramic-based approach were investigated and evaluated by analyzing the measured responses.