Displacement Time Histories Research Articles

Research on a multi-point impact test of single-layer spherical reticulated shell

In order to study the impact resistance performance of reticulated shell under multi-point impact load, the multi-point elastic-plastic impact and destructive impact test of Kiewitt 6 (K6) single-layer spherical reticulated shell are carried out. The dynamic strain of key members, the dynamic displacement and the dynamic acceleration of key nodes are recorded during the multi-point impact process. The equivalent size numerical model of the test model is established by using the ANSYS/LS-DYNA simulation program, and the test conditions are simulated and analyzed. The peak strain and strain time history curves of the key members, the peak displacement and displacement time history curves of the key nodes, and the peak acceleration and acceleration time history curves of the key nodes in the test and simulation conditions are compared and analyzed. The energy transformation and energy transmission in the entire process of impact are studied at the same time. Results indicated a good agreement between the simulation results and the experimental results, and the dynamic response of the structure has the same trend. The number and position of impact points have a greater influence on the dynamic response of the impact area, while the influence on the non-impact area is smaller. With the increase of the number of impact points, the strain and displacement of the impact area of the reticulated shell gradually increase, but the acceleration change has no clear trend. At the same time, the experiment verified three deformation modes of the reticulated shell structure under the multi-point impact load.

Nonlinear Dynamic Analysis of a Masonry Arch Bridge Accounting for Damage Evolution

This study investigates the nonlinear dynamic response of the masonry bridge ‘Ponte delle Torri’ in Spoleto, aiming at assessing the seismic performance of the structure and evaluating the occurring damaging mechanisms. A 3D Finite Element (FE) macromechanical procedure implemented in the FE program FEAP is adopted to model the bridge. To reproduce the typical nonlinear microcracking process evolving in masonry material when subjected to external loads, an isotropic damage model is used. This is based on a scalar damage variable introduced in the stress-strain constitutive law and equally degrading all the components of the elastic constitutive operator. A nonlocal integral definition of the damage associated variable, that is the equivalent strain measure governing its evolution, is adopted to overcome the mesh dependency problems of the FE solution typically occurring in the presence of strain softening behavior. Based on the results of a recent study by some of the authors, a single equivalent pier is analyzed, whose geometry and boundary conditions are selected so that its response can provide useful information on the out-of-plane dynamic behavior of the overall bridge. To perform the seismic assessment, a set of recorded accelerograms is properly selected to simulate the seismic history of the Spoleto site. The nonlinear dynamic response of the structure is evaluated and monitored in terms of top displacement time history, evolution of the global damage index, and distribution of the damage variable. First, a set of analyses is performed by imposing the selected ground motions one by one on the initial undamaged configuration for the structure with the aim of emphasizing the damaging effects on its dynamic response. Then, the accelerograms are arranged in sequence to reproduce the seismic history of the site and analyze the influence of accumulated damage on the dynamic amplification of the response. A critical comparison of the bridge response to the sequence of accelerograms and the single records is made, and the interaction between the damaged structure dynamic response and the signal characteristic is highlighted, as well.

Measurement of loads acting on a hydropower unit during stationary and transient operations

Load measurements on hydropower turbines and generators are usually expensive and troublesome due to the need of sensor installation on the rotating parts. Often a longer stop of the unit is required when the sensors are to be installed or removed from the turbine in the waterway, which results in production disturbances.This article presents a methodology and results for estimation of loads acting on turbines and generators during operation without installation of sensors on the turbine or in the generator. The measured quantities are shaft displacement in the bearings and the bending moment on the shaft between the turbine guide bearing and the lower generator guide bearing.To enable calculation of the loads acting on the turbine and the generator, it is necessary to have as many measurement points as the number of unknown loads acting on the shaft train. By using the displacement in the three radial bearings, six of the eight unknown forces can be determined. The remaining two unknown loads can be calculated by measuring the bending moment in the shaft connecting the generator to the turbine. In journal bearings the stiffness is a non-linear function of eccentricity, which means that the obtained stiffness function must be integrated to achieve the force as a function of the shaft eccentricity.By applying the measured time history of displacement and moment on the numerical model, the time history for the unknown loads can be solved. The time history of the external loads is then converted into load spectra.

Behavior of steel-concrete-steel sandwich beams with blot connectors under off-center impact load

The structural performance of the Steel-Concrete-Steel (SCS) sandwich beams with bolt connectors subjected to off-center impact load was investigated in this paper. Nine SCS sandwich beams were tested under impact loading by employing a drop hammer impact test system. The influences of concrete core depth, steel plate thickenss, top to bottom steel plate thickness ratio, and impact location on the responses of SCS sandwich beams were experimentally studied. The experiments revealed three failure modes of the SCS sandwich beams, i.e., flexure, flexure-shear, and fracture of the bottom steel plate. The separation of steel plates was prevented, and the structural integrity of the SCS sandwich beams was maintained despite the impact near the support, owing to the presence of bolt connectors. An analytical model was developed to predict the displacement–time histories of the SCS sandwich beams subjected to impact load. The predicted displacement responses showed a good match with the experimental results. Moreover, the force–displacement curves obtained from the analytical model and impact tests were compared, and good agreement between them could be observed.

A rate-independent internal friction to describe the hysteretic behavior of pantographic structures under cyclic loads

The cyclic behavior and dissipative properties of pantographic fabrics are analytically and experimentally investigated in this paper. To these purposes, a two-dimensional second-gradient continuum model, already developed in the literature, is taken as representative of the elastic behavior of the homogenized system. However, to take into account the dissipative phenomena, the model is enriched with a suitable Dahl-like phenomenological dissipation law, accounting for friction effects at the interconnecting pivots, that it is shown to be effective in describing the macroscopic hysteretic behavior under cyclic and quasi-static imposed displacement time-histories. Numerical results are compared against experimental ones, thus showing the effectiveness of the continuum model in reproducing the real behavior of pantographic fabrics.

Towards the vulnerability assessment of low-code RC frame buildings at precarious slopes subjected to rainfall induced landslide hazard

The presence of capillary pressure in unsaturated soils may, under certain circumstances, significantly affect the stability of a slope and the safety of a structure standing in the vicinity of its crest. More specifically, the change in saturation induced by a rainfall event may directly influence the value of apparent soil cohesion potentially resulting in excessive permanent slope displacements and subsequent structural damages for the structure standing on it. Under these considerations, this study aims at the vulnerability assessment of typical reinforced concrete (RC) buildings on precarious slopes subjected to rainfall induced landslide hazard. The proposed methodology is based on a two-step numerical analysis procedure. First, the permanent absolute and differential ground displacement time histories at the assumed building location within the landslide zone are evaluated performing coupled mechanical-flow analysis of a generic initially unsaturated natural slope configuration considering the employment of different rainfall events of increased intensity at the surface of the slope model. Different soil properties of the surface layer are considered to represent silty and soft clayey soil material. Appropriate analytical relationships between the rainfall intensity and its duration and the computed permanent displacements are extracted. Then, a series of nonlinear static time history analyses are performed for reference low and mid-rise, low-code RC bare and infilled frame buildings located next to the slope's crest to assess their fragility. The buildings’ response assessed using appropriate engineering demand parameters (EDPs) to account for both the global flexural and local shear demand are statistically correlated with predefined damage limit states to construct the fragility functions. To facilitate the applicability of the proposed methodology within a quantitative landslide risk assessment framework, vulnerability curves are finally developed to quantify the expected direct losses in normalised cost terms.

Full-Scale Experimental Study of a Reinforced Concrete Bridge Pier under Truck Collision

Recently, vehicle collisions with bridge piers have occurred frequently, which have attracted the attention of researchers. However, as one of the most important research methods, vehicle collision tests for reinforced concrete (RC) bridge piers are limited. The impact responses and damage characteristics of full-scale RC piers have seldom been studied in actual truck collision tests, which hinders further development in this research area. Therefore, a medium-duty truck will be employed to carry out a full-scale crash test on an RC single-column pier 1 m in diameter in this study. The crashing truck had a mass of 7.76 t and a collision speed of 81 km/h. The damage and failure level of the vehicle and RC pier will be examined, and some critical data will be obtained from the test, such as the displacement, velocity, and acceleration time histories of the truck, and the deflection, acceleration, and reinforcement strain time histories of the RC pier. In addition, based on the accelerations of the cargo box and engine, the vehicular impact forces will be derived approximately and discussed. This will provide valuable experimental data for the study of vehicular collisions with RC bridge piers, especially in the calibration and validation of the finite element (FE) models for truck collisions and the impact resistance evaluation of RC bridge piers.

Experimental Investigation of Vortex-Induced Vibrations on Yawed and Inclined Flexible Cylinders

Abstract An experimental setup was built to investigate the vortex-induced vibration (VIV) phenomenon on yawed and inclined flexible cylinders, in which five yaw angles θ = 0 deg, 10 deg, 20 deg, 30 deg, and 45 deg and five azimuth angles β = 0 deg, 45 deg, 90 deg, 135 deg, and 180 deg were combined. The experiments were carried out in a towing tank facility at Reynolds numbers from 1800 to 18,000, comprising vibrations up to the eighth natural mode. Time histories of displacements were recorded using a submerged optical system that tracks 17 reflective targets. A modal decomposition scheme based on Galerkin’s method was applied, aiming multimodal behavior investigations. Such an approach allowed the analysis of the modal amplitude throughout time, revealing interesting results for such a class of VIV tests. The flexible cylinder total response is generally a combination of two or more modes. Only for azimuths 0 deg, 90 deg, and 180 deg, a unimodal response was observed for the two first lock-in regimes. The frequency response showed that when the response was multimodal, non-dominant modes can follow the vibration frequency of the dominant one. Assuming a priori the independence principle (IP) valid to define the reduced velocities (Vr), it was observed that the resonance region was restricted to 3 ≤ Vr ≤ ~8 for the tested cases, indicating that the IP can be at least partially applied for flexible structures. As the literature scarcely explores the simultaneous yawed and inclined configurations, the present work may contribute to further code validation and improvements regarding the design of slender offshore structures.

Dynamic Response Characteristics of Aeolian Sediment along Sichuan-Tibet Railway

In order to reduce the damage of liquefaction of aeolian sand along the Sichuan-Tibet railway, the dynamic response characteristics of saturated aeolian sand in the study area were discussed by using shaking table test. The results show that the macroscopic characteristics of saturated aeolian sand in the study area are subsidence, water flow and fracture. The displacement time history shows that the surface displacement increases with increasing the input ground motion acceleration. When the acceleration is small (0.1g), the vibration in the soil layer has an obvious tendency to enlarge continuously from bottom to top. With the increase of the acceleration (0.2g), the amplification trend basically disappeared. When the acceleration increases to 0.3g, the ground motion increases first and then decreases.

Experimental study of the motion characteristics of a bubble near a flexible structure with an attached air bubble

The strong coupling effect between an air bubble attached to a flexible plate and a nearby spark-generated bubble is investigated experimentally while also considering the structural deformation of the plate. Numerous interesting and complex bubble behaviors that depend strongly on the attached air bubble and deformation effect of the structure are observed with a high-speed camera. A multitude of bubble jet behaviors are observed during the two bubble pulsations, such as bubble splitting, jets away from the plate, jets toward the plate, upward and downward movement after splitting, and splitting after the coalescence of the two bubbles. Moreover, the attached air bubble can appear hemispherical, mushroom-shaped, or cup-shaped. To gain a better understanding of the interaction between the bubble and the flexible boundary, the displacement time history curves at the center point of the flexible structure and at the top and bottom vertices of the bubble are obtained. The motion response mode of the flexible plate under bubble loading is explored to summarize the deformation pattern of the flexible structure and the motion characteristics of the bubble. An air bubble attached to the flexible structure can change the motion characteristics of the spark-generated bubble to some extent.

Experimental Study on Seismic Response of Buried Oil and Gas Pipeline Soil Layers under Lateral Multipoint Excitation

The seismic response of buried oil and gas pipelines is mainly influenced by the site soil. In this paper, a bidirectional laminar shear continuum model box is developed for the site response of buried oil and gas pipelines under transverse multipoint seismic excitation. By comparing the acceleration response of the soil and pipeline, monitoring the soil displacement, and analyzing the acceleration coefficient and Fourier spectrum, the seismic response characteristics of the soil at different excitation modes and peak seismic acceleration and its laws were investigated. The test results show that the soil under transverse excitation undergoes the process of soil compaction to nonlinear characteristics and finally soil damage, and the course of multipoint excitation develops faster and causes more serious soil damage. The peak Fourier spectrum of both the pipe and the soil appears at the frequency of 4–6 Hz, and in general, the acceleration of the pipe is greater than that of the soil; the difference between the two gradually decreases with the increase of loading level. Compared with the uniform excitation, the increase in the loading level during the lateral multipoint excitation will result in a decrease of the consistency of the acceleration time history curve at each measurement point and a decrease of the peak of the spectrum. The effect of laminar shear between soil bodies becomes more obvious with the increase of acceleration peaks on the shaking table. It is also found out that the excitation method has little effect on the displacement time history curve, but the multipoint excitation may cause fluctuations in the displacement time history curve.

Time domain nonlinear kinematic seismic response of composite caisson-piles foundation for bridge in deep water

As a new type of bridge foundation in deep water, the composite caisson-piles foundation (CCPF) is inevitability suffered from multiple hazards such as earthquake, scouring and erosion, especially for sea-crossing bridges in ocean environment. In this study, a simplified method in time domain is proposed for predicting the nonlinear kinematic seismic response of CCPF and the effect of scouring on the seismic response is investigated. The proposed method is verified by the results of centrifuge shaking-table tests. It is found that compared with frequency domain method, the proposed time domain method can give good estimations of the acceleration and the displacement of the CCPF, especially at high frequencies. Moreover, the kinematic seismic response of CCPF designed for the Qiongzhou Strait Bridge is analyzed. The results indicate that the proposed model can effectively simulate the soil nonlinearity, and the soil nonlinearity significantly affects the kinematic seismic response of CCPF, especially the acceleration and displacement time histories. Finally, the scouring effect is considered by an analytical model and the performance of the CCPF under combined earthquake and scouring is investigated.

Nanopore creation in MoS2 and graphene monolayers by nanoparticles impact: a reactive molecular dynamics study

In this work, extensive reactive molecular dynamics simulations are conducted to analyze the nanopore creation by nanoparticles impact over single-layer molybdenum disulfide (MoS2) with 1T and 2H phases. We also compare the results with graphene monolayer. In our simulations, nanosheets are exposed to a spherical rigid carbon projectile with high initial velocities ranging from 2 to 23 km/s. Results for three different structures are compared to examine the most critical factors in the perforation and resistance force during the impact. To analyze the perforation and impact resistance, kinetic energy and displacement time history of the projectile as well as perforation resistance force of the projectile are investigated. Interestingly, although the elasticity module and tensile strength of the graphene are by almost five times higher than those of MoS2, the results demonstrate that 1T and 2H-MoS2 phases are more resistive to the impact loading and perforation than graphene. For the MoS2nanosheets, we realize that the 2H phase is more resistant to impact loading than the 1T counterpart. Our reactive molecular dynamics results highlight that in addition to the strength and toughness, atomic structure is another crucial factor that can contribute substantially to impact resistance of 2D materials. The obtained results can be useful to guide the experimental setups for the nanopore creation in MoS2or other 2D lattices.

Displacement Monitoring during the Excavation and Support of Deep Foundation Pit in Complex Environment

Taking a super large deep foundation pit project as an example, the horizontal displacement of crown beam and driveway, surface settlement, axial force of anchor cable, and underground water level in the construction process of the foundation pit are dynamically monitored and analyzed. The excavation deformation rule of the deep foundation pit and the influence of excavation on surrounding buildings are analyzed. The results show that, with the excavation of the foundation pit, the crown beam and driveway of the foundation pit incline towards the direction of the pit and eventually tend to be stable. The variation of axial force of the prestressed anchor cable in the first layer of the foundation pit is basically consistent with the variation of horizontal displacement time history. The variation trend of the groundwater level at each side of the foundation pit is different but tends to be stable in a short time. In the whole monitoring period, the cumulative settlement value of each area of the foundation pit is within the controllable range, but the surface settlement of the north side of the foundation pit and a surrounding building has not reached stability, so it is suggested to extend the monitoring time of settlement in the relevant area.

A Comparative Study of the Optimum Tuned Mass Damper for High-rise Structures Considering Soil-structure Interaction

The present paper focuses on the optimum design of tuned mass damper (TMD) as a device for control of the structures. The optimum free vibration parameters such as period and damping ratio depend on the soil condition. For this reason, the seven meta-heuristic algorithms namely colliding bodies optimization (CBO), enhanced colliding bodies optimization (ECBO), water strider algorithm (WSA), dynamic water strider algorithm (DWSA), ray optimization (RO) algorithm, teaching-learning-based optimization (TLBO) algorithm and plasma generation optimization (PGO) are used to find the TMD parameters considering soil-structure interaction (SSI) effects. These optimization methods are applied to a benchmark 40-story structure. For comparison, the obtained results of these algorithms are compared. The capability and robustness of the algorithms are investigated through the benchmark problem. The results are shown that the soil type affects the optimum values of the TMD parameters, especially for the soft soil. To evaluate the performance of the obtained parameters in both the frequency and time domains, time history displacement and acceleration transfer function of the top story of the structure are calculated for the model with and without considering the SSI effects.

Dynamic analysis of railway track on variable foundation under harmonic moving load

This paper aims to investigate the effect of variable foundation stiffness on the dynamic response of an infinite railway track under the action of a harmonic moving load. In this study, harmonic variation in the foundation stiffness along the track length is considered. Here, the dynamic response of the finite element (FE) model is obtained with the help of Newmark Beta method using programming in MATLAB. It is ascertained that in the central region of the long FE model, the response is repetitive, thereby ensuring that boundary conditions do not influence the response in the central region. In this problem, two frequencies, i.e. frequency of moving load and the spatial frequency of variable foundation stiffness, are involved. Their combined influence on the dynamic characteristics such as resonant frequency, critical velocity, displacement time history, displacement below load, and bifurcation curve are investigated. It is shown that the dynamic response is qualitatively and quantitatively affected by the wavelength (λ) and amplitude (ε) of the variation of foundation stiffness. It is also shown that the important dynamic properties, i.e., the critical velocity and the resonant frequency reduce with the increase in wavelength of stiffness variation. This reduction is significant for the large amplitude of harmonic stiffness variation.

Optimal Design of Negative Stiffness Devices for Highway Bridges Using Performance-Based Genetic Algorithm

Parameter optimization analysis on the negative stiffness device (NSD) installed in the benchmark highway bridge is carried out in this study. Key parameters and constrain conditions are determined in accordance with the characteristics of NSD, and an objective function is designed with safety and comfort being considered. Individual fitness value–related cross and mutation operators are designed to protect excellent chromosome and improve the efficiency and convergence of computation. The genetic algorithm is used to realize parameter optimization of the NSD used in a benchmark highway bridge. Dynamic responses of structure without NSD, with random NSD, and with optimal designed NSD are compared. By analyzing the time history of displacement and acceleration, it can be concluded that dynamic responses of the structure decrease obviously when the NSD is added, and a better seismic reduction effect can be reached when the NSD is designed optimally in accordance with the optimization method and different earthquake excitations have slight influence on the optimization results.

Absolute Displacement-Based Formulation for Peak Inter-Story Drift Identification of Shear Structures Using Only One Accelerometer.

Only one accelerometer is used in this paper for estimating the maximum inter-story drifts and time histories of the relative displacements of all stories of multi-degree-of-freedom (MDOF) shear structures under seismic excitation. The calculation based on the data of one sensor using a conventional method is unstable, and when modal coordinates are used, higher modes should be included, which is different from the estimation based on the responses recorded by many accelerometers. However, the parameters of the higher modes of structures are difficult to obtain from structures under small excitations. To overcome this difficulty, the recorded absolute acceleration is converted into the absolute displacement, and a state-space equation is formulated. Numerical simulations of a nine-story structure were conducted to check the applicability, robustness against environmental noise, and optimal installation location of the accelerometer of the proposed approach. In addition, the effects of the higher modes were analyzed in terms of the number of accelerometers and type of response. Finally, the proposed approach was validated in a simple experiment. The results indicate that it can accurately estimate the time histories of the relative displacements and maximum inter-story drifts of all floors when one accelerometer is used and just the first two modal parameters are incorporated in the model. Furthermore, the approach is robust against environmental noise.

TDOF model for evaluating the global and local impact response of steel-plate composite panels

Steel-plate composite (SC) panel consists of two steel plates, a core concrete, and various types of mechanical connectors. The SC panel demonstrates the high bearing capacity and energy dissipation capacity against impact loading. This paper put forwards an improved two-degree-of-freedom (TDOF) mass–spring–damper model to evaluate the impact response of the SC panel. The derivations and verifications are discussed and results show that the model gives satisfactory predictions on both failure modes and impact responses, including impact force, projectile displacement, and panel deflection time histories.

Extracting full-field subpixel structural displacements from videos via deep learning

Conventional displacement sensing techniques (e.g., laser, linear variable differential transformer) have been widely used in structural health monitoring in the past two decades. Though these techniques are capable of measuring displacement time histories with high accuracy, distinct shortcoming remains such as point-to-point contact sensing which limits its applicability in real-world problems. Video cameras have been widely used in the past years due to advantages that include low price, agility, high spatial sensing resolution, and non-contact. Compared with target tracking approaches (e.g., digital image correlation, template matching, etc.), the phase-based method is powerful for detecting small subpixel motions without the use of paints or markers on the structure surface. Nevertheless, the complex computational procedure limits its real-time inference capacity. To address this fundamental issue, we develop a deep learning framework based on convolutional neural networks (CNNs) that enable real-time extraction of full-field subpixel structural displacements from videos. In particular, two new CNN architectures are designed and trained on a dataset generated by the phase-based motion extraction method from a single lab-recorded high-speed video of a dynamic structure. As displacement is only reliable in the regions with sufficient texture contrast, the sparsity of motion field induced by the texture mask is considered via the network architecture design and loss function definition. Results show that, with the supervision of full and sparse motion field, the trained network is capable of identifying the pixels with sufficient texture contrast as well as their subpixel motions. The performance of the trained networks is tested on various videos of other structures to extract the full-field motion (e.g., displacement time histories), which indicates that the trained networks have generalizability to accurately extract full-field subpixel displacements for pixels with sufficient texture contrast.