This paper presents an experimental study on the structural behavior of low yield point steel(LYP225)under both monotonic and cyclic loading under the triaxial stress states.A total of 14 tensile tests on four types of specimens with various initial geometries were conducted to establish monotonic mechanical properties of LYP225 steel under triaxial stress states,and 10 ultra-low cyclic tests(cycles＜100)were carried out to study the ultra-low cycle fatigue of LYP225 steel under various loading systems.A combination of both isotropic and kinematic hardening characteristics can be observed from the hysteresis curves of LYP225 steel.The A-F(Armstrong-Frederick)plasticity constitutive model of LYP225 steel including the characteristics of both isotropic and kinematic hardening was calibrated based on an optimization algorithm.The influences of stress triaxiality and Lode angle on fracture ductility of LYP225 steel were investigated.A fracture criterion with combination of stress triaxiality and Lode angle was proposed.An ultra-low cyclic fracture criterion,termed as CBW was established based on the accumulation of ductile damage and the attenuation of ductile damage threshold.
A self-centering concrete-filled square steel tubular frame with slit steel plate shear walls (SC-SSPSW) adopting post-tensioned (PT) column base was proposed in this study. The slit steel plate shear walls (SSPSWs) with edge stiffeners were only connected to the beams. Cycle tests were conducted on three 1/3 scaled single-span twostory specimens to investigate their mechanical and seismic behavior, and the formulas for beam and column internal forces considering frame expansion effects were derived. Finite element (FE) models of SC-SSPSWs were established, and the FE results agreed well with the test results. The results indicate that the hysteretic curves of specimens exhibit a typical double flag shape, and the residual deformation of all specimens is approximately 0.20% when loading to 2.0% story drift. Damage is primarily concentrated in the SSPSWs while other structural members remain elastic, and favorable resilience and energy dissipation capacity are achieved. With an increase in the width-to-thickness ratio or aspect ratio of links while keeping the thickness of SSPSWs unchanged, the energy dissipation decreases, and the self-centering capacity improves. The column that expands in the same direction as the loading direction bears more lateral loads. The theoretical gap opening value, PT tendons force, as well as beam axial force agree well with the test and FE results.
The combination of polyvinyl chloride(PVC)with corrosion resistance,carbon fiber(CFRP of lightweight and high-strength)and spiral reinforcement(with strong plastic-ity)can make full use of the characteristics of the materials,in order to provide constraints for concrete and effectively improve the mechanical properties of reinforced concrete.To study the seismic performance of concrete columns with spiral reinforcement embedded in CFRP-PVC tubes,low cycle repeated loading tests were completed on 9 specimens with va-rying parameters of spiral reinforcement diameter,spiral reinforcement spacing and shear span ratio.The results show that most of the specimens fail due to compression bending of CFRP-PVC composite tube,and the spiral reinforcement provides significant restraint in the compression area after reaching the ultimate load,which may improve the seismic perform-ance of concrete columns;The failure ductility of all specimens may range from 2.76 to 4.35,and the ultimate inter-story drift ratio is greater than 1/30,which meets the require-ments of the specification;The decrease of the diameter of spiral stirrups,the increase of the spacing of spiral reinforcements and the increase of the shear span ratio all reduce the shear capacity and ductility of the specimens,but they have little effect on the stiffness deg-radation.The refined finite element model built by ABAQUS software can accurately predict the bearing capacity and internal concrete cracking behavior of CFRP-PVC spiral reinforce-ment composite confined concrete columns under constant axial force and repeated horizontal force.The parameter analysis shows that the axial compression ratio and concrete strength significantly affect the bearing capacity and deformation capacity of the specimen,and the strength and diameter of the longitudinal reinforcement have a great impact on the bearing capacity and the stiffness of the specimen after its yielding.The relationship between the con-crete strength and the limit value of the axial compression ratio is proposed.The proposed calcu-lation method of the compression bending bearing capacity of the concrete columns has good ap-plicability for the cases with low axial compression ratio and ordinary strength grade concrete.
An eccentrically braced composite frame with a low-yield-point (LYP) steel shear link is an efficient energy dissipation system that exhibits good mechanical properties. However, existing experimental studies have not fully demonstrated the superiority and applicability of the structural system. We present a structural mechanics and finite element model analysis of an eccentrically braced composite frame with a vertical shear link. The effect of the design parameters on the seismic performance of the structure is analyzed. First, a theoretical model of the mechanics of the structural system is established to provide a comprehensive description of the key parameters. Then, a finite element model is developed using the computer program ABAQUS to analyze the mechanical and energy dissipation mechanisms. Finally, the beam-to-column stiffness ratio, shear link web thickness, shear link web width and length, and diagonal brace stiffness are analyzed to determine their effects on the mechanical properties of the structural system. Furthermore, some design parameter values are suggested.
This paper presents the results of field tests performed to investigate the installation effects of pre-bored grouted planted (PGP) pile in deep clayey soil. The variation of horizontal soil displacements, excess pore water pressures and lateral soil pressures was measured in the PGP pile installation process. The test results show that the drilling and grouting process induced large horizontal soil displacements in the soil within a radial distance of 2 d ( d is pile diameter), and the maximum horizontal soil displacements induced by the drilling and grouting process were smaller than 15.9 mm when the radial distance reached 4–5 d. Moreover, the horizontal soil displacements decreased along the soil layer depth, as the superficial soil layers were of small deformation modulus and lateral soil pressure. The drilling process brought large excess pore water pressures in the soil when the radial distance was less than 3 d, and the excess pore water pressures induced by the drilling stage were all less than 55 kPa when the radial distance reached 4–6 d. The drilling process also induced some lateral soil pressure increases in the soil within a radial distance of 3 d, while the measured maximum lateral soil pressure increases were smaller than 10.9 kPa when the radial distance increased to 4 d. On the whole, the excess pore water pressures and lateral soil pressure increases induced by the installation of PGP pile were much smaller than that induced by the installation of driven PHC pile. Moreover, the horizontal soil displacements, excess pore water pressures and lateral soil pressure increases induced by the installation of PGP pile all recovered rapidly after the installation of pile, as the cemented soil in the pile hole was in liquid state after the drilling and grouting stage.
The new frame ventilated anchor is an independently developed flexible support structure for frozen soil slope,which has a broad application prospect.In order to explore the cooling effect and mechanical effect of the new frame ventilation anchor,a multifunctional frozen soil laboratory box was designed,and it has the functions of loading,changing angle,adapting to the shaking table,and can simultaneously measure the temperature,moisture,wind speed and internal force.And a permafrost model slope reinforced by the frame ventilation bolts was erected to investigate the variation laws of temperature,moisture,wind speed and internal force of frame ventilation bolts subjected to freeze-thaw cycles.The closer to the anchor bolts,the more obvious the change of soil temperature and moisture.The frame ventilation anchor can absorb the cold energy and transmit and diffuse it along the axial and radial direction,with good cooling effect and maintaining the frozen state of frozen soil slope.The wind speed change law in the anchor bolt is consistent with the change of the external wind speed.A large external wind speed can motivate a more remarkable cooling effect of the frame ventilation anchor bolt.In a freeze-thaw cycle,the axial force of the anchor bolt changes in a parabola,and the axial force in the freezing period is greater than that in the thawing period.And the internal force of frame in freezing period is 2-3 times that in thawing period.The results can provide guidance for the design and engineering application of the new frame ventilation anchor.
Sea-crossing cable-stayed bridges may encounter the combined effects of earthquakes, waves, and currents, seriously threatening their service safety. In this paper, the effects of the combined earthquake and wave action on the dynamic responses of a sea-crossing cable-stayed bridge are investigated through underwater shaking table tests. The coordinative similitude law is adopted to fulfil the similarity of hydrodynamic and wave force. The test results indicate that, the combined action of earthquake and wave has significant effects on the dynamic response of the bridge tower, while the effects vary with the height of the tower and the type of ground motion. The side pier exhibits greater dynamic responses at the same loading cases and is more susceptible to the combined earthquake and wave action than the auxiliary pier. On the whole, in comparison to earthquake excitation alone, the combined earthquake and wave action significantly enhances the dynamic responses of sea-crossing cable-stayed bridge in most cases, with a maximum magnitude up to 20%–30%. By comparing with previously conducted underwater shaking table tests of substructures, the effects of water-structure interaction on the full cable-stayed bridge and bridge tower foundation are close, both of which are obviously lower than the effects on bridge pier.
To describe the nonlinear mechanical behavior of the metallic dampers more accurately, this paper developed a novel restoring force model incorporating performance degradation. The force-deformation relationship of the restoring force model was divided into two stages: small and large deformation stages. During the small deformation stage, the combined hardening model was used to predict the restoring force of dampers as plastic deformation increased. The kinematic and isotropic hardening variables were adopted to reflect the Bauschinger effect and cyclic hardening of metallic dampers, respectively. When accumulative plastic deformation exceeded a specific value, the restoring force model entered the large deformation stage. During this stage, exponential and sigmoid functions were adopted to modify the combined hardening model to reflect the stiffness and strength degradation due to the significant out-of-plane deformation and fracture of metallic dampers, respectively Furthermore, test results from literature on slit dampers were compared with the simulation results from the proposed model and other commonly used models. The results indicate that the proposed model can accurately predict the nonlinear behavior of metallic dampers across the entire deformation range.
The wave impeding block(WIB)is commonly used to control vibration pollution caused by vibration sources such as power machines,rail transit,and construction.However,the WIB is limited by the cut-off frequency of soil layer,resulting in a narrow vibration isolation frequency band,making targeted vibration isolation in a specific frequency range difficult.Based on the principle of phononic crystal,a three-dimensional periodic structural wave impeding block(PSWIB)is proposed.The band gaps of the three-dimensional PSWIB with cube and sphere scatters are calculated using the COMSOL finite element method.The effects of structural and material parameters on band gap characteristics are discussed,and orthogonal test optimization design is conducted.Both theoretical and numerical calculation show that the attenuation zone obtained by the three-dimensional finite periodic structure wave impeding block is consistent with the band gap range of the infinite periodic wave impeding block,which has band gap.The maximum amplitude attenuation in the vibration attenuation zone can reach 54 dB.Compared to traditional wave impeding block,the three-dimensional periodic structure wave impeding block broadens the vibration isolation frequency band,overcomes the cut-off frequency limitation,and allows for the design of material parameters based on the vibration source characteristics to meet target frequency isolation requirements.
To study the fracture propagation mechanism driven by expansible polymer grouting materials in soil, considering the characteristics of expansible polymer materials, a 2D elastoplastic model is established based on the extended finite element method (XFEM). In this method, the initiation location of fractures is judged by the maximum tensile stress criterion, and the propagation of fractures is controlled by fracture criteria applicable to the soil. The pressure on the fracture surface is solved iteratively by an experimentally derived relationship between expansion pressure and density of polymer grout. The correctness of the method is confirmed by contrasting the previous experimental results. On this basis, numerical computations with different grouting quantities are simulated to present the polymer grout's variation characteristics, including length, width, expansion pressure, and density. The method can reflect the interaction between the polymer and soil, which lays the foundation for the further study of the fracture propagation mechanism of expansive polymer grouting materials in soil.
Excess pore water pressure (EPWP) induced by shield tunneling has a significant influence on the stability of the tunnel face, post-construction settlement, and the mechanical behavior of the tunnel lining. However, the three-dimensional and unsaturated property of the soil field is seldom considered in current research. Considering the non-uniform radial convergence model, a modified three-dimensional displacement solution induced by shield tunneling was established first. Then based on unsaturated EPWP elastic theory, a reliable and efficient method is developed to expedite the evaluation of EPWP distribution in three-dimensional saturated and unsaturated clay soil. The validity of the method is confirmed through comparison with field test and numerical outcomes. The analysis examples demonstrate that negative and positive EPWP are generated above the tunnel crown and beneath the tunnel invert, respectively. In the vertical direction, the negative EPWP exhibits a decreasing trend ahead of the heading face and an increasing trend behind it. Along the longitudinal direction, the influence zone of EPWP extends to 1D ahead of and 6D behind the heading face. With the decrease of soil saturation, the EPWP values tend to diminish. The maximum EPWP values observed in saturated conditions can be 16.13 times higher than those under unsaturated conditions.
Wind-induced vibration has become increasingly prominent for long-span bridges. Flutter instability, as a type of divergent vibration, is a key component in the wind resistance investigation of long-span bridges. Based on the principle of instantaneous power balance (IPB) of the flutter critical state of bridge girders, an algorithm for predicting the flutter critical wind speed of long-span bridges was proposed by utilizing a nonlinear optimization strategy. For the bending-torsional coupling two-dimensional (2-D) motion system, the contribution of the wind-induced self-excited and structural elastic forces of the 2-D bridge section to the energy of the system was revealed to be dependent on some key parameters, such as reduced frequency, wind speed, amplitude ratio, and phase lag between vertical and torsional motions. Therefore, according to the principle of IPB during the critical flutter state, the prediction of flutter onset wind velocity can be transformed into an extreme value optimization problem of the IPB objective function. The feasibility and accuracy of the IPB algorithm were verified by comparing them with those obtained from segmental model wind tunnel tests and previous 2-D flutter prediction algorithms. Compared with the traditional methods characterized by force balance, the proposed method clearly and quantitatively presents the contribution relationship among multiple self-excited aerodynamic components on the flat plate and bridge section while flutter occurs; the method first evaluates the flutter critical state as an alternative algorithm from an energy perspective.
This paper explores modified strategies for the Modified Metropolis-Hastings (MMH) algorithm in the subset simulation (SS) for structural reliability assessment in ocean engineering. To improve sampling efficiency in complex distributions comprising correlated or non-normal variables, this study proposes a modified approach involving a two-stage delayed rejection and an adaptive standard deviation (STD) for the proposal distribution based on MMH. The acceptance rate of candidate samples in two-stage delayed rejection approach is derived based on the reversibility condition of Markov chain to reduce the repeated samples. Additionally, the STD for all accepted samples is used as the STD for the normal proposal, which dominates the sampling scale and increases the acceptance rate. The Neal's normal distribution, the Banana-shaped bivariate distribution, and the correlated joint distribution for wind and wave are adopted to study the sampling efficiency and ergodicity for the MMH with delayed rejection (MMHDR), the adaptive STD for MMH with delayed rejection (AMMHDR), and the adaptive STD for MMH with two-stage delayed rejection (AMMHDDR). Furthermore, three sampling algorithms are employed to generate conditional samples for estimating the probabilities of base shear failure and system failure in jacket platforms. The results indicate that the AMMHDDR can enhance sampling efficiency, especially for complex distributions with correlated variables. Also, the AMMHDDR can be used in SS to improve the accuracy and reduce the variation when estimating failure probabilities of offshore structures.
Bridge towers are important load-bearing components in cable bridges, especially multi-tower cable-stayed bridges with high requirements on the stiffness and strength of the middle tower.The concrete-filled steel-plate composite（referred to as CSC） bridge tower is a new choice of bridge tower structure, which is of great significance for solving key engineering problems such as insufficient stiffness of the middle tower in the multi-tower cable-stayed bridge.This study summarizes the research work of composite structure research team of Tsinghua University with respect to the development and application the CSC bridge tower from interface connection level, tower wall level to tower level.Based on model tests of connectors and tower walls, the force mechanism, structure behaviors, and design method of the proposed CSC tower structure and its constructions were investigated.Furthermore, a multi-scale model for nonlinear time-history analysis of the multi-tower cable-stayed bridge with composite towers was established.The CSC tower has been successfully applied to Nanjing No.5 Bridge, and the seismic performance of its tower was investigated as an example.The applications of composite bridge tower were investigated and summarized.Composite structure is mostly used as partial structure in tower or is used in small and medium-span bridges in the form of concrete-filled steel tube.Research on large-section tower of large-span cable-supported bridge is still absent.The CSC structure and the connectors that ensure the compatible functioning of composite tower are investigated and analyzed.However, the existing results from small-scale composite structure tests and the research of conventional connectors are not applicable to composite bridge towers.Hence, further research on the force mechanism of the composite bridge tower is still necessary.The CSC bridge tower and thin-rib perfobond connectors were proposed, and then its force mechanism and design theory were studied by experiments and theoretical analysis.Finally, based on the research results and design process of Nanjing No.5 Bridge, the design methods for the CSC bridge tower were developed.Compared with the reinforced concrete bridge tower and steel bridge tower, the CSC bridge tower exhibits its advantages in the mechanical performance, economic performance and construction convenience.Both research and practice suggested that the proposed CSC tower system has significant advantage of performance and satisfying economic benefits, resulting in new ideas and options for the large-span cable-supported bridge structure and greatly promoting the application of steel-concrete composite structures to bridge engineering.
Thin plates are analogized to gridwork systems, and the compatibility and incompatibility between these two structures in static and dynamic analyses are discussed.It is proved that the plates with simply supported and/or clamped boundaries are compatible problems, for which the gridwork results converge to the theoretical solutions of plates as the grids being refined, whereas the plates with free boundaries do not have such convergence property.By employing the 3D Structural Mechanics Solver as the solution tool, the requisite algorithms and a wide range of numerical examples, including rectangular, triangular, and circular plates, are provided to verify the validity of the proposed theory and algorithms and show the great performance of the 3D Solver for such extended functions.
以往的研究表明桩-土相互作用和水-桥墩相互作用均会显著影响桥墩的地震响应。本文通过离心机振动台试验研究了同时考虑桩-土相互作用和水-桥墩相互作用的桩基桥墩地震响应特征，以1:50为比例缩尺设计了试验桩基桥墩模型，模型安装在层状剪切箱中，土层为200mm厚度软土，土层上部的水深为300mm。试验考虑了小震、中震和大震三种强度地震激励，获得了模型的位移、弯矩、动土压力和动水压力试验数据。通过试验结果分析表明，桥墩的位移增长率随地震强度的增大而减小；桥墩和桩基的最大弯矩分别在墩底和桩顶位置，且桥墩水下部分沿墩身的弯矩增长率明显大于水上部分；动土压力在桩中和桩底较大，在桩顶较小；动水压力在桥墩上部变化较小，在桥墩底部和承台明显增大。
针对目前扁平钢箱梁日照作用下温度场的研究方法集中在数值模拟和长期实桥测试，尚未采用解析法研究的现状，根据热传导原理，建立带铺装层的扁平钢箱梁顶板与腹板的温度场模型，利用积分变换法求得二者温度场的解析解。以某独塔斜拉桥为例，重点对比分析了顶板温度场实测值与解析解中太阳辐射强度分别采用实测法、Hottle法、幂指数法获得时的计算值，并根据太阳辐射强度采用实测法获得的解析解，进一步探究了桥面防撞护栏与钢箱梁断面形式对顶板温度分布的影响。结果表明：在实测法获得的太阳辐射强度下，解析解计算结果与实测值最贴近，其计算值与实测值最大偏差不超过3℃，两者平均绝对误差最大不超过1.3℃；而在幂指数法与Hottle法中，太阳辐射强度采用幂指数法获得时，解析解计算准确度要更高；日照下，桥面防撞护栏遮挡引起的扁平钢箱梁顶板不均匀温度分布主要发生在该护栏附近的顶板位置，且引起的温度不均匀程度与太阳辐射强度呈正相关；PK断面钢箱梁与封闭断面钢箱梁顶板温度分布差异体现在是否开口位置，钢箱梁封闭箱室的保温效果导致箱内外环境温度不同是造成该差异的主要原因。
为研究多模块高温气冷堆核岛厂房基底隔震结构的抗震性能，设计了缩尺比为1/20的核岛厂房振动台试验模型，分别进行了抗震、摩擦摆支座隔震、橡胶支座隔震三种工况下的振动台试验，对比分析了不同工况下核岛厂房隔震结构的动力响应规律，包括结构动力特性、加速度和位移响应、楼层反应谱等。试验结果表明：采用隔震措施后，结构自振周期明显延长，隔震效果显著；三向地震动输入时，隔震上部结构的加速度放大系数在四层以上会突然增大，这是由于结构中部T形墙高度仅至四五层之间，结构在此处被削弱；结构整体刚度较大，抗震结构和隔震后上部结构的相对位移均较小，基本处于平动；隔震措施能明显减小核岛厂房结构在其自振频率处的水平向加速度反应谱峰值，而在隔震频率处隔震模型加速度反应谱值有所增加；在三向地震动输入下，隔震模型的竖向楼层加速度谱较抗震结构的竖向加速度谱有明显放大。
西安地裂缝是一种典型的城市地质灾害,对供水管道造成严重危害.以西安市球墨铸铁供水管道穿越地裂缝为工程背景,通过供水管道45°斜穿地裂缝足尺模型试验,研究地裂缝作用下供水管道的变性特征、结构应力与土压力变化规律.试验结果表明,在地裂缝作用下,上盘管道产生差异沉降,远离地裂缝管道末端沉降量最大,上盘管道顶部和底部部分区域形成脱离区或脱空区.管道表现出复杂的伸臂梁受力模式,竖向上上盘管道产生"上凸下凹"的弯曲变形,顶部形成受拉区,底部形成受压区;下盘管道产生"上凹下凸"的弯曲变形,导致其顶部形成受压区,底部形成受拉区;水平方向上,地裂缝与管道锐角侧形成受拉区,钝角侧形成受压区.管道柔性T型接口可一定程度调节管道两侧差异沉降,且在管道接口处轴向应力最大.基于模型试验结果,提出地裂缝发育区管道建设应合理规划线路、浅埋或架空布设、采用柔性接口并增加接口强度和刚度、布设监测装置的工程措施.
现行水工混凝土规范对高水头闸门门槽的受剪承载力复核提出了要求,但尚缺乏对应的计算方法.为此,文章针对检修闸门门槽的受剪承载力展开了试验和数值研究.以拉西瓦水电站进水口检修闸门门槽原型为对象,设计了 2组13个缩尺检修闸门门槽试件的静力试验,研究剪跨比、混凝土强度与横向配筋率对检修闸门门槽破坏模式和极限受剪承载力的影响.通过三维有限元数值仿真,补充说明对应参数的影响规律.最后基于试验结果通过回归分析提出计算检修闸门门槽的受剪承载力计算公式,并给出算例进行应用展示.研究结果表明检修闸门门槽的受剪破坏机理为斜压破坏,混凝土的抗剪贡献占主导地位,受剪承载力提升幅度和混凝土轴心抗压强度增幅相同.尽管提高配筋率可提升受剪承载力,但该方法经济性差.检修门门槽的名义剪压比在剪跨比0.16～0.22随剪跨比的增加而近似线性增加,但在剪跨比0.22～0.28基本保持不变.
