Mechanical Behavior of Rectangular Steel-Reinforced ECC/Concrete Composite Column under Eccentric Compression

In order to improve the seismic performance, deformation ability and ultimate load-carrying capacity of columns with rectangular cross section, engineered cementitious composite （ECC） is introduced to partially substitute concrete in the edge zone of reinforced concrete columns and form reinforced ECC/concrete composite columns. Firstly, based on the assumption of plane remaining plane and the simplified constitutive models, the calculation method of the load-carrying capacity of reinforced ECC/concrete columns is proposed. The stress and strain distribu- tions and crack propagation of the composite columns in different states of eccentric compressive loading are ana- lyzed. Then, nonlinear finite element analysis is conducted to study the mechanical performance of reinforced ECC/concrete composite columns with rectangular cross section. It is found that the simulation results are in good agreement with the theoretical results, indicating that the proposed method for calculating the load-carrying capacity of concrete/ECC composite columns is valid. Finally, based on the proposed method, the effects of ECC thickness, com- pressive strength of concrete and longitudinal reinforcement ratio on the mechanical performance of reinforced ECC/ concrete composite columns are analyzed. Calculation results indicate that increasing the thickness of ECC layer or longitudinal reinforcement ratio can effectively increase the ultimate load-carrying capacity of the composite column with both small and large eccentricity, but increasing the strength of concrete can only increase the ultimate Ioad- carrying capacity of the composite column with small eccentricity.

Experimental investigation of p-section concrete beams under contact explosion and close-in explosion conditions

This paper presents the results of an experimental investigation on explosive breaching of p-section concrete beams. Twenty three p-section concrete beams with a 100 cm length were tested. TNT charges were placed at three positions: contact detonation in the center, contact detonation above the web and close-in detonation in the center. The external and internal breach parameters of the panels were evaluated by measuring the diameter of the ejection crater, spalling crater and breach hole created by the charge detonation. The experimental results were compared to predict values obtained by the analytical models proposed by McVay, Morishita and Remennikov. A modified breach with crater limit line and breach without crater limit line were put forward based on the experimental results. The maximum cross-sectional destruction area ratio(MCDAR) values were used to evaluate the damage degree. The maximum value of MCDAR reached 0.331 corresponding to the C5 experimental condition, of which explosion occurred above the web.

Weakening effect of vertical prestress ducts on web sections in prestressed concrete box girder bridges

Based on experimental study result of two simply supported beams,which were prestressed by vertical tendons,the research on the weakening effect of vertical prestress ducts on web sections of prestressed concrete box girder was carried out. The test result shows that in the condition without grouting into the ducts,the cracking load is evidently lower than that with full grouting,and stirrup stress and principal tension stress at the surface of concrete will increase obviously. Meanwhile,the finite element analysis has been done to the test beams. The research is consistent with the analysis of finite element. The research tells that strengthening the detection and management for the vertical prestress grouting quality have an important meaning to prevent cracking of the prestressed concrete box girder bridge and provide one theoretical and testing basis for analyzing the cracking reason of such bridges.

Mechanical properties and influence mechanism of confined concrete arches in high-stress tunnels

Deep underground projects(e.g., coal mines), are often faced with complex conditions such as high stress and extremely soft rock. The strength and rigidity of the traditional support system are often insufficient,which makes it difficult to meet the requirements of ground control under complex conditions. As a new support form with high strength and rigidity, the confined concrete arch plays an important role in controlling the rock deformation under complex conditions. The section shape of the tunnel has an important impact on the mechanical properties and design of the support system. However, studies on the mechanical properties and influence mechanism of the new confined concrete arch are rarely reported. To this end, the mechanical properties of traditional U-shaped steel and new confined concrete arches are compared and comparative tests on arches of circular and straight-leg semicircular shapes in deep tunnels are conducted. A large mechanical testing system for underground engineering support structure is developed. The mechanical properties and influence mechanism of confined concrete arches with different section shapes under different loading modes and cross-section parameters are systematically studied. Test results show that the bearing capacity of the confined concrete arch is 2.10 times that of the U-shaped steel arch, and the bearing capacity of the circular confined concrete arch is 2.27 times that of the straight-leg semicircular arch. Among the various influencing factors and their engineering parameters,the lateral stress coefficient has the greatest impact on the bearing capacity of the confined concrete arch,followed by the steel pipe wall thickness, steel strength, and core concrete strength. Subsequently, the economic index of bearing capacity and cost is established, and the optimization design method for the confined concrete arch is proposed. Finally, this design method is applied to a high-stress tunnel under complex conditions, and the deformation of the surrounding rock is effectively controlled.

Crack elongation and its width of large depth reinforced concrete beams

In order to meet the requirement of structural inspection,the crack spacing and crack width at various heights in the tensile zone of six large depth reinforced concrete beams were measured under several loading levels of serviceability state.The effects of the depth of normal section beams on the crack spacing and crack width were analyzed,and the modified model is proposed for calculating the average crack spacing by thinking about the depth of normal section,the reinforcement arrangement and the effective reinforcement ratio.The relationships of crack widths at any position in the tensile zone and at the reinforcement level on the side surface of beam were studied.By theoretical and statistical analysis,a method is proposed to calculate the ratios of crack widths between any position and the reinforcement level on the side surface of large depth reinforced concrete beams.

Concrete-Filled Steel Tube Arch Bridges in China

In the past 20 years, great progress has been achieved in China in the construction of concrete-filled steel tube （CFST） arch bridges and concrete arch bridges with a CFST skeleton. The span of these bridges has been increasing rapidly, which is rare in the history of bridge development. The large-scale construction of expressways and high-speed railways demands the development of long-span arch bridges, and advances in design and construction techniques have made it possible to construct such bridges. In the present study, the current status, development, and major innovative technologies of CFST arch bridges and concrete arch bridges with a CFST skeleton in China are elaborated. This paper covers the key con- struction technologies of CFST arch bridges, such as the design, manufacture, and installation of steel tube arch trusses, the preparation and pouring of in-tube concrete, and the construction of the world＇s longest CFST arch bridge-the First Hejiang Yangtze River Bridge. The main construction technologies of rein- forced concrete arch bridges are also presented, which include cable-stayed fastening-hanging cantilever assembly, adjusting the load by means of stay cables, surrounding the concrete for arch rib pouring, and so forth. In addition, the construction of two CFST skeleton concrete arch bridges-the Guangxi Yongning Yong River Bridge and the Yunnan-Guangxi Railway Nanpan River Bridge--is discussed. CFST arch bridges in China have already gained a world-leading position; with the continuous innovation of key technologies, China will become the new leader in promoting the development of arch bridges.

Experimental research on mechanical properties of prestressed truss concrete composite beam encased with circular steel tube

Tests of 4 simply supported unbonded prestressed truss concrete composite beams encased with circular steel tube were carried out. It is found that the ratio of the stress increment of the unbonded tendon to that of the tensile steel tube is 0.252 during the using stage,and the average crack space of beams depends on the ratio of the sum of the bottom chord steel tube's outside diameter and the secondary bottom chord steel tube's section area to the effective tensile concrete area. The coefficient of uneven crack distribution is 1.68 and the formula for the calculation of crack width is established. Test results indicate that the ultimate stress increment of unbonded tendon in the beams decreases in linearity with the increase of the composite reinforcement index β0. The pure bending region of beams accords with the plane section assumption from loading to failure. The calculation formula of ultimate stress increment of the unbonded tendon and the method to calculate the bearing capacity of normal section of beams have been presented. Besides,the method to calculate the stiffness of this sort of beams is brought forward as well.

Progressive collapse resisting capacity of reinforced concrete load bearing wall structures

Reinforced concrete(RC) load bearing wall is widely used in high-rise and mid-rise buildings. Due to the number of walls in plan and reduction in lateral force portion, this system is not only stronger against earthquakes, but also more economical. The effect of progressive collapse caused by removal of load bearing elements, in various positions in plan and stories of the RC load bearing wall system was evaluated by nonlinear dynamic and static analyses. For this purpose, three-dimensional model of 10-story structure was selected. The analysis results indicated stability, strength and stiffness of the RC load-bearing wall system against progressive collapse. It was observed that the most critical condition for removal of load bearing walls was the instantaneous removal of the surrounding walls located at the corners of the building where the sections of the load bearing elements were changed. In this case, the maximum vertical displacement was limited to 6.3 mm and the structure failed after applying the load of 10 times the axial load bored by removed elements. Comparison between the results of the nonlinear dynamic and static analyses demonstrated that the "load factor" parameter was a reasonable criterion to evaluate the progressive collapse potential of the structure.

Identification of Connection Flexibility Effects Based on Load Testing of a Steel-Concrete Bridge

In the case of composite girders, an effective cooperation of both parts of the section is influenced by deformability of connectors. Limited flexural stiffness of welded studs, used commonly in bridge structures, does not provide full interaction of a steel beam and a concrete slab. This changes strain distribution in cross-sections of a composite girder and results in redistribution of internal forces in steel and concrete element. In the paper partial interaction index defined on the basis of a neutral axis position, which can be used for verification of steel-concrete interaction in real bridge structures rather than in specimens is proposed. The range of the index value changes, obtained during load testing of a typical steel-concrete composite beam bridge, is presented. The investigation was carried out on a motorway viaduct, consisting of two parallel structures. During the testing values of strains in girders under static and quasi-static loads were measured. The readings from the gauges were used to determine the index, characterizing composite action of the girders. Results of bridge testing under movable load, changing position along the bridge span is presented and obtained in-situ influence functions of strains and index values are commented in the paper.

Ultimate flexural strength of normal section of FRP-confined RC circular columns

Numerical analysis is carried out to study the sectional properties of the fiber-reinforced polymer（FRP）-confined reinforced concrete（RC）circular columns. The axial load ratio, the FRP confinement ratio and the longitudinal reinforcement characteristic value are the three main parameters that can influence the neutral axis depth when concrete compression strain reaches an ultimate value. The formula for computing the central angle θ, corresponding to the compression zone, is established according to the data regression of the numerical analysis results. The numerical analysis results demonstrate that the concrete stress enhancement from transverse confinement and strain hardening of the longitudinal reinforcement can cause a much greater flexural strength than that defined by the design code. Based on the analytical studies and the test results of 36 large scale columns, the formula to calculate the flexural strength when columns fail under seismic loading is proposed, and the calculated results agree well with the test results. Finally, parametric studies are conducted on a typical column with different axial load ratios, longitudinal reinforcement characteristic value and FRP confinement ratios. Analysis of the results shows that the calculated flexural strength can be increased by 50% compared to that of unconfined columns defined by the code.

混凝土实体重力坝断面拓扑与形貌优化研究

为了获得较为经济的混凝土实体重力坝断面形式,提出了基于拓扑与形貌优化方法的混凝土实体重力坝优化设计方法。该方法以混凝土实体重力坝断面柔度最小为优化目标,对混凝土实体重力坝断面先进行拓扑优化设计,然后进行形貌优化设计,进而有效地减小混凝土实体重力坝断面面积。某混凝土实体重力坝断面优化结果表明:优化得到的断面与一般传统方法设计的断面较为接近,但断面面积有所减少,且抗滑稳定安全系数与应力等均满足相关规范要求,证明了优化设计方法的合理性;重力坝形貌优化断面优于拓扑优化断面与传统设计断面;随着形貌优化迭代次数的增加,抗滑稳定安全系数逐渐增大,坝体竖向应力与竖向位移最大值逐渐减少;重力坝形貌优化断面在坝体上部存在颈缩现象,该现象可为重力坝设计提供新的思考方向。

火灾后设置加劲钢筋的十字形薄壁钢管混凝土短柱轴压机理

利用ABAQUS软件,建立了火灾后设置加劲钢筋的十字形薄壁钢管混凝土短柱的有限元分析模型,在验证模型有效性的基础上,研究了钢管和核心混凝土协同作用机理、钢管对核心混凝土法向接触力分布规律、核心混凝土的纵向应力分布规律。研究结果表明,火灾后钢管混凝土短柱受压过程分为弹性阶段、弹塑性阶段、强化阶段或下降阶段;钢管承担的荷载比例随受火时间延长而增大;钢管与混凝土之间的相互作用随升温时间、宽厚比和加劲钢筋间距的增加而减小。

地震作用下变截面钢管混凝土单桩动力响应

为了探讨桥梁位于软弱土层时,地震作用下变截面钢管混凝土桩基的动力响应,以翔安大桥为工程背景,利用FLAC3D有限元软件建立地震作用下桩-岩土体-钢管耦合作用的数值模型,研究在不同强度的地震作用下变截面钢管混凝土单桩和变截面普通混凝土单桩桩身加速度、桩身位移、桩身弯矩的动力响应差异.结果表明:变截面钢管混凝土单桩在地震作用下的桩身加速度、加速度放大系数、桩身位移最大值和桩身弯矩变化规律均同变截面普通混凝土单桩类似,但钢管使桩身加速度放大系数降低、桩基抵抗侧向变形能力和抗弯承载力提高.因此,钢管可以显著提升地震作用下桩基的抵抗变形能力和承载力.

工艺孔对大截面部分包覆钢-混凝土组合梁受弯性能影响的试验研究

为了研究工艺孔对大截面部分包覆钢-混凝土组合梁(大截面PEC梁)受弯性能的影响,对5根不同构造的大截面PEC梁进行了静力试验,研究了两点加载下主次梁腹板开洞方式对大截面PEC梁试件受弯性能、延性、破坏模式等的影响规律。结果表明:静力荷载作用下,各个试件均表现出良好的延性,腹部开孔对试件承载力和截面刚度有一定的削弱作用。型钢主钢件的应变与混凝土的应变沿截面高度方向均大致呈线性变化,符合平截面假定。按《部分包覆钢-混凝土组合结构技术规程》(T/CECS 719—2020)计算得到的开工艺孔的大截面PEC梁抗弯承载力与试验值误差较小,受尺寸效应影响不大,所采用计算公式安全可靠。

开口断面钢-混结合梁悬索桥颤振特性及颤振形态研究

为准确评估开口断面双边箱钢-混结合梁悬索桥颤振性能和多模态参与效应,以典型的钢-混结合梁悬索桥——怀化洞庭溪沅水特大桥为背景,开展风洞模型试验及数值分析。制作加劲梁节段缩尺模型并进行风洞试验,测试原桥梁结构的颤振性能,分析调整阻尼比、采用气动措施及结构措施对结构颤振特性的影响,采用三维和二维两种颤振分析方法分析设置中央扣结构的颤振模态及多模态参与效应。结果表明:该钢-混结合梁悬索桥存在较明显的颤振起振点,且颤振是以扭转为主的弯扭耦合振动,同时其扭转和竖向振动存在较大的相位差(相差近180°);节段模型风洞试验与三维和二维颤振分析一致得到设置中央扣后,频率较高的正对称扭转模态先于反对称模态发生颤振,且三维与二维颤振分析结果相差不大。

思政元素融入混凝土结构课程的教学设计--以单筋矩形截面梁承载力设计为例

土木工程专业的课程思政设计是实现专业课程与思政课同步前行的重要举措,也是培养社会主义合格建设者和可靠接班人的重要途径。混凝土结构系列课程在土木工程专业课程体系中占据重要的地位,因此,在对课程教学背景和学情分析的基础上,以单筋矩形截面梁承载力设计知识单元为例,基于OBE理念,将思政元素融入教学设计,从课程目标、课程内容、教学过程设计、教学方法设计及教学实施方面,探讨如何合理、科学地将思政元素引入课程教学设计的各个环节,不仅解决专业教育和思政教育“两张皮”问题,且对学生的自主学习能力和学习热情起到积极的促进作用,另外对其他专业课具有引领和示范作用。

内置型钢增强钢管约束钢筋混凝土柱节点抗震性能研究

采用钢管对传统RC柱进行约束,形成的钢管约束钢筋混凝土(STRC)柱具有优越的承载能力和耗能性能。但在应用于框架节点设计时,节点区域仍采用普通RC柱节点构造,可能导致节点处没有钢管约束而成为薄弱点,节点处易发生破坏。针对钢管约束钢筋混凝土(STRC)柱节点抗震性能薄弱的现状,提出采用内置型钢短柱进行增强。基于ABAQUS工作平台对具有相同构造形式的节点试件建立有限元模型,将数值计算所得破坏形态、滞回曲线和型钢应力与试验结果进行对比,两者吻合较好,在此基础上探讨内置型钢长度和轴压比对节点抗震性能的影响。结果表明:随着内置型钢长度增加,试件的承载力、极限位移和延性均有不同程度的提升,轴压比对试件受力性能影响较小。通过进一步分析试件的破坏机制、型钢抗弯性能和耗能能力,提出满足节点抗震性能需求的内置型钢长度建议取值。

型钢高强自密实混凝土黏结滑移性能试验研究

为研究型钢与高强自密实混凝土界面黏结滑移性能,以混凝土保护层厚度和型钢埋置长度作为变化参数,设计9个型钢高强自密实混凝土试件进行推出试验.观察试件破坏过程、破坏形态和裂缝发展形式,获取了试件加载端的荷载-滑移全过程曲线,研究不同变量对其黏结性能的影响规律.结果表明:型钢高强自密实混凝土组合结构界面极限黏结强度随着混凝土保护层厚度的增加而增大,最大增幅94.9%;随着型钢埋置长度的增加而减小,最大削减为38.1%.界面黏结抗剪刚度随着型钢保护层厚度的增加先增大后减小,最大增幅85.1%;随着型钢埋置长度的增加而增大,最大增幅为30%.界面耗能能力与其变量相关性较弱;随保护层厚度的增加,界面损伤发展越晚;随型钢埋置深度的增加,界面损伤速度减缓.提出了型钢高强自密实混凝土黏结强度的计算公式,能较好地预测型钢高强自密实混凝土黏结强度.

马鞍山长江公铁大桥Z3号桥塔施工关键技术

巢马城际铁路马鞍山长江公铁大桥主航道桥为(112+392+2×1120+392+112)m三塔钢桁梁斜拉桥,Z3号桥塔为超高多肢钢-混组合塔,高308 m。上塔柱钢结构高87.5 m,分13个吊装节段,最重505 t;中、下塔柱混凝土结构高217.5 m,分38个节段液压爬模施工;钢-混结合段高3 m,内部采用PBL键+剪力钉+高强度钢锚杆+高强度混凝土结构形式。在中塔柱设置钢管临时横撑控制塔柱线形及应力;下横梁采用落地支架法分层施工,与对应塔柱同步浇筑;钢-混结合段混凝土采用C60细石补偿收缩混凝土+高强度灌浆料,保证了混凝土施工质量;采用工厂“2+1”立体匹配制造、“提升站+运输栈桥”钢塔节段转运等技术,并研制15000 t•m超大型塔吊,实现了钢塔柱大节段的制造、整体滩地运输和吊装;钢塔节段间采用栓焊组合连接形式,通过设置工艺隔板、双面坡口等措施控制了钢塔焊接变形;利用定位桁架临时锁定钢塔合龙段实现了钢塔的精确合龙,定位桁架受力及变形均满足要求。

钢-UHPC结合段PBL连接件和钢梁端面承压联合作用研究

钢-混结合段作为钢梁和混凝土连接为整体的核心部件,传力构件的受力性能对钢-混凝土结合段至关重要。为提高结合段的力学性能,进行了仅有钢梁端面承压、PBL连接件和两者联合作用3组推出试验。结果表明:3组试件最终破坏裂缝分布均为钢梁端面侧“八”字形分布;钢梁端面组(D组)试件破坏模式主要为UHPC的开裂破坏而失效,PBL连接件组(P组)和联合作用组(PD组)试件主要是贯穿钢筋的剪切破坏而失效;3组试件的荷载-滑移曲线均包含弹性、裂缝开展和屈服3个阶段,由于贯穿钢筋和UHPC榫具有抑制裂缝开展和提高试件延性的作用,P组和PD组试件裂缝开展阶段较D组更短,屈服阶段更长,延性更好。对荷载-滑移曲线进行拟合,提出了钢梁端面承压和PBL连接件承载力随滑移量变化的计算公式。引入滞后滑移差Δs_(j),当Δs_(j)=0~0.67 mm时,联合作用计算结果偏于安全。计算得到钢梁端面承压传力比例约为43%,PBL连接件传力比例约为57%,两者传力效果基本相当。利用UHPC作为外包混凝土时,建议采用承载能力更高的贯穿钢筋,以充分发挥UHPC的高强性能,从而提高结合段的承载能力。