Negative Moment Zone Research Articles

Numerical and FEA investigation of sectional capacity and moment redistribution behavior of steel fiber reinforced concrete (SFRC) beam

Steel fiber reinforced concrete exhibits ductility. This ductility behavior enables redistribution of moment between the negative and positive moment zones in continuous SFRC beams. The sectional capacity of an SFRC continuous beam was determined using the moment-curvature response, the ductility of the member using the moment-rotation response, and the moment redistribution behavior using the moment-rotation response. The theoretical moment-curvature response was calculated from the stress-strain response for 0 % (control), 0.5 %, 0.75 %, and 1.5 % SFRC member which served the basis for the defined moment-rotation behavior and FEA. A five-point bending test was conducted on a two-span beam using a constitutive material model in the FEA software package, Abaqus/CAE. The concrete damage plasticity (CDP) model was used to conduct a displacement-controlled analysis on the fully integrated 3D hexahedral element (C3D8). The numerical study revealed that the flexural behavior, bending capacity, rotation capacity, and ductility of the 0.5 %, 0.75 %, and 1.5 % SFRC beams were significantly enhanced than the 0 % SFRC beam. However, as steel fiber volume increased, the quantity of moment redistribution in the SFRC beam decreased. The amount of moment redistribution obtained was 21.8 % for 0 %, 19 % for 0.5 %, 18.1 % for 0.75 %, and 13.9 % for 1.5 % SFRC.

Moment Redistribution in Continuous Externally CFRP Prestressed Beams with Steel and FRP Rebars.

This paper assesses the impact of adopting carbon- or glass-fiber-reinforced polymer (CFRP or GFRP) instead of steel rebars on the redistribution of moments in prestressed concrete beams (PCBs) with external CFRP tendons. A numerical program is introduced, and numerical simulations are performed on two-span continuous beams with steel, CFRP or GFRP rebars of various areas, i.e., Ar2 = 360–3560 mm2, and Ar1/Ar2 = 1.5, where Ar1 and Ar2 are areas of tensile rebars over the positive and negative moment zones, respectively. The results show the moment redistribution is contributed by concrete cracking only for the beams with fiber-reinforced polymer (FRP) rebars, and by concrete cracking and steel yielding for the beams with steel rebars. As a result, the use of FRP rebars leads to a substantially lower moment redistribution than in steel rebars. It is also demonstrated that Eurocode 2, CSA A23.3-04 and ACI 318-19 fail to reflect the rebar influence on moment redistribution in PCBs with external tendons. A simplified equation for the quantification of moment redistribution in externally PCBs with steel and FRP rebars is recommended, which yields accurate and conservative predictions.

Cracking Analysis of Pre-stressed Steel–concrete Composite Girder at Negative Moment Zone

In order to analyze the influence position of prestressed reinforcement on the cracking load of composite girders, scale test and finite element analysis is used to study the cracking behavior of prestressed composite beam at the negative moment zone. The load–deflection curve and the cracking law of the scaled beam are obtained from the experimental results. The influence law of the resultant point and the spacing change through the symmetrical axis of the prestressed beam on the cracking load is obtained through the finite element parametric analysis. Furthermore, the prototype beam's mechanical performance in terms of the scaled test data is verified by both similarity theory and the prototype beam's numerical model. The results show that: Under different prestress levels, the cracking load is larger when the resultant force point of prestressed reinforcement on one side of the concrete slab's symmetric axis is within 0.15 ~ 0.50 times of slab half-width. In addition, the cracking moment of the specimen can be increased up to 11% by optimizing the prestress arrangement.

Negative bending behavior of novel U-shaped steel and concrete composite beams

The negative bending behavior of the reinforcement stiffened cold-formed U-shaped steel and concrete composite beams (RCUCBs) is discussed in this paper. The negative bending experiment on 10 specimens were conducted and the damage process and corresponding static behavior in each stage are defined. Two distinct failure modes were found depending on whether the inverted U-shaped reinforcement (IUR) is installed or not: ductile plastic failure mode (when installed) and sudden elastic failure mode (when not installed). The IUR was observed to keep the integral action and improve the bending capacity and ductility of a RCUCB. The closed steel U-section stiffened by the reinforcement truss provides significant confinement to the encased concrete and even maintains its integrity after concrete cracking. The study results show that the plastic hinging of the framed RCUCB under vertical loads spreads to about L0/6 and suggest that the effective slab width in the negative moment zone is Be = L0/3 + b, similar to the positive moment case. Lastly, a negative bending design method was proposed, giving good calculations compared to the test results. The slab reinforcement, steel U-section, encased concrete, and bottom reinforcement contribute 53%, 30%, 11%, and 6% respectively to the negative bending capacity on average.

Behaviour of steel–concrete composite girders under combined negative moment and shear

Existing studies on the behaviour of steel–concrete composite girders under combined bending and shear are mainly focused only on composite beams with compact sections; very few studies have investigated slender composite girders, which are commonly used in bridge engineering. Therefore, the behaviour of slender steel–concrete composite girders under combined negative moment and shear were investigated experimentally and numerically in this paper. With the moment/shear ratio, reinforcement ratio, connection type, and web thickness as the experimental parameters, experiments were conducted on seven simply supported composite girders subjected to negative moment and shear. The test results showed that reinforcement ratio, concrete slab thickness, and web thickness significantly influence the ultimate load and failure modes of composite girders; the connection type only slightly influences the behaviour of full-shear-connection composite girders under combined negative moment–shear interaction. All the test data were outside the moment–shear interaction curve of Eurocode 4, i.e. Eurocode 4 tended to produce safer results. Then, a nonlinear finite element model was developed and validated using the experimental results. The factors affecting the moment–shear interaction of composite girders in negative-moment zones were studied through numerical simulation. The numerical results showed that the negative moment–shear interaction law changes little with the change of reinforcement ratio or slab thickness. The web depth-to-thickness ratio significantly influences the negative moment–shear interaction law, and the higher the web depth-to-thickness ratio, the earlier the composite girder begin experiencing moment–shear interaction.

Research on bearing jacking method of steel -concrete composite beam bridge

There are plenty of advantages of steel-concrete composite beam bridges. However, Transverse cracks in composite girder bridges usually appear because of the tensile stresses in the concrete deck induced by the negative bending moment at the intermediate supports of continuous composite girder bridges. There are many measures to reduce the tensile stress in the negative bending moment zone of steel-concrete composite beams. At present, bearing jacking method is used more and more in engineering. Taking a composite beam in Shaanxi province as an example, this paper studies the influence of jacking amount and span number on the improvement effect of bearing jacking method. The improvement of stress level of concrete deck in negative moment zone of steel-concrete composite beam bridge is studied and analysed which puts forward relevant engineering solution.

Strengthening of one-way reinforced concrete slabs using near surface mounted bars

Abstract In this work, an experimental study on the performance of continuous one-way RC slabs strengthened by near surface mounted (NSM) bars under static loads is presented. Seven slab specimens were cast, with 2400 mm total length, 500 mm width and 100 mm thickness. The first specimen slab had no strengthening and considered as a control and the other six specimens were strengthened with either different embedded lengths of near-surface mounted bars; or using different bars spacing, adopting a different material type of bars, or adding steel plates as end-anchors to prevent unexpected premature failure that may occur by debonding of steel bars. Tests were done under two-point loads that centered at each span of 1100 mm length. Results were discussed in terms of ultimate loads, crack width, cracking, failure mode and load–deflection curves. The results of experimental tests showed enhancement on the ultimate load when using near-surface mounted steel bars ranged between (156.25%) and (225%). Steel bars may give a more efficient contribution than CFRP bars when it is used in the strengthening of the negative moment region of continuous one-way slabs due to its high ductility. In addition, adopting end-anchors considerably restricted the crack width in the negative moment zone.

Analysis of shear lag effect in the negative moment region of steel-concrete composite beams under fatigue load

Shear lag effect was a significant mechanical behavior of steel-concrete composite beams, and the effective flange width was needed to consider this effect. However, the effective flange width is mostly determined by static load test. The cyclic vehicle loading cases, which is more practical, was not well considered. This paper focuses on the study of shear lag effect of the concrete slab in the negative moment region under fatigue cyclic load. Two specimens of two-span steel-concrete composite beams were tested under fatigue load and static load respectively to compare the differences in the negative moment region. The reinforcement strain in the negative moment region was measured and the stress was also analyzed under different loads. Based on the OpenSees framework, finite element analysis model of steel-concrete composite beam is established, which is used to simulate transverse reinforcement stress distribution as well as the variation trends under fatigue cycles. With the established model, effects of fatigue stress amplitude, flange width to span ratio, concrete slab thickness and shear connector stiffness on the shear lag effect of concrete slab in negative moment area are analyzed, and the effective flange width ratio of concrete slab under different working conditions is calculated. The simulated results of effective flange width are compared with calculated results of the commonly used specifications, and it is found that the methods in the specifications can better estimate the shear lag effect in concrete slab under static load, but the effective flange width in the negative moment zone under fatigue load has a large deviation.

Moment redistribution versus neutral axis depth in continuous PSC beams with external CFRP tendons

The neutral axis depth is adopted by many codes of practice as an indicator of flexural ductility to quantify moment redistribution in continuous prestressed concrete (PSC) beams. Moment redistribution, however, does not only depend on ductility but also on differences in stiffness along the length of the beam. Therefore, the effectiveness of using solely the neutral axis depth for redistribution quantification needs to be further evaluated. This study examines moment redistribution against neutral axis depth in two-span PSC beams with external CFRP tendons by applying a validated finite element model. The main variable is the content of non-prestressed reinforcement either at the positive or negative moment zone to generate varying stiffness differences between critical sections. The study shows that the use of neutral axis depth as a key parameter is inadequate when quantifying moment redistribution in these beams. Modifications of CSA, BSI and EC2 equations are proposed by introducing a parameter reflecting the impact of stiffness difference. The proposed equations show much better fit to the actual redistribution than that provided by equations in current design codes.

Ageing models and maintenance strategy for road tunnels

The ageing and deterioration of road tunnels due to environmental factors are inevitable. Rational and appropriate maintenance methods are required to maintain safety. This study aims to establish a maintenance strategy that determines the timing, location, and methods adopted to repair a degraded tunnel. Ageing models, which are key to maintenance strategy decision-making, are established with three failure modes—flexural capacity, convergence, and crack. Carbonation penetration from the inner surface of the lining, as well as chloride erosion from the outer surface of the lining, is considered in formulating the time-variant deterministic models for the three failure modes. Then, considering the uncertainties of the variables, statistical models of the structural response are obtained to calculate the probability characteristics. Also, the statistical model for the load effect is established based on the modified routine method. The probability of structural failure is determined based on the statistics of the structural response and the load effect models. Furthermore, a maintenance strategy is proposed at the system level with the performance-based maintenance method. Finally, an example is illustrated to demonstrate the application of the proposed method. The result shows that the standard deviation of the flexural capacity of the lining has similar time-variant characteristics as the means. The coefficient of vibration changes remains in a narrow interval during the ageing process. The tunnel lining crown and bottom have a higher probability of failure than other zones. Moreover, the failure probability of the positive moment zones is higher than that of the negative moment zones. The result can assist tunnel managers in developing a rehabilitation or replacement plan for the operation of tunnels.

Research on Global Stability Design of Lower Flange of Steelconcrete Composite Beams under Negative Bending Moment

The instability problem of steel-concrete composite beams will occur in the negative bending area, and different codes get different results for beams bearing negative bending moment on stability. In this paper, the MATLAB is used to analyse the parameter sensitivity of span, height-thickness ratio of web, concrete slab reinforcement ratio based on Eurocode 4 and China Code for Design of Steel and Concrete Composite Bridges. Through a large number of data analysis, the influence degree of each parameter on the stability is determined, and the results of the two codes are compared, which provides a basis for the design of the stability in the negative moment zone of steel-concrete composite beams.

Innovative strengthening schemes of concrete cantilever beams using CFRP sheets: End anchorage effect

The potential of improving the structural performance of concrete cantilever beams by extending the attachment of CFRP sheets to concrete columns through chamfers and fillets, constructed at beam-column connection, was investigated; while considering the impact of end anchorage using U-wraps of CFRP sheets or steel plates. Fourteen beam-column connections with 750-mm-long cantilever beams (150 × 150 mm) were load-tested after the attachment of CFRP sheets along the negative moment zone of the cantilever beams only or extending attachment of CFRP sheets’ to the columns surface through creating chamfers and fillets at the beam-column joints. CFRP sheets’ bond length and width, and number of layers, as well as the use of concrete chamfers and fillets had a significant impact upon mechanical performance of tested cantilever beams. End anchorage using U-wrap CFRP sheets was beneficial in delaying CFRP sheets debonding hence improving mechanical response; especially for cantilever beams, repaired by extending CFRP sheets to columns surface using large concrete chamfers.

Comparative Study on Stability Theory of Steel-concrete Composite Beams in Negative Moment Region Based on ABAQUS

Steel-concrete composite beams are environmentally-friendly and cost-effective. The existing specifications have no calculation method for the stability of steel-concrete composite beams in the negative bending area, and the computational results of different theories put forward by scholars are quite different. In this paper, the finite element method is used to compare the three theoretical methods of the critical load in the negative bending moment zone when the length, thickness ratio and the reinforcement ratio of the concrete plate are changed. The analysis shows that the bending energy method is inaccurate and insecure, and the beam on elastic foundation method is more concise and simpler than the bending torsion energy method.

Properties of Fiber Reinforced High Performance Cement Based Overlays for Orthotropic Bridge Decks

An increasing number of steel orthotropic bridge deck require repair due to fatigue cracking. For the rib to deckplate joint, a possible solution is to reinforce the fatigue prone deck plate by replacing the asphalt wearing surface with a reinforced high strength concrete overlay. However, this process has been proven very difficult to execute and consequently has some issues regarding durability. This paper reports on high performance fiber reinforced cement based overlay solutions with gritted epoxy interlayer for a typical orthotropic deck with inadequate plate dimensions. Material characterization of three mix designs includes determination of compressive and tensile strength, and post crack behavior by CMOD registration from 3PBT. Furthermore, the steel-concrete composite action in the positive and negative bending moment zone is investigated experimentally. The test results give insight into the concrete behavior, the steel-epoxy-concrete interaction, and are used as input for a 3D finite element model of the reinforced deck under fatigue loading. The results indicate that fiber reinforced cement based overlay solutions may be a viable option for orthotropic decks with inadequate deck plate thicknesses

Numerical investigation of continuous composite girders strengthened with CFRP

Nonlinear behavior of two-span, continuous composite steel-concrete girders strengthened with Carbon Fiber Reinforced Polymers (CFRP) bonded to the top of concrete slab over the negative moment region was evaluated using a non-linear Finite Element (FE) model in this paper. A three-dimensional FE model of continuous composite girder using commercial software ABAQUS simulated and validated with experimental results. The interfacial regions of the composite girder components were modeled using suitable interface elements. Validation of the proposed numerical model with experimental data confirmed the applicability of this model to predict the loading history, strain level for the different components and concrete-steel relative slip. The FE model captured the different modes of failure for the continuous composite girder either in the concrete slab or at the interfacial region between CFRP sheet and concrete slab. Through a parametric study, the thickness of CFRP sheet and shear connection required to develop full capacity of the continuous composite girder at negative moment zone have been investigated. The FE results showed that the proper thickness of CFRP sheet at negative moment region is a function of the adhesive strength and the positive moment capacity of the composite section. The shear connection required at the negative moment zone depends on CFRP sheet\'s tensile stress level at ultimate load.

노출강연선으로 보강된 하이브리드 건축용 OCB보의 실물모형 재하실험연구

최근 국내의 건축구조는 공간활용을 극대화할 수 있도록 계획되고 있다. 공간활용의 장점을 실현하기 위하여 하이브리드 OCB (Optimized Composite Beam)를 개발하였다. 건축용 OCB는 부모멘트 구간에서는 노출강연선으로 보강된 H형강으로 구성되고 정모멘트 영역에서는 PSC 구조로 구성된다. 본 연구에서는 건축용 OCB의 휨성능을 조사하였다. 15 m 길이의 시험체를 제작하여 3점 휨실험을 실시하였다. 실험을 통해서 다음과 같은 결과를 얻었다. 1) 시험체의 재하실험에서 초기 균열은 설계사용 하중의 171%에서 발생되었다. 2)사용하중에서 처짐은 건축구조설계기준에 제시된 허용처짐량 이내로 만족하였다. 3)균열의 양상, 파괴모드 그리고 극한하중 등의 실험결과는 3D 수치해석결과와 비교검토되었다. 본 실험연구 결과로부터 건축용 OCB보의 구조적인 신뢰성이 입증되었다.

장경간 건축구조를 위한 하이브리드 OCB보의 개발

최근 국내의 건축구조는 공간활용을 극대화할 수 있도록 계획되고 있다. 공간활용의 장점을 실현하기 위하여 하이브리드 OCB(Optimized Composite Beam)보를 개발하였다. 본 논문에서 개발된 건축용 OCB보는 부모멘트 구간에서는 노출강연선으로 보강된 H형강으로 구성되고 정모멘트 영역에서는 프리텐션 방식의 PSC 구조로 구성된다. 본 연구에선 유한요소법을 이용하여 건축용 OCB보의 휨성능을 조사하였다. 15m, 20m, 30m 길이의 OCB모형을 구성하여 재료 및 기하학적 비선형 정적해석을 수행하였다. 해석결과로부터 다음과 같은 결과를 얻었다. 1)해석모델의 초기균열은 사용하중이상에서 모두 발생되었다. 2)사용하중단계에서 처짐은 건축구조설계기준에 제시된 허용처짐량 이내로 만족하였다. 3)유한요소모델의 파괴 시 극한하중은 모두 단면의 설계공칭강도 이상에서 발생되었다. 해석결과로부터 건축용 OCB보의 구조적인 신뢰성이 입증되었다. The building structure in Korea is planned to maximize the use of space in recent. The hybrid OCB(Optimized Composite Beam) beam is developed to take advantage of using the space. The OCB beam is composed of the steel H-beam section reinforced by open strands in negative moment zone and the pretensioned PSC concrete section in positive zone. Flexural behavior of typical architectural hybrid OCB beam section was investigated by F.E.M. The 15m, 20m, 30m OCB models were tested on nonlinear material and geometry under static loading system. Following results are obtained from the analysis; 1)The OCB beam develop initial flexural cracking over full service loading. 2)Overall deflections of OCB beam under the service loads are less than those of the allowable limits in KCI Code(2012). 3)The ultimate load capacity get over the nominal strength of the OCB main section. The OCB beam is verified of structural reliability from the finite element analysis.

Using Steel Fiber Reinforced Concrete (SFRC) as an Alternative to Negative Reinforcement in Continuous RC Slab Panels

This study presents an experimental investigation performed to investigate the using of steel fiber reinforced concrete (SFRC) as an alternative to negative reinforcement in continuous RC thin slab panels. More rational way has been used by replacing negative reinforcement near interior supports by steel fiber reinforced concrete (SFRC). Tests were carried out on four slab panels, simply supported under single point loading. One of which were made fully with NSC, and the others were made partially with SFRC in negative moment zone. Experimental results show that the ultimate load capacity are increased (23% -58%) and the cracking loads are increased (25% -62.5%) for tested specimens strengthened with SFRC, in comparison with the reference specimens. Crack arrest mechanism of steel fibers limits crack propagation, improves the ultimate and tensile strength. So, more practical technique can be concluded from this study and employed in manufacturing of thin slabs.

Experimental characterisation of load responses to failure of a RC frame and a NSM CFRP RC frame

This paper presents results from loading to failure of two reinforced concrete (RC) frames. The control frame was under-reinforced with 0.94% tension steel in the beams. The second frame was as the control, with the additional use of up to 0.35% near-surface mounted tension CFRP reinforcement in the positive moment zone and one negative moment zone of the beams. Each frame was indeterminate to the degree four and comprised a rectangular bay with tip-restrained cantilever extensions to represent beam lengths to contraflexure in adjacent bays. The FRPRC frame was found to carry 37% more load than the control. Using the recorded beam reinforcement strains in nonlinear constitutive RC models, the peak moments along the frame were quantified and shown to satisfy equilibrium. Accordingly, the control frame, but not the FRPRC frame, was found to exhibit significant beneficial moment redistribution at higher loads which offset the adverse moment redistribution due to cracking at lower loads. The control frame exhibited ductile failure. Surprisingly, the brittle separation of the FRP which defined failure of the FRPRC frame started in the beam’s midspan zone without evidence of local yield. This non-yield midspan separation mode, normally associated with extensive steel yield, should be studied further.

Distortional and local buckling of steel-concrete composite box-beam

Distortional and local buckling are important factors that influences the bearing capacity of steel-concrete composite box-beam. Through theoretical analysis of distortional buckling forms, a stability analysis calculation model of composite box beam considering rotation of steel beam top flange is presented. The critical bending moment calculation formula of distortional buckling is established. In addition, mechanical behaviors of a steel beam web in the negative moment zone subjected separately to bending stress, shear stress and combined stress are investigated. Elastic buckling factors of steel web under different stress conditions are calculated. On the basis of local buckling analysis results, a limiting value for height-to thickness ratio of a steel web in the elastic stage is proposed. Numerical examples are presented to verify the proposed models.