Horizontal Excitation Research Articles

NONLINEAR FIELD RESPONSE OF PILES UNDER HORIZONTAL HARMONIC EXCITATIONS

Harmonic excitation of pile foundation is a typical dynamic phenomenon occur during machine vibration. In many cases, it has been found that the horizontal excitations are the critical ones which governs the design for machine-based pile foundations. In order to comprehend the horizontal frequency-amplitude responses of pile foundation under various harmonic excitation forces, forced vibration tests on several sets of small-scale single and pile groups have been carried out. According to the test results, there are two resonant peaks within the operational frequency range shown by the dynamic frequency-amplitude responses, where the first peak is found to be more dominant as compared to the second peak. Additionally, as resonant frequencies are lowered and resonant amplitudes grow for an increase in excitation forces, the response curves demonstrate the nonlinear patterns of pile groups. The basic dynamic characteristics of all soil-pile setups are found to be similar to each other. As the group's pile count increases, the resonant frequency values increase while the resonant amplitude values decrease, increasing the dynamic stiffness of the soil-pile systems. The resonant amplitude for the first peak of 2х2, 2х3, and 3х3-pile groups are found to be approximately 13%, 36% and 79% lower, respectively, as contrasted with the single pile under the same dynamic load.

The interfacial swirling motion of two-layer liquids in a tank under orbital excitations

The swirling motion of the interface of two-layer liquids in a tank with a square base is investigated experimentally in this study. The tank was fixed on a platform and the horizontal circular-orbital excitations were applied. Both resonant and off-resonant responses were studied and the profiles of the interface between two liquids along two adjacent vertical walls of the tank were recorded and processed by using the image processing method. When the frequency of the excitation was equal to the lowest natural frequency of the interface between two liquids, the resonant response of the interface was triggered as swirling waves with strong nonlinearity. Instead of being a circular shape, the parametric curve was more of a triangular shape. The modal analysis revealed that this is caused by the additional contributions from the nonlinear modes of sloshing waves with multiple times of the natural frequency. For off-resonant cases, the nonlinear contribution is weaker and the parametric curve is, thus, close to a circular shape.

Effects of biaxial loading path on seismic behavior of T-shaped RC walls

T-shaped RC walls are commonly used to resist horizontal seismic excitations in multiple directions. The differences in the intensity and spectrum characteristics of two translational components of ground motion cause the wall to experience complex biaxial displacement trajectories, resulting in a different seismic response compared to the behavior derived from conventional uniaxial cyclic tests. Thus, to investigate the seismic behavior and coupling mechanism of T-shaped walls subjected to different magnitudes of loading in two orthogonal directions, this study examined five identical T-shaped walls under uniaxial loading along two principal axes and biaxial loading with three represented loading paths. The test results showed that the mechanical behavior in one direction was significantly affected by the loading in the orthogonal direction owing to internal force redistribution and additional stress caused by the biaxial coupling bending, resulting in vertical variation and intersection of their hysteretic curves. Further, the biaxial loading primarily aggravated the cracking and damage of the flange, which accelerated the performance degradation in the flange direction and reduced the effective flange width involved in resisting web direction load. In addition, the biaxial coupling effect was found to be proportional to the area enclosed by the loading path. Thus, the most severe damage and performance degradation were observed under a square path, followed by an eight-shaped path, with the cruciform path exhibiting the least effect. Consequently, based on the results, recommendations for the design of T-shaped walls subjected to biaxial earthquake actions were proposed.

Free-field soil response to bidirectional horizontal non-uniform ground motion from multi-point shaking table tests

This paper assesses the dynamic nonlinear free-field soil response under bidirectional horizontal non-uniform excitation through a series of multi-point 1 g shaking table tests. The soil bed was enclosed in an innovative soil box supported by an array of three shake tables and was subjected to uniform and non-uniform ground motions in one and two horizontal directions. The soil response to bidirectional and unidirectional excitations, both uniform and non-uniform, were compared and discussed in terms of acceleration time history and Fourier spectra, acceleration amplification factor and soil settlement, as well as shear stress-shear strain behavior. The comparison of soil responses to bidirectional and unidirectional shakings demonstrated that the trends of peak acceleration amplification factors are similar along soil depth, but the response difference between bidirectional and unidirectional shaking is increasingly higher along the soil profile towards the ground surface, especially for uniform excitation. The variations of soil settlement, shear stress-strain and damping curves under bidirectional excitation were similar to that under longitudinal excitation; however, they were much different than those observed under transverse excitation. The soil damping in Y-direction under non-uniform excitation was larger than that under uniform excitation. These different site response characteristics under bidirectional non-uniform excitation should be properly considered in the seismic design of extended infrastructure.

Response of rigid circular plate in a functionally graded transversely isotropic, half-space under horizontal steady-state excitation

In this paper, a theoretical formulation for lateral translation and impedance function of a rigid foundation in a transversely isotropic functionally graded (TIFG) half-space is presented. Accordingly, the medium is considered linear elastic and the elastic constants of materials vary exponentially along the depth for considering the effect of elastic non-homogeneities. The relax treatment of mixed boundary value problem is formulated by integrating the transformed kernels of Green’s functions over the foundation area with unknown surface traction function, which can be reduced into a Fredholm integral equation. The results represent the impact and treatment of frequency and material anisotropy through the medium. In addition, static and dynamic distribution of tangential shear stress over the foundation for surface and embedded disk extracted and the cutoff frequency is found out explicitly. The proposed solution gives exact concurrence with accessible literature.

Investigation on Rupture Initiation and Propagation of Traffic Tunnel under Seismic Excitation Based on Acoustic Emission Technology.

Traffic tunnels are important engineering structures in transportation, and their stability is critical to traffic safety. In particular, when these tunnels are in an earthquake-prone area, the rupture process under seismic excitation needs to be studied in depth for safer tunnel design. In this paper, based on a construction project on the Nairobi-Malaba railway in East Africa, a laboratory shaking table test with 24 working cases of seismic excitation on a mountain tunnel is designed, and the acoustic emission (AE) technique is employed to investigate the tunnel rupture process. The results show that the high frequency components between 20 and 30 kHz of AE signals are the tunnel rupturing signals under the seismic excitation under such conditions. The tunnel vault and the arch foot are prone to rupture during the seismic excitation, and the initial rupture in the arch foot and vault of the tunnel occur under the horizontal and vertical Kobe wave seismic excitation, respectively, with a maximum acceleration of 0.4 g. After the rupture initiation, the tunnel arch foot continues to rupture in the subsequent working cases regardless of whether the excitation direction is horizontal or vertical, while the tunnel vault does not rupture continuously with the implementation of the subsequent excitations. Moreover, the Kobe seismic wave has a higher degree of damage potential to underground structures than the El seismic wave.

Shaking table test and numerical simulation of shallow foundation structures in seasonal frozen soil regions

Frozen soil layers can induce the formation of double or multilayers on the ground. Such formations change the dynamic characteristics and dominant period of soil, directly influencing the stability and safety of upper buildings. In this paper, the frozen soil-structure interaction system in cold region was taken as the research object. The frozen soil model was made artificially, and two kinds of structure models with different natural frequencies were designed. Through a series of shaking table tests and finite element numerical analysis, the influence of structural natural frequency, soil frozen depth and seismic load on the seismic response of structure were studied. The test results indicate that ground freezing could weaken the soil–structure interaction effect and the nonlinear development of the soil structure. The effect of peak acceleration amplification of the ground is significant close to the surface. The acceleration amplification coefficient of the ground surface decreased as the freezing depth increased, which indicates that the freezing of the ground suppressed the acceleration response of the ground surface. If the peak frequency of a seismic wave is close to the natural frequency of the structure, it can significantly amplify the seismic response of the structure. Owing to the interaction between the soil and the structure, the degree of liquefaction of the ground varied with different structures. Subsequently, a nonlinear finite element program was used to establish the dynamic calculation model of frozen soil-structure interaction system considering the gradient change of stiffness of frozen soil and the dependence of frozen soil on temperature. On the basis of validating the numerical model rationality, the seismic response of frozen soil-structure interaction system under horizontal seismic excitation was further explored. The analysis results show that the seismic response of the structure increased significantly with amplitude excitation; the greater was the freezing depth, the greater was the seismic response of the structure, The influence of the frozen ground on the seismic response of structures in the design of structures in frozen soil areas should be considered. In summary, the dominant period of the ground, the dynamic characteristics of a structure, the type of seismic wave, and the degree of liquefaction of the ground have a significant influence on the seismic response of the structure. Therefore, considerable attention should be paid to the influence of frozen soil on the dynamic characteristics of a structure in frozen soil regions.

A Multi-Modal Energy Harvesting Device for Multi-Directional and Low-Frequency Wave Energy

As a kind of sustainable energy source, ocean wave energy has always attracted attention. Because of the characteristics of multi-directions and low-bandwidth, the harvesting of wave energy has always been difficult. To harvest broadband wave energy in multiple directions and improve power generation efficiency, we present a multi-modal energy harvesting device by using a cross beam structure. In this device, the efficient harvesting of multi-frequency vibration energy in multiple directions has been achieved by using the multi-modal characteristics of the structure and the high power generation efficiency of the electric device based on liquid metal. Contrast experiments in multiple directions show that the device not only has the capability of multi-directional energy harvesting but also can work in the sweep range of the full band. Under horizontal excitation, compared with traditional single cantilever structure, the peak power density increased to 2227% and the working frequency band increased to 6.25 times. The experimental results show that the device significantly improves the efficiency of low-bandwidth multi-directional energy harvesting, providing a new method for vibration energy harvesting.

Comparative Assessment of Dynamic Behavior of Vertical and Batter Pile Groups under Horizontal Excitation

Comparative Assessment of Dynamic Behavior of Vertical and Batter Pile Groups under Horizontal Excitation

Research on wave suppression mechanism of porous baffle based on vortex dynamics

The exciting force generated by the violent sloshing of the fluid in the tank will damage the tank structure and even affect the attitude stability of a vehicle. Hence, this study constructs a numerical model and horizontal excitation experimental platform. Taking the porous baffle in the tank of a specific type of tanker as an example, the current study analyses the impact pressure response law at different frequencies. On the basis of vortex dynamics, this work explores the evolution mechanism of the vortex flow structure under different excitation amplitudes and frequencies. The exploration reveals the law of dimensionless vortex intensity and energy change. In addition, the relationship between vortex intensity and wall pressure is determined. Studies have shown that the non-linear violent sloshing causes the resonance frequency of the tank to deviate from the natural frequency. When the excitation amplitude intensifies, the range and strength of the vortex structure and the distance from the vortex core to the baffle increase. In such circumstance, the accumulation and dissipation of energy are more evident. In addition, when the excitation frequency is closer to the resonance frequency, the distance between the vortex core and the baffle widens. Moreover, the energy intensity around the baffle increases as the frequency increases. The time history of wall pressure P and vortex intensity Q shows a certain regularity, and the ratio of Pmax to Qmax decreases with the increase of the excitation frequency.

Reduced and test-data correlated FE-models of a large timber truss with dowel-type connections aimed for dynamic analyses at serviceability level

The rise of wood buildings in the skylines of cities forces structural dynamic and timber experts to team up to solve one of the new civil-engineering challenges, namely comfort at the higher levels, in light weight buildings, with respect to wind-induced vibrations. Large laminated timber structures with mechanical joints are exposed to turbulent horizontal excitation with most of the wind energy blowing around the lowest resonance frequencies of 50 to 150 m tall buildings. Good knowledge of the spatial distribution of mass, stiffness and damping is needed to predict and mitigate the sway in lighter, flexible buildings. This paper presents vibration tests and reductions of a detailed FE-model of a truss with dowel-type connections leading to models that will be useful for structural engineers. The models also enable further investigations about the parameters of the slotted-in steel plates and dowels connections governing the dynamical response of timber trusses.

Seismic Performance Assessment of a Single-Layer Spherical Lattice Shell Structure with Multifunctional Friction Pendulum Bearings

The dynamic responses of spatial lattice shell structures with friction pendulum bearings (FPBs) under multidimensional seismic excitations are complex. In addition, FPBs may experience uplift and separation of the bearing components owing to excessive displacements. In this study, a novel multifunctional FPB (MFPB) with a multi-defense system was developed, and its effectiveness in reducing and controlling the seismic responses of spherical lattice shell structures was evaluated. The proposed MFPB comprises an FPB, superelastic shape-memory alloy cables, and sleeve restrainers. A mechanical model of the MFPB was established, and its isolation and control behaviors were investigated through numerical simulations. Furthermore, the main characteristics and advantages of the isolation system were analyzed. Subsequently, the MFPB system was applied to a single-layer spherical lattice shell structure with surrounding columns. A computational model of the controlled structure was developed using the OpenSees software. Finally, nonlinear time history analyzes were conducted to analyze the seismic performance of controlled and uncontrolled lattice shells. The results demonstrate that the MFPB isolation system can effectively control the structural responses of isolated spatial lattice shell structures under horizontal and vertical seismic excitations and improve their seismic resilience.

Surface wave of the r-3/2 order terms due to the point-like horizontal excitation source (L).

In this letter, the higher order terms of the time-harmonic solution due to a point-like horizontal excitation source are investigated. It is found that the surface wave of the r-3/2 order terms polarize in both radial and azimuthal directions, complementing the findings in the literature. At the ground surface, the polarization distribution of this wave demonstrates similar behavior as the magnetic induction lines of a magnetic dipole. It suggests that this wave can be observed near the horizontal excitation source, and thereby its effects need to be taken into account for the application of shallow buried object detection.

Progressive Collapse of the Base-Isolated Frame Structures Supported by Stepped Foundation in Mountainous City

Base-isolated frames supported by stepped foundation in mountainous areas possess their own particularity, so its progressive collapse dynamic response performance and dynamic effect propagation path are very different from those of ordinary flat ground-isolated structural systems. In order to study the progressive collapse performance of the base-isolated frames supported by stepped foundation in mountainous areas under two-directional coupled dynamic excitation, a four-span three-story plane frame demolition column test was simulated to verify the reliability of the computing platform. The common ground motion and three types of long-period ground motions were selected, and the two-dimensional dynamic coupling of the ordinary flat ground isolation structure and the base-isolated frames supported by stepped foundation in mountainous areas was obtained based on the demolition method. First, the seismic isolation structure was subjected to the collapse dynamic response under the vertical unbalanced load, then the collapse dynamic response under the vertical unbalanced load and the horizontal seismic coupling excitation was made; the two were compared and analyzed. It can be used as a reference for the design of progressive collapse of the frame structure of the base-isolated frames supported by stepped foundation in mountainous areas.

Effective excitation conditions for the intense motion of the ginkgo seed-stem system during mechanical vibration harvesting

The degree of the motion response of the ginkgo seed-stem system during mechanical vibration harvesting determines the separation force obtained, thereby affecting harvesting efficiency. A response surface stochastic finite element analysis was carried out to obtain the modal shapes and the ranges of natural frequencies of the seed-stem systems. It was initially based on model updating and considering the inherent variability of the ginkgo seed-stem system, Then, the dynamic response of the ginkgo seed-stem system under different vibration parameters was investigated. The results showed that at low frequencies, only the first-order natural frequency could lead to resonance and cause the seed-stem system to swing violently. Under horizontal excitation, the main resonance of the system was at an excitation frequency close to the natural frequency. Under vertical excitation, the system had a sub-harmonic resonance at an excitation frequency close to twice the natural frequency. When the excitation acceleration was much larger than the gravitational acceleration, due to the increase in the excitation frequency, the seed-stem bounced violently, significantly increasing the inertial forces on the ginkgo seeds. Thus, vertical excitation is more likely to cause the intense motion response of the seed-stem system than horizontal excitation. This study not only proposes a correction method for the finite element model of the seed-stem system but it also clarifies the effective excitation conditions for the ginkgo seed-stem system. It provides theoretical guidance for the efficient mechanical vibration harvesting of ginkgo seeds.

Slosh Suppression Blocks - A concept for mitigating fluid motions in floating closed containment fish pen in high energy environments

This paper introduces a new concept to mitigate sloshing, tailored to floating closed containment aquaculture systems, termed slosh suppression blocks. Slosh suppression blocks reduce the natural sloshing periods by contracting the free surface area without any significant reduction of the operational volume in the tank, and can be retrofitted to existing systems. Sloshing energy is dissipated on top of the suppression blocks, out of the fluid domain to reduce the spread of turbulent eddies in the main body of the fluid in the tank. The fundamental idea and working principles of the slosh suppression blocks are illustrated with an experimental setup representing a 2D section of a closed-containment system under forced horizontal excitations. Maximum sloshing amplitudes are compared for 25 different test configurations for a range of excitation frequencies, using cubic slosh suppression blocks of different sizes. For selected configurations, the fluid's internal state is assessed by visualising the fluid motions both experimentally and by numerical simulations. The results show that slosh suppression blocks reduce the total sloshing volume and turbulent motions within the main body of the tank, providing a much calmer, controlled low energy living environment which is regarded essential for fish wellbeing. It is found that positioning the top of the slosh suppression blocks flush with the still water level mitigates and damps internal sloshing most effectively. To quantify and characterise the degree of slosh suppression, three slosh indices are introduced; the slosh reduction index, the slosh overtopping index and the slosh free surface index. The novel concept of slosh suppression blocks provides an important step forward in the design of floating closed containment aquaculture tanks that minimises sloshing and internal fluid motions in energetic environments.

Seismic fragility analysis of RC bridges in high seismic regions under horizontal and simultaneous horizontal and vertical excitations

Although Himalayan highways generally pass close to the active seismic faults, near field excitation effects are not considered in designing bridges. The effect of strong vertical shaking is known to cause several anomalous and remarkable consequences in bridges. To this end, we present a comparative seismic vulnerability analysis of representative RC bridge from Nepal Himalaya, considering horizontal only and simultaneous horizontal and vertical excitations. Fragility functions for longitudinal and transverse directions considering horizontal only and simultaneous horizontal and vertical excitations are constructed for representative RC bridge from Asian Highway-2 (AH-2) in Nepal that generally follows the same alignment as that of one of the very active seismic faults in the region. Analytical fragility functions are also compared with the existing empirical fragility functions. The results depict that the horizontal response parameters, such as displacement and displacement ductility, are not considerably affected by the vertical ground motion, but the effect is found to be increasingly significant for higher damage states. The sum of observations highlights that the transverse direction of the bridge will be more prominent while considering vertical excitation effects together with the horizontal excitation rather than the horizontal excitation only.

Dynamic Behavior of Single Steel-Driven Vertical and Batter Piles under Horizontal Excitations: Field Model Testing

AbstractThis paper presents the field test results for 3.3-m-long driven steel pipe single vertical piles, P1 (β=0°), and single batter piles, P2 (β=10°) and P3 (β=20°), for four different eccentri...

Numerical simulation of a heterogeneous liquid in a 2-D rectangular container

We study the sloshing of a heterogeneous liquid in a 2-D rectangular container in the presence of a horizontal dynamic excitation. The governing equations of the motion of the liquid are presented in the case of a liquid of low heterogeneity. The study of these equations shows the existence of a small zone of instability. A two-dimensional numerical method is carried out to simulate the mathematical model. The effect of liquid heterogeneity on the changing aspects of the free surface was studied. As shown in the results, the free surface profiles increase by increasing the heterogeneity parameter. The analysis of sloshing in the stable zone shows significant effects on the evolution of the free surface of the liquid, these effects depend on the coefficient of heterogeneity and frequency of excitation.

Nonlinear 3D finite element analysis of suction caisson-tower-soil system subjected to horizontal earthquake excitation

Suction caissons are widely used as a promising alternative foundation for offshore structures under variable offshore environments. In this paper, a three-dimensional finite-element model which takes into account soil inelasticity was used to study the seismic behavior of a suction caisson in clay taken from an offshore wind farm in the Hebei province of China. The results showed that the maximum acceleration of the soil increases with increasing depth. There is a distinct amplification for relatively low frequencies when seismic waves propagate to the mudline. It was assumed that the rotation center of suction caisson under seismic conditions was virtually existent. The rotation center was obtained by the proposed formula to explore the movement positions of the suction caisson. The time of the peak of lateral displacement of the lid did not synchronize with that of the changing direction of displacement of the rotation center, indicating that the displacement of the lid can not be a comprehensive assessment of the position state of suction caissons. The point mass at the top of the wind turbine tower was responsible for the increase of vibration amplitude of displacement. • In this paper, a three-dimensional finite-element model taking account of soil inelasticity was carried out to study the seismic behavior of suction caisson in soft clay. A 3D equivalent viscous-spring artificial boundary element is applied to eliminate the unnatural reflection wave generated on the boundary. • It is assumed that the rotation center of suction caisson under seismic conditions is virtually existent and it was obtained by the proposed formula to explore the movement positions of the suction caisson. • The time histories of lateral and vertical displacement were extracted for studying seismic behavior of the wind turbine tower under seismic loading.