Apparent Wave Velocity Research Articles

Experimental investigation on the deterioration of the physical and mechanical properties of autoclaved aerated concrete at elevated temperatures

Abstract Autoclaved aerated concrete (AAC) has been extensively studied and applied in the past decades because of its excellent thermal insulation and acoustic performance, energy efficiency, and outstanding structural performance. To investigate the deterioration characteristics of AAC under high temperatures, the physico-mechanical properties of AAC at different temperatures were tested by mass loss, wave velocity, and compressive tests, and the deterioration mechanism was discussed by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) tests. The results showed that after exposure to elevated temperatures, the parameters of apparent form, mass loss, wave velocity, and compressive strength underwent remarkable conversions. It was observed that the ultrasonic behavior of AAC was affected at high temperatures as it increased at 100°C and decreased beyond 100°C. In addition, the compressive strength exhibited a two-stage transformation, slightly increased from ambient temperature to 300°C, and exhibited a rapid reduction beyond 300°C. At 900°C, the specimen lost its strength. By XRD, TGA, and SEM, it was confirmed that a series of physicochemical changes in AAC, such as the water escape and evaporation, decomposition of calcium silicate hydrate and calcium carbonate, and structural damage, were the primary reasons for the deterioration of the physical and mechanical properties at elevated temperatures.

Experimental study on the influence of the wall cavity on stability of kerosene two-phase rotating detonation combustion

The combustion characteristics in annular rotating detonation combustors are experimentally investigated using 40% oxygen-enriched air and liquid aviation kerosene (RP-3) as reactants. Two types of wall cavity structures, embedded in the outer and the inner wall of the annular combustors, respectively, are employed to study their effects on detonation stability. The characteristics of typical combustion modes observed in the annular combustors are firstly analyzed. The regime of typical combustion modes is then examined in the phase diagram of mass flow rate and equivalence ratio. Results show that the single-wave mode regime can be affected by wall cavities. In comparison to the combustor without a wall cavity, the regime is expanded for the cavity in the inner wall but significantly reduced for the cavity in the outer wall. When the single-wave mode forms, the cavities embedded in the inner wall result in a significant increase of the apparent detonation wave velocity but a decrease in the velocity fluctuation rate, and the effects are changed with the axial location of the cavities. The axial location of the cavity can also influence the stable propagation location of the detonation wave. Additionally, the analysis of the high-frequency pressure signals indicates that the cavities do not affect the initiation process of the formation of the combustion modes. However, the variation of oxidizer mass flow rate and equivalence ratio can evidently influence the evolution time of different combustion modes after ignition. The study indicates that the wall cavity structure has a significant influence on the propagation behavior of rotating detonation waves in the combustors, and it can be utilized to enhance the stability of kerosene two-phase rotating detonation combustion.

Fragility Assessment of a Long-Unit Prestressed Concrete Composite Continuous Girder Bridge with Corrugated Steel Webs Subjected to Near-Fault Pulse-like Ground Motions Considering Spatial Variability Effects

Prestressed concrete composite girder bridges with corrugated steel webs (PCCGBCSWs) are extensively employed in bridge construction because of their low dead weight, fast construction, and high prestressing efficiency. Moreover, PCCGBCSWs will experience deformation and failure of the corrugated steel webs, including steel fatigue and fracture, during earthquakes. These changes will introduce safety hazards, which can be addressed via bridge disaster prevention and mitigation. Because near-fault pulse-like ground motions (NFPLGMs) have high peak accelerations, these motions can easily cause damage to a bridge. Therefore, in this study, a seismic fragility assessment is performed for long-unit PCCGBCSWs subjected to NFPLGMs considering spatial variability effects, and a sensitivity evaluation of the seismic fragility is conducted considering girder type, bearing type, ground motion type, and apparent wave velocity to offer a point of reference for seismic design. The results show that PCCGBCSWs are less vulnerable than concrete bridges. The shock absorption effect of the friction pendulum bearing is better than that of the viscous damper. The impact of NFPLGMs on bridges is greater than that of near-fault non-pulse-like ground motions (NFNPLMs) and far-fault ground motions (FFGMs). The seismic fragility under nonuniform excitation conditions is greater than that under uniform excitation conditions, showing an increasing trend with decreasing apparent wave velocity.

Nonlinear seismic responses of a long-span railway suspension bridge crossing strike-slip fault rupture zones

This study investigates the nonlinear seismic responses of a long-span suspension bridge crossing strike-slip faults and their sensitivity to various parameters and conditions. Four sets of near-fault records containing forward directivity and fling-step effects are selected, corrected, and then incorporated into the analysis. Empirical model is proposed using 54 sets of near-fault records to evaluate the dynamic pulse amplitude in fling-step ground motions. A simplified scaling method considering self-consistency is proposed to obtain the fling-step ground motions with different fault static offset and dynamic pulse amplitude. Multiple parameter method is derived by theoretical analysis to consider the effect of the fault-crossing angle on the wave propagation distance and direction. A nonlinear numerical model of double-pylon suspension bridge is established to study the seismic responses of near-fault and fault-crossing suspension bridges. The results indicate that the girder of the fault-crossing bridge rotates around the midspan, and the most of the bridge responses are slightly smaller than those of the near-fault case, whereas the transverse residual bearing displacement is substantially higher. Moreover, the sensitivity of the bridge responses to the fault-crossing angle, fault-crossing location, permanent displacement, dynamic pulse-type motion, ground motion component, and wave passage effect is comprehensively investigated. The minimum bridge responses, except for the residual bearing displacement, are obtained at a fault-crossing angle of 90° and a fault-crossing location at the middle span. The effects of the permanent displacement, dynamic pulse-type motion, and apparent wave velocity on the bridge responses are highly correlated with the fault-crossing angle. At a fault-crossing angle of 90°, all bridge responses are insensitive to these parameters, except that the peak transverse pylon drift and the bearing displacement increase considerably with the increasing amplitude of dynamic pulse-type motion. When the fault-crossing angle is acute (obtuse), the wave passage has a considerable effect on the bridge responses, and most of the bridge responses increase significantly with an increase in the permanent displacement and dynamic pulse-type motion. However, the permanent displacement has a negligible effect on the peak pylon drift. The wave passage effect is more reasonable to be considered using the apparent wave velocity in the horizontal direction than in the bridge axis direction because the influence of the fault-crossing angle on the propagation distance and direction is taken into account. Furthermore, the vertical component exerts a minimal effect on all bridge responses, whereas the fault-normal component significantly affects most bridge responses.

Martian Crustal Model From a Joint Inversion of Receiver Functions and Apparent Shear Wave Velocity

AbstractReceiver function (RF) based studies have revealed the layered Martian crustal structure beneath the InSight, which provides key information about the composition, rock alteration, deep magma process, and thermal history of Mars. However, limited by the frequency and trade‐offs between the thickness and velocity, inverting RFs alone is difficult to provide an accurate image of the upper crust. Here, by achieving stable measurements of the apparent shear wave velocities (Vs,app) at different frequencies for high‐quality marsquakes, we perform a joint inversion with the RFs. Our crustal models differ from previous models by having an extra shallow structure in the upper crust, either a layer with a sharp velocity jump interface at 4.0 ± 0.9 km depth or a gradient structure with shear wave velocity increasing gradually from 1.1 ± 0.1 km/s near the surface to 1.7 ± 0.3 km/s at a depth of 6.0 ± 2.4 km, which well fits both RFs and Vs,app's. Such an upper crustal velocity model is consistent with the depth‐decaying of porosity due to self‐compaction and indicates a highly porous upper crust with a surface porosity larger than 30%. Besides, a much higher porosity decay constant than that scaled from the Moon is derived, suggesting different mechanical properties of the lunar and Martian crust.

Axial transmission technique for screening bucked shin in a horse leg

For the safe and simple screening of equine leg bones, we applied an ultrasonic axial transmission (AT) technique to an equine bone sample with a weak bucked shin. Both experimental and simulation studies have been conducted. To simulate by the finite-difference time-domain method, a digital model of the equine leg bone was fabricated. The experimental and simulation results showed a similar tendency. The obtained apparent wave velocities in the axial direction were almost constant in the healthy part but strongly fluctuated in the bucked shin part because of the small surface irregularities. The standard deviation values of the wave velocities in the bucked shin parts were large. These results indicate that a weak bucked shin in the equine leg bone may be screened clinically by a simple evaluation of velocity fluctuation using the AT technique.

Wavenumber unwrapping for the automated analysis of SASW measurements

Depending upon the surface wave methods, the measured surface wave velocities are either normal mode velocities or apparent velocities. The “apparent” surface wave velocity corresponds to the superposed mode combining the available normal modes. The spectral-analysis-of-surface-waves (SASW) method measures the apparent surface wave velocities. However, these velocities render SASW analyses complicated because of the mode jumps induced by several wave groups. To alleviate such complications, this study introduced a beamforming technique and produced information on unwrapped wavenumber, which is vital in unwrapping the phase spectrum in SASW analyses. Additionally, incoming wave directions filtered out multiple reflections and irrelevant seismic events to retain only the surface waves directly propagating from the source. The proposed wavenumber unwrapping technique was verified using field applications, dynamic finite element analysis, and synthetic waveforms. Notably, an optimal beamforming technique was established using four geophones, making the proposed method highly practical and reducing the hardware requirements. Hopefully, the proposed analysis technique provides SASW practitioners with a more comfortable and more reliable environment in utilizing the SASW technique.

Application of Microtremor Survey Technology in a Coal Mine Goaf

Goafs are one of the main factors that endanger the safety of mining areas, leading to roadway collapse, land subsidence and other problems. Determining how to detect goafs accurately and efficiently is an important issue faced in engineering geophysical exploration research. Microtremor survey technology, used in geophysical exploration, has been developed in recent years. It has the advantages of low cost, flexible construction, low topographic impact and high efficiency. In this paper, microtremor survey technology was applied to the detection of a coal mine goaf. In the known goaf area of the Taiyuan Nanling Coal Mine, a linear observation array was arranged to conduct rolling acquisition, observe the microtremor signal records of the natural field and use the extended spatial autocorrelation method to extract the dispersion curve. Through the inversion of the extracted dispersion curve, the apparent shear wave velocity profile of the underground medium clearly showed the location of the goaf. It was roughly consistent with the goaf data obtained from the mine. The detection results reflect the advantages of micromotion exploration. The goaf of Coal Seam #2 and its affected area appear to be due to an obvious low-speed phenomenon. The boundary of the apparent shear wave velocity profile of micromotion detection was basically consistent with the boundary of the goaf, indicating that micromotion exploration technology has good technical advantages and application prospects in the detection of coal mine goafs. The rolling acquisition method of linear arrays can be used to ensure accurate detection, improve exploration efficiency and save on exploration costs.

Apparent wave velocity inverse analysis method and its application in dynamic pile testing

AbstractThis study proposes a new non‐destructive testing method (NDT), namely the apparent wave velocity of piles (AWVP) inverse analysis method, to solve the detection problem of the gradually varying cross‐sectional defects (cracks or necking) and the material property defects (concrete disintegration or steel corrosion) of piles. The analytical solution of the AWVP has been derived based on the additional mass model. The rationality and accuracy of the theoretical model have been validated through the comparisons with the experiment results. The variation mechanism of the AWVP due to the presence of soil has been clarified. A parametric study is conducted to investigate the major factors to determine the variation tendency of the AWVP. The optimum working conditions and parameter combinations of the AWVP based NDT method have been recommend. The main conclusions can be drawn as: (1) the variation of the AWVP with respect to frequency can be generally divided into a sensitive zone and a stable zone. (2) The AWVP initially decreases rapidly with the frequency within the sensitive zone, however when the frequency decreases to the stable zone, this decline tendency becomes much smaller. (3) Although the AWVP varies extensively within the frequency sensitive zone, this frequency range can substantially relax the requirements for the parametric accuracy of the pile–soil system.

Traveling wave effect of multi-span through concrete-filled steel tubular arch bridge

To explore the different influence of traveling wave effect on the isolated and non-isolated arch bridge, the isolated and non-isolated multi-span arch bridges models are established respectively, three measured seismic waves were selected, and under the eight kinds of apparent wave velocities and multi-point consistent excitations, the structural responses of the two models, including the internal force response, the arch rib velocity, the pier’s internal force response, the bridge deck acceleration, and the shear force and displacement of isolation support of different arch ribs position in two models, were compared and analyzed. The results show that the isolation effect of the isolation structure is obvious; the wave effect of the isolated structure is significant, the two structures show different internal force response curves in different positions of the arch ribs under the influence of different seismic waves; the influence of the apparent wave velocity on the force and shock reduction rate of the arch ribs is complex; the vertical acceleration of the arch ribs and bridge deck of the isolated structure is reduced significantly; and the shear force and displacement of the isolation bearing increase with the increase of the apparent wave velocity. The study shows the traveling wave effect of multi-span through a concrete-filled steel tubular arch bridge with and without isolation, and the results will be used for the seismic design and analysis of structural diseases caused by the wave effect.

Laterally Varying Crustal and Quaternary Fault-Zone Structures in the Seoul Metropolitan Area, South Korea, from a Joint Inversion Analysis Based on Dense Seismic Arrays

ABSTRACTThe crustal structure and Quaternary fault zone in the Seoul metropolitan area are investigated using a joint inversion that combines the receiver functions with surface wave dispersion based on seismic data from densely deployed seismic arrays. The uncertainty in inverted models is examined through a set of synthetic experiments. Synthetic tests and comparisons with other methods verify the observed crustal features. The Moho depths range between 28.7 and 32.8 km with relatively shallow depths in the central Seoul metropolitan area. The VP/VS ratio varies between 1.67 and 1.85. A Conrad discontinuity with a weak velocity increase is present at a depth of ∼20 km. Low-velocity anomalies appear at depths of ∼3 km and 6–11 km along the Chugaryeong fault zone and fault junction area. Apparent shear wave velocity contrasts appear across the fault zone where earthquakes are clustered. A high-velocity block in the western Seoul metropolitan area lies along the fault zone that runs across western Seoul. The shear wave velocity models and seismicity suggest that the fault zone may extend to a depth of 11 km at least.

Pounding performance between a seismic-isolated long-span girder bridge and its approaches

For girder bridges located in regions with a high seismic hazard level, seismic isolation bearings are usually implemented to suppress the seismic inertial force transmitted from the superstructure to substructures, protecting the pier columns and foundations. However, whether potential pounding might happen for these seismic-isolated girder bridges between adjacent units are scarcely considered, especially for those with long spans where the wave passage effect is of great significance. This paper develops a finite element model for a typical hybrid truss-girder bridge, based on which the influence of seismic isolation bearings and wave passage effect on pounding is carefully investigated considering both near-fault and far-field motions. The results show that for this type of long-span girder bridges, pounding may happen even the adjacent bridge units are designed with similar vibration periods, especially when near-fault motions and wave passage effect are considered. Under non-uniform excitations with different apparent wave velocities, the employment of isolation bearings can reduce the relative displacement at expansion joints and thus improve the performance against pounding; however, the occurrence of pounding and damage of expansion joints cannot be fully avoided. Further, motions with lower apparent velocities generally correspond to more devastating responses, which should be particularly considered during engineering practice.

Shaking Table Test Investigation on Seismic Performance of Joint Model of Immersed Tunnel

Critical for the seismic safety of immersed tunnels is the magnitude of deformation and force developing in the segment joints. To investigate the seismic performance of segment joint in immersed tunnel, this paper presents a series of shaking table arrays tests that were performed on a microconcrete tunnel model embedded in the soil. The tests take account of the uniform and wave passage effect in different apparent wave velocity of longitudinal seismic excitation. The result showed that the wave passage effect had a great impact on axial force, bending moment and deformation of joints. The comparison showed that structural response under nonuniform earthquake excitation is larger than that under uniform excitation. The simplified model was established in ABAQUS for numerical analysis. The soil around the tunnel was simplified as spring-damper, the tunnel was simplified as beam element, and the joint was simulated by nonlinear spring. The numerical simulation results were in good agreement with the experimental data. In addition, the model was analyzed by changing input apparent wave velocity, joint stiffness and joint number. The results showed that the deformation of joints was smaller and the deformation of flexible joints was greater under high apparent wave velocity.

Investigation on Seismic Response of Long‐Span Special Steel Truss Cable‐Stayed Bridge

Due to the small self‐weight of the steel truss cable‐stayed bridge with a single tower and a single cable plane, the torsional stiffness and wind stability of the structure are reduced. The arrangement of the deck type makes the mechanical properties of the cable‐stayed bridge more complicated while reducing the cost and increasing the aesthetics. The effects of structural parameter variations and traveling wave effects on the seismic response of this steel truss cable‐stayed bridge with a single tower and a single cable plane were investigated by the nonlinear time‐history analysis method and nonuniform seismic analysis method. The results show that the displacement of the floating system under seismic action is larger than the other three systems, but its internal force is significantly smaller than the other three systems. The spectral characteristics and the duration of ground shaking have a greater influence on the maximum bending moment values corresponding to the height of the cable tower and the maximum axial force values of the main girder bars corresponding to the length of the bridge under earthquake action. The effect of steel truss girder stiffness parameter variations on structural internal forces of bridges and the effect of traveling wave effects on structural displacements of bridges in specific apparent wave velocity intervals do not exist universally.

Building a geothermal formation model using microtremor array measurement

A comprehensive understanding of the internal structure and construction of a geomechanical formation model play an important role in developing and using geothermal resources. Formation models help in identifying the channel and cycling modes of the heat flow. Due to urban sprawl and development, constructing a formation model of geothermal resources based on data from traditional geophysical methods is challenging. The microtremor survey method (MSM) is adopted to obtain critical information in Jimo, which is famous for rare seawater geothermal resources in China. Three microtremor survey lines are deployed to identify subsurface structures up to 2 km into the ground. Dispersion curves of Rayleigh waves with frequencies from 0.4 to 10 Hz are extracted using the spatial autocorrelation method. An empirical equation is adopted to obtain the apparent shear wave (S-wave) velocity of each survey point and to plot the apparent S-wave velocity sections. The obtained sections reveal the development of two interacting faults, which form a channel for the heat-flow cycle. Two conceptual models are established to depict the formation and cycling modes of seawater geothermal resources in Jimo, based on the results and analysis. Our model will help verify the geothermal system and scientifically guide the development of unique geothermal resources. Moreover, the developed model verifies that MSM is effective and dependable for identifying fracture zones and strata.

Study of seismic-responses and semi-active control of Taizhou Bridge under multi-support excitation

PurposeIn order to make sure of the safety of a long-span suspension bridge under earthquake action, this paper aims to study the traveling wave effect of the bridge under multi-support excitation and optimize the semi-active control schemes based on magneto-rheological (MR) dampers considering reference index as well as economical efficiency.Design/methodology/approachThe finite element model of the long-span suspension bridge is established in MATLAB and ANSYS software, which includes different input currents and semi-active control conditions. Six apparent wave velocities are used to conduct non-linear time history analysis in order to consider the seismic response influence in primary members under traveling wave effect. The parameters α and β, which are key parameters of classical linear optimal control algorithm, are optimized and analyzed taking into account five different combinations to obtain the optimal control scheme.FindingsWhen the apparent wave velocity is relatively small, the influence on the structural response is oscillatory. Along with the increase of the apparent wave velocity, the structural response is gradually approaching the response under uniform excitation. Semi-active control strategy based on MR dampers not only restrains the top displacement of main towers and relative displacement between towers and girders, but also affects the control effect of internal forces. For classical linear optimal control algorithm, the values of two parameters (α and β) are 100 and 8 × 10–6 considering the optimal control effect and economical efficiency.Originality/valueThe emphasis of this study is the traveling wave effect of the triple-tower suspension bridge under multi-support excitation. Meanwhile, the optimized parameters of semi-active control schemes using MR dampers have been obtained, providing relevant references in improving the seismic performance of three-tower suspension bridge.

Seismic Response and Vibration Reduction Analysis of Suspended Structure under Wave Passage Excitation

In order to study the influence of traveling wave effect on the seismic response and damping effect of suspended structure, a series of shaking table tests of the 1 : 20 suspended structure have been carried out to compare and analyze the dynamic responses of suspended structures under two points and a consistent input. The vibration damping effect and vibration reduction law of suspended structure are discussed at different apparent wave velocity and in the different connection. The research shows that the damping suspended structure has a good damping effect, and the amplitude reduction of the top displacement peak response is up to 15%, which corresponds to smaller apparent velocities. Moreover, the upper bound of the maximum acceleration response at the structures’ top under nonuniform input motions equals that of the uniform motion. However, there is a hysteresis in the acceleration response under wave travelling excitations, and the smaller the apparent wave velocity, the more obvious the hysteresis.

Influence of soil mass on the vertical dynamic characteristics of pipe piles

A new soil-pile interaction model, called ‘additional soil model’, is developed to investigate the influence of soil mass on the dynamic characteristics of pipe piles. The soil is divided into an inner region in the immediate vicinity of the pile and an outer region at greater distance. The soil in the inner region is assumed to be an additional mass to participate with the vibration of the pile. The viscous damping at the soil-pile interface and the radiation damping in the soil are calculated separately. An analytical solution for the vertical dynamic characteristics of pipe piles is derived based on the developed soil model. The rationality and accuracy of the additional soil model have been validated through comparison of theoretical predictions and measured driving stress waves of two full-scale steel pipe piles. The approach makes a useful improvement of soil-pile interaction modeling for dynamic applications. The main conclusions of this study include: 1) both the soil plug and the pile surrounding soil can lead to a decrease of apparent wave velocity of pipe piles; 2) the pile impedance will be underestimated if calculated without the consideration of soil mass; 3) the resonant amplitude and natural frequency of piles will be significantly overestimated if the soil mass is ignored.

Seismic Analysis of a Curved Bridge Considering Soil-Structure Interactions Based on a Separated Foundation Model

A separated foundation model was proposed in order to reduce the calculation scale of the numerical model for analyzing soil-bridge structure dynamics. The essence of the wave input analysis model considering soil-structure interaction was analyzed. Based on the large mass method, a one-dimensional time-domain algorithm of the free field was derived. This algorithm could simulate the specified ground motion input well. The displacement expansion solution of the free wave field was solved based on the propagation law of waves in a medium. By separating the soil foundations around the pile foundations of the bridge, the ground motion was transformed into an equivalent load applied on an artificial boundary. The separated foundation model could consider the incoherence effect and soil-structure interaction simultaneously; the number of model elements were reduced, and the computational efficiency was improved. In order to investigate the seismic response of a curved bridge considering soil-structure interaction under spatially varied earthquakes, a curved bridge with small radius was adopted in practical engineering. Spatially correlated multi-point ground motion time histories were generated, and the nonuniform ground motion field was simulated based on the wave input method on an artificial viscoelastic boundary. The effects of different apparent wave velocities, coherence values, and site conditions on the seismic response of the bridge were analyzed. The results showed that the spatial variation of seismic ground motion had a considerable effect on the bending moment and the torsion of the girder. The site effect had great influence on the bending moment of the pier bottom. When considering soil-structure interaction, the spatial variation of ground motion should be fully considered to avoid underestimating the structural response.

Influence of Random Multi-Point Seismic Excitations on the Safety Performance of a Train Running on a Long-Span Bridge

The increasing use of bridges in high-speed railway (HSR) lines raises the possibility of train derailment on bridges under seismic excitations. In this paper, the influence of random multi-point earthquakes on the safe running of a train on a long-span bridge is studied in terms of the dynamic reliability, considering spatial seismic effects, and randomness of ground motions and train locations. The equations of motion for the train and the track/bridge as time-invariant subsystems under earthquakes are established, separately. The two subsystems are connected via the wheel–rail interface, for which a nonlinear contact model and detachment are considered. The time-history samples of nonstationary multi-point random earthquakes considering wave passage effects and incoherence effects are generated by the autoregressive moving average (ARMA) model. The ground motions are imposed on the bridge support points in terms of displacement and velocity. The train location at the time of earthquake is considered a uniformly distributed random variable. The running safety reliability of a train moving on a long-span bridge under earthquakes is determined by combining subset simulation (SS) with a prediction-based iterative solution method. Under different seismic components, train speeds, apparent seismic wave velocities and seismic intensities, the most unfavorable train location intervals are determined, which provides a reference for the safety performance assessment of trains traveling on bridges under earthquakes. Numerical results show that the influence of the lateral seismic component on the wheel derailment coefficient (WDC) is greater than the vertical seismic component, and the earthquake that occurs before the train’s arrival at 70% length of the bridge will significantly reduce its running safety.