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The acoustic response has many important roles in seismic exploration and nondestructive testing. It enables the development of fracture classification and sizing. In this paper, we combined Hudson’s effective medium scheme and finite-difference time-domain modeling method to simulate acoustic wave propagation in fractured media. Fractures are represented by discrete fracture networks, allowing for a state-of-the-art representation of natural fracture networks by a negative Exponential Law length distribution. The propagation of acoustic waves that are emitted by a point source and reflected from a fractured area in a 2D digital rock model are examined numerically with the purpose of developing an acoustic inference of fracture properties. In these fractured models, we vary the number and mean length of fractures to explore the relation between internal structure of rock and acoustic wave field characters. The modeling results indicate that acoustic wave field is more sensitive to the fracture number than to the mean of the fracture length. Moreover, a fracture-dependent attenuation analysis of the reflection records of discrete stochastic fractured models is obtained. The frequency- and time- dependent attenuation profiles feature two parts in frequency, (1) fracture-to-background at lower frequencies and (2) fracture-to-fracture at higher frequencies. Our results indicate that accounting for attenuation effects may not only allow for improving estimation of fracture number, but also provide information about geometrical characteristics of length distribution. Such an approach can be used to estimate nature fracture network properties with given acoustic records.

Energy storage is becoming indispensable for increasing renewable energy integration, and it is critical to the future low-carbon energy supply. Large-capacity, grid scale energy storage can support the integration of solar and wind power and support grid resilience with the diminishing capacity of baseload fossil power plants. With the development of thermal energy storage (TES) for concentrating solar power systems, standalone TES for grid integration becomes attractive due to the declining renewable generation cost and an increasing need for energy storage. The standalone TES system introduced in this paper can play a big role in the carbon-free energy future with capacity larger than batteries and cost likely lower than other energy storage methods such as pumped storage hydropower and compressed air energy storage, both of which also have geological limitations. To this end, we describe a TES system that uses stable, inexpensive solid particles as a TES media to provide scalable, low cost energy storage. The particle-based TES has the ability to drive various thermal power cycles including conventional steam-Rankine, air Brayton turbine with combined-cycle ability, or the emerging supercritical carbon dioxide Brayton power cycle. This work describes the containment design method including a concrete silo and an internal-insulation layer for the particle-TES system. The economic analysis shows significantly low storage cost when the particle-TES is integrated with Brayton combined-cycle power generation. The paper shows the design approach of the particle-TES system and its economic potential for bulk energy storage. The advantage of the particle-TES system as a promising bulk energy storage method is its ability to economically support dispatchable renewable grid penetration for larger capacity and longer discharging hours than current battery storage technologies.

Solid particles can operate at higher temperature than current molten salt or oil, and they can be a heat-transfer and storage medium in a concentrating solar power (CSP) system. By using inexpensive solid particles and containment material for thermal energy storage (TES), the particle-TES cost can be significantly lower than other TES methods such as a nitrate-salt system. The particle-TES system can hold hot particles at more than 800°C with high thermal performance. The high particle temperatures increase the temperature difference between the hot and cold particles, and they improve the TES capacity. The particle-based CSP system is able to support high-efficiency power generation, such as the supercritical carbon-dioxide Brayton power cycle, to achieve >50% thermal-electric conversion efficiency. This paper describes a solid particle-TES system that integrates into a CSP plant. The hot particles discharge to a heat exchanger to drive the power cycle. The returning cold particles circulate through a particle receiver to absorb solar heat and charge the TES. This paper shows the design of a particle-TES system including containment silos, foundation, silo insulation, and particle materials. The analysis provides results for four TES capacities and two silo configurations. The design analysis indicates that the system can achieve high thermal efficiency, storage effectiveness (i.e., percentage usage of the hot particles), and exergetic efficiency. An insulation method for the hot silo was considered. The particle-TES system can achieve high performance and low cost, and it holds potential for next-generation CSP technology.

This study presents fundamental response features of seismic shear motion in multi-story buildings with a continuous–discrete model and its degenerated ones, and shows their applications in inverse parametric identification. In particular, the building is modeled as a series of continuous shear-beams for inter-story columns/walls and discrete lumped-masses for rigid floors. Shear motion response at one location of the building is then obtainable to an impulsive motion at another location in the time and frequency domains, termed here as generalized impulse and frequency response functions (GIRF and GFRF). The GIRF and GFRF are not only fundamental in relating seismic responses at the two locations of a building structure subjected to ground seismic excitation that is not fully known due to the complicated soil–structure interaction. They also play a key role in characterizing structural responses, as well as in identifying dynamic parameters of the building. For illustration, this study examines response features of ten-story building of Millikan Library in Pasadena, California with the Yorba Linda earthquake of September 3, 2002. With the use of the continuous–discrete model as well as its degenerated ones, structural responses are interpreted from the perspective of wave propagation, and more importantly validated with the pertinent recordings and discrete-model-based results. Parametric identification of the building with a pair of seismic recordings is then presented. This study finally comes up a conclusion that the proposed approach with continuous–discrete modeling is efficient and robust in forward predicting analysis and inverse system identification.

a b s t r a c t This study examines one-dimensional wave propagation in a multi-story building with seismic excitation. In particular, the building is modeled as a series of shear beams for columns/walls and lumped masses for floors. Wave response at one location of the building is then derived to an impulsive motion such as displacement and acceleration at another location in time and frequency domains, termed here as wave- based or generalized impulse and frequency response function (GIRF and GFRF), which is dependent upon the building characteristics above the impulse location. Not only does this study illustrate features of GIRF and GFRF in terms of building properties, it also shows broad-based applications of the modeling. Two examples are presented with the use of the modeling. One is wave-based characterization of ten-story Millikan Library in Pasadena, California with the recordings of Yorba Linda earthquake of September 3, 2002. The other is analysis for influence of stochastic floor-to-column mass ratio, story-height and seismic input in seismic wave responses.

This study proposes a conceptual framework of in-situ vibration tests and analyses for quality appraisal of non-slender, cast-in-place piles with irregular cross-section configuration. It evaluates a frequency index from vibration recordings to a series of impulse loadings that is related to total soil-resistance forces around a pile, so as to assess if the pile achieves the design requirement in terms of bearing capacity. In particular, in-situ pile-vibration tests in sequential are carried out, in which dropping a weight from different heights generates series impulse loadings with low-to-high amplitudes. The high-amplitude impulse is designed in way that the load will generate equivalent static load that is equal to or larger than the designed bearing capacity of the pile. This study then uses empirical mode decomposition and Hilbert spectral analysis for processing the nonstationary, short-period recordings, so as to single out with accuracy the frequency index. Comparison of the frequency indices identified from the recordings to the series loadings with the design-based one would tell if the total soil resistance force remains linear or nonlinear and subsequently for the quality appraisal of the pile. As an example, this study investigates six data sets collected from the in-situ tests of two piles in Taipu water pump project, Jiangshu Province of China. It concludes that the two piles have the actual axial load capacity higher than the designed bearing capacity. The true bearing capacity of the piles under investigation can be estimated with accuracy if the amplitude of impact loadings is further increased and the analyses are calibrated with the static testing results.

This study proposes a recording-based approach to characterize and quantify earthquake-induced site nonlinearity, exemplified as soil nonlinearity and/or liquefaction. Alternative to Fourier spectral analysis (FSA), the paper introduces time–frequency analysis of earthquake ground motion recordings with the aid of so-called Hilbert-Huang transform (HHT), and offers justification for the HHT in addressing the nonlinear features shown in the recordings. With the use of the 2001 Nisqually earthquake recordings, this study shows that the proposed approach is effective in characterizing site nonlinearity and quantifying the influences in seismic ground responses. Major results from this study are listed below. 1. HHT-based site amplification is defined as the ratio of marginal Hilbert amplitude spectra, similar to the Fourier-based one that is the ratio of Fourier amplitude spectra. The HHT-based site amplification can be used in effectively quantifying site nonlinearity in terms of frequency downshift in the low-frequency range and amplification-reduction factor in intermediate-frequency range in comparison with the Fourier-based one. 2. Instantaneous damping, and Hilbert and marginal Hilbert damping spectra are defined in ways similar to instantaneous frequency, and Hilbert and marginal Hilbert amplitude spectra, respectively. Consequently, the HHT-based site damping is found as the difference of marginal Hilbert damping spectra, which can be used as an alternative, complementary index to measure the influences of site nonlinearity in seismic ground responses.

Reconnaissance reports and pertinent research on seismic hazards show that liquefaction is one of the key sources of damage to geotechnical and structural engineering systems. Therefore, identifying site liquefaction conditions plays an important role in seismic hazard mitigation. One of the widely used approaches for detecting liquefaction is based on the time-frequency analysis of ground motion recordings, in which short-time Fourier transform is typically used. It is known that recordings at a site with liquefaction are the result of nonlinear responses of seismic waves propagating in the liquefied layers underneath the site. Moreover, Fourier transform is not effective in characterizing such dynamic features as time-dependent frequency of the recordings rooted in nonlinear responses. Therefore, the aforementioned approach may not be intrinsically effective in detecting liquefaction. An alternative to the Fourier-based approach is presented in this study, which proposes time-frequency analysis of earthquake ground motion recordings with the aid of the Hilbert-Huang transform (HHT), and offers justification for the HHT in addressing the liquefaction features shown in the recordings. The paper then defines the predominant instantaneous frequency (PIF) and introduces the PIF-related motion features to identify liquefaction conditions at a given site. Analysis of 29 recorded data sets at different site conditions shows that the proposed approach is effective in detecting site liquefaction in comparison with other methods.

This paper presents the implementation of a method for nonlinear, nonstationary data processing, namely the Hilbert–Huang transform (HHT) in traditional vibration-based approaches to characterizing structural damage and shows the frequency signature of local structural damage in nonstationary vibration recordings. In particular, following the review of traditional approaches to characterizing structural damage from nonstationary vibration recordings, this study first offers the justifications of the HHT as an alternative and complementary data process in addressing the nonstationarity of the vibration. With the use of recordings from controlled field vibration tests of substructures in the Trinity River Relief Bridge in Texas in its intact, minor- and severe-damage pile states, this study then shows that the HHT-based approach can single out some natural frequencies of the structure from a mixed frequency content in recordings that also contain the time-dependent excitation and noise frequencies. Subsequently, this study exposes that the frequency downshift for the damaged pile relative to the undamaged one is an indicative index for the damage extent. The above results are also validated by an ANSYS model-based analysis. Finally, a comprehensive HHT-based characterization of structural damage is discussed, and the potential use for cost-effective, efficient structural damage diagnosis procedures and health-monitoring systems is provided.

This paper examines the rationale of a method of nonstationary data processing and analysis, referred to as the Hilbert-Huang transform (HHT), for its application to a recording-based approach in quantifying influences of soil nonlinearity in site response. In particular, this paper first summarizes symptoms of soil nonlinearity shown in earthquake recordings, reviews the Fourier-based approach to characterizing nonlinearity, and offers justifications for the HHT in addressing nonlinearity issues. This study then uses the HHT method to analyze synthetic data and recordings from the 1964 Niigata and 2001 Nisqually earthquakes. In doing so, the HHT-based site response is defined as the ratio of marginal Hilbert amplitude spectra, alternative to the Fourier-based response that is the ratio of Fourier amplitude spectra. With the Fourier-based approach in studies of site response as a reference, this study shows that the alternative HHT-based approach is effective in characterizing soil nonlinearity and nonlinear site response.

This paper aims to identify the natural frequencies and modal damping ratios of the Tsing Ma suspension bridge during Typhoon Victor using the newly emerged empirical mode decomposition (EMD) method in conjunction with the Hilbert-transform (HT) technique. Stationary tests on the acceleration responses of the bridge recorded at different locations and in different directions during Typhoon Victor are first carried out to classify the recorded response data. Natural frequencies and modal damping ratios identified by the EMD-HT method are then compared with those obtained by the traditional fast Fourier transform (FFT)-based method. The modal parameters identified by the EMD-HT method from the bridge responses recorded at different locations are compared with each other to check their consistency. Furthermore, the variations of natural frequency and total modal damping ratio with vibration amplitude and mean wind speed are examined. The results demonstrated that the EMD-HT method is applicable to modal parameter identification of large civil structures under typhoons. The EMD-HT method and the FFT-based method produced almost the same natural frequencies but the FFT-based method gave higher modal damping ratios than the EMD-HT method in general.

Modal vibration tests combined with the Hilbert-Huang transform (HHT) method were used to analyze previous recordings of controlled field-vibration tests of one concrete pile substructure of the Trinity River Relief Bridge No. 4 in Texas in its intact, minor-, and severe-damage states. Piles were excavated and broken to simulate flood and earthquake damage to a bridge substructure. The HHT algorithm is unique in that it reveals the quantitative difference in the instantaneous frequency of sound and damaged structures, consistent with the damage states and with what a simple structural model predicts. This approach differs from traditional modal vibration analyses in that a short-lived shift downward in resonant frequency can be seen in the HHT from a damaged member, whereas this shift is often lost due to the averaging effect of a modal vibration analysis. Accordingly, an HHT-based structural health monitoring procedure is discussed.

This study examines the rationale of Hilbert-Huang transform (HHT) for analyzing dynamic and earthquake motion recordings in studies of seismology and engineering. In particular, this paper first p...

This paper presents galloping analysis of multiple-degree-of-freedom (MDOF) structural roofs with multiple orientations. Instead of using drag and lift coefficients and/or their combined coefficient in traditional galloping analysis for slender structures, this study uses wind pressure coefficients for wind force representation on each and every different orientation roof, facilitating the galloping analysis of multiple-orientation roof structures. In the study, influences of nonlinear aerodynamic forces are considered. An energy-based equivalent technique, together with the modal analysis, is used to solve the nonlinear MDOF vibration equations. The critical wind speed for galloping of roof structures is derived, which is then applied to galloping analysis of roofs of a stadium and a high-rise building in China. With the aid of various experimental results obtained in pertinent research, this study also shows that consideration of nonlinear aerodynamic forces in galloping analysis generally increases the critical wind speed, thus enhancing aerodynamic stability of structures.

This study presents vibration analysis of medical devices by a finite element analysis (FEA) model calibrated with test data. The medical device under investigation is the Lifepak500 automated external defibrillator (AED), a product that is frequently exposed to vibration and shock in transportation means such as ambulances and medical-evacuation helicopters. In structure, the AED is a plastic case that contains a printed circuit board (PCB) with various attached electronic components such as capacitors, resistors, inductors, and integrated circuits. In this study, an FEA model of the AED is established with the use of ANSYS based on design specifications and static tests. The model is first calibrated with various static and dynamic tests to verify that the static displacements at selected locations, PCB twist angles, and first three natural frequencies predicted by the FEA model are consistent with those obtained by the tests. The model is then used to examine the dynamic characteristics and vibration transmissibility of the PCB within both rigid and flexible medical device cases. Finally, random vibration analysis of the PCB is presented. This study shows that the predicted frequency data favorably agrees with test data (within 7% error range), while predicted vibration amplitudes are in a reasonable range at major PCB locations when compared with the test data, but do not always agree well at the locations where the PCB has more complicated structural features and boundary conditions. The established FEA model predicts the reliability and functionality of current design of the AED from a vibration viewpoint. It can also help engineers improve the PCB mechanical design and product reliability when used in harsh vibration environments.

A generalized earthquake-wave-motion model is established in this study, which considers 3D random heterogeneous media, together with existing models for sources and realistic geological profiles for sedimentary basins and irregular topography. The model can be used not only to examine the influences of random heterogeneous media, but also to explore the multiple interactions of source, site (irregularity and/or heterogeneity), and wave interference on spatial variations of ground motion. Specifically, the earth is modeled as a layered half-space with 3D weak-random heterogeneity media. Seismic waves are generated by a shear-dislocation source buried in one of the layers and then propagated through the modeled earth medium. A first-order perturbation approach together with wave propagation theory is used to solve the problem at hand for wave motion response, which is found as the superposition of the mean and scattered wave responses. The mean wave response is obtained as a wave-motion solution for a layered half-space without heterogeneity subjected to a buried seismic dislocation source. The scattered wave field is obtained as a wave-motion solution for the same layered half-space without heterogeneity subjected to virtual distributed body forces that mathematically replace the heterogeneity. The explicit expressions for the responses in both mean and scattered wave fields are derived in this study. A computational procedure for the wave-motion responses is also presented in detail.

Wind exposure categories are currently used to account for the influences of different ground roughness in wind profile that is fundamentally important to wind-resistant structural analysis and design. Since each classified exposure covers a relatively large spectrum of ground roughness with constant indices such as exponent α , this results in uncertain wind-resistant structural analysis and design for structures in each and every categorized exposure. More importantly, such an uncertainty is not only quantitative but also qualitative in terms of aerodynamic instability assessment and economic loss estimation. This study proposes a method to determine the actual wind exposure and estimates its influences in wind-resistant structural analysis and design. In particular, this paper first provides a review of wind exposure categories and illustrates the shortcomings of its use in structural analysis and design. It then introduces a method to determine the actual ground exposure. The method generates formulae for the actual ground roughness exponent on the basis of measured data from both wind-tunnel and full-scale experiments, of ground roughness-induced wind energy dissipation analysis, as well as of current Chinese and other countries’ loading codes. Through examples of hypothetical and practical engineering projects, this study finally investigates the influences of the actual ground roughness exponent on wind-induced structural responses and economic loss estimation, as well as on wind-resistant structural design.

The objective of the present approach is to determine the structural response to external force excitations and earthquake excitations with consideration of soil-structure interaction. The physical model concerned is a flexible structure resting on a rigid or flexible foundation embedded in a layered soil medium. The vibration of the structure is first analyzed using the wave propagation approach without reference to the interaction. The interaction effect is recovered by the impedance relationship developed in the companion paper. In an attempt to use the free field ground motion as the input to the system, the Maxwell's reciprocal theorem is applied to remedy the difference between the free field and real ground motion in the presence of the structure. As a result, the structural response at any location is written as a linear function of external excitations and the seismic source. Such a solution form is convenient for statistical analysis.

This study presents system identification of ten-story Millikan Library in Pasadena, California with a pair of the Yorba Linda earthquake of September 3, 2002. The fundamental of the proposed approach is based on wave features of generalized impulse and frequency response function (GIRF or GFRF), i.e., wave response at one structural response location to an impulsive motion at another reference location in time and frequency domains respectively. With a pair of seismic recordings at the two locations, GIRF/GFRF is obtainable. With a continuous-discrete model for the structure, a closed-form solution of GFRF, and subsequent GIRF with Fourier transformation of GFRF, can also be found in terms of structural physical properties above the impulse location. Matching the two sets of GIRF/GFRF from recordings and the model helps identify system parameters such as wave velocity or shear modulus.

An analysis is given of the statistical and probabilistic properties of the random time at which a piece of essential equipment, such as a computer, may topple under base excitations. The equipment is modeled as a rigid block, and the base accelerations are modeled as white noises in horizontal and vertical directions. The cases of a free-standing block and an anchored block are both considered. The white-noise idealization resembles the most intense portion of ground acceleration in a seismic event, which is an acceptable simplification provided that the correlation times of actual base excitations are short compared with the dominant quasi-periods of the induced rocking motion. It is shown that the presence of vertical excitation about one-half the level of horizontal excitation may increase the probability of toppling by 30–40%, and that larger blocks are more stable than the smaller ones of the same geometrical proportion. The analysis is applicable to such objects as monuments, storage tanks, and certain types of tall buildings.

This paper develops a theory to compute the statistical properties for the response of a surface-mounted pipeline to seismic excitations. Making the assumptions that the pipeline behaves like an infinitely long Euler-Bernoulli beam on evenly spaced supports, that the seismic motion is statistically stationary in time but nonhomogeneous in space, and that the seismic inputs are fed into the pipeline system through the supports, solutions are obtained for the spectral densities of the structural response. The theory developed is aplicable to any spectral distributions.

This study examines rationale of correction factor β in the formula of thickness resonant frequency, fundamental to the thickness estimation of impact-echo (IE) approach in nondestructive testing (NDT) for integrity appraisal and damage diagnosis of infrastructure systems. It shows the role of the factor in the formula from the perspective of testing equipment setup, wave propagation, and resonant frequency identification, much broader than what was first introduced empirically for shape correction of a structure under test. Emphasis is laid in wave-based interpretation of resonant frequency, typically obtained from traditional fast Fourier transform (FFT) data analysis of IE recordings. Since the FFT data analysis provides average, not true, characteristic of resonant frequency shown in the nonstationary IE recordings, it typically distorts the thickness estimation from the formula if the correction factor is not used. An adaptive time-frequency data analysis termed Hilbert–Huang transform (HHT) is then introduced to overcome the shortage of FFT analysis in identifying the resonant frequency from noise-added IE recordings. With FFT and HHT analyses of five data sets of sample IE recordings from sound and damaged concrete structures and comparison with referenced ones, this study reveals that the proposed IE approach with HHT data analysis not only eliminates the subjective use of correction factor in the formula, but it also improves greatly the accuracy in the thickness estimation.