Sequential Motions Research Articles

Performance of sheet pile to mitigate liquefaction-induced lateral spreading of loose soil layer under the embankment

Liquefaction-induced lateral spreading of the foundation soil severely damages the embankment resting on it. Installation of sheet pile near the toe of the embankment is a widely used technique to mitigate lateral spreading of the foundation soil and the associated damage to the embankment. However, several factors, such as the thickness of the liquefiable foundation, width of sheet pile, and softening of the bearing stratum, that may affect the performance of the sheet pile as seismic retrofit against lateral spreading has not been well studied yet. Therefore, a series of dynamic centrifuge experiments are carried out to study the applicability of sheet pile in various thickness of liquefiable foundation under moderate to strong sequential ground motions. It is found that the sheet pile effectively mitigates the lateral spreading of a thin as well as thick liquefiable foundation. Besides, the sheet pile having the same width of the target area performs equally as a wider sheet pile in mitigating channel-ward ground movement. The experimental results also show that the liquefaction-induced softening of the dense bearing stratum immediately below the loose foundation significantly influences the performance of the sheet pile.

Random field model of sequential ground motions

This paper proposes a random field model of sequential ground motions, which considers the spatial correlation between the mainshock and aftershock from a stochastic view. First, the correlation between the mainshock and aftershock under the physical “source–path–local site” mechanism is explained. Based on the mechanism, the point-source model, and uniform-isotropic-medium model, a random single-point model for sequential ground motion with eight basic parameters is presented. Second, the random field model is developed from the random single-point model. More than 1000 pairs of sequential ground motions are used to identify and analyze statistically the basic parameters in the model. Moreover, the probability distribution of each parameter is presented based on copula functions. The results show that the spatial correlations of sequential ground motions can be effectively simulated based on the proposed random field model. Furthermore, it is possible to realistically reproduce the attenuation and time lag of ground motions at a local site.

Electricity−free electroluminescence excited by droplet impact driven triboelectric field on Solid−Liquid interface

In this work, a novel solid-liquid interfacing triboelectric field based electroluminescence (SL-TEL) was proposed. When a droplet impacting on the SL-TEL device surface, specific textured green SL-TEL emit on the two cross-overlapped areas. As a coupling effect of solid-liquid interfacing triboelectrification and electroluminescence, the SL-TEL is essentially a type of electroluminescence excited by changing triboelectric field of droplet impact or wave scour. It relies on converting mechanical energy of liquid to visual light energy utilizing an intermediate triboelectric field without any electric power consumption. The SL-TEL device simply consist of three thin flexible layers, which is greatly simplifying the electroluminescent device structure and showing strong advantages of simple structure such as easy to integration and excellent stability. Experiments show that sequential liquid motions of droplet impinging on solid surface excite a corresponding dynamic series of electroluminescence patterns. The features of the electroluminescence also reveal the characteristics of liquid and the solid surface, such as contour edge of liquid spreading, capillary convergence of liquid at the apex (advancing and receding), and so on, which is capable of both sensing liquid leakage and displaying the dynamic information of droplets impacting motions.

Seismic response of partially saturated soils beneath shallow foundations under sequential ground motions

Induced partial saturation is an innovative soil improvement technique intended to mitigate earthquake-induced liquefaction. Historical records indicate that successive earthquakes may occur in high seismic areas. Therefore, a comprehensive understanding of the response of partially saturated soils to sequential ground motions is of great significance for the rational design and execution of this method in engineering practice. In this study, a series of dynamic centrifuge experiments were conducted to investigate the impacts of sequential ground motions on the behavior of partially saturated soils beneath shallow foundations. Two different shallow foundation models with a bearing pressure of 135 kPa and 50 kPa were examined. Three seismic simulations, in order of increasing amplitudes, were sequentially applied to loosely-packed partially saturated sand models prepared with air injection technique. The assessment of the test results indicated that shallow foundations resting on saturated models of loose sand did suffer excessive settlements with each event, producing a large embedment of the foundation. However, much smaller settlements were recorded for partially saturated ground, and the level of the foundation embedment remained limited in this case. The deformation vector fields also indicated that different displacement mechanisms were observed for each successive event.

Centrifuge model test and numerical interpretation of seismic responses of a partially submerged deposit slope

Abstract Partially submerged deposit slopes are often encountered in practical engineering applications. However, studies on evaluating their stability under seismic loading are still rare. In order to understand the seismic behavior of partially submerged deposit slopes, centrifuge shaking table model tests (50g) were employed. The responses of horizontal accelerations, accumulated excess pore pressures, deformation mode, and failure mode of the partially submerged deposit slope model were analyzed. In dynamic centrifuge model tests, EQ5 shaking event was applied numerically. The results indicated that in the saturated zone of the deposit slope, liquefaction did not occur, and the measured horizontal accelerations near the water table were amplified as a layer-magnification effect. It was also shown that the liquefaction-resistance of the deposit slope increased under multiple sequential ground motions, and the deformation depth of the deposit slope induced by earthquake increased gradually with increasing dynamic load amplitude. Except for the excessive crest settlement generated by strong shaking, an additional vertical permanent displacement was initiated at the slope crest due to the dissipation of excess pore pressure under seismic loading. The result of particle image velocimetry (PIV) analysis showed that an obvious internal arc-slip was generated around the water table of the partially submerged deposit slope under seismic loading.

Centrifuge testing to investigate effects of partial saturation on the response of shallow foundation in liquefiable ground under strong sequential ground motions

Induced partial saturation is one of the novel techniques to increase the liquefaction resistance of saturated sandy ground. Nonetheless, a limited number of experimental studies are available on the delineation of this method. Moreover, the performance of induced partial saturation under sequential ground motions is poorly understood. Dynamic centrifuge experiments are carried out to investigate the effects of partial saturation on the response of shallow foundation resting on liquefiable ground under sequential ground motions. Centrifuge models consist of two distinct shallow foundations and associated superstructures resting on a liquefiable uniform sand layer. The drainage-recharge method is used to induce partial saturation in the model ground. Compressibility change of pore fluid and alteration of ground permeability because of induced air voids, affect the deformation mechanism of the ground-foundation system. Assessment of maximum potential volumetric compressibility of pore fluid because of induced air voids is essential to understand the effectiveness of induced partial saturation. Centrifuge test results signify that the induced partial saturation reduces the overall deformation of the foundation-structure system. However, slightly amplified kinematic seismic demand is observed at superstructures in case of partially saturated ground in comparison with fully saturated ground.

Study on nonlinear crawling locomotion of modular differential drive soft robot

This paper presents a novel modular differential drive soft robot (MDDSR) and its nonlinear model of crawling locomotion. The MDDSR consists of three differential drive soft modules (DDSMs) in series, and each module constructed by two bending soft actuators in parallel to be equally or differentially driven can achieve inchworm-like crawling movement with capability of straight and steering motion. Through sequential motions of the DDSMs, the MDDSR can perform straight and steering crawling locomotion. A nonlinear state-space kinematic model with the principle of minimum frictional work is built to characterize the MDDSR’s crawling locomotion, which is based on nonlinear bending behavior of the soft actuator and nonlinear crawling motion of the DDSMs. Feasibility of the proposed robot and its model is verified through locomotion experiments.

Nonlinear dynamic analysis of frame-core tube building under seismic sequential ground motions by a supercomputer

Historical records indicate that strong earthquakes are usually accompanied by aftershocks with large peak ground accelerations. Structural damages caused by the mainshock can be further aggravated by aftershocks, which can lead to structural collapse. Current structure seismic design practices generally consider the mainshock effects only. Performance of buildings subjected to mainshock-aftershock sequential ground motions, should always be fully investigated. Previous studies on sequential ground motion focus mainly on frame structures. There is a paucity of publications addressing other type of structures, especially high-rise buildings. The reasons for this omission have been identified as the complexity of high-rise buildings and unaffordable computational costs related to the nonlinear dynamic analysis of long duration sequential ground motions. Thus, the authors used Tianhe-2, once known as the fastest supercomputer in the world, to conduct nonlinear dynamic analysis of a typical 20-story frame-core tube building subjected to sequential mainshock-aftershock ground motions. This study focuses on 104 mainshock and aftershock ground motions from four different sites. Thus, the effects of mainshocks only and sequential ground motions of a frame-core tube structure at four different sites were analyzed in this study's finite element model. The Performance of these structures under mainshocks and sequential ground motions are compared in terms of inter-story displacement ratio, hysteretic energy and damage index based on the Park-Ang model. The results prove that a supercomputer can be used to solve the computational cost issue in structural engineering and emphasize that the effects of sequential earthquakes need to be considered in structural design, even for frame-core tube structures with lateral resisting members.

Regional short-term and long-term risk and loss assessment under sequential seismic events

A framework for building-portfolio risk and loss assessment under mainshock-aftershock earthquake sequences is presented. The temporary increase in seismic hazard after a large-magnitude mainshock coupled with the reduction in the structural capacity under sequential ground motions are considered in the framework. For the inventory of reinforced concrete frames located in Los Angeles County, the annual rate of “complete” (HAZUS) damage and expected annual losses are, on average, 10% and 25% higher, respectively, when aftershocks are incorporated in the risk assessment procedure. A short-term risk and loss analysis after a hypothetical scenario mainshock on the Puente Hills fault showed a substantial increase in the seismic risk within one week after the mainshock. The aftershock-induced losses during the same period were estimated at approximately 30% of the losses due to the mainshock.

Design and analysis of a metamorphic mechanism for automated fibre placement

Automated fibre placement (AFP) integrates the functions of fibre feeding, transmission, clamping, restarting, cutting, heating, and compacting, which is characterised by multiple degrees-of-freedom, multiple drives, constraints, and strong coupling. To simplify the complexity of mechanical structures, a metamorphic design to achieve above functions is introduced in this study based on the relationship of the motion sequence and constraints in the effort to realise sequential motions. Furthermore, the variable topological structures and the mobility of this mechanism are analysed in different phases of the operation, and a kinematics model is proposed to reveal the dynamic performance. Specifically, a mechanical model is presented for convenient analysis of the transformation of phases. The global optimisation process is then introduced using global and other configuration optimisers that ultimately obtain global optimisation results. Corresponding simulation results demonstrate that the proposed mechanism can effectively realise stable operation.

Bioinspired 3D structures with programmable morphologies and motions

Living organisms use spatially controlled expansion and contraction of soft tissues to achieve complex three-dimensional (3D) morphologies and movements and thereby functions. However, replicating such features in man-made materials remains a challenge. Here we report an approach that encodes 2D hydrogels with spatially and temporally controlled growth (expansion and contraction) to create 3D structures with programmed morphologies and motions. This approach uses temperature-responsive hydrogels with locally programmable degrees and rates of swelling and shrinking. This method simultaneously prints multiple 3D structures with custom design from a single precursor in a one-step process within 60 s. We suggest simple yet versatile design rules for creating complex 3D structures and a theoretical model for predicting their motions. We reveal that the spatially nonuniform rates of swelling and shrinking of growth-induced 3D structures determine their dynamic shape changes. We demonstrate shape-morphing 3D structures with diverse morphologies, including bioinspired structures with programmed sequential motions.

Risk factors identification and visualization for work-related musculoskeletal disorders with wearable and connected gait analytics system and kinect skeleton models

Abstract Risk factors, such as overexertion, awkward postures, excessive repetition, and the combination of these factors are main causes of work-related musculoskeletal disorders (WMSDs). In this paper, we proposed an automatic WMSDs risk factors identification and visualization method based on Wearable and Connected Gait Analytics System (WCGAS) and Kinect skeleton models. WCGAS was capable of recording plantar pressure from which postures, force exertions, and repetitions could be recognized with algorithms such as sequential minimal optimization (SMO) algorithm and long short term memory (LSTM) network. Kinect skeleton models were used to make the WMSDs risk factors visualized. Experiments with quasi-static and sequential postures were designed to evaluate the recognition performance of work-related motion type (i.e. “lifting”, “carrying”, “bending”, “pulling”, and “pushing”). A load variable (with/without 10 kg load) was introduced for evaluating the performance of force exertions recognition. 5 repetitions of each motion were used for evaluating the performance of repetitions recognition. Results showed that quasi-static postures could be recognized with 100% accuracy and the accuracy for sequential motions recognition were 74%, 79%, 92%, 99% and 99% for “bending”, “carrying”, “lifting”, “pulling” and “pushing”, respectively. Force exertions were recognized with 100% accuracy. For repetitions recognition, except the accuracy in the “bending” motion was 80%, the repetitions of other motions could be recognized correctly. Kinect skeleton model showed its ability of making the WMSDs risk factors vivid which would contribute to the accuracy of WMSDs risks evaluation. These results indicated that it is possible to use WCGAS and Kinect skeleton models for WMSDs risk factors identification and visualization applications.

A machine learning framework for assessing post-earthquake structural safety

A machine learning framework is presented to assess post-earthquake structural safety. The concepts of response and damage patterns are introduced and incorporated into a systematic methodology for generating a robust dataset for any damaged building. Incremental dynamic analysis using sequential ground motions is used to evaluate the residual collapse capacity of the damaged structure. Machine learning algorithms are used to map response and damage patterns to the structural safety state (safe or unsafe to occupy) of the building based on an acceptable threshold of residual collapse capacity. Predictive models including classification and regression tree and Random Forests are used to probabilistically identify the structural safety state of an earthquake-damaged building. The proposed framework is applied to a 4-story reinforced concrete special moment frame building. Distinct yet partially overlapping response and damage patterns are found for the damaged building classified as safe and unsafe. High prediction accuracies of 91% and 88% are achieved when the safety state is assessed using response and damage patterns respectively. The proposed framework could be used to rapidly evaluate whether a damaged building remains structurally safe to occupy after a seismic event and can be implemented as a subroutine in community resilience evaluation or building lifecycle performance assessment and optimization.

Estimating aftershock collapse vulnerability using mainshock intensity, structural response and physical damage indicators

This paper describes statistical models for estimating aftershock collapse vulnerability of buildings using mainshock intensity, structural response and physical damage indicators. The performance of mainshock-damaged buildings is assessed by performing Incremental Dynamic Analyses to collapse using sequential ground motions. Alternative dependent variables are suggested including the ratio of the conditional collapse probability for the damaged and intact buildings. Challenges arising from strong correlation among predictors are addressed using more advanced methods, including Best Subset Regression, Least Absolute Shrinkage and Selection Operator, Principal Components Analysis and Gaussian Kernel Ridge Regression. The models are evaluated based on their accuracy and stability while dealing with issues stemming from high dimensionality. Overall, Gaussian Kernel Ridge Regression is the most favorable model based on the accuracy and stability of its predictions. Of the three types of predictors, those related to observable physical damage to key structural components produced the most accurate and stable estimates of aftershock collapse vulnerability.

Effects of frequency contents of aftershock ground motions on reinforced concrete (RC) bridge columns

This study focuses on exploring effects of frequency contents of aftershock ground motions on seismic responses of reinforced concrete (RC) bridge columns. It has been well recognized that RC columns damaged by a sizeable main shock event become more vulnerable to following aftershocks. Therefore, it is essential to use proper main shock-aftershock sequential ground motions in seismic analyses to ensure the safety and integrity of infrastructure. Using frequency –invariant scaling factors, conventional methods were developed in the past. These methods combine sequential ground motions when performing a time history nonlinear analysis. However, these conventional methods neglect frequency contents of aftershock ground motions, which are usually different from those of main shock ground motions.This research demonstrates the importance of properly representing aftershock ground motions in estimating seismic responses of RC columns, and presents the differences in frequency contents between aftershock ground motions and the corresponding main shock ground motions. Time history seismic analyses using a finite element analysis program OpenSees are carried out. First, main shock motions recorded during the 1994 Northridge, California, the United States of America, the 1997 Umbria-Mache, Italy and the 1999 Chi-Chi, Taiwan earthquakes are used in this study. Then, the corresponding aftershock motions are selected or obtained: (1) from recordings during the seismic events, (2) by scaling main shock (which represents a traditional method), or (3) by spectrally matching main shock motions to the aftershock motions. The peak displacements and residual displacements of the RC columns using the spectrally matched motions are closer to those results using real aftershock motion records, as opposed to using the scaled motions. This demonstrates that the frequency contents of aftershock ground motions have significant impacts on the seismic responses of RC columns.

Sequential Ground Motion Effects on the Behavior of a Base-Isolated RCC Building

The sequential ground motion effects on the dynamic responses of reinforced concrete containment (RCC) buildings with typical isolators are studied in this paper. Although the base isolation technique is developed to guarantee the security and integrity of RCC buildings under single earthquakes, seismic behavior of base-isolated RCC buildings under sequential ground motions is deficient. Hence, an ensemble of as-recorded sequential ground motions is employed to study the effect of including aftershocks on the seismic evaluation of base-isolated RCC buildings. The results indicate that base isolation can significantly attenuate the earthquake shaking of the RCC building under not only single earthquakes but also seismic sequences. It is also found that the adverse aftershock effect on the RCC can be reduced due to the base isolation applied to the RCC. More importantly, the study indicates that disregarding aftershocks can induce significant underestimation of the isolator displacement for base-isolated RCC buildings.

Reciprocal Learning for Robot Peers

This paper proposes a robot peer reciprocal learning system in which robot peers can not only cooperatively accomplish a difficult task but also help each other to learn better. In this system, each robot is an independent individual and has the ability to make individual decisions. They can communicate about image information, individual decisions, and current state to formulate mutual decisions and motions. For learning a new concept, we propose a mutual learning method, which allows the robots to learn from each other by exchanging weights in their neural network concept learning system. The simulation results show that the robots can learn from each other to build general concepts from limited training, while improving both of their performances at the same time. Finally, we design two cooperative tasks, which require the robots to formulate mutual sequential motions and keep communicating to manage their motions. The robotic experiments demonstrate that the proposed robot peer reciprocal learning system can help robots achieve difficult tasks in appropriate and cooperative ways, just as humans do.

Ap/Vp specific inelastic displacement ratio for the seismic response estimation of SDOF structures subjected to sequential near fault pulse type ground motion records

This research study is focused on an improved statistical equation proposed to estimate the inelastic displacement ratio, C1, of structures subjected to sequential (pre-shock, main shock, after shock) pulse type near fault (NF) ground motions. Proposed equation considers the effects of fundamental vibration period of the structure, T, lateral strength ratio, R, and frequency content of the design earthquake on the variation of the response. Frequency content of the design earthquake, represented by the Ap/Vp ratio (i.e. ratio of peak ground acceleration (Ap) to peak ground velocity (Vp)), is a function of the earthquake magnitude, distance to fault, faulting mechanism and site class. In scope of the study statistical analyses were conducted to develop a simple and accurate statistical equation to estimate the C1 of structures subjected to sequential pulse type NF ground motions. From the results of the study it was observed that the C1 values obtained from the proposed equation are, generally, in good agreement with the calculated exact results. Also, the C1 and T relationships were plotted together with those of a former study to clearly show the detrimental effect of the sequential ground motion loading in terms of amplified displacement demands.

Redundant Input Safety Tracking for Omnidirectional Rehabilitative Training Walker with Control Constraints

AbstractThis study proposes a nonlinear safety tracking method based on redundant degrees of freedom with control constraints to protect against actuator faults that may occur in robotic rehabilitative walkers. A redundant input model with uniform actuator faults is constructed by separating the corresponding columns of the control matrix, and an adaptive robust control method is presented to deal with the separated term that is considered in relation to the extrinsic bounded interference that occurs on robotic walker systems. Based on the backstepping technique, an adjustable control law can be designed to maintain stability in terms of solving linear matrix inequality. The trajectory tracking error performance and the velocity tracking error performance are derived simultaneously. As an application, simulation results confirm the effectiveness of the proposed method and verify that the walker can provide safe sequential motions even when one wheel actuator fails.

Damage accumulation of a base-isolated RCC building under mainshock-aftershock seismic sequences

Strong aftershocks have the potential to induce greater structural damage for Nuclear Power Plant (NPP) structures. In order to ensure the safety and integrity of Reinforced Concrete Containment (RCC) buildings, the base isolation technique is proposed to reduce the effects of earthquake shaking on the building and avoid damage of the structure under earthquakes. However, none of the studies have focused on evaluating the seismic performance of RCC buildings with isolation for mainshock-aftershock seismic sequences. The aim of this paper is to thoroughly investigate the structural damage of RCC buildings with Lead Rubber Bearing (LRB) isolators under seismic sequences. A suite of twenty-eight artificial sequential ground motions is developed to investigate the influence of considering aftershocks on the performance of RCC buildings with isolators for both design-basis and beyond-designbasis earthquake shaking. The results reveal that strong aftershocks would induce greater damage for the RCC without isolators under beyond-design-basis earthquake. While, base isolation can protect the RCC building under seismic sequences being damaged even for beyond-design-basis earthquake shaking. The IDA results of base-isolated RCC demonstrate that isolators also increase the structural reserve capacity significantly under both mainshocks and seismic sequences.