Effect of small axial force on flexural ductility design of high‐strength concrete beams

Small axial forces may appear in beams in a reinforced concrete (RC) structure. The presence of compressive axial force, even at a low level, has an adverse effect on the flexural ductility of RC beams, which is a key attribute for seismic design. For example, Eurocode EN1998‐1 explicitly specifies such minimum flexural ductility, whereas Chinese code GB50011 limits the depth of equivalent rectangular stress block at peak resisting moment to achieve indirectly a certain nominal flexural ductility. Therefore, ignoring the presence of compressive axial force may be risky. In this study, the effect of small compressive axial force on the flexural ductility performance of both normal‐strength and high‐strength concrete beams is evaluated on the basis of a rigorous full‐range moment–curvature analysis. An effective strategy for flexural ductility design of RC beams with small compressive axial force is identified so that various flexural ductility requirements can be satisfied. The essential control parameter proposed is the maximum difference of tension and compression reinforcement ratios. Empirical formulae and tables are developed for convenient implementation. Copyright © 2015 John Wiley & Sons, Ltd.     

Flexural ductility design of high‐strength concrete columns

In the seismic design of a structure, it is necessary to provide not only sufficient strength, but also a minimum level of flexural ductility for reinforced concrete (RC) columns. Eurocode EN1998‐1 directly specifies such minimum flexural ductility, while Chinese code GB50011 limits the normalized design axial force to achieve a nominal minimum flexural ductility. American code ACI 318‐08 uses the tension steel strain at peak resisting moment to control the failure mode. To provide the required flexural ductility, a much lower axial strength reduction factor is assigned to compression‐controlled failure than to tension‐controlled failure. To develop an effective strategy for flexural ductility design of RC columns, it is necessary to identify the essential parameters and control them properly. This is particularly important to those cast of high‐strength concrete that is inherently more brittle. The essential parameters identified include the maximum normalized axial force and maximum normalized neutral axis depth at peak resisting moment, as they help to guarantee various flexural ductility requirements. Their relationship with the flexural ductility is studied using a rigorous full‐range moment‐curvature analysis procedure. Empirical formulae and tables are also developed to facilitate flexural ductility design of RC columns. Copyright © 2010 John Wiley & Sons, Ltd.     

Concurrent flexural strength and ductility design of RC beams via strain‐gradient‐dependent concrete stress–strain curve

The authors have previously conducted an experimental study that showed that strain gradient would improve the maximum concrete stress and strength of reinforced concrete (RC) members under flexure. As a continued study, the authors herein will extend the investigation of strain gradient effect on flexural strength and ductility of RC beams to higher strength concrete up to 100 MPa by theoretical analysis. In this study, the flexural strength of RC beams is evaluated using nonlinear strain‐gradient‐dependent stress–strain curves of concrete applicable to both normal‐strength and high‐strength concrete. On the basis of this, a parametric study is conducted to investigate the combined effects of strain gradient and concrete strength on the flexural strength and ductility of RC beams. It was evident from the results that both the flexural strength and ductility of RC beams would be improved with strain gradient considered. From the results, two formulas are proposed for the strain‐gradient‐dependent concrete stress block parameters α and β. A constant value of 0.0032 is proposed for the ultimate concrete strain in flexural strength design with strain gradient effect considered. Lastly, for practical engineering design purpose, design formulas and charts have been presented for flexural strength and ductility of RC beams incorporating strain gradient effect. Copyright © 2015 John Wiley & Sons, Ltd.     

单调荷载作用下高强混凝土梁受弯性能尺寸效应研究

进行了不同截面尺寸高强混凝土梁的弯曲试验,研究了梁高对其受弯性能的影响。试件采用C70高强混凝土,纵向受力钢筋采用HRB400级钢筋。试件截面尺寸不同,截面长宽比、剪跨比和配筋率等参数保持一致。分析了不同截面尺寸对高强混凝土梁的名义开裂弯矩、名义屈服弯矩、名义极限弯矩、延性以及塑性转动能力的影响。研究结果表明,高强混凝土梁的名义开裂弯矩、名义屈服弯矩和名义极限弯矩无明显尺寸效应,而试件的位移延性系数和塑性铰区的塑性转动能力则表现出明显的尺寸效应,随截面尺寸的增大梁的位移延性系数和塑性铰区塑性转动能力有所降低。     

Theoretical analysis of post‐peak flexural behaviour of normal‐ and high‐strength concrete beams

A new method of analysing the post‐peak flexural behaviour of reinforced concrete beams has been developed and applied to normal‐ and high‐strength concrete beams. It was revealed that at the post‐peak stage the neutral axis depth keeps on increasing, and at a certain point the strain in the tension reinforcement starts to decrease, even though the curvature is increasing monotonically. Such strain reversal in the tension reinforcement occurs in all concrete beams and has significant effects on the post‐peak behaviour and flexural ductility of concrete beams. Therefore, the stress path dependence of the tension reinforcement needs to be taken into account in the analysis. By means of a parametric study, the variation of ultimate concrete strain with tension steel ratio and the effects of various structural parameters on flexural ductility have been studied. Based on the numerical results, design values of ultimate concrete strain that are independent of tension steel ratio have been recommended and a simple formula for predicting the flexural ductility of reinforced normal‐ and high‐strength concrete beams has been developed. Copyright © 2003 John Wiley & Sons, Ltd.     

钢管约束钢筋混凝土压弯构件滞回性能试验研究与分析

进行了4个圆钢管约束钢筋混凝土(CTRC)和4个方钢管约束钢筋混凝土(STRC)压弯构件滞回性能的试验研究,并进行了两个钢筋混凝土(RC)对比试件的试验研究。试验中的主要参数为轴压比(0.34、0.65和0.80)和混凝土强度等级(C30和C60)。试验结果表明,由于钢管对核心混凝土的有效约束,核心高强混凝土柱的承载力、延性和耗能能力得到了显著提高。随轴压比和混凝土强度的提高,CTRC压弯构件的受弯承载力提高;但轴压比和混凝土强度对试件的延性无明显影响。随轴压比和混凝土强度的提高,STRC压弯构件的受弯承载力提高,但延性下降。相同轴压比条件下,CTRC压弯构件的受弯承载力和延性明显优于STRC构件。根据试验结果,建议了钢管约束钢筋混凝土柱截面受弯承载力的计算方法。建立了钢管约束钢筋混凝土压弯构件的纤维模型数值计算方法,计算中采用随荷载的增加而不断增大钢管对核心混凝土的约束效应的方法,数值计算结果与试验结果吻合良好。     

Optimization of ductility in concrete ductile frames

An investigation of the parameters affecting the optimization of curvature ductility in a reinforced concrete frame sub-assembly subjected to seismic loads is presented using the feasible directions method. Four design variables are used: width of beam cross section, effective depth of cross-section, area of tension steel, and compression reinforcing. Two other variables, concrete compressive strength and yield strength of reinforcement, are treated as preassigned parameters because these quantities are usually taken as integer values in structural design. Curvature ductility is maximized for a given demand moment when one uses: the lowest yield strength steel and the highest concrete strength available; equal amounts of tension and compression reinforcement; the largest cross-section permitted by the design constraints; and sufficient reinforcement to just satisfy the demand moment constraint.     

Seismic behavior of T‐shaped steel reinforced concrete shear walls in tall buildings under cyclic loading

Shear walls are often used as the primary lateral load resisting elements in high‐rise buildings because of their large in‐plane stiffness and strength. It is a common practice to combine rectangular walls to form T‐shaped, I‐shaped and L‐shaped walls for functionality and esthetic reasons. Three relatively slender steel reinforced concrete (SRC) shear walls with T‐shaped cross‐sections were constructed and tested to failure under cyclic lateral loading. This research was conducted to assess the failure mechanism, hysteretic behavior, ductility and energy dissipating capacity of SRC T‐shaped walls under various axial load ratios. All the specimens exhibited a flexural mode characterized by crushing of the concrete and buckling of the steel at the free web boundary. The experimental results showed good hysteretic characteristics without pinching phenomena. The ductility coefficient varied from 2.3 to 4.1, and the deformation capability decreased with the increasing of axial load ratios. The stiffness, strength and ductility of T‐shaped walls are dependent upon the direction of the applied lateral loads. Higher stiffness and strength and lower ductility are achieved when the flange is in tension. The failure mechanism suggested that special attention should be paid to the design of the free web boundary to prevent premature failure under compression. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental study on the static performance of steel reinforced concrete columns with high encased steel ratios

The static performances of 12 steel reinforced concrete (SRC) columns with high encased steel ratios subjected to biaxial bending and axial loadings are studied experimentally. The main design parameters of the specimens in this experiment are the encased steel ratio, axial compression ratio, and shape distribution of the encased steel section. The crack formation, failure processes, bearing capacity, and ductility of the specimens are investigated in detail. The experimental results show that the increased encased steel ratio results in the increased bearing capacity and ductility of the specimens. Furthermore, the stiffness of the specimen degenerates gradually beyond the peak point with an increase in the axial compression ratio. In addition, a more extensive shape distribution of an encased steel section has a positive influence on the ductility of the specimen. A comparison of the yield and peak bearing capacities between the experimental results of this study and the predictions according to Eurocode 4 also highlights that Eurocode 4 would underestimate the bearing capacity of SRC columns with high encased steel ratios because it ignores the confinement of concrete provided from the steel section and hoops and it employs a conservative prediction approach for the simplified axial load‐bending moment curve.     

细晶粒高强钢筋混凝土梁受弯性能试验与参数分析

为研究配置了细晶粒高强钢筋混凝土梁的受弯性能,制作了HRBF400、HRBF500级钢筋混凝土矩形截面梁各4根进行静力抗弯试验。研究表明HRBF筋混凝土梁在短期荷载作用下的最大裂缝宽度实测值满足规范要求,但计算值不满足。HRBF400级钢筋混凝土梁在正常使用条件下的挠度能满足规范要求,HRBF500级钢筋混凝土梁不能够满足规范要求。推导了HRBF筋混凝土梁在裂缝/挠度控制条件下的承载力计算公式,提出了构件承载力利用系数的概念,分析了钢筋强度、钢筋直径、混凝土强度、配筋率、混凝土保护层厚度、高跨比对构件承载力利用系数的影响。在经济配筋率范围内,HRBF筋混凝土梁的延性基本满足要求。HRBF筋混凝土梁的耗能能力在较低配筋率时与普通钢筋混凝土梁相近,但随着配筋率的提高,其耗能能力较普通钢筋混凝土梁降低的快。同配筋率下,HRBF筋混凝土梁在弹性阶段的耗能能力较普通钢筋混凝土梁要高,且随着配筋率的增大而提高。     

Behavior and strength of circular tube confined reinforced-concrete (CTRC) columns

An experimental investigation was conducted on the behavior of circular tube confined reinforced-concrete (CTRC) columns. Eighteen CTRC stub columns were tested under cyclic or monotonic axial compression. The results of the elastic-plastic analysis on the steel tubes indicate that the steel tube yields at the peak load point in the stub columns under axial compression. In addition, a design equation to calculate the axial load strength of CTRC columns is proposed. A total of five columns including one circular reinforced-concrete (CRC) column and four CTRC columns have been studied under combined axial compression and lateral cyclic load. The test results indicate that CTRC columns exhibit much higher flexural strength, displacement ductility and greater energy dissipation ability than CRC columns confined with hoop ties. The flexural strength increases as the axial load ratio or concrete compressive strength increases for CTRC columns, while the ductility is barely affected by the increase in axial load or concrete compressive strength. It is proposed that the moment strength of the cross section of CTRC columns can be calculated using a modified ACI code method.     

超高强钢筋ECC梁受弯性能试验及承载力分析

为进一步研究工程用水泥基复合材料(ECC)与超高强钢筋组合成的超高强钢筋ECC梁(UHSRRE梁)的受弯性能,对3根UHSRRE梁、1根普通强度钢筋增强ECC梁(RECC梁)和1根普通强度钢筋增强混凝土梁(RC梁)进行弯曲试验,分析弯曲试验现象、ECC应变、延性性能和特征弯矩,并研究纵筋配筋率对UHSRRE梁承载力的影响。结果表明:UHSRRE梁和RECC梁的控裂能力比RC梁的控裂能力强; 与RECC梁相比,UHSRRE梁并未因采用超高强钢筋而使其控裂能力明显下降; UHSRRE梁截面应变基本符合平均应变的平截面假定,梁受拉区边缘的ECC应变小于ECC单轴受拉极限应变,梁受拉区的ECC始终不退出工作; UHSRRE梁受拉区和受压区边缘ECC应变的最大值、受压区高度和特征弯矩(除开裂弯矩)都随纵筋配筋率增加而变大; 随纵筋配筋率增加,UHSRRE梁的能量延性系数先增后减; 当UHSRRE梁具有适当纵筋配筋率时,其延性性能可优于RECC梁的延性性能。     

Moment redistribution in continuous plated RC flexural members. Part 1: neutral axis depth approach and tests

It is now common practice to retrofit or rehabilitate existing reinforced concrete beams and slabs by adhesively bonding fibre reinforced polymer (FRP) or metal plates to their surfaces. Advanced design rules are available for quantifying the various plate debonding mechanisms and consequently the shear and flexural capacities of the plated sections. These design rules show that even though the required increase in strength can be obtained by plating, plate debonding can severely reduce the ductility of a flexural member to such an extent that plating guidelines often exclude moment redistribution. This exclusion may reduce the application of plating, in particular to retrofitting buildings where ductility is often a requirement, or it may require the development of a radically different approach to design that does not rely implicitly on ductility. In this paper, it is shown that the commonly used neutral axis depth approach for moment redistribution in RC flexural members cannot be used for most plated structures because plate debonding often occurs before the concrete crushes. Tests on plated flexural members are also reported which show that moment redistribution can occur. In Part 2 of this paper, a moment redistribution analysis procedure is developed that can cope with plate debonding of externally bonded plates.     

钢筋混凝土联肢墙小跨高比复合斜筋连梁抗震性能试验研究

钢筋混凝土联肢剪力墙或联肢筒壁中的连梁是抗震关键部件。小跨高比连梁因剪弯比高,不采取特殊措施难以满足高剪力条件下的高延性要求。现行设计规范使用的交叉暗撑连梁虽抗震抗剪能力及延性好,但钢筋绑扎困难;现行规范降低抗剪能力上限、强化配箍要求的普通配箍连梁则抗剪能力过低、延性依然不足。为此,有必要寻求一种不需内力或刚度折减的高抗剪能力、高延性、便于施工的小跨高比连梁设计方案。为适应小跨高比连梁的受力特征,提出一种由交叉斜筋和上、下L形斜筋复合配筋的新设计方案,并完成19个具有不同跨高比、不同剪压比和不同斜筋———L形筋用量比的足尺连梁低周交变加载试验。试验结果表明,采用新方案配筋的连梁具有良好的抗震性能;剪跨比为0.8~2.5的该类连梁在剪压比高达0.3及以上时仍具有不低于4.0~5.0的位移延性,且施工难度明显降低。这种新的连梁配筋方案可推荐用于高抗震等级的建筑结构。     

Seismic behavior of high strength concrete beams

Experimental and analytical studies on the seismic performance of high strength concrete beams in moment resisting frame structures are described. Two large-scale high strength concrete and two counterpart normal strength concrete model beams with a length-to-depth ratio of 6·0 (i.e. shear span ratio of 3·0) have been tested under cyclic shear and double bending. The beams were heavily reinforced in the longitudinal direction to study beam performance under high seismic shear demand. Transverse reinforcement was designed following the seismic design provisions of current ACI 318–95 code. Both high and normal strength concrete beams developed ductile flexural responses. High strength concrete beams exhibited increased capacity and improved hysteretic performance compared to normal strength concrete beams. A simple analytical approach based on the moment curvature characteristics of critical sections has also been developed to predict the flexural behavior of structural beams. Comparison of analytical and test results indicates that behavior of high strength concrete beams can be satisfactorily predicted. © 1998 John Wiley & Sons, Ltd.     

Seismic performance evaluation of staggered wall system structures with middle corridors

In this paper, the seismic performance of reinforced concrete (RC) staggered wall structures with middle corridor was evaluated. To this end, 6‐, 12‐ and 18‐storey structural models were designed and were analyzed to investigate the seismic load‐resisting capacity. The response modification factors were computed based on the overstrength and the ductility capacities obtained from pushover curves. The effect of a few retrofit schemes on the enhancement of strength and ductility was also investigated. The pushover analysis results showed that the response modification factors ranged between about 4.0 and 6.0 with the average value around 5.0. When the bending rigidity of the link beams increased up to 100%, the overall overstrength increased by only about 25%. When the rebar ratio of the link beams was increased by 50%, the overstrength increased by about 40%. The replacement of the RC link beams with steel box beams resulted in superior performance of the structures with reduced beam depth. The displacement time histories of the model structures subjected to the earthquake ground motions scaled to the design seismic load showed that the maximum interstorey drifts were well below the limit state specified in the design code. Based on the analysis results, it was concluded that the staggered wall systems with a middle corridor had enough capacity to resist the design seismic load. Copyright © 2012 John Wiley & Sons, Ltd.     

四级抗震等级时钢筋混凝土异形柱轴压比限值的研究

考虑到二、三级抗震等级时异形柱轴压比限值显著低于矩形柱轴压比限值的实际情况,需对四级抗震等级时异形柱轴压比限值问题进行深入研究。本文基于理论分析和非线性全过程分析,取曲率延性比μ_(φc)=6作为四级抗震等级时异形柱应具有的曲率延性水平,不仅可以保证异形柱具有与矩形柱相当的延性性能,而且满足抗震结构位移延性的需求;然后以此为标准,确定了四级抗震等级时异形柱的轴压比限值及相应的构造要求。本文研究成果将为工程设计中四级抗震等级时钢筋混凝土异形柱轴压比限值的确定提供参考,也为完善相关设计规范提供理论依据。     

HRB500级钢筋配箍的混凝土梁受剪性能试验研究

利用一刚性试验系统,进行18根钢筋混凝土梁受剪性能试验。试验分为两个系列,一个系列采用HRB500级钢筋配箍的有腹筋梁,另一个系列为对应的无腹筋梁。主要试验变量为剪跨比和混凝土抗压强度。试验得到试验梁包括下降段在内的荷载-挠度全曲线。根据试验结果,分析梁的斜截面受剪承载力和变形性能。通过试验数据分析,发现剪跨比对有腹筋梁和无腹筋梁的受剪性能都有较大的影响,有腹筋梁的剪切破坏不属于脆性破坏,有腹筋梁使用阶段的斜裂缝宽度可以满足正常使用极限状态的要求。不同强度等级和不同剪跨比的混凝土梁,采用HRB500级钢筋作为箍筋,其受剪承载力可按现行《混凝土结构设计规范》有关公式进行计算,所得结果偏于安全。     

Experimental study of reinforced concrete beams strengthened with a GFRP channel and CFRP sheets

Glass–fibre‐reinforced polymer (GFRP) jackets were used in several different ways to improve the flexural and shear performance of reinforced concrete (RC) beams. The GFRP jacket increased both the ductility and the strength of the concrete beam. An experimental and analytical study was conducted to investigate the use of the GFRP channel in improving the flexural capacity of a RC wide beam. The parameters investigated in this study were stiffness, ductility and failure mode. The test results showed that using the GFRP channel significantly improved the flexural capacity of the RC wide beams. The tensile stresses due to the inclined cracks in the concrete can be transferred to the concrete on each side of the crack when interfacial bond activities are maintained. Copyright © 2008 John Wiley & Sons, Ltd.     

Seismic behavior and strength of square tube confined reinforced-concrete (STRC) columns

A steel tube confined reinforced-concrete (STRC) column is an ordinary RC column where most of the lateral ties are in the form of a thin steel tube. Twenty-three square tube confined concrete stub columns were tested in this paper under cyclic or monotonic axial compression. A design equation to calculate the axial load strength of square tube confined concrete stub columns is proposed in this paper. A total of five beam-columns have been studied under combined axial compression and lateral cyclic loads. The test results indicate that the columns confined with square steel tubes exhibit much higher flexural strength, displacement ductility, and energy dissipation ability than common RC columns confined with lateral ties. Fiber models were also developed for STRC beam-columns in this paper.