双层箍筋约束高强混凝土柱轴心受压力学特性研究

为了解双层箍筋对混凝土变形性能的影响,对6根双层箍筋约束混凝土圆形柱与2根普通箍筋圆柱进行了轴心受压试验,研究了其破坏过程和破坏形态。并基于试件的荷载-应变曲线,分析了箍筋形式、内外层箍筋间距以及配箍特征值对试件变形性能的影响。结果表明:双层箍筋比普通箍筋能更有效地提高高强混凝土的延性,从而提高了混凝土柱的变形性能。     

高强箍筋高强混凝土短柱抗震性能试验研究

为了研究高强箍筋约束高强混凝土短柱在地震作用下的抗震性能,采用建研式加载装置,通过14根高强混凝土短柱试件的低周反复加载试验,研究了高强箍筋约束高强混凝土短柱的破坏形态、滞回性能、骨架曲线、变形和延性、耗能能力以及高强箍筋的应力发挥水平和受剪承载力计算等,分析了轴压比、剪跨比、箍筋强度、箍筋间距、箍筋形式和混凝土强度等因素对短柱破坏形态、滞回性能和承载力的影响。结果表明：短柱破坏形态受设计参数的影响有剪切破坏和剪切黏结破坏两类;与普通强度箍筋混凝土短柱相比,高强箍筋高强混凝土短柱在节约材料的同时具有优越的抗震性能和抗倒塌能力;达到极限荷载后,箍筋的应变发展较快,高强箍筋的强度发挥充分,短柱的抗震性能明显改善;通过对高强箍筋应力取值进行适当修正,采用GB 50010-2010规范公式计算高强箍筋高强混凝土短柱的受剪承载力是可行的。     

钢筋混凝土连续梁火灾下抗剪性能的试验研究

对8根足尺钢筋混凝土两跨连续梁和4根足尺钢筋混凝土简支梁进行了标准火灾试验,研究了钢筋混凝土连续梁高温下的抗剪性能,并与简支梁进行了对比,得到了连续梁混凝土及钢筋的温度变化、竖向挠度、轴向变形及最终破坏形态,分析了不同荷载比、剪跨比、混凝土保护层厚度、配箍率及受火工况对连续梁耐火极限的影响。结果表明:火灾下荷载比和混凝土保护层厚度对钢筋混凝土连续梁的抗剪性能影响显著。随着剪跨比和箍筋间距的减小,连续梁发生受剪破坏的耐火极限显著提高。与单跨受火工况相比,两跨同时受火的连续梁耐火极限降低。高温下连续梁由于发生了内力重分布,其抗剪性能优于简支梁。     

高强箍筋高强混凝土梁受剪试验研究

设计了一套使钢筋混凝土梁剪切破坏稳定可控的刚性试验系统,利用该试验系统,完成了19根剪跨比为3的钢筋混凝土简支梁的剪切破坏试验,得到了荷载-挠度曲线。根据试验结果,分析了混凝土强度等级、箍筋的强度和倾角、纵筋配筋率和纵向分布钢筋等因素对试验梁破坏形态、剪切延性系数和受剪承载力的影响,并将受剪承载力试验值与我国混凝土结构设计规范和美国ACI规范计算值进行了对比。结果表明:对于剪跨比等于3的梁,适当配置腹筋,可以改善其延性性能;在高强混凝土梁中应用高强箍筋,可使两种材料的强度充分发挥,不仅增加了梁的剪切延性,而且提高了梁的受剪承载力;高强箍筋高强混凝土梁的受剪承载力仍可采用我国现行混凝土结构设计规范公式进行计算,但是对于高强混凝土无腹筋梁、纵筋配筋率低的梁和配有高强箍筋的普通强度混凝土梁安全度偏低。     

基于简化修正压力场理论的钢筋混凝土柱荷载-变形分析

基于简化修正压力场理论对钢筋混凝土柱抗剪机理进行了分析,并考虑核心混凝土膨胀对箍筋抗剪承载力贡献的影响,计算了骨料咬合作用及受压区的抗剪承载力贡献,获得受拉区和受压区的抗剪强度,从而建立箍筋屈服后柱构件抗剪强度计算方法;结合传统截面纤维分析法,同时引入弯曲变形、剪切变形及滑移变形3种变形分量,在箍筋屈服前对柱构件进行抗弯分析,最终得出压弯剪作用下钢筋混凝土柱荷载-变形曲线,并与所收集的15个钢筋混凝土柱低周反复试验结果进行了对比。研究结果表明:采用该方法计算的荷载-变形曲线与试验骨架曲线吻合较好,对发生弯曲破坏、弯剪破坏及剪切破坏3种不同破坏类型的钢筋混凝土柱均有较好的分析效果,可用于压弯剪作用下钢筋混凝土柱的荷载-变形分析。     

非对称配钢型钢混凝土柱受剪机理及承载力研究

为揭示非对称配钢型钢混凝土柱的受力特点和破坏机理,考虑剪跨比、轴压比、体积配箍率以及拉结筋等4个影响因素,对12个T形配钢型钢混凝土柱进行了水平低周往复荷载试验。对水平低周反复荷载和恒定轴向荷载作用下试件的受力过程和破坏特征进行了研究,结果表明:非对称配钢型钢混凝土柱具有较好的滞回性能和耗能性能,其破坏形态主要有弯曲破坏、剪切斜压破坏和剪切粘结破坏三类。通过实测型钢腹板和箍筋的荷载-应变滞回曲线,分析了试件在不同阶段的受力状态,基于破坏机理和力学分析推导了T形配钢型钢混凝土柱发生剪切斜压破坏和剪切粘结破坏时的受剪承载力计算公式,计算结果与实测值符合较好。     

CFRP筋增强混凝土梁抗剪性能试验

针对桥梁和建筑结构中大量使用的碳纤维筋（CFRP）增强混凝土梁的抗剪性能,首先设计箍筋间距和剪跨比不同的3根足尺CFRP筋增强混凝土梁试件,并基于规范要求给出其抗剪性能测试试验工况和加载模式;然后,观察不同工况下试件在两点集中加载方式下裂缝的产生和发展过程,总结归纳CFRP筋混凝土梁的失效模式和破坏特征;同时,由CFRP纵向受力筋和CFRP箍筋的荷载—应变关系及CFRP筋混凝土梁的开裂荷载、极限荷载和荷载–挠度曲线的变化情况,分析研究CFRP筋混凝土梁承载能力和变形性能随剪跨和箍筋间距的变化规律;最后,分析剪跨、箍筋间距等影响因素对CFRP筋混凝土梁裂缝宽度、裂缝数量及裂缝分布的影响规律。结果表明：CFRP筋混凝土梁的剪切破坏由弯剪区贯通斜裂缝发展所致;CFRP箍筋配箍率和剪跨比对开裂荷载影响显著,箍筋配箍率越大,试验梁斜截面开裂荷载和极限破坏荷载增加,但是剪跨比越大梁承载力则明显降低;梁的挠度–荷载曲线呈两段式线性分布,梁破坏时变形明显,裂缝宽度较大,高强CFRP箍筋强度发挥有限。     

不同配箍率下HRB500级纵筋活性粉末混凝土梁抗剪性能研究

为研究配箍率对HRB500级钢筋活性粉末混凝土(RPC)梁的抗剪性能的影响,对5根配置不同箍筋的梁进行四分点加载试验。通过观察试验过程,以及对加载过程中挠度、箍筋应变等数据进行分析,得出配箍率对HRB500级钢筋RPC梁抗剪承载力与剪切延性的影响规律。试验结果表明:随着配箍率的增加,试件斜裂缝的分布规律由少而集中变为多而分散,同级荷载下的裂缝宽度变得更细,箍筋起到抑制裂缝开展的作用;梁中配置箍筋不仅可以提高构件抗剪承载力,同时还能提高其剪切延性,效果明显;钢纤维在抵抗由温度、弯曲等荷载形成的均匀应力时作用明显,当抵抗由集中荷载为主所产生的剪应力时则不能取代箍筋的作用。     

高温下再生混凝土梁受剪性能试验研究

对9根不同再生骨料取代率的再生混凝土梁试件进行了常温和标准升温条件下的受剪试验,研究再生粗骨料取代率和初始荷载比等对试件的破坏特征、耐火极限、温度场分布和变形性能等的影响。利用ABAQUS有限元分析软件分别进行试件的温度场分析和顺序热-应力耦合全过程分析,并提出高温下再生混凝土梁受剪承载力理论计算方法和设计计算式。研究结果表明:高温下再生混凝土梁的剪切破坏特征与普通混凝土梁相似,但再生骨料取代率越大,火灾试验后试件表面混凝土的剥落程度越明显;试件的耐火极限随再生粗骨料取代率的增大而提高,随初始荷载比的增大而降低,再生混凝土梁的耐火极限受初始荷载比的影响比普通混凝土梁低;再生粗骨料取代率越大,相同时刻试件内部测点的温度越低,箍筋温度也越低,再生混凝土梁比普通混凝土梁具有更好的耐火性能;在火灾试验末期,随着再生粗骨料取代率的增加,试件的变形增长速率逐渐减小,说明高温下再生混凝土梁斜截面破坏时的延性比普通混凝土梁好;有限元分析结果与试验结果吻合较好;提出的高温下再生混凝土梁受剪承载力理论计算方法和设计计算式均具有可行性,能够分别满足试验和工程设计的要求。     

带斜裂缝变截面混凝土梁剪切变形试验研究

为研究腹板斜裂缝对混凝土梁剪切变形的影响程度,特开展了根小剪跨比变截面混凝土约束梁的剪切变形试验研究。设计制作了用于梁段应变测量的组合式测量框格,提出了变截面梁弯曲、剪切变形分离测量计算方法,实现了基于框格应变测量值的斜裂缝开展前、后的弯曲与剪切变形分离。试验研究表明,在斜裂缝开裂前,剪跨比是影响剪切变形比例的主要因素;斜裂缝开裂后,配箍率对剪切变形的影响更为明显;第一条主斜裂缝出现和箍筋屈服是荷载-剪切变形曲线的两个转折点;提高混凝土强度等级和配箍率能增加开裂后抗剪刚度,抑制开裂后剪切变形的发展。     

Experimental bending tests of partially encased beams at elevated temperatures

This paper presents the result of an experimental research about the lateral torsional buckling instability during bending tests of Partially Encased Beams (PEB) at elevated temperature. A set of twenty seven four-point bending tests, grouped in ten series, were carried out to analyse the influence of relative slenderness, beam temperature and the shear bond conditions between concrete and steel in bending. In addition, this study compares the behaviour of PEB and bare steel beam under bending at room temperature.PEB specimens are based on IPE100 steel profiles, with two different lengths 2.4 m (medium series) and 3.9 m (large series), tested in bending using simple supporting conditions and exposed to different temperatures levels of 200 °C, 400 °C, and 600 °C.Two different shear bond conditions, between steel profile and lateral concrete, were analysed at 400 °C: one series with connectors formed by welded stirrups to the web and another series with natural adherence between steel and concrete, not welded stirrups.PEB attained lateral torsional buckling as deformed failure mode at the ultimate limit state, except for the case of PEB tested at 600 °C that results in a plastic hinge failure. The bending resistance was determined for the maximum load event (F_u) and for the displacement limit corresponding to L/30 (F_L/30) and compared with the results of the Eurocode 3 part 1–2 simple calculation method, considering an adaptation of its formulae to PEB. The expected reduction in bending resistance at elevated temperature is in good agreement with the experimental reduction factor, when the deformation criterion is used.     

小剪跨比锈蚀钢筋混凝土梁受剪性能试验研究

钢筋混凝土梁中箍筋发生锈蚀易导致结构性能退化以至失效破坏,为此针对HRB400钢筋作为箍筋发生锈蚀后的钢筋混凝土梁在剪跨比为1.5以下的受剪性能的退化展开研究。制作8根钢筋混凝土梁,按照剪跨比为1.5和1分成两组进行试验,比较未锈蚀和锈蚀后的钢筋性能以及试验梁的受剪承载力。研究结果表明：定义箍筋竖直段的锈蚀率为有效锈蚀率进行分析,能够有效反映箍筋的锈蚀程度和截面损失情况;箍筋有效锈蚀率小于20%时钢筋混凝土梁受剪承载力下降并不明显,当箍筋有效锈蚀率大于30%时受剪承载力下降明显,且剪跨比越大钢筋混凝土梁受剪承载力下降幅度越大;剪跨比小于1.5时,锈蚀箍筋性能退化对钢筋混凝土梁受剪承载力下降影响较小,受剪承载力下降主要是因为箍筋锈蚀引发锈胀裂缝导致核心混凝土整体性下降所致。     

Behaviour of headed stud shear connectors for composite steel-concrete beams at elevated temperatures

The behaviour of composite steel-concrete beams at elevated temperatures is an important problem. A three-dimensional push test model is developed herein with a two-dimensional temperature distribution field based on the finite element method (FEM) and which may be applied to steel-concrete composite beams. The motivation for this paper is to increase the awareness of the structural engineering community to the concepts behind composite steel-concrete structural design for fire exposure. The behaviour of reinforced concrete slabs under fire conditions strongly depends on the interaction of the slabs with the surrounding elements which include the structural steel beam, steel reinforcing and shear connectors. This study was carried out to consider the effects of elevated temperatures on the behaviour of composite steel-concrete beams for both solid and profiled steel sheeting slabs. This investigation considers the load-slip relationship and ultimate load behaviour for push tests with a three-dimensional non-linear finite element program ABAQUS. As a result of elevated temperatures, the material properties change with temperature. The studies were compared with experimental tests under both ambient and elevated temperatures. Furthermore, for the elevated temperature study, the models were loaded progressively up to the ultimate load to illustrate the capability of the structure to withstand load during a fire. It is concluded that finite element analysis showed that the shear connector strength under fire exposure was very sensitive. It is also shown that profiled steel sheeting slabs exhibit greater fire resistance when compared with that of a solid slab as a function of their ambient temperature strength.     

Behavior of Steel–Concrete Partially Composite Beams Subjected to Fire—Part 2: Analytical Study

This paper presents the development of an analytical model of steel–concrete partially composite beams subjected to fire. The model includes consideration of temperature dependent material properties, temperature dependent interface slip between concrete and steel, non-uniform temperature distributions throughout the cross-section and the effect of different rates of thermal expansion at the concrete–steel interface. Model predictions showed good agreement with the results of fire tests on two composite beams reported in an earlier companion paper as well as with limited experimental data published in literature. An extensive parametric study was undertaken by using the proposed model. Parameters considered in this study included geometric dimensions of the composite beam, material grades of steel and concrete, shear connection ratio, reinforcing steel ratio in the concrete slab, and load level on the beam. The parametric study clearly shows that shear connection ratio and load level significantly influence the fire performance of partially composite beams. The critical temperatures with shear connection ratio of 50%, 75% and 100% are 645°C, 602°C and 548°C, respectively, under load level of 0.6. The critical temperatures under load ratio of 0.5, 0.6 and 0.7 are 468°C, 553°C and 633°C respectively, with a shear connection ratio of 50%.     

受长度约束钢柱的耐火性研究

火灾下钢柱的性能取决于其与包覆结构的相互作用。为提高对该现象的认识,对受长度约束的钢柱进行了一系列耐火性试验。设计了新的试验方法,并采用该方法进行试验。试验柱的长细比分别为50.6和63.3,初始荷载分别为室温下钢柱屈曲荷载设计值的30%和70%,包覆结构轴向刚度分别为13,45,63.3kN/mm。初始荷载偏心度与截面的弱惯性轴或两个主惯性轴的长度相等。结果显示,包覆结构刚度增加并不能够降低柱的临界温度,这是由于轴向刚度增加导致扭转刚度增加,而扭转刚度增加将使得柱的临界温度升高。结果还指出,初始应力水平越高,柱的临界温度越低。     

Behavior of prestressed stayed steel columns under fire conditions

Prestressed stayed steel columns experience loss of strength and stiffness when exposed to fire conditions. This paper presents results from experimental studies on the behavior of prestressed stayed circular steel columns under fire conditions. Two full scale prestressed stayed steel columns were tested by subjecting the columns to simultaneous gravity (mechanical) loading and fire conditions. In these fire tests, the varied parameters include load level and level of prestressing. Cross sectional temperatures, axial deformations, as well as fire resistance during the fire tests were recorded and measured. The results indicate that prestressed stayed steel columns undergo various failures modes under different combinations of load and prestress ratios. Specifically, load level significantly influence the fire response of prestressed stayed steel columns with higher load level leading to higher contraction and lower fire resistance.     

偏心荷载作用下细长型高强钢管混凝土柱的防火性能

对受偏心轴向荷载作用、分别填充普通混凝土和高强混凝土的细长型圆钢管柱进行24组防火试验。此试验是对Romero等人在2011年所做的中心荷载柱的试验结果的进一步探讨。此防火试验中的试验参数涵盖了混凝土标准强度(30MPa和90MPa)、填充类型(普通混凝土、钢筋混凝土和钢纤钢筋混凝土)、轴向荷载水平(20%和40%)和荷载偏心(20mm和50mm)。在固定的边界条件下对室内温度下相对长细比高于其他样本0.5的柱进行试验。研究这种内填混凝土组合件偏心荷载的影响。试验结果显示,与填充普通混凝土的柱相比,添加钢纤维并不能提高细长型柱在偏心荷载下的防火性能。然而,在混凝土柱中添加钢筋则可以提高防火性能。在中空圆钢管中填充混凝土可以提高其防火性能,钢管高强混凝土柱在承载力方面有更明显的提升。与欧洲规范4-1-2中的简易计算模型相比较,尽管此方法可用于偏心荷载作用下的柱,但在预测内填普通混凝土柱和钢纤维钢筋混凝土柱时仍会产生较大的误差。     

Experimental study on local buckling of axially compressed steel stub columns at elevated temperatures

There are few design provisions in codes and standards on local buckling of steel columns under fire conditions. To examine the local stability of steel stub columns at elevated temperatures, 12 stub columns were tested under simultaneous application of load and fire conditions. The test variables included Grade (type) of steel, buckling resistance, temperature and load levels. During fire tests, cross sectional temperatures, axial displacement, buckling deflection, and local buckling failure modes of flange and web of stub columns were recorded at various temperatures. Data from the tests is utilized to evaluate buckling resistance of flange and web both at room and elevated temperatures by applying the ultimate strain method and curve inflexion point method. Results from fire tests are utilized to validate a finite element model developed for tracing local buckling of steel columns at elevated temperatures. Results from fire tests and finite element analysis show that failure mode of columns at room and elevated temperatures follow similar pattern but the load bearing capacity of Q460 steel columns degrade much more rapidly under fire conditions than that of Q235 steel columns. Further, Eurocode 3 provisions for local buckling produce non-conservative results in certain situations.     

Fire behavior of eccentrically loaded slender high strength concrete-filled tubular columns

This paper describes a series of 24 fire tests conducted on slender circular hollow section columns filled with normal and high strength concrete, subjected to eccentric axial load. It is a continuation of a previous research paper (Romero et al., 2011 [1]), where test results on centrally loaded columns were presented. The test parameters covered in this fire testing program were the nominal strength of concrete (30 and 90 MPa), the infilling type (plain, bar-reinforced and steel fiber reinforced concrete), the axial load level (20% and 40%) and the load eccentricity (20 and 50 mm). The columns were tested under fixed-pinned boundary conditions, with a relative slenderness at room temperature higher than 0.5 for all the specimens. The aim of this paper is to study the influence of eccentricity in combination with the type of concrete infill. The results show that the addition of steel fibers does not improve the fire resistance of slender columns under eccentric loads, as compared to columns filled with plain concrete. However, the addition of reinforcing bars increases the fire resistance of the columns in this situation. Filling the steel hollow section columns with concrete increases their fire resistance, the increase in load bearing capacity being more noticeable for columns filled with high strength concrete. A comparison with the current simple calculation model in Eurocode 4 Part 1.2 shows that, although the method is safe for eccentrically loaded columns, it produces a high error in the predictions for columns filled with plain or steel fiber reinforced concrete.     

Zur maximalen Durchstanztragfähigkeit von Flachdecken.

Maximum Punching Capacity of Flat Slabes Eurocode 2 assesses the maximum punching shear resistance by limiting the nominal shear stress at the periphery of the load to a value equal to the web‐crushing limit of beams with shear reinforcement. However, experimental investigations revealed that in most cases the failure occurs at a lower load level once the compression zone at the column face crushes. Furthermore, Eurocode 2 overestimates the maximum shear punching capacity for slabs including stirrups compared to the capacity of slabs including double‐headed studs according to construction approvals. To verify the maximum punching shear capacity according to Eurocode 2, at the Institute of Structural Concrete, RWTH Aachen University a punching test on a slab including stirrups as shear reinforcement was conducted and, in addition, tests from literature were evaluated.