Inconsistent effect of dynamic load waveform on macro-and micro-scale responses of ballast bed characterized in individual cycle:a numerical study

Cyclic load is widely adopted in laboratory to simulate the effect of train load on ballast bed.The effectiveness of such load equivalence is usually testified by having similar results of key concerns of ballast bed,such as deformation or stiffness,while the consistency of particle scale characteristics under two loading patterns is rarely examined,which is insufficient to well-understand and use the load simplification.In this study,a previous laboratory model test of ballast bed under cyclic load is rebuilt using 3D discrete element method(DEM),which is validated by dynamic responses monitored by high-resolution sensors.Then,train load having the same magnitude and amplitude as the cyclic load is applied in the numerical model to obtain the statistical characteristics of inter-particle contact force and particle movements in ballast bed.The results show that particle scale responses under two loading patterns could have quite deviation,even when macro-scale responses of ballast bed under two loading patterns are very close.This inconsistency indicates that the application scale of the DEM model should not exceed the validation scale.Moreover,it is important to examine multiscale responses to validate the effectiveness of load simplification.

Development of combined transitional pavement structure for urban tram track-road grade crossings

The grade crossings and adjacent pavements of urban trams are generally subjected to complex load conditions and are susceptible to damage.Therefore,in this study,a novel pavement structure between tram tracks and roads constructed using polyurethane(PU)elastic concrete and ultra-high-performance concrete(UHPC),referred to as a track-road transitional pavement(TRTP),is proposed.Subsequently,its performance and feasibility are evaluated using experimental and numerical methods.First,the mechanical properties of the PU elastic concrete are evaluated.The performance of the proposed structure is investigated using a three-dimensional finite element model,where vehicleinduced dynamic and static loads are considered.The results show that PU elastic concrete and the proposed combined TRTP are applicable and functioned as intended.Additionally,the PU elastic concrete achieved sufficient performance.The recommended width of the TRTP is approximately 50 mm.Meanwhile,the application of UHPC under a PU elastic concrete layer significantly reduces vertical deformation.Results of numerical calculations confirmed the high structural performance and feasibility of the proposed TRTP.Finally,material performance standards are recommended to provide guidance for pavement design and the construction of tram-grade crossings in the future.

A Simplified Nonlinear Model of Vertical Vortex-Induced Force on Box Decks for Predicting Stable Amplitudes of Vortex-Induced Vibrations

Wind-tunnel tests of a large-scale sectional model with synchronous measurements of force and vibration responses were carried out to investigate the nonlinear behaviors of vertical vortex-induced forces （VIFs） on three typical box decks （i.e., fully closed box, centrally slotted box, and semi-closed box）. The mechanisms of the onset, development, and self-limiting phenomenon of the vertical vortex-induced vibration （VlV） were also explored by analyzing the energy evolution of different vertical VIF components and their contributions to the vertical VIV responses. The results show that the nonlinear components of the vertical VIF often differ from deck to deck; the most important components of the vertical VIF, governing the stable amplitudes of the vertical VIV responses, are the linear and cubic components of velocity contained in the self-excited aerodynamic damping forces. The former provides a constant negative damping ratio to the vibration system and is thus the essential power driving the development of the VIV amplitude, while the latter provides a positive damping ratio proportional to the square of the vibration velocity and is actually the inherent factor making the VIV amplitude self-limiting. On these bases, a universal simplified nonlinear mathematical model of the vertical VIF on box decks of bridges is presented and verified in this paper; it can be used to predict the stable amplitudes of the vertical VIV of long-span bridges with satisfactory accuracy.

Characterizing dynamic load propagation in cohesionless granular packing using force chain

When dynamic load is applied on a granular assembly,the time-dependent dynamic load and initial static load(such as gravity stress)act together on individual particles.In order to better understand how dynamic load triggers the micro-structure's evolution and furtherly the ensemble behavior of a granular assembly,we propose a criterion to recognize the major propagation path of dynamic load in 2D granular materials,called the“dynamic force chain”.Two steps are involved in recognizing dynamic force chains:(1)pick out particles with dynamic load larger than the threshold stress,where the attenuation of dynamic stress with distance is considered;(2)among which quasi-linear arrangement of three or more particles are identified as a force chain.The spatial distribution of dynamic force chains in indentation of granular materials provides a direct measure of dynamic load diffusion.The statistical evolution of dynamic force chains shows strong correlation with the indentation behaviors.

Analytical and numerical analysis on the torsional performance of elliptically deformed tunnels in the longitudinal direction

Segmental lining structures constructed by shields/tunnel boring machines are often subjected to uneven longitudinal cross-section torsion as a result of eccentric external loads,which is extremely adverse to tunnel safety but has not received sufficient attention for a long time.To figure out the torsional performance of segmental tunnels,it is essential to assess the longitudinal torsional stiffness and active-torsion-rejection capability of a segmented tunnel.The aim of the paper is to derive an analytical solution to the longitudinal torsional stiffness of a segmental tunnel with existing elliptical deformation.The longitudinal torsional stiffness under different internal force combinations is deduced considering the longitudinal axial force and bending moment based on the equivalent continuous model and force balance equation.The validity of the analytical solution is verified by comparing it with finite element method results.Then,a parametric analysis,using the new analytical solution,is included to investigate the effect of the key parameters on torsional behaviors,including the segment size,the bolt size and the transverse bending stiffness,etc.It is found that:(1)the longitudinal torsional stiffness efficiency(LTSE)of the segmental tunnel decreases with the rise of segment thickness to diameter ratio but increases with the ring width to diameter ratio;(2)the LTSE reduces with the increase of the effective shear length but rises with the diameter of bolts;(3)the LTSE increases rapidly with the ratio of compression-torsion or bending-torsion.Furthermore,the envelope curve of the critical load(N0,M0)for a tunnel to actively resist a certain internal torque is given.The proposed solution can be easily utilized to determine the longitudinal torsional stiffness of segmental tunnels and is an effective tool for tunnel design and maintenance.