Finite element analysis of a concrete slab under various non-uniform support conditions

A two-dimensional finite element program was utilised to study the effects of a non-uniform subgrade support on the tensile stresses in a concrete slab. A total of 12 subgrade support conditions were created including uniform and non-uniform support. The modulus of subgrade reaction (k-value) for different support areas was arbitrarily assigned either a soft (50 psi/in., 13.5 MPa/m) or stiff (500 psi/in., 135 MPa/m) value. Three axle types (single, tandem and steer-drive), three linear temperature differentials (none, negative and positive), two loading paths (edge and wheel path) and multiple longitudinal wheel positions were considered in the analysis. The theoretical analyses showed that several non-uniform subgrade support cases had a profound impact on the peak tensile stresses in a slab. In particular, a stiff subgrade with soft edges and a subgrade with randomly assigned soft and stiff locations significantly increased the peak tensile stresses relative to a uniformly soft support condition by at least 30%.     

Impact of different approaches to modelling rigid pavement base layers on slab curling stresses

The magnitude of stresses induced in jointed concrete slabs due to thermal loads is influenced by the stiffness of the underlying foundation layers (base, sub-base and subgrade). The layer that most significantly affects the Portland cement concrete (PCC) slab responses is the base. Field observations have demonstrated the increase in reflecting cracking of PCC slabs placed over relatively stiff base layers. To predict thermo-mechanical responses of jointed PCC slabs accurately, appropriate idealisation of foundation layers in finite element (FE) analysis is required. Several modelling methods have been proposed to idealise the effect of the base layer. These methods differ in the structural contribution assigned to the base layer in the pavement concrete system. Four approaches for modelling the base layer in FE analysis of jointed concrete pavements are presented in this paper. The first and second approaches involve modelling the base as a plate separate from the other foundation layer(s). In the third and fourth approaches, the base layer is modelled as part of a Winkler or Vlasov foundation, respectively. A series of parametric studies are carried out to evaluate the capability and feasibility of each modelling approach proposed in this study in reflecting the effect of the base course rigidity on the PCC slab responses under thermal loads.     

温度与荷载耦合下农村公路混凝土路面板结构尺寸优化

为优化农村公路混凝土路面板结构设计以减小路面板厚度和节约成本,对板尺寸和板厚进行研究,分析正负温度梯度下交通荷载的最不利位置,建立文克尔地基上的板模型进行计算。研究结果表明:相比于板厚,板平面尺寸对板中最大拉应力的影响更大,因此可通过减小板尺寸来减小板厚;将只承受轮载作用的板称为小尺寸板,其临界板尺寸为2.0 m×2.0 m;路基反应模量对小尺寸路面板的板中最大拉应力影响不大;考虑疲劳损伤的小尺寸板在正、负温度梯度(15℃和-15℃)下的临界板厚分别为19 cm和16 cm。研究结果可以为节约成本为目的农村公路路面板结构尺寸优化设计提供参考。     

Simplified plastic settlement analysis of non-ballasted slab railroad track foundations on fine-grained soil

ABSTRACT

This study develops a simplified method to evaluate the plastic settlement of non-ballasted slab rail-road track foundations on fine-grained soil. Instead of three-dimensional finite element analysis, a two-dimensional plane strain model is utilized to deduce approximate deviatoric stress profiles for use in plastic strain calculation. From analysis examples of an embedded slab railroad track foundation, it is shown that the proposed method is computationally efficient, greatly reducing the computation load and time. This study also applies this method to perform a parametric study to investigate the design of prepared subgrade in reducing the plastic settlement. The results show that with the same prepared subgrade width as the foundation slab, the effect of the thickness or stiffness of the prepared subgrade on the reduction of settlement is limited; however, once the width of the prepared subgrade is larger than that of the slab, the effect of the prepared subgrade thickness or stiffness can be significantly enhanced.     

一类非均布荷载作用下厚壁圆筒平面问题的应力解析解

采用复变函数法给出了表面受有一类非均布径向压力作用下厚壁圆筒平面问题的应力解析解,发现随着非均布侧压系数的增大,圆筒内某些区域的切向应力和(或)径向应力会由压应力变为拉应力,并且切向拉应力远大于径向拉应力。在不存在拉应力区的前提下,反算出非均布侧压系数的范围,并给出了不同非均布侧压系数所对应的拉应力区。最后用数值方法验证了理论分析的正确性。     

混合结构房屋现浇楼板裂缝的有限元数值分析

现浇钢筋混凝土楼板开裂是混合结构房屋常遇到的问题之一。在对若干实际工程调查和模型试验的基础上,用有限元程序建立了两种结构形式的有限元模型,进行了非线性有限元分析。有限元分析结果包括混凝土主拉应力及应变、楼板裂缝和钢筋应力的分布和发展,其与试验结果符合程度较好。研究结论给出了楼板开裂的主要原因和影响因素、裂缝形态及范围、控制裂缝的关键措施和楼板钢筋合理的布置方式。     

水泥混凝土路面角隅应力分析

鉴于我国刚性、半刚性基层上水泥混凝土路面角隅断裂多发的状况,采用弹性地基上不等平面尺寸双层结构模型分析其产生的原因。面层角隅在单个矩形荷载作用下,讨论了半空间地基和文克勒(Winkler)地基路面结构应力、弯沉的异同,给出了按应力和弯沉等效时地基参数之间的换算关系;分析了基层超宽对面层和基层自身应力的影响,得到了基层超宽系数的近似回归式;探讨了面层角隅受荷和温度梯度共同作用下面层应力采用叠加方法和耦合方法计算的差异;初步给出了板底脱空(层间接触刚度弱化)、邻板接缝传荷对路面结构应力、弯沉影响的规律。     

Stress intensity factors evaluation for through-transverse crack in slab track system under vehicle dynamic load

The stress intensity factors (SIFs) for through-transverse crack in the China Railway Track System (CRTS II) slab track system under vehicle dynamic load are evaluated in this paper. A coupled dynamic model of a half-vehicle and the slab track is presented in which the half-vehicle is treated as a 18-degree-of-freedom multi-body system. The slab track is modeled as two continuous Bernoulli–Euler beams supported by a series of elastic rectangle plates on a viscoelastic foundation. The model is applied to calculate the vertical and lateral dynamic wheel–rail forces. A three-dimensional finite element model of the slab track system is then established in which the through-transverse crack at the bottom of concrete base is created by using extended finite element method (XFEM). The wheel–rail forces obtained by the vehicle-track dynamics calculation are utilized as the inputs to finite element model, and then the values of dynamic SIFs at the crack-tip are extracted from the XFEM solution by domain based interaction integral approach. The influences of subgrade modulus, crack length, crack angle, friction coefficient between cracked surfaces, and friction coefficient between faces of concrete base and subgrade on dynamic SIFs are investigated in detail. The analysis indicates that the subgrade modulus, crack length and crack angle have great effects on dynamic SIFs at the crack-tip, while both of the friction coefficients have negligible influences on variations of dynamic SIFs. Also the statistical characteristics of varying SIFs due to random wheel–rail forces are studied and results reveal that the distributions of dynamic SIFs follow an approximately Gaussian distribution with different mean values and standard deviations. The numerical results obtained are very useful in the maintenance of the slab track system.     

Prediction of stresses in concrete pavements subjected to non-linear gradients

This paper presents a technique for analyzing the residual stresses in concrete pavements subjected to non-linear (stress or strain) gradients throughout the slab thickness. The analysis is separated into two parts. In the first part, an expression is presented for calculating the self equilibrated stresses within a cross-section due to internal restraints (i.e. satisfying equilibrium conditions and continuity of the strain field within the cross-section). These stresses are independent of slab dimensions and boundary conditions. In step two, the stresses due to external restraints (i.e. self-weight and subgrade reaction) are calculated using an equivalent linear temperature gradient obtained from the first part and using existing closed form solutions by Westergaard [Westergaard, H. M., Computation of stresses in concrete roads. In Proc. of the 5th Annual Meeting, Vol. 5, Part I, Highway Research Board, 1926, pp. 90–112] or Bradbury [Bradbury, R. D., Reinforced Concrete Pavements. Wire Reinforcement Institute, Washington D.C., 1947.]. The solution to this step includes slab length and boundary conditions. Total internal stresses due to non-linear gradients are obtained using the superposition principle. The proposed method has been applied to field data from another study for varying temperature profiles within a 24 h period and compared to results from conventional analysis assuming linear gradients. Significant differences were found between the two methods for night-time and early-morning conditions. A linear gradient solution sometimes underestimates tension in the bottom of a slab prior to vehicle loading by a factor of three.     

基于高速铁路路基冻胀的轮轨动力响应研究

季节性冻土区高速铁路无砟轨道路基冻胀,影响了列车运行的安全性、舒适性以及无砟轨道主体结构的服役性能。为研究路基冻胀和高速行车荷载组合效应下的轮轨动力响应,建立了车辆-轨道-路基冻胀耦合动力学模型,对路基不同冻胀幅值、冻胀位置和行车速度下CRTSⅠ型板式无砟轨道轮轨动力响应及轨道结构受力进行分析。结果表明:冻胀发生区段轮轨动力响应增大,列车以350 km/h运行时的安全性和舒适性满足冻胀管理标准要求,但轮轨力随冻胀幅值和速度的增加而增大;轨道板和底座板振动加剧,在计算冻胀波长和幅值范围内,离缝处轨道板振动加速度峰值超过动态验收标准要求,容易引起离缝处CA砂浆层及路基基床表层伤损破坏,且轨道板、底座板振动加速度随行车速度增加而增大;轨道结构动应力和列车荷载传递关系密切,路基冻胀状态下列车荷载引起轨道板和底座板处于交替和交变的拉压受力状态,需要在设计中提出控制裂纹的措施,行车速度对短波冻胀时轨道结构受力影响较小。     

路基注浆对高速铁路轨道-路基体系动力特性的影响

翻浆冒泥是多雨地区铁路路基的常见病害,土颗粒流失后导致道床脱空,影响列车安全运行.高聚物注浆是修复路基翻浆冒泥的有效手段,但注浆对轨道-路基体系动力特性的影响有待进一步研究.通过全比尺无砟轨道路基模型试验重现了列车运行荷载下路基的翻浆冒泥现象,开展了路基注浆驱水和填充加固试验,测定了正常路基、路基翻浆冒泥及注浆修复后等...     

水泥混凝土路面脱空振动映象测试技术

脱空是水泥混凝土路面常见病害,是造成路面板断裂的主要原因。当混凝土面板在受到瞬态激振后,其振动频率主要与其脱空与否、脱空程度等因素有关,因此提出了基于振动频率的混凝土路面脱空振动映象检测技术。简述了振动映象测试原理。对混凝土路面板进行了模态分析和模型试验,获得了面板在不种脱空情况下的振动特性。在对路面板振动频率因子分析和路面瞬态振动信号频谱分析的基础上,发现路面板的振动频率对于脱空是很敏感的,从而建立了基于振动频率的混凝土路面脱空判别标准。研究表明：当路面板下严重脱空时,其振动基频值通常小于50 ;当路面板良好支承时,其振动基频值一般在200 以上;当有低于50 与高于200 的两个振动基频同时存在时,表明路面板下存在轻微脱空。     

Impact of fibre orientation on tensile,bending and shear behaviors of a steel fibre reinforced concrete

Fibre orientation and density are known to have a significant influence on steel fibre reinforced concretes (SFRC) mechanical properties. In practice, parameters such as fresh state properties, restriction to concrete flowability and placing methods are likely to induce different fibre orientations in characterisation specimens and structural components. This difference in fibre orientation can impact the mechanical behavior of the structural component and therefore provide an unsafe design if not considered. This project consisted to produce a large SFRC slab, extract specimens with different fibre orientations, and submit specimens to tensile, bending and shear tests to evaluate the impact of fibre orientation and density on mechanical and post-peak strengths. Test results have shown that tensile and bending behaviors are mainly influenced by the fibre orientation, while the shear behavior is mainly impacted by fibre density. Test results were processed to allow comparison between tensile and bending tests. Linear correlations between tensile residual stresses and fibre orientation where found, linear or power type correlations according to bending residual stresses, as well as linear correlations between shear stresses and fibre density.     

钢-混凝土组合梁剪力滞效应弹性解析解

基于Gjelsvik组合梁滑移模型中引入描述组合梁翼板横向非均匀分布的纵向位移翘曲函数,建立一个能够同时考虑界面滑移和翼板剪力滞双重效应的钢-混凝土组合梁模型,并推导了简支组合梁在均布荷载和悬臂梁在梁端集中荷载作用下的解析解。采用有限元方法验证了理论模型和解析解的正确性和适用性。算例分析表明,组合梁的剪滞效应和界面滑移...     

虚拟裂缝尖端拉应力大小的确定

针对单边切口的混凝土轴心受拉构件,基于虚拟裂缝模型提出一种计算极限承载力的解析模型,并在此基础上确立了虚拟裂缝尖端拉应力与混凝土轴心抗拉强度之间的关系。结果表明:二者的比值随初始缝高比的增大呈线性增加,但对混凝土强度等级的变化不敏感。其原因是由于所有的混凝土试件都存在初始缺陷,导致截面上存在明显的应力梯度,因而得到的混凝土轴心抗拉强度值是截面应力的平均值,而虚拟裂缝尖端拉应力为截面上的最大应力。很显然,轴心受拉构件的初始缺陷越长,截面的应力梯度越大,虚拟裂缝尖端拉应力与平均应力的比就越大。通常情况下,虚拟裂缝尖端的拉应力大小约为混凝土轴心抗拉强度值的1.22倍,约等于混凝土的抗折强度。     

应力状态下离心成型钢纤维混凝土耐硫酸腐蚀研究

对1组未受侵蚀和4组不同初始拉应力状态下经受侵蚀后的离心成型钢纤维混凝土环形截面试件进行了侧压力学试验,对经受侵蚀后的试件进行了化学分析研究.结果表明硫酸的腐蚀降低了构件的极限荷载和极限变形,降低了钢纤维混凝土的弹性模量,初始拉应力的增加使腐蚀破坏进一步加剧,受拉区的硫酸根含量和中性化深度随之增加.根据试验结果分析,推导了应力状态下离心成型钢纤维混凝土中性化深度与硫酸根含量的数学预测模型.     

Punching shear strength of interior concrete slab–column connections reinforced with steel fibers

A theoretical study was performed to investigate the punching shear strength of interior slab–column connections made of steel fiber reinforced concrete (FRC). In the steel FRC slab–column connection, the shear force applied to the critical section is resisted by both the compression zone and the tension zone at the critical section. The shear capacity of the compression zone was defined by considering the interaction between the shear and the normal stresses developed at the critical section. The shear capacity of the tension zone was defined by considering the post-cracking tensile strength of FRC. By using the shear capacity, a new strength model for the punching shear strength of steel FRC slab–column connections was developed. The proposed strength model was verified using existing test results and showed very good accuracy. For convenience in design, a simplified design equation was also developed.     

An acoustoelastic determination of the stress tensor in textured metal sheets using the birefringency of ultrasonic shear waves

A theory is formulated that enables one to determine two-dimensional applied stresses in sheets of slightly orthotropic material by means of ultrasonic shear waves To relate experimental acoustic data to a stress tensor we introduce an acoustoelastic tensork. Our experimental technique can measure times of flight and polarization angles within 1/3 nsec and 1°, respectively. In the case of the aluminium 2024-T351 alloy we used, calibration using tensile specimens leads to a nonisotropick tensor. Measurements of the complete (plane) stress tensor are carried out in the center of a compressed disc.     

预应力混凝土简支箱梁足尺模型试验及 非线性全过程分析

对一片30 m跨的预应力混凝土小箱梁和一片20 m跨的预应力混凝土空心板进行了足尺模型试验,破坏模式分别为预应力筋拉断和顶板混凝土压碎,按能量的观点计算的延性指标小箱梁和空心板分别为1.99和1.23。基于平截面假定,采用有限条带法编制了非线性计算程序,可对包括卸载过程在内的全过程受力性能进行分析。重点介绍了卸载曲线如何在应力-应变、弯矩-曲率及荷载-位移3个层面进行计算。通过程序,对预应力筋配筋量、预应力筋张拉系数对结构的承载力、变形性能及延性的影响进行了分析。     

THE EFFECT OF GRAIN SIZE, STRAIN AND TEMPERATURE ON THE MONOTONIC STRESS-STRAIN BEHAVIOUR OF POLYCRYSTALLINE ALUMINIUM AND AL ALLOYS

Abstract The tensile yield and flow stresses of aluminium, A1-2.63Mg alloy and A1-2.07Li alloy at room temperature are shown to depend on the inverse square root of the polycrystal grain size and are described empirically by the Hall-Petch relation. The same relation describes the flow stress-grain size dependence for A1-2.07Li alloy at temperatures ranging from - 196°C to 400°C. The strain hardening in the friction stress of each material at 20°C is independent of the grain size, is approximately parabolic and is greatest for the precipitation strengthened A1-2.07Li alloy. The grain size contribution to the tensile flow stress is dependent on both the tensile strain and composition. The friction stress, σ₀, and slip band stress intensity parameter, k_ε, at yield, k_y, are both dependent on temperature. Low temperature suppresses dislocation annihilation and recovery processes, leading to planar pile-ups at grain boundaries and a hardening that is linear with strain. Weak hardening is observed at 250°C and 400°C due to extensive annihilation and recovery. The value of k_ε, at all temperatures falls following initial yielding with the generation of freshly unlocked sources.