3D Finite Element Analysis of Temperature‐Induced Stresses in Dowel Jointed Concrete Pavements

A detailed 3D finite element (3DFE) model is developed to investigate the applicability of Westergaard’s curling stress equations to doweled jointed concrete pavements. The model does not rely on any of Westergaard’s assumptions and is capable of handling nonlinear and/or time‐dependent temperature profiles. However, only linear gradient is applied to facilitate the comparison with Westergaard’s results. The transverse stress calculated using Westergaard’s formula was found to be within 10% of that computed using 3DFE. Westergaard’s longitudinal stress equation required a correction. The 3DFE results confirm Westergaard’s finding that the slab curling stresses are independent of slab length. Thus, curling stress does not explain the field‐observed dependency of mid‐slab cracking on the slab length. Through the examination of the mechanism of dowel‐concrete interaction, it is shown that uniform temperature changes play the major role in mid‐slab transverse cracking of relatively long slabs. Due to built‐in slab curling as well as temperature or moisture curling, the dowel bars bend restricting the slab from free contraction due to uniform temperature drop. This gives rise to a large component of stress that has not been considered in previous investigations. Application of a combined temperature gradient and uniform temperature drop to slabs of different lengths revealed the dependency of mid‐slab transverse cracking on slab length.     

3D Finite Element Study on Load Transfer at Doweled Joints in Flat and Curled Rigid Pavements

This study examines load transfer across doweled joints in rigid pavements using 3D finite element analysis. A recently developed dowel modeling strategy is employed that allows the efficient and rigorous consideration of dowel/slab interaction. Parametric studies on the response of a typical, dowel‐retrofitted pavement system subjected to axle loads and varying degrees of slab curling are conducted. To examine the effect of slab support on pavement response, the studies consider two different foundation types: layered elastic with an asphalt‐treated base and a dense liquid foundation. The results of the studies are discussed with emphasis on the effect of slab curling and foundation type on joint load transfer and the potential for joint distress. While there are significant differences in response for the ATB‐supported slabs and the slabs founded on a dense liquid, slab curling does generally increase dowel shears and dowel/slab bearing stresses. However, further examination of the parametric study results that accounts for compressive fatigue of the concrete at the dowel/slab interface indicates that slab curling may not significantly increase the potential for damage to the slab concrete surrounding the dowels.     

Mesoscale Approach to Modeling Concrete Subjected to Thermomechanical Loading

Concrete subjected to combined compressive stresses and temperature loading exhibits compressive strains, which are considerably greater than for concrete subjected to compressive stresses alone. This phenomenon is called transient thermal creep or load induced thermal strain and is usually modeled by macroscopic phenomenological constitutive laws which have only limited predictive capabilities. In the present study a mesoscale modeling approach is proposed in which the macroscopically observed transient thermal creep results from the mismatch of thermal expansions of the mesoscale constituents. The mesostructure of concrete is idealized as a two-dimensional three-phase material consisting of aggregates, matrix, and interfacial transition zones. The nonlinear material response of the phases is described by a plasticity interface model. The mesoscale approach was applied to analyze compressed concrete specimens subjected to uniform temperature histories and the analysis results were compared to experimental results reported in the literature.     

Service and Ultimate Load Behavior of Bridge Deck Reinforced with Carbon FRP Grid

A full-scale model of a bridge deck slab isotropically reinforced with 0.3% fiber-reinforced plastic reinforcement (FRP) was tested. The 6 m long slab of 185 mm thickness had two 2 m spans and 1 m cantilevers on both sides. The slab was first loaded monotonically to crack the concrete. Then it was loaded cyclically in three stages of 4 million cycles each at a frequency of 5 Hz, with the load varying between 0 and 100 kN in the first two stages and between 0 and 125 kN in the last. Finally, it was loaded monotonically to failure. Constructability of the slab with the FRP reinforcement was found satisfactory. The cyclic load test shows that deflections and stresses are small, and the degradation indicated by increases in deflections and stresses after 4 million cycles of overloading by 25% is very small. The ultimate load capacity of the slab was at least five times the maximum wheel load of 100 kN, and the failure mode was punching shear. Long-term performance of FRP reinforcement under the combined effect of exposure to the chemical environment and loading needs to be studied to evaluate durability, and hence cost-effectiveness, of FRP reinforcement in bridge decks.     

Temperature Correction and Strut Loads in Central Artery Excavations

Braced excavations for the Central Artery/Tunnel project in Boston, Mass. include several instrumented sections for monitoring excavation and ground response to soil removal and strut installation. Strain in the struts is measured using thermally matched vibrating wire strain gauges (VWSGs). This paper presents issues related to strut load interpretation using VWSG data and the difficulties and errors associated with the use of field data. The paper presents a procedure to separate earth load from thermal load induced in the struts due to temperature variation throughout construction. The procedure is an extension of a method that uses measured temperature and corresponding load change to compute a thermal (temperature change induced) load coefficient. The procedure emphasizes the dependency of the thermal load coefficient on construction stage and support configuration. A numerical analysis is presented to support the proposed procedure. Strut loads and earth pressure diagrams are presented using the proposed procedure. Comparisons are made between measured loads and the design load envelope developed prior to construction.     

Thickness measurement of soft tissue biomaterials: a comparison of five methods

Thickness measurement in soft connective tissues is a continuing problem due to the apparent compression of the tissue by micrometer-type gauges. We have compared five methods for the measurement of thickness: (1) a Mitutoyo non-rotating thickness gauge; (2) a custom-built, instrumented thickness gauge which was strain-gauged to measure contact force; (3) a commercial Hall effect probe (Panametrics Magna-Mike); (4) a custom-built electrical resistance probe; and (5) measurement of fresh frozen histological sections under polarized light. Using bovine pericardium as a test material, all the methods examined were adequate to assess sample-to-sample and location-to-location differences in thickness. The resistance gauge gave significantly greater thicknesses than did the other methods, with little or no compression; indeed, extrapolation to zero load of thickness readings from the instrumented gauge yielded identical thickness. Thicknesses measured by frozen sections were indistinguishable from those measured with the non-rotating gauge, the instrumented gauge under 0.5-1.2 g compressive load, or the Hall effect probe. With the correct technique, the simple and inexpensive non-rotating gauge remains a pragmatic choice for thickness measurement in planar soft tissue.     

Curved Sandwich Panels Subjected to Temperature Gradient and Mechanical Loads

The results of a detailed study of the nonlinear response of curved sandwich panels with composite face sheets, subjected to a temperature gradient through the thickness combined with mechanical loadings, are presented. The analysis is based on a first-order shear-deformation Sanders-Budiansky-type theory, including the effects of large displacements, moderate rotations, transverse shear deformation, and laminated anisotropic material behavior. A mixed formulation is used with the fundamental unknowns consisting of the generalized displacements and the stress resultants of the panel. The nonlinear displacements, strain energy, principal strains, transverse shear stresses, transverse shear strain energy density, and their hierarchical sensitivity coefficients are evaluated. The hierarchical sensitivity coefficients measure the sensitivity of the nonlinear response to variations in the panel parameters, the effective properties of the face sheet layers and the core, and the micromechanical parameters. Numerical results are presented for cylindrical panels subjected to combined pressure loading, edge shortening or extension, edge shear, and a temperature gradient through the thickness. The results show the effects of variations in the loading and the panel aspect ratio, on the nonlinear response, and its sensitivity to changes in the various panel, effective layer, and micromechanical parameters.     

Behavior, Analysis, and Design of an Instrumented Link Slab Bridge

Presented in this paper are the results of a research project on the monitoring and assessment of the first link slab jointless bridge in the state of North Carolina. The structure was instrumented with a remote data acquisition system and monitored for over a year. In addition, a controlled load test was conducted in an effort to determine the demand on the link slab under known loads. A procedure for the limit-states design of a link slab system is also presented. Results indicate that while the crack size in the link slab exceeded the design level, the link slab fulfilled its function. Furthermore, the rotational demand from the large controlled loads as well as the traffic loads was similar in magnitude to the thermal induced rotations due to the difference in temperature between the top and bottom of the bridge.     

Long-Term Behavior of Continuous Precast Concrete Girder Bridge Model

Continuous concrete box girder bridges composed of precast reinforced and prestressed concrete beams with a U cross section and a cast-in-place top slab are frequently used for medium spans due to their competitiveness. The service behavior of such bridges is very much influenced by their segmental construction, due to time-dependent materials behavior that makes it difficult to accurately predict the stresses, strains, and deflections at long term. A 1:2 scale model of a two-span continuous bridge was tested in order to study its behavior during the construction process and under permanent loads. Time-dependent concrete properties, as well as support reactions, deflections, and strains in concrete and steel, were measured for 500 days. Important time-dependent redistributions of stresses and internal forces throughout the bridge were also measured. The test results were compared with analytical predictions obtained by means of a numerical model developed for the nonlinear and time-dependent analysis of segmentally erected, reinforced and prestressed concrete structures. Generally good agreement was obtained, showing the adequacy of the model to reproduce the structural effects of complex interactive time-dependent phenomena.     

Optimisation of pinch roll load in thin slab continuous casting

Abstract

An analytical model has been developed to investigate the optimum pinch roll load and resulting slab thickness reduction during the continuous casting of a steel, constant thickness, thin slab in a vertical type machine. The pinch roll unit consists of two driven pinch roll sets, which support the solidified hot slab weight and take up the reaction forces owing to slab bending. Optimum conditions prevail when the pinch roll load required to support the slab weight is equally shared by the two pinch roll sets and has the minimum value allowing proper control of the slab speed. The model has been validated by being applied to an existing industrial case of thin slab continuous casting of plain low carbon steel. The effects of pinch roll radius, slab thickness, casting speed, slab temperature and the material carbon percentage on minimum pinch roll load and slab thickness reduction ratio have been investigated. Expressions have been obtained for the optimum pinch roll load and slab thickness reduction ratio related to the prevailing process parameters, which the plant operator can directly apply to determine the optimum pressure setting for the hydraulic cylinders fitted to the movable roll ends. The bending roll load is shown to be small compared with the pinch roll load.     

Analysis of non-uniform thermal behaviour of special steel during slab continuous casting

A full-scale finite-element model of a slab and its mould was developed to reveal the complex thermal behaviour of special steel in a vertical caster during slab continuous casting. An inverse algorithm was applied to calculate the heat flux and combined with the temperatures measured using thermocouples that were buried in different positions of the mould. The model was validated by comparing the calculated temperature with the measured ones. The temperature distribution of the slab is not symmetric, reflecting the non-uniform nature of heat transfer and hence the necessity of formulating a full-scale model. It will provide a helpful tool for further improving the casting parameters and operations for special steel.     

宽厚板坯连铸结晶器流场、温度场及应力场的耦合数值模拟

利用Pro CAST软件对2400 mm×400 mm宽厚板坯结晶器建立三维动态模型,采用移动边界法实现结晶器内流场、温度场及应力场的耦合模拟.结果表明:考虑凝固坯壳的影响,下回流区位置向铸坯中心靠拢,真实反映了钢液在连铸结晶器内的流动情况.自由液面的钢液从窄面流向水口,速度先增大后减小,距水口约0.7 m处,出现最大表面流速,约为0.21 m·s^-1.结晶器出口坯壳窄面中心厚度最小且由中心向两侧逐渐增大,最小厚度约为10.4 mm;受流股冲击影响较弱的宽面坯壳与窄面相比生长更均匀,宽面偏角部和中心的坯壳厚度分别为18.9 mm和27.6 mm.铸坯坯壳应力变化趋势与温度基本保持一致,表明初凝坯壳应力主要是热应力.结晶器内铸坯宽窄面上的等效应力均沿着结晶器高度下降方向呈增大趋势,铸坯角部、宽面中心及窄面中心位置的最大应力各约为200、100和25 MPa.     

Model for Reinforced Concrete Members under Torsion, Bending, and Shear. I: Theory

A model has been developed that can predict the load-deformation response of a reinforced concrete (RC) member subjected to torsion combined with bending and shear to spalling or ultimate capacity. The model can also be used to create interaction surfaces to predict the failure of a member subjected to different ratios of applied torsion, bending, and shear. The model idealizes the sides of an reinforced concrete member as shear “wall panels.” The applied loads are distributed to the wall panels as uniform normal stresses and uniform shear stresses. The shear stress due to an applied torsional moment and shear force are summed over the thickness of the shear flow zone. Stress-strain relationships are adopted for tension stiffening and softened concrete in compression. The crack alignment rotates to remain normal to the principal tensile stress and the contribution of concrete in shear is neglected. The model has been validated by comparing the predicted and experimental behavior of members loaded under torsion combined with different ratios of bending and shear. The torque-twist behavior, reinforcement stress, and concrete surface strain predicted by the model were in agreement with experimental results.     

Plastic Rotation of an RCC T-Beam Bridge Girder under the Combined Influence of Flexure and Torsion

In the nonlinear analysis of a reinforced concrete T-beam bridge superstructure using grillage idealization, and rigid–plastic idealization of moment–curvature and torque–twist relationships of the resulting grillage members, it becomes necessary to compute the plastic rotation capacity of the resulting T-beam bridge girder due to limited ultimate strain capacity of concrete. An idea about the plastic rotation capacity of these members enables one to determine the true ultimate load carrying capacity of this type of structure, extent of redistribution of stresses at failure, and ductility of the structure. This paper presents analytical methods to determine the plastic rotation capacity of reinforced cement concrete T-beam bridge girder (or grillage member) under the combined influence of flexure and torsion. The methods have been validated by experiments. The analytical methods are based on skew-bending and space truss theories. The tests have been carried out on 1:6 microconcrete models. The salient conclusions have been enumerated.     

Modeling of Steel-Concrete Composite Beams under Negative Bending

Negative bending moments acting in the support regions of continuous composite beams generate tensile stresses in the concrete slab and compressive stresses in the lower steel profile. As a result the mechanical behavior of these beams is strongly nonlinear even for low stress levels, due not only to the slip at the beam-slab interface, but also to cracking in the slab. Therefore, an adequate theoretical modeling should take account of the interactions between the structural steel and the concrete slab by shear connectors and also between steel rebars and concrete in tension by bond phenomenon. In this paper a model of steel and concrete composite beams subjected to negative bending is presented. It accounts for the slip occurring at both the beam-slab interface and the steel reinforcement-concrete interface. Some numerical results, obtained using a suitable numerical procedure, are discussed to show the capacity of the model.     

Behavior of FRP Jacketed Concrete Columns under Eccentric Loading

This paper describes a study on the behavior of fiber-reinforced polymer (FRP) jacketed square concrete columns subjected to eccentric loading. The effect of strain gradient on the behavior of concrete columns confined by the FRP jacket was investigated through experimental and numerical analysis methods. Nine (108 × 108 × 305 mm) square concrete column stubs with zero, one, and two plies of unidirectional carbon FRP fabric were tested under axial compressive loading. In addition to the FRP jacket thickness, the effects of various eccentricities were examined. The nonlinear finite-element analysis results were compared and validated against the experimental test results. The results show that the FRP jacket can greatly enhance the strength and ductility of concrete columns under eccentric loading and that the strain gradient reduces the retrofit efficiency of the FRP jacket for concrete columns. Therefore, a smaller enhancement factor should be used in designing FRP-jacketed columns under eccentric loading. Furthermore, the nonlinear finite-element models established in this study can be used as templates for future research work on FRP-confined concrete columns.     

Thermomechanical finite-element model of shell behavior in continuous casting of steel

A coupled finite-element model, CON2D, has been developed to simulate temperature, stress, and shape development during the continuous casting of steel, both in and below the mold. The model simulates a transverse section of the strand in generalized plane strain as it moves down at the casting speed. It includes the effects of heat conduction, solidification, nonuniform superheat dissipation due to turbulent fluid flow, mutual dependence of the heat transfer and shrinkage on the size of the interfacial gap, the taper of the mold wall, and the thermal distortion of the mold. The stress model features an elastic-viscoplastic creep constitutive equation that accounts for the different responses of the liquid, semisolid, delta-ferrite, and austenite phases. Functions depending on temperature and composition are employed for properties such as thermal linear expansion. A contact algorithm is used to prevent penetration of the shell into the mold wall due to the internal liquid pressure. An efficient two-step algorithm is used to integrate these highly nonlinear equations. The model is validated with an analytical solution for both temperature and stress in a solidifying slab. It is applied to simulate continuous casting of a 120 mm billet and compares favorably with plant measurements of mold wall temperature, total heat removal, and shell thickness, including thinning of the corner. The model is ready to investigate issues in continuous casting such as mold taper optimization, minimum shell thickness to avoid breakouts, and maximum casting speed to avoid hot-tear crack formation due to submold bulging.     

Model Validation for Bridge-Road-Vehicle Dynamic Interaction System

The dynamic response of highway bridges subjected to moving truckloads has been observed to be dependent on (1) dynamic characteristics of the bridge; (2) truck configuration, speed, and lane position on the bridge; and (3) road surface roughness profile of the bridge and its approach. Historically, truckloads were measured to determine the load spectra for girder bridges. However, truckload measurements are either made for a short period of time [for example, weigh-in-motion (WIM) data] or are statistically biased (for example, weigh stations) and cost prohibitive. The objective of this paper is to present results of a 3D computer-based model for the simulation of multiple trucks on girder bridges. The model is based on the grillage approach and is applied to four steel girder bridges tested under normal truck traffic. Actual truckload data collected using a discrete bridge WIM system are used in the model. The data include axle loads, truck gross weight, axle configuration, and statistical data on multiple presence (side by side or following). The results are presented as a function of the static and dynamic stresses in each girder and compared with code provisions for dynamic load factor. The study provides an alternate method for the development of live-load models for bridge design and evaluation.     

Finite-element analysis of effects of the laser-processed bimaterial component size on stresses and distortion

Processing of bimaterial parts via a moving laser beam has been investigated using three-dimensional (3D) finite-element modeling. Effects of the size of parts on the temperature distribution, thermal transient stresses, residual stresses, and distortion have been evaluated. The result indicates that the size of the part to be processed has strong influence on the transient temperature, transient stresses, residual stresses, and distortion of the part. Distortion is small when the size of the part is small and can be predicted from the thermal-expansion mismatch between the two materials. However, when the size of the part becomes large, both distortion and residual stresses increase. Furthermore, both distortion and residual stresses, in this case, cannot be predicted based on the thermal-expansion mismatch alone. The distortion, in this case, is mainly determined by the asymmetrical plastic deformation driven by transient thermal stresses, while residual stresses are dictated by the thermal-expansion mismatch and the temperature gradient of the part before and during cooling.     

Thermal Buckling Analysis of SMA Fiber-Reinforced Composite Plates Using Layerwise Model

Thermal buckling analysis of laminated smart composite plates subjected to uniform temperature distribution has been presented. Shape memory alloy (SMA) fibers whose material properties depend on temperature have been used as a smart material. A three-dimensional layerwise plate model has been employed in developing the system equations using variational approach. Finite-element method has been adopted for discretization of the laminate. Lagrangian interpolation functions have been used to approximate the displacement components along the thickness as well as in the in-plane direction. The actual variation of prebuckling stresses has been accounted for in the derivation of the geometric stiffness matrix of the laminates. An incremental load technique has been used in the analysis to take into account the nonlinearity in the material properties of the SMA arising due to their temperature dependence. The effects of thickness ratio, orthotropic ratio, fiber orientation, aspect ratio, stacking sequence and various boundary conditions on the critical buckling temperature have been examined in details. The results have been validated with those available in the literature.