Chen Ming Kuo

Development of 3-d finite element model for flexible pavements

Abstract This paper presents the development of a three‐dimensional finite element model for flexible pavements. The procedures of building a model and performing static analysis with the ABAQUS package are introduced. The element shapes and the size of the finite element model were studied to improve analysis efficiency. The model should at least consist of finite elements up to three times the loading diameter if centered at the loading position. Infinite elements should be used beyond the boundary of finite elements. The accuracy was justified with Boussnisq solutions for semiinfinite elastic solids. The surface layer was modeled with creep test data of asphalt concrete. Viscoelastic behavior was observed for the pavement model under wheel loading. The model was also validated by comparing computed results and field test data. Further calibrations of the model are underway by comparison with laboratory test results. The application of the model is introduced with an example of placing a geogrid‐reinforced asphalt concrete overlay on rigid pavement. Significant reduction of tensile strains at the bottom of the asphalt concrete was found with geogrid reinforcement.

DERAILMENT RISK ANALYSIS OF A TILTING RAILWAY VEHICLE MOVING OVER IRREGULAR TRACKS UNDER WIND LOADS

Based on the nonlinear creep model and Kalkers linear theory, this paper studies the governing differential equations of motion for a tilting railway vehicle moving over irregular curved tracks under wind loads. The tilting vehicle is modeled by a 24-degree-of-freedom (24-DOF) car system, considering the lateral, roll and yaw motions of each wheelset, the lateral, vertical, roll and yaw motions of each bogie frame and the car body. The derailment quotients of the tilting railway vehicle with the wheelsets moving over irregular rails in the lateral direction and the car body acted upon by the wind loads are investigated for various tilting angles. The analysis results show that in general, the derailment quotient of the wheelset increases as the tilting angle of the railway vehicle increases. When the railway vehicle moves at low speeds, the derailment quotient calculated for the case with rail irregularities is greater than that for the case with no rail irregularities. Moreover, the derailment quotient of a wheelset moving over curved tracks of various radii is presented. Finally, the derailment quotient computed for the case under wind loads is greater than that free of wind loads. As a result, the influence of rail irregularities and wind loads on the derailment risk of a tilting vehicle cannot be ignored.

An analytical study of axle load equivalency factors of concrete pavements

Abstract The concept of single axle load equivalency factor (LEF) was reviewed and an in‐depth study was focused on the idea of equivalent single axle load (ESAL) developed by AASHO. A three‐dimensional finite element model was utilized to simulate the AASHO Road Test pavement sections and to develop LEF formulae for various pavement sections under various axle loadings. The characteristics of the mechanistic‐empirical derived LEF formula were examined through the viewpoints of pavement structural factors. Comparing the current AASHTO Design Guide, additional effect of base courses, joint devices, and subgrade strength were found for rigid pavement thickness design. Adequacy of traffic equivalency estimation for a specific pavement structure can be enhanced through the proposed LEF regression formula. The design example also showed that the current LEF tables in the AASHTO Design Guide resulted in lower ESALs than those from the proposed LEF formula for pavements with cement‐treated and bituminous‐treated bases.

Significance of subgrade damping on structural evaluation of pavements

Falling weight deflectometer (FWD) has been widely adopted to generate a transient loading on pavement to simulate a moving wheel on the road. Although damping effects influence structural responses in dynamic conditions, it was usually neglected in pavement evaluations. This study presented the pavement responses of finite element analysis of FWD tests on various types of subgrade soils. The results showed that subgrade damping significantly influences pavement deflections of FWD tests. Among the types of soils, the analysis results revealed that gravel subgrade has the least damping effect on dynamic deflections. Deflection velocity was found to be crucial to interpret the importance of soil damping in FWD tests. The simulated pavement responses of the FWD test are different from that caused by a moving vehicle.

Analysis of derailment criteria

Rail transportation systems include high-speed rail, rapid mass transit systems, rail freight, and light-rail transit systems are generally wheel-on-rail systems. Thus, the evaluation of the risk of a derailment is a critically important issue; consequently, researchers have endeavored to identify and evaluate an effective derailment criterion. Nadal’s quotient has been widely adopted in the assessment of the risk of a derailment. Although recent studies have sometimes criticized that criterion, it still serves as the basis of new criteria and regulations. This article seeks to investigate the problems associated with various derailment criteria, to point out their features and deficiencies, and then to theoretically illustrate their differences. It has been found that the Weinstock criterion does not correctly consider the tread contact wheel, with the revised Weinstock criteria being proposed to correct that problem. Furthermore, a new criterion has been developed that solely considers the horizontal component of the tangential force at the contact point. The new criterion was compared with the wheel load reduction criterion and other criteria to justify the derailment assessments.

Experiments on Vibration Mitigation with a Mass-Spring-Series Model

A mass-spring-series model was assembled and tested using displacement-controlled vibrations in order to validate the effectiveness of vibration mitigation in a floating slab track. A mass mounted with an accelerometer was isolated with a spring from the vibration source. The other end of the mass was also connected with a spring to the fixed end. The spring coefficients and natural frequencies of the mass-spring combinations were carefully measured and verified. The time histories of the vibration amplitude indicated that properly tuned combinations of mass and springs effectively retain vibrations in the designated mass and reduce propagation and reflection. It was determined that using a stiff spring to confront the excitation source and a soft spring as a foundation isolator may alleviate vibration propagation and reduce vibrations reflected to the excitation. The drawbacks of the experimental design were also discussed.

STUDY OF LOAD TRANSFER PARAMETER IN AASHTO DESIGN GUIDE FOR CONCRETE PAVEMENT

Some of the design parameters in AASHTO’s Guide for Design of Pavement Structures require experienced engineering judgment to obtain adequate designs. The load transfer parameter for concrete pavements in the AASHTO Guide is reviewed. A set of equations was developed to assist in choosing a J-factor for various pavement conditions. With three-dimensional finite element analysis, factorial runs were conducted to find the relationships between the critical stresses and joint design parameters—that is, joint width, diameter, length, and spacing of dowel bars. Extended procedures that incorporate dowel parameters into the J-factor were proposed. Conclusions were made to clarify the load transfer concept in the current AASHTO Guide and the effects of joint parameters on pavement performance.