Auto-parametric resonance of a continuous-beam-bridge model under two-point periodic excitation:an experimental investigation and stability analysis

The auto-parametric resonance of a continuous-beam bridge model subjected to a two-point periodic excitation is experimentally and numerically investigated in this study.An auto-parametric resonance experiment of the test model is conducted to observe and measure the auto-parametric resonance of a continuous beam under a two-point excitation on columns.The parametric vibration equation is established for the test model using the finite-element method.The auto-parametric resonance stability of the structure is analyzed by using Newmark's method and the energy-growth exponent method.The effects of the phase difference of the two-point excitation on the stability boundaries of auto-parametric resonance are studied for the test model.Compared with the experiment,the numerical instability predictions of auto-parametric resonance are consistent with the test phenomena,and the numerical stability boundaries of auto-parametric resonance agree with the experimental ones.For a continuous beam bridge,when the ratio of multipoint excitation frequency(applied to the columns)to natural frequency of the continuous girder is approximately equal to 2,the continuous beam may undergo a strong auto-parametric resonance.Combined with the present experiment and analysis,a hypothesis of Volgograd Bridge's serpentine vibration is discussed.

Fluid Flow and Solidification Simulation in Beam Blank Continuous Casting Process With 3D Coupled Model

Based on turbulent theory, a 3D coupled model of fluid flow and solidification was built using finite difference method and used to study the influence of superheating degree and casting speed on fluid flow and solidification, analyze the interaction between shell and molten steel, and compare the temperature distribution under different technological conditions. The results indicate that high superheating degree can lengthen the liquid-core depth and make the crack and breakout possible, so suitable superheating should be controlled within 35℃ according to the simulation results. Casting speed which is one of the most important technological parameters of improving production rate, should be controlled between 0. 85 m/min and 1.05 m/min and the caster has great potential in the improvement of blank quality.

NUMERICAL SIMULATION OF THE THERMO-MECHANICAL PROCESS FOR BEAM BLANK CONTINUOUS CASTING

The aim of this study was to simulate the solidification process of beam blank continuous casting, and then find the reasons for the typical defects of the beam blank. A two-dimensional transient coupled finite element model has been developed to compute the temperature and stress profile in beam blank continuous casting. The enthalpy method was used in the heat conduction equation. The thermo-mechanical property in the mushy zone was taken into consideration in this calculation. It is shown that at the mold exit the thickness of the shell had its maximum value at the flange tip and its minimum value at the fillet. The temperature had a great fluctuation on the surface of the beam blank in the secondary cooling zone. At the unbending point, the surface temperature of the web was in the brittleness temperature range under the present condition. To ensure the quality, it is necessary to weaken the intensity of secondary cooling. At the mold exit the equivalent stress and strain have higher values at the flange tip and at the web. From the spray 1 to the unbending point, the maximum values of stress and strain gradually moved to the internal section of the flange tip and the web. However, whenever, there were bigger stress and strain values near the flange tip and the web than in the other parts, it must be very easy to generate cracks at those positions. Now, online verification of this simulation has been developed, which has proved to be very useful and efficient to instruct the practical production of beam blank continuous casting.

THEORETICAL MODEL ON INTERFACE FAILURE MECHANISM OF REINFORCED CONCRETE CONTINUOUS BEAM STRENGTHENED BY FRP

Fiber reinforced polymer （FRP） composites are increasingly being used for the re-pair and strengthening of deteriorated concrete structural components through adhesive bonding of prefabricated strips/plates and the wet lay-up of fabric. Interfacial bond failure modes have attracted the attention of researchers because of the importance. The objective of the present study is to analyse the interface failure mechanism of reinforced concrete continuous beam strength-ened by FRP. An analytical solution has been firstly presented to predict the entire debonding process of the model. The realistic bi-linear bond-slip interfacial law was adopted to study this problem. The crack propagation process of the loaded model was divided into four stages （elastic,elastic-softening,elastic-softening-debonded and softening-debonded stage）. Among them,elastic-softening-debonded stage has four sub-stages. The equations are solved by adding suitable stress and displacement boundary conditions. Finally,critical value of bond length is determined to make the failure mechanism in the paper effective by solving the simultaneously linear algebraic equations. The interaction between the upper and lower FRP plates can be neglected if axial stiffness ratio of the concrete-to-plate prism is large enough.

Structural Behavior of Continuous Prestressed Steel Fiber Reinforced High Strength Concrete Beam

The flexural behaviors of continuous fully and partially prestressed steel fiber reinforced high strength concrete beams are studied by experiment and nonlinear finite element analysis. Three levels of partial prestress ratio （PPR） are considered, and three pairs of two-span continuous beams with box sections varying in size are designed. The major parameters involved in the study include the PPR and the fiber location. It is concluded that the prestressed high strength concrete beam exhibits satisfactory ductility; the influences of steel fiber on the crack behaviors for partially prestressed beams are not as obvious as those for fully prestressed ones; steel fibers can improve the structural stiffness after cracking for fully prestressed high strength concrete beams; the moment redistribution from mid-span to intermediate support in the first stage should be mainly considered in practical design.

Impact coefficient and reliability of mid-span continuous beam bridge under action of extra heavy vehicle with low speed

To analyze the dynamic response and reliability of a continuous beam bridge under the action of an extra heavy vehicle, a vehicle–bridge coupled vibration model was established based on the virtual work principle and vehicle–bridge displacement compatibility equation, which can accurately simulate the dynamic characteristics of the vehicle and bridge. Results show that deck roughness has an important function in the effect of the vehicle on the bridge. When an extra heavy vehicle passes through the continuous beam bridge at a low speed of 5 km/h, the impact coefficient reaches a high value, which should not be disregarded in bridge safety assessments. Considering that no specific law exists between the impact coefficient and vehicle speed, vehicle speed should not be unduly limited and deck roughness repairing should be paid considerable attention. Deck roughness has a significant influence on the reliability index, which decreases as deck roughness increases. For the continuous beam bridge in this work, the reliability index of each control section is greater than the minimum reliability index. No reinforcement measures are required for over-sized transport.

EFFECT OF SUBMERGED ENTRY NOZZLE (SEN) PARAMETERS AND SHAPE ON 3-D FLUID FLOW IN MOULD FOR BEAM BLANK CONTINUOUS CASTING

According to turbulent theory and characteristics of beam blank continuous casting, 3-D model to represent the flow of beam blank mould is established. The predicted results indicate that the exit obliquity of up 15°(+15°) should be adopted, which will benefit the floatation of non-metallic inclusion and purification of the molten steel. When the nozzle angle is 120°, the flow pattern is reasonable. Proper nozzle depth can be 200mm. Turbulent kinetic of meniscus can be reduced by adopting the square nozzle and suitable area of side outlet when casting speed increases. The results are consistent with those of water model experiment, so the model is exact and reasonable. The model can provide important information for design of SEN and defining of immersion depth.

Analysis on plastic properties of reactive powder concrete continuous beams reinforced with GFRP bars

To study the plastic properties of reactive powder concrete continuous beams reinforced with GFRP bars,the calculation programs for moment redistribution coefficients are prepared by using nonlinear analysis methods such as moment-curvature,conjugate beam method and so on. By comparing the test results of existed FRP bars reinforced concrete continuous beams with simulation results,the accuracy of the calculation program is verified. Then 18 simulated GFRP bars reinforced reactive powder concrete continuous beams are selected whose change parameters are reinforcement ratio of mid-span and middle support. Through the nonlinear analysis of simulated beams,moment redistribution coefficients under mid-span concentrated loads,one-third point loads and uniformly distributed loads are obtained respectively. Thus the formula of moment redistribution coefficients is obtained by fitting moment redistribution coefficients and factors. The results show that the reactive powder concrete continuous beams reinforced with GFRP bars have good plastic properties.

Damage assessment of a continuous beam bridge based on the strain mode

Based on the method of strain mode, damage identification of continuous beam bridges by comparing the variance of several curves of strain modes difference is studied. Three cases of numerical simulation demonstrate that the proposed method is applicable to detecting many a damage in a continuous beam bridge, which accurately identifies the damaged positions of the bridge, and detects the damage severity of an element by its according peak value of the curve of strain modes difference that is found to increase with the increasing damage severity.

Longitudinal force in continuously welded rail on long-span tied arch continuous bridge carrying multiple tracks

Considering arch rib, lateral brace, suspender, girder, pier and track position, the model for the interaction between long-span tied arch continuous bridge and multiple tracks was established by using steel-concrete composite section beam element to simulate concrete-filled steel tube(CFST) arch rib, using the beam element with rigid arm to simulate the prestressed concrete girder and using nonlinear bar element to simulate longitudinal constraint between track and bridge. Taking a(77+3×156.8+77) m tied arch continuous bridge with four tracks on the Harbin-Qiqihar Passenger Dedicated Line as an example, the arrangement of continuously welded rail(CWR) was explored. The longitudinal force in CWR on the tied arch continuous bridge, the pier top horizontal force and torque due to the unbalance load case, were analyzed under the action of temperature, vertical live load, train braking and wind load.Studies show that, it can significantly reduce track displacement to set the track expansion devices at main span arch springing on both sides; the track stress due to arch temperature variation can reach 40.8 MPa; the track stress, pier top horizontal force and torque are related to the number of loaded tracks and train running direction, and the bending force applied to unloaded track is close to the loaded track, while the braking force applied to unloaded track is 1/4 to 1/2 of the loaded track; the longitudinal force of track due to the wind load is up to 12.4 MPa, which should be considered.

Longitudinal forces of continuously welded track on high-speed railway cable-stayed bridge considering impact of adjacent bridges

A proven beam-track contact model was used to analyze the track-structure interaction of CWR (continuously welded track) on bridge. Considering the impact of adjacent bridges, the tower-cable-track-beam-pier-pile finite element model of the cable-stayed bridge was established. Taking a bridge group including 40-32m simply-supported beam and (32+80+112)m single-tower cable-stayed bridge and 17-32m simply-supported beam on the Kunming-Shanghai high-speed railway as an example, the characteristics of CWR longitudinal force on the cable-stayed bridge were studied. It is shown that adjacent bridges must be considered in the calculation of the track expansion force and bending force on cable-stayed bridge. When the span amount of adjacent bridges is too numerous, it can be simplified as six spans; the fixed bearing of adjacent simply-supported beams should be placed on the side near the cable-stayed bridge; the track expansion device should be set at the bridge tower to reduce the track force near the bridge abutment.

Applicability of small resistance fastener on long-span continuous bridges of high-speed railway

Ballastless tracks have been widely applied in high-speed railway (HSR). The adaptability research between continuous welded rails (CWR) and long-span bridges of HSR is of great practical engineering significance. Based on the HSR long-span continuous bridges, the integrative spatial finite element model of track-bridge-pier-foundation system was established with the nonlinear spring element simulating the longitudinal resistance between track and bridge. Comparative study on the various additional longitudinal forces of CWR using the common fasteners and small resistance fasteners was carried out. Analysis results indicate that the additional expansion forces and additional rail-breaking forces in long-span ballastless continuous girders can be reduced evidently by 40% 50% after adopting small resistance fasteners, but lead to greater rail broken gap. The small resistance fasteners have little influence on the additional force only caused by vertical load, but can reduce the additional force caused by vertical load combined with braking load by over 10%. Besides, transient analysis method is proved to be more accurate and safe in calculating additional longitudinal forces when the train running or braking on the bridge, compared with the traditional static method.

基于主线不停运的城轨支线独立运营改造方案

研究目的:针对大型铁路枢纽接入已运营轨道交通线路引起的既有线改造问题,为满足铁路枢纽与轨道交通连接线(支线)施工期间主线正常运营不受影响,对改造涉及的运营组织与接轨方案进行系统研究,并对土建、轨道、接触网、环网与动照、信号、综合监控与通信系统等的改造及影响进行深入研究,在统筹考虑实施难度、投资与工期的基础上,提出实施风险最小、技术最合理的改造方案。研究结论:(1)既有高架车站与区间改造影响范围较小且相对可控,可以实现主线正常运营情况下新建独立运营支线工程;(2)为最大程度保障主线运营不受影响,在现有技术水平条件下,应尽量避免在运营天窗期进行各类信号光电缆的割接;(3)支线独立运营方案,应尽量采取措施将新建支线与主线相连、轨道相通,以利于车辆检修与维保,并增加运营的灵活性;(4)桥梁浇筑与平移施工临近既有线,实施阶段应进一步完善工艺衔接与交叉施工统筹,增加既有线安全保障;(5)本研究结论可为类似工程提供参考。

多跨连续梁自由振动的动态有限元分析

利用Timoshenko梁振动微分方程的奇次解,建立了单元任意截面的变形、内力与节点位移的关系,取端点截面的内力得到动态有限元列式。动态有限元具有不依赖网格密度、可准确计算任意阶次的振动频率和振型等特点。利用动态有限元刚度矩阵行列式为0条件,推导出1~4跨等截面等跨径的多跨连续梁频率方程。分析了多跨连续梁的自由振动,结果表明:相应简支梁的频率为多跨连续梁的固有频率、振型经扩展后为多跨连续梁的振型;多跨连续梁的振动有第二频谱现象和梁高振动特征。

高铁大跨连续梁不同约束体系下抗震性能研究

研究目的:为研究不同抗震约束体系对高速铁路大跨度连续梁桥纵向抗震性能的影响,以一座跨径为(60+100+60)m的高速铁路连续梁桥为工程背景,采用OpenSees软件对全桥进行非线性时程分析,针对不同的设防烈度,从动力特性、桥墩墩底弯矩、墩顶位移、支座位移等方面,对比分析延性体系、减隔震体系以及延性体系加减隔震体系多种工况下桥梁的地震响应差别。研究结论:(1)与延性体系相比,减隔震体系可显著延长桥梁结构的自振周期;(2)对于墩高20 m的高铁大跨连续梁按罕遇地震设防时,在Ⅶ度区及以上设防烈度时,桥墩会进入延性,且位移延性系数随着桥墩配筋率的增大而减小;(3)采用延性抗震体系的高铁大跨连续梁,可通过增大配筋率和设置阻尼器减轻桥墩的损伤,使其处于轻微损伤状态;(4)采用减隔震体系可保证高烈度地震区的高铁大跨连续梁桥的桥墩在罕遇地震下处于弹性工作状态,结构的减震率可超过80%;(5)摩擦摆支座体系减震效果好,但需要通过附加阻尼的形式来满足支座设计位移需要;(6)本研究成果可为高铁大跨连续梁桥的抗震设计提供参考。

基于绝对位移法的曲线梁桥多维多点弹塑性随机响应分析

针对当前地震动空间效应下曲线梁桥地震反应分析多采用确定性激励输入且忽略桥梁非线性的情况,采用了多维多点非平稳随机激励对曲线连续梁桥进行弹塑性响应分析。建立非线性有限元模型并降维解耦非平稳地震动非平稳演化功率谱(energy power spectral density,EPSD)矩阵,采用绝对位移法对桥梁进行非线性时程分析。考虑不同视波速、场地条件、相干性以及平稳与非平稳地震激励,综合分析了曲线连续梁桥的随机响应及其频域特性和时域特性。结果表明,地震动空间效应和地震动的非平稳性对曲线梁桥随机响应影响很大,其中地震动空间效应对桥梁随机响应大小及其频域分布有显著影响,而非平稳性会对随机响应大小及其时变响应趋势产生重要影响。因此,在曲线连续梁桥抗震分析中需充分考虑地震动空间效应和地震动非平稳性,以避免错误估计桥梁抗震性能。提供了全面的分析结果,对加强曲线连续梁桥的抗震设计和评估,从而提高其抗震性能和可靠性具有重要意义。

曲线条件大跨度拱加劲连续梁设计

贵南高铁二塘双线特大桥采用90m+180m+90m拱加劲连续梁结构。大桥位于曲线上,梁体采用分段直线布置,悬灌施工;主拱采用哑铃形钢管混凝土拱肋,钢管直径1 m,拱肋全高3 m;采用整束挤压钢绞线吊杆,全桥拱设置36组吊杆;纵向活动主墩通过构造措施,形成半刚构结构体系,限制部分梁体问题伸缩变形;大桥采用先悬灌主梁,后在主梁上搭设支架原位拼装拱肋方式施工。经论文研究计算结果表明,该桥刚度、受力等均满足规范要求。

钢-混凝土组合简支桥面连续结构横向应力分析

针对钢-混组合简支梁桥的桥面连续结构开裂等病害,围绕桥面连接板的横桥向应力问题,采用线弹性理论和板的偏微分方程进行分析,得出了桥面连接板挠度和应力的分布函数,建立非线性有限元模型,并进行实桥荷载试验.通过比较理论解、有限元解和实测试验结果,证实了理论解和有限元的有效性.根据得到的分布函数,发现横桥向和纵桥向上的最大拉应力出现在钢梁端部位置的连接板的上缘.此外,还分析了连接板区域尺寸变化对横桥向应力峰值的影响,包括纵梁端部距支座的长度、纵梁的间距以及连接板区域整体尺寸变化.结果表明:较小的纵梁间距和较长的纵梁端部与支撑之间的距离会导致连接板中的横向拉应力峰值增加,并提高横向拉应力在总应力中的占比,从而导致桥面连接板早于设计荷载开裂.因此对于纵梁间距较小、梁端长度较长的钢-混组合简支梁桥桥面连续结构,仅计算其纵桥向受力性能会导致计算结果偏危险,建议按照本文方法考虑横桥向应力对桥面连接板的影响.

连续梁桥跳车冲击下车桥耦合振动响应分析

为研究车辆在不同工况下发生跳车对变截面连续梁桥动力响应的影响,文章选用1/4车辆模型,采用D’Alembert原理建立车辆振动平衡方程;基于Euler-Bernoulli梁理论将变截面连续梁划分成多个微段并进行受力分析,建立桥梁振动平衡方程,采用模态坐标法考虑振型的正交性对方程进行简化,与车辆振动方程联立得到车桥耦合振动方程,最终理论推导出跳车冲击过程中的车桥耦合振动平衡方程;利用MATLAB自编程序求解车桥耦合振动方程,得出车桥耦合动力响应。研究表明:当跳车高度不断增加时,桥梁动力响应持续加重,位移最大值逐渐增加;当不同桥跨跨中发生跳车时,跳车跨位移响应最大,距离跳车跨越远,位移响应越小。