Toe Load Research Articles

Effect of facing stiffness on reinforced soil walls under surcharge loading–unloading

This paper investigates the impact of facing stiffness on the performance of geosynthetic-reinforced soil (GRS) walls under surcharge loading–unloading. Data from two instrumented physical model tests are utilised considering two different facing stiffness (block and wrapped). After the end of construction, the physical models were loaded and unloaded, step by step. They were comprehensively instrumented to monitor the load mobilised along with the reinforcements, horizontal toe load, lateral facing displacement, vertical displacement at the top of the walls and horizontal stress behind the block faces. In addition, a simplified analytical procedure is proposed in order to determine the maximum reinforcement load under surcharge unloading. The results show that the toe restraint at the base of the block-faced wall highly controls the performance of GRS wall with structural facing. When there is no toe resistance, irrespective of the facing type, the walls present, in general, similar performance under surcharge loading and unloading. Furthermore, during surcharge unloading, the reinforcement load near the face may be less affected in the wrapped-face wall compared with the block-faced wall. Moreover, the maximum reinforcement loads during surcharge unloading were fairly captured using the proposed procedure.

PREDICTION OF FATIGUE IN RAIL PAD USING SIMULATION

The primary purpose of rail pads is to prevent cracking in concrete connections, which is assumed to be caused by the passing train's impact and vibration generated by movement from its wheels. To ensure the usage of rail pads is reliable and safe, failure and fatigue life prediction need to be done. However, analyzing fatigue conditions using the experimental method is time-consuming and costly. Thus, this work aims to develop a finite element model for analyzing the fatigue and life cycle of the rail pads. By using the simulation method, the time and cost of analyzing the process can be reduced. The rail fastening system model comprises a steel rail, rail pad, and concrete sleeper. The Mooney-Rivlin model was used to develop the rail pad, and the isotropic elasticity model was used for the steel rail and concrete sleeper. Using the modified Goodman theory, this study was able to estimate the fatigue life of the rail pad in terms of the number of cycles for a range of compressive forces and toe load. The findings show that a toe load at 18 kN shows more life cycle compared to a higher toe load of 35 kN with more than 50% difference. The life cycle also reduces as the load applies increases. This concludes that the fatigue life of the rail pad is greatly dependent on the toe load condition and compressive load. The rail pad is less durable under greater compressive load circumstances.

Evaluating the Nonlinear Dynamic Stiffness of Rail Pad using Finite Element Method

Unwanted vibration and noise from railroads have a significant negative impact on the environment, causing damage to roads, buildings, and other structures. To mitigate this condition, rail pads have been installed as dampers to lessen the impact of vibration and shock on the railway track. The rail pad is made of a polymeric substance having nonlinear properties. This research examined the dynamic stiffness of rail pads made of thermoplastic elastomers (TPEs). ANSYS software was used to estimate the impact of temperature, toe load, and frequency under dynamic loading. The three-dimensional (3D) finite element model (FE) was created based on hyperelastic theory. The dynamic stiffness of the interlayer decreases with increasing temperature. For the effect of peak load and frequency, both parameters were directly proportional to dynamic stiffness. An increase in either parameter results in a stiffening of the interlayer. Frequency has the least effect on the dynamic stiffness of the track bed compared to temperature and peak load, with the average percentage difference between high and low being 28.31%, 55.57%, and 21.9%, respectively.

Do visual and step height factors cause imbalance during bipedal and unipedal stances? A plantar pressure perspective.

Objective: The plantar pressure analysis technique was used to explore the static balance ability and stability of healthy adult males under the influence of visual and step height factors during bipedal and unipedal stances. Methods: Thirty healthy adult males volunteered for the study. Experiments used the F-scan plantar pressure analysis insoles to carry out with eyes open (EO) and eyes closed (EC) at four different step heights. The plantar pressure data were recorded for 10s and pre-processed to derive kinematic and dynamic parameters. Results: For unipedal stance, most of kinematic parameters of the subjects' right and left feet were significantly greater when the eyes were closed compared to the EO condition and increased with step height. The differences in toe load between right and left feet, open and closed eyes were extremely statistically significant (p < 0.001). The differences in midfoot load between the EO and EC conditions were statistically significant (p = 0.024) and extremely statistically significant between the right and left feet (p < 0.001). The difference in rearfoot load between EO and EC conditions was extremely statistically significant (p < 0.001) and statistically significant (p = 0.002) between the right and left feet. For bipedal stance, most of kinematic parameters of the subjects' EO and EC conditions were statistically significant between the right and left feet and increased with step height. The overall load's difference between EO and EC states was statistically significant (p = 0.003) for both feet. The overall load's difference between the right and left feet was extremely statistically significant (p < 0.001) in the EC state. The differences between the right and left feet of the forefoot and rearfoot load with EO and EC suggested that the right foot had a smaller forefoot load, but a larger rearfoot load than the left foot (p < 0.001). The differences between the forefoot and rearfoot load of the subjects' both feet with EO and EC were extremely statistically significant (p < 0.001). Conclusion: Both visual input and step height factors, even the dominant foot, act on kinematic and dynamic parameters that affect the maintenance of static balance ability.

Assessing the nonlinear static stiffness of rail pad using finite element method

The unintended vibrations and sound emerging from train operation have an enormous impact on the surroundings, including the deterioration of the way, buildings, and structures. One proposed strategy to mitigate this issue is the installation of a rail pad inserted between the steel rail and concrete sleeper to provide flexibility to the track and cushion the shocks and vibrations generated by the train wheels’ movement. These materials have nonlinear, dissipative properties influenced by service conditions like temperature and toe load. This work aimed to investigate the static stiffness of different materials of the rail pad under the influence of temperature and toe load. The rail pads were categorized as soft, medium, or hard based on static stiffness. A 3D model of the rail pad, steel rail, and concrete sleeper was simulated and analyzed using ANSYS FEM software. The neo-Hookean model was used to model the rail pad, and the isotropic elasticity model was used for the steel rail and concrete sleeper. The static stiffness of the ethylene propylene diene monomer (EPDM) pad was 85.42 kN/mm, lower than that of thermoplastic elastomers (TPEs, 138.98 kN/mm) and ethylene vinyl acetate (EVA, 303.70 kN/mm) under reference conditions (20 °C), without including toe load effects. Increasing the temperature decreased the rail pad’s static stiffness, with the highest reduction of 53.49 %. However, increasing the toe load contributed up to a 23.53 % increase in the static stiffness of the rail pad.

Experimental study on load sharing characteristics of long-short CFG pile composite foundation adjacent to rigid retaining wall rotating about its base

Practicing geoengineers and researchers generally consider the load sharing behavior in multi-type pile composite foundation as an important design aspect. On the other hand, due to urbanization, such foundation system in cities will inevitably appear next to supported excavation. This paper discusses the result from relatively large-scale indoor experiment conducted to investigate the load sharing behavior of loaded long-short CFG pile composite foundation behind a neighboring rigid retaining wall undergoing rotation around the bottom. It was found that with progression of wall movement, the hidden load from soil displacement was borne by the piles with marked reduction in soil load sharing. At the end of wall rotation, the percentage of long piles’ head load increment needed to arrive at a new static equilibrium was about 12.57~32.22% while the end bearing increased by more than 97%. The consequences on the short piles, however, were manifested with an increasing pile head (13.42%) and toe (28.9%) load for the pile far from the wall whereas the closest one experienced a certain increment up to 15×10-4rad wall rotation and finally the head load and end bearing decreased to 8.28% and 12.63%, respectively. The 3D numerical back analysis conducted using FE software ABAQUS yielded the pile – soil stress ratio lower than the value obtained from the experiment but provided great insight into pile settlement characteristics during wall rotation.

Interpretation of bi-directional tests on piles with the evaluation of stress relief at the pile toe

This paper presents the interpretation of bi-directional load tests performed on three auger piles, in the city of São Paulo, Brazil, using a method based on transfer functions for the shaft and toe. Elastic shortenings of the shaft were directly measured through a displacement indicator at the pile top and two telltales at the upper and bottom plates of the expansive cell. The equivalent top-down load-settlement curves were estimated and compared with two other methods from the literature, one which considers the pile infinitely rigid; and the other, which takes the pile elastic shortening into account. The curves resulted in good agreement considering the pile compressibility. Yet for the infinitely rigid pile, the settlements resulted in up to 75% smaller. Furthermore, the influence of stress relief on the toe behavior due to shaft lifting was investigated. For the cases studied, involving bored and auger piles with the slenderness ratio (Ls/r) greater than 20, the percentage of this effect was generally small, up to 5% of the toe load, being negligible for practical uses.

Lateral stability of CWR tracks in transition zones of open-deck steel bridges

Relative displacements are introduced between steel bridges and continuously welded rail (CWR) tracks as the bridge girders expand and contract due to thermal effects. Depending on the fastening profile between the CWRs and steel bridge girders, additional forces are transmitted between the two, and when rail compression forces are introduced at the transition zones of the bridge, the risk of track buckling increases. The aim of this paper is to study CWR track buckling in the transition zones of open-deck steel bridges. In order to do so, a finite element model of a bridge is developed and calibrated based on the available literature. The model is then used to study the effects of a number of fastening profiles between the CWRs and steel bridge girders on track buckling in transition zone of the bridge. It is shown that girder temperature and location of lateral misalignment are the primary factors affecting the buckling temperature in the transition zone. Increasing the girder temperature by 45 °C can cause a drop of 11.5 °C in the track buckling temperature, a reduction of almost 19% compared to that of the open track. Furthermore, using zero toe load fasteners can reduce the risk of track buckling in the transition zone, yet there is little to resist the increasing gap size in case of a broken rail in winter.

Thermal challenges of replacing jointed rails with CWR on steel railway bridges

Visual inspection of a steel truss railway bridge revealed multiple fatigue cracks on the top member of the truss, mainly attributed to the high impact loads of crossing wheels over the jointed tracks on the bridge. The proposal to replace the jointed track with continuously welded rails (CWR) raised a number of issues regarding the thermal interaction of track and bridge, as the bridge experiences extreme temperature variations. Incorporation of high longitudinal restraint between the track and the bridge may increase the risk of track buckling failure at bridge transition zones, since bridge thermal expansion during hot weather can add compressive forces to the rails transferred through the track fastening system. Reducing longitudinal restraining may increase the size of a rail gap resulting from cold weather rail fracture. An optimal longitudinal fastening profile between the bridge and the track therefore is required to mitigate thermal issues both in hot summers and cold winters. To do so, a numerical model of the track and the bridge is developed in Abaqus and calibrated based on the results of tests performed on the bridge and available literature. Three longitudinal fastening profiles between the bridge and the track are introduced in the numerical model and performance of each for mitigating buckling failure and cold weather rail fracture are analyzed. It is concluded that providing 50% resilient fasteners and 50% zero toe load fasteners on each span can be a sound approach to mitigating the thermal issues of CWR tracks on the bridge.

Analysis of Toe Function during Single-Leg Standing Using Toe Load and Center of Pressure Acceleration

Analysis of Toe Function during Single-Leg Standing Using Toe Load and Center of Pressure Acceleration

Influence of malposition on the performance of elastic rail clip: Toe load, stress, and friction

In ballastless railway track, the elastic rail clip (i.e., an e-clip) is applied to maintain the rail in proper positions by mean of a compressive toe load. If the e-clip moves from its initial installation; the toe load may deviate from its specification. In this study, the influences of e-clip’s malposition on the toe load, stress, and friction were numerically evaluated. The toe loads of inner and outer e-clips increased with the increase in malposition. However, the toe loads of inner e-clip were slightly higher than those of outer e-clip because of the geometries of base plate and cast shoulder. The maximum von Mises stresses and the maximum normal stresses were found at the inner curvature of e-clip’s rear arch. The friction forces on the contacts between heel of e-clip vs. cast shoulder and toe of e-clip vs. insulator were slightly influenced by the malposition. However, the influence of malposition on the friction force on the contact between leg of e-clip vs. slot of cast shoulder was more prominent. The findings can be used as a guideline for the installation and maintenance to ensure the structural safety of a ballastless railway track.

Prediction of mechanical properties of rail pads under in-service conditions through machine learning algorithms

Train operations generate high impact and fatigue loads that degrade the rail infrastructure and the vehicle components. Rail pads are installed between the rails and the sleepers in order to damp the transmission of vibrations and noise and to provide flexibility to the track. These components play a crucial role in maximizing the durability of the railway assets and minimizing maintenance costs. Rail pads can be fabricated with different polymeric materials that exhibit non-linear mechanical behaviours, which strongly depend on the service conditions. Therefore, it is extremely difficult to estimate their mechanical properties, in particular the dynamic stiffness. In this work, several machine learning methodologies (multilinear regression, K nearest neighbours, regression tree, random forest, gradient boosting, multi-layer perceptron and support vector machine) were used to determine the dynamic stiffness of rail pads depending on their in-service conditions (temperature, frequency, axle load and toe load). 720 experimental tests, under different realistic operating conditions, were performed to produce a dataset that was then used for the training and testing of the machine learning methods. The optimal algorithm was gradient boosting for EPDM (R2 of 0.995 and mean absolute percentage error of 5.08% in the test dataset), TPE (0.994 and 2.32%) and EVA (0.968 and 4.91%) pads. This model was implemented in an application, available for the readers of this journal, developed on the Microsoft .Net platform that allows the dynamic stiffness of the pads study to be estimated as a function of the temperature, frequency, axle load and toe load.

3D modelling of strip reinforced MSE walls

This paper reports the results of 3D numerical modelling of a 6-m-high mechanically stabilised earth (MSE) wall constructed with concrete panels and steel or polymeric strip reinforcement. These systems pose numerical challenges as a result of the discontinuous reinforcement arrangement which is not the case for MSE walls constructed with continuous reinforcement layer configurations. Details of the numerical approach including modelling of the reinforcement strips, concrete facing panels and compressible bearing pads between panels are described. Examples of numerical predictions for facing deformations, toe loads due to soil down-drag behind the panels, soil and reinforcement settlements, and reinforcement tensile loads are presented. The influence of reinforcement stiffness is demonstrated by comparing numerical predictions for the same MSE wall with relatively inextensible steel strips and with relatively extensible polymeric strips. Of particular interest are the results showing the disruption of earth pressures along vertical and horizontal planes in the reinforced soil zone as a result of the discontinuous strip inclusions, and the vertical load that accumulates on the reinforcement strips close to the connections due to soil settlement behind the facing. The details of the modelling approach used here and the lessons learned provide a benchmark for future similar lines of investigation and for practitioners, particularly as the computational power of desktop computers continues to increase.

Influence of the operational conditions on static and dynamic stiffness of rail pads

The track has a crucial role in the performance of the rail network as it provides support and guidance to the rolling stock. During train operation, the vehicle-track interaction generates high impact loads and fatigue, which lead to degradation of vehicle components and rail infrastructure. These loads tend to increase the maintenance needs and, consequently, the life-cycle-costs of the rail assets. In order to minimize these consequences, rail pads are generally used between the rails and the sleepers in order to provide flexibility to the track and to damp the transmission of noise and vibrations. In ballasted tracks, this flexibility is a combination of the mechanical properties of the ballast and the rail pads. However, in slab tracks, the flexibility of the infrastructure is almost exclusively dependant on the rail pads. These materials exhibit non-linear, dissipative characteristics that are affected by the service conditions such as temperature, frequency, toe load and axle load. This work aims to investigate experimentally different pad materials widely used in the rail industry in order to characterize the influence of these factors on the mechanical characteristics of the rail supporting elements. The detailed characterization of the rail pads enables not only better understanding of their performance in realistic service conditions, but also provides good perspectives for use of these well quantified mechanical properties in studying the vehicle-track dynamic behaviour in different scenarios and predicting the long-term performance of the infrastructure components.

Immediate effect of visual, auditory and combined feedback on foot strike pattern

BackgroundA growing body of literature supports the promising effect of real-time feedback to re-train runners. However, no studies have comprehensively assessed the effects of foots trike and cadence modification using different forms of real-time feedback provided via wearable devices. Research questionThe purpose of the present study was to determine if a change could be made in foot strike pattern and plantar loads using real-time visual, auditory and combined feedback provided using wearable devices. MethodsVisual, auditory and combined feedback were provided using wearable devices as fifteen recreational runners ran on a treadmill at self-selected speed and increased cadence. Plantar loads and location of initial contact were measured with a flexible insole system. Repeated measures ANOVAs with Bonferroni adjusted pair-wise comparisons were used to assess statistical significance. Results and significanceA significant effect of condition was noted on location of center of pressure (p < 0.01). Bonferroni-adjusted post-hoc comparisons showed that feedback conditions differed from baseline as well as the new cadence conditions, however did not differ from each other. A significant interaction effect (region x feedback) was found for plantar loads (maximum force P < 0.001). Significant effects of feedback were noted at the heel (P < 0.001), medial midfoot (P < 0.001), lateral midfoot (P < 0.001), medial forefoot (P = 0.003), central forefoot (P = 0.003), and great toe (P = 0.004) but not at the lateral forefoot (P = 0.6) or lateral toes (P = 0.507). SignificanceThe unique findings of our study showed that an anterior shift of the center of pressure, particularly when foot strike modification was combined with 10% increased cadence. We found lower heel and midfoot loads along with higher forefoot and great toe loads when foot strike modification using real-time feedback was combined with increased cadence. Our findings also suggest that auditory feedback might be more effective than visual feedback in foot-strike modification.

Experimental evaluation of the effects of surcharge width and location on geosynthetic-reinforced soil walls

Surcharge width and location effects on the behaviour of geosynthetic-reinforced soil walls were experimentally evaluated using the results of three physical models (constructed at the COPPE/UFRJ Geotechnical Laboratory). The walls were well-instrumented in order to monitor the values of the toe load, the reinforcement load, the horizontal facing displacement and the horizontal stress at the back of the block facing. The walls were similar except for the surcharge conditions. At the end of the loading, the toes of the walls were gradually released to verify the influence of the different toe restraints. The results clearly showed the influence of the surcharge width and location on the wall behaviour, and also showed that the surcharge location and toe release can change the position of the maximum mobilised tension along the reinforcement. Furthermore, the location of the surcharge may affect the shape of the facing displacement profile.

A Force-Sensing Insole to Quantify Impact Loading to the Foot.

Lower leg injuries commonly occur in frontal automobile collisions, and are associated with high disability rates. Accurate methods to predict these injuries must be developed to facilitate the testing and improvement of vehicle safety systems. Anthropomorphic test devices (ATDs) are often used to assess injury risk by mimicking the behavior of the human body in a crash while recording data from sensors at discrete locations, which are then compared to established safety limits developed by cadaveric testing. Due to the difference in compliance of cadaveric and ATD legs, the force dissipating characteristics of footwear, and the lack of direct measurement of injury risk to the foot and ankle, a novel instrumented insole was developed that could be applied equally to all specimens both during injury limit generation and during safety evaluation tests. An array of piezoresistive sensors were calibrated over a range of speeds using a pneumatic impacting apparatus, and then applied to the insole of a boot. The boot was subsequently tested and compared to loads measured using ankle and toe load cells in an ATD, and found to have an average error of 10%. The sensors also provided useful information regarding the force distribution across the sole of the foot during an impact, which may be used to develop regional injury criteria. This work has furthered the understanding of lower leg injury prediction and developed a tool that may be useful in developing accurate injury criteria in the future for the foot and lower leg.

Nonlinear finite element analysis for structural capacity of railway prestressed concrete sleepers with rail seat abrasion

Abstract Prestressed concrete sleepers are the most commonly used type of railway sleepers in ballasted railway track. They have a strong influence on track performance, track stiffness and railway safety. Reportedly in many railway lines (especially in heavy-rail networks), many prestressed concrete sleepers have failed due to rail seat abrasion (RSA). RSA is a wear deterioration of the concrete underneath the rail that results in various problems such as loss of fastening toe load, gauge variation, improper rail cant, and eventually loss of rail fastening. In addition, the RSA will directly decrease the capacity of worn concrete sleepers. However, to the best of authors' knowledge, there were very few studies that quantitatively examined the effects of RSA on the structural capacity of the prestressed concrete sleepers. In this paper, a numerical study is executed to evaluate the load-carrying capacity of a prestressed concrete sleeper using LS-DYNA. The nonlinear model was validated firstly based on both theoretical analyses and experimental results in accordance with Australian Standard. Using the validated finite element model, the influences of different wear depth of RSA are investigated; and different compression strength and tensile strength of concrete and the prestress losses are highlighted. The outcomes of this study lead to better insight into the influences of RSA more clearly and improve track maintenance and inspection criteria.

Analysis of the fatigue performance of elastic rail clip

Abstract As a critical component of railway fastening system, the elastic rail clip (e-clip) maintains the rail position using its specified toe load. To understand its fatigue failure during service, the fatigue experiment, finite element analysis (FEA) and failure analysis were performed on e-clip. Under normal wheel load, e-clips with high, normal, and low toe loads were run-out at 5 × 106 cycles. Under the contribution of impact on wheel load, the fatigue lives were reduced to 5468 cycles and 16,839 cycles for e-clips with high and normal toe loads, respectively. For low toe load, the e-clip under the contribution of impact could withstand >5 × 106 cycles. The stress distribution on e-clip was numerically evaluated using FEA, which was in good agreement with that of experiment. Goodman diagram could be used for the fatigue evaluation of e-clip. The mean stresses and stress amplitudes of run-out e-clips were under the line of Goodman equation, while those of failed e-clips were on and above the line of Goodman equation. The fatigue crack was found to nucleate at the location of highest stress. The severity at fatigue crack front increased with the crack length until the overload fracture.

Influence of toe load on the fatigue resistance of elastic rail clips

As a critical component of the fastening system, the elastic rail clip maintains the rail in the vertical, lateral, and longitudinal positions using its specified toe load. In this study, the influence of toe load on the deformation and fatigue resistance of the clip was studied. Finite element analysis, static load experiments, and fatigue experiments were performed to evaluate the deformation and fatigue resistance of the clip. With the contribution of the lateral wheel load, the deformation range of the outer clip was higher than that of the inner clip. Therefore, the cyclic deformation of the outer clip was used for the fatigue experiment. The toe load had no influence on the fatigue resistance of the clip under normal wheel load, i.e. the clips under high, normal, and low toe loads were run-out at 5 × 106 cycles. However, with the contribution of impact on the wheel load, the fatigue lives were reduced to 5468 cycles and 16,839 cycles for the clips under high and normal toe loads, respectively. In the case of low toe load, the clip under the contribution of impact could withstand more than 5 × 106 cycles. Accordingly, the reduction of toe load may enhance the fatigue resistance of the clip under impact.