Longitudinal Bar Ratio Research Articles

Experimental and numerical study on the shear behavior of hybrid BFRP/steel RC beams without shear reinforcement

Hybrid reinforcement of steel and fiber-reinforced polymer (FRP) bars can provide a balanced performance between strength, ductility, and durability for the concrete structures. Thus, this study evaluates the shear performance of hybrid Basalt-FRP/steel-RC beams without shear reinforcement. A total of twelve RC beams were cast and tested under four-point loading to study the influence of reinforcement type, reinforcement ratio of longitudinal bars and axial stiffness between FRP and steel ratio, [Formula: see text] on the shear performance of the concrete beams. The test results were analyzed in terms of crack patterns, load-midspan deflection, load-crack width relationships, and shear strength of the tested beams. In addition, the experimental results were compared with the theoretical results obtained from various design codes and guidelines. Moreover, a finite element (FE) model was created, and the experimental results were used for validation of the FE model. The test results revealed that the beam specimens designed with similar effective reinforcement ratio exhibits comparable shear strength nevertheless the tensile reinforcement used. Experimental test results demonstrated that using hybrid bars increased the shear capacity of the beams by 11% compared to steel bars. Furthermore, the energy absorption of the hybrid-RC beams was enhanced by 16% compared to FRP-RC beams. By elevating the [Formula: see text] ratio from 1.25 to 2.44, both the shear strength and energy absorption improved by approximately 10%. Notably, there was a significant decrease in the deflection and crack width of the hybrid-RC beams in comparison to the FRP-RC beams. The design codes ACI440.11-22 and CSA S806-12 displayed accurate enough estimates of the shear strength, while JSCE-1997 showed a conservative result.

Experimental and numerical study on the dynamic behavior of prestressed concrete panels suffering low speed impact

Bidirectional bonded prestressed concrete (PS) structures have widely been used in energy infrastructure. Nonetheless, there has been a lack of research on the effect of the reinforcement ratio of longitudinal bars on the dynamic behavior of PS structures. Therefore, the low speed drop hammer impact tests of 4 reinforced concrete (RC) and 9 PS panels have been completed in this paper, firstly. The dynamic behavior of PS panels has been investigated through work, and the working mechanism of the reinforcement ratio on the dynamic behavior of the specimens is analyzed. The results show that the panels mainly suffer plastic deformation and damage; increasing the reinforcement ratio could improve the membrane force effect and enhance the confinement effect on concrete to improve the load-bearing capability of the specimen. However, it will expand the plastic damage area of the specimen. Furthermore, the numerical models have been verified based on the test results to deeply analyze the influence of reinforcement ratio on the energy dissipation mechanism of the specimen. It shows that the increase of the reinforcement ratio of reinforcement will increase the energy dissipation proportion of reinforcement, but it will also increase the plastic energy consumption ratio of the concrete and increase the prestress loss after the impact; while, the rise of the reinforcement ratio of prestressing bars is beneficial to use the reinforcement, prestressing bars and tensile bars fully. Therefore, the designer could increase the reinforcement ratio of prestressing bars to enhance the impact resistance of the structures.

Unified two-way shear model for steel and FRP-RC slabs: Evaluation and reliability calibration

This study introduces a unified two-way (punching) shear design model for concrete reinforced with slabs steel- and FRP bars. The unified model modifies the ACI 318-19 punching shear model to account for the axial stiffness of the tension reinforcement by providing a new modification term (ρ Ereft./Ec)0.26, representing the reinforcing ratio multiplied by the elastic modular ratio of longitudinal bars to concrete. The unified punching shear model has been evaluated over two experimental datasets: FRP-RC slabs (145 specimens) and steel-RC slabs (390 specimens). A comparative assessment between the ACI 440.11-22, CSA S806, JSCE, and the proposed models to test their generalization abilities for slabs reinforced with both types of reinforcement. The assessment indicated a similar performance of the proposed model to the CSA S806 and the JSCE models in terms of statistical measures. The ACI 440.11–22 showed a high average strength ratio (Vexp./Vpred.) of 1.83. However, the ACI and the CSA models showed reduction in the conservatism at high reinforcement axial stiffness. In addition, reliability analysis using Monte Carlo simulation indicated that the unified punching shear model provides a satisfactory safety level with a reliability index between 3.5 and 4.0 for both steel and FRP-RC slabs.

Deformation capacity of flexural-controlled SRC columns under lateral cyclic load

Steel reinforced concrete (SRC) columns are widely used in high-rise buildings of earthquake prone regions. Fewer studies have been carried out on the estimation method of deformation capacity for flexural-controlled SRC columns, which is vital for the seismic evaluation. In the paper, 110 SRC columns with a failure dominated by flexural effect were collected from literatures to establish a test database firstly. The data of yield rotation and ultimate rotation of column specimens were extracted from the backbone curves. Then, an analytical method to predict the yield rotation was developed based on the classical model for flexural-dominated RC columns, showing a reasonable accuracy. The influences of shear-span ratio, axial load, steel shape ratio, transverse reinforcement, total longitudinal bar ratio and concrete compressive strength on the ultimate rotation were discussed with the test results of database specimens. Based on the parameter study, an empirical equation, related to shear-span ratio, axial compression ratio and concrete compression strength, was proposed via the regression analysis. In addition, the machine-learning methods, including the artificial neural network and support vector machine, were adopted to evaluate the accuracy of proposed empirical equation further. Finally, some useful recommendations were provided for the seismic evaluation of SRC columns.

Analytical Study of Geometric Parameter Effect on the Behavior of Horizontally Curved Reinforced Concrete Deep Beam

Nonlinear finite element simulation was once employed to look into the behavior of horizontally curved reinforced concrete deep beams under concentrated load at its mid-span. The study focused on the parametric impact of span length-to-depth (L/D) and span length-to-radius (L/R) ratios. In addition, the effect of longitudinal and spacing of shear reinforcement on the behavior of the beam has been investigated. The study considered sixteen beam specimens. Three of these specimens were straight beams as a control, and others were curved beams. The concrete-damaged plasticity model has been used to model the beam with C-25 grade concrete and steel reinforcements having diameters of ∅ 4 mm, ∅ 10 mm, and ∅ 12 mm with 568 MPa, 596 MPa, and 643 MPa steel grade, respectively. Reduced twenty-noded brick (C3D20 R) and two-noded (T3D2) elements have been used for modeling concrete and steel, respectively. The ultimate load capacity, the strain distribution, the load-deflection curve, and the load-twisting curve are the main outputs of the FE simulation. The study confirmed a considerable decrease in load-carrying capacity by up to 8.74% and 27.95% as the (L/R) ratio increased from 0 to 1.57 and the L/D ratio increased from 2.4 to 3, respectively. However, as the longitudinal steel ratio increased from 0.02042 to 0.02608 and the spacing of shear reinforcement decreased from 100 mm to 50 mm, the ultimate load capacity is increased up to 9.28% and 4.3%, respectively. Sensitivity evaluation was also conducted to see how much the independent variables (L/D ratio, L/R ratio, longitudinal bar ratio, and spacing transverse reinforcement) affect the dependent parameter (ultimate load capacity).

Experimental Study on SSRC under Eccentric Compression

The use of stainless steel bars can improve the durability and sustainability of building materials. Through the static performance test, this research analyzes the failure pattern and bearing performance of bias stainless steel reinforced concrete (SSRC) column. The influence of reinforcement ratio of longitudinal bars and eccentricity on the mechanical performance of specimens was studied. Different constitutive models of stainless steel bars were used to calculate the ultimate bearing capacity of the section of the column under eccentric compression column. Based on the experimental results, a method to modify the expression of the design specification is proposed. And, the results were compared with the test results. The results showed that the damage patterns and failure modes of SSRC columns are essentially the same as those of traditional reinforced concrete columns. The bearing capacity of SSRC columns rises with the increase in the longitudinal reinforcement ratio, and the ductility of the specimens is enhanced. The ultimate load of the specimen decreases with the rise in eccentricity but the deflection increases gradually. The strain distribution of the mid-span section of the SSRC column conforms to the plane section assumption. The bearing capacity of the specimen can be analyzed by referring to the calculation method of the specification, and some parameters in the calculation formula of the specification are modified to adapt to the design and calculation of the SSRC column.

Overstrength requirements to avoid brittle shear failure in RC slender beams with stirrups

In this paper, an analytical expression was derived for the prediction of shear strength of slender reinforced concrete (RC) beams with stirrups. Failure occurring in B and D regions of the beam was considered. An expression derived for the shear strength of the B region was based on a modified constant angle truss model. The analytical expression derived consisted of three terms. The first one was related to the concrete contribution expressed through the square root of cylindrical compressive strength, geometrical ratio of longitudinal bars, and depth to width ratio. The second one was relating to the shear force induced by yielding of stirrups to the inclination of the main crack occurring at failure; its value depends on the geometrical ratio of longitudinal bars, transverse stirrups, and modulus of elasticity of concrete and steel. Finally, the third term was related to the shear force induced by the flexure of steel bars in the space s, which depends on the number and diameter of longitudinal bars and the yielding stress of longitudinal bars. Failure of the D regions was related to the concrete crushing of loaded joint under the effect of shear force and compressive force, including also the confinement effect due to the presence of stirrups. The original contribution of the paper was the determination of the maximum mechanical ratio of longitudinal and transverse steel to ensure over strength in shear respecting the strength hierarchy.Expressions derived and some of those available in the codes were verified against three different data banks of experimental researches available in the literature. The data banks utilized considered a very wide range of the geometrical ratio of longitudinal bars and transverse stirrups, including, for the latter, very high value producing brittle failure due to concrete crushing.

Fractional factorial design model for seismic performance of RC bridge piers retrofitted with steel-reinforced polymer composites

This study explores the effects of key design parameters on the performance of seismically deficient rectangular cross-section reinforced concrete (RC) bridge piers strengthened with steel-reinforced polymer (SRP) composites. Nonlinear response of bridge piers was modeled using fiber-based section discretization. Three-level fractional factorial design of experiments at 5% significance level was used to capture the effects of design parameters and their interactions, including concrete compressive strength, yield strength of steel bars, geometric ratio of longitudinal bars, internal transverse reinforcement spacing, pier aspect ratio, and number of retrofitting SRP layers. A parametric study was used to examine the main influence of and interactions between these factors on the seismic performance of SRP-strengthened piers at different damage limit states, including concrete core crushing, longitudinal reinforcement yielding and buckling, and ductility performance. Results show that the lateral load-carrying capacity and ductility performance of SRP-confined RC bridge piers were significantly influenced by the pier aspect ratio, materials properties, and amount of transverse and longitudinal reinforcement. Moreover, the resistance to buckling base shear, and overall ductility increased with increasing number of SRP layers.

Numerical and experimental investigations on the Park-Ang damage index for high-speed railway bridge piers with flexure failures

The traditional Park–Ang damage index (DI) has been devised for damage assessment of highway bridge piers and building columns, but is inappropriate for high-speed railway (HSR) bridge piers, which have much larger cross sections, much less longitudinal bar ratios and much less axial compression ratios under earthquakes. In order to obtain the appropriate DI for HSR bridge piers, numerical analysis of 144 HSR bridge piers and quasi-static test of 7 HSR bridge piers was done to study the combined parameter β in DI. The new DI was subsequently validated by pseudo-dynamic testing of 2 HSR bridge piers. The results show that the new combined parameter β in the reformulated DI for HSR bridge piers is mainly affected by the longitudinal bar ratio and the axial compression ratio; however, the traditional β and DI are significantly affected by the stirrup reinforcement ratio. It is suggested that 0.06, 0.19, 0.49 and 1.0 should be taken as the critical values for the new DI among the basically intact, slightly damaged, moderately damaged, severely damaged and collapsed states of HSR bridge piers with axial compression ratio being less than 0.09, while values 0.11, 0.33, 0.49 and 1.0 are recommended for HSR piers with an axial compression ratio from 0.09 to 0.15. As for the collapse state of HSR bridge piers, the cumulative damage contribution of energy to the full DI (of value 1.0) is 0.64. It is a large value and indicates that HSR bridge piers are vulnerable to cumulative damage from earthquakes.

Bearing behavior of reinforced concrete column-isolated footing substructures

Under earthquake, the interaction between column footing and soil foundation directly affects the failure mechanism of reinforced concrete (RC) structures. In the present study, to investigate the structural performance and failure mode of RC column-isolated footing substructures on foundation, a test device using rubber-wooden composite blocks to simulate elastic foundation based on Winkler foundation model was designed. Three RC column-isolated footing substructures were tested to evaluate the load-carrying capacity, failure mode, crack distribution, footing slab deflection, subgrade reaction, and rebar strain. The test parameter was the longitudinal bar ratio of the footing slab, and axial compression and lateral loading were applied to the column. Nonlinear analysis model was established in ABAQUS, and the predictions agreed well with the test results. On the basis of the analysis model, a parametric study was performed to investigate the effects of foundation coefficient, axial compression ratio, and footing slab thickness on the load-carrying capacity, failure mode, and column constraint.

Cracking strut-and-tie model for shear strength evaluation of reinforced concrete deep beams

In the present study, a novel cracking strut-and-tie model (CSTM) is developed to better predict the shear strength of a deep beam. The principal concept of strut-and-tie model (STM), the experimental phenomena of diagonal crack patterns, and strain distribution of longitudinal bars in shear span are considered. The proposed CSTM divides the diagonal strut into two parts according to the location of the critical shear crack (CSC). In the part above the CSC that is not affected by the flexural-shear cracks, its effective compressive strength is defined as the ultimate compressive strength of normal concrete strut. On the other hand, the effective compressive strength of the part below the CSC is derived from the forces transferred by the aggregate interlock, web reinforcement, and dowel action of longitudinal bars on the CSC surface. The improvement of aggregate interlock action under compression on the CSC surface, and the interaction between transverse reinforcement and longitudinal bars are theoretically considered. For practical design, a simplified CSTM is also proposed. The proposed method is applied to 355 existing test specimens, to predict the shear strength of deep beams with and without web reinforcement. The predicted results are compared with existing test results and the predictions of existing STMs. The results show that the prediction of the proposed model is better than those of other models. Further, parametric analysis is performed to evaluate the contribution of each shear mechanism. The analysis results show that the proposed CSTM is able to describe well the influences of main design parameters (i.e., shear span-to-depth ratio, longitudinal bar ratio, web reinforcement ratio, concrete compressive strength, and effective depth) on the shear resistance of deep beams.

Axial compression analysis of square concrete-encased concrete filled steel tubular stub columns after fire exposure

Abstract This paper presents results from numerical studies on the effect of critical factors governing the axial compression mechanical properties on square cross-section concrete-encased concrete filled steel tubular (CFST) stub columns after fire exposure. A validated three dimensional finite element model is applied for evaluating axial compression mechanical properties of concrete-encased CFST stub columns after fire exposure under different factors. The factors varied in the parametric study include fire exposure time, steel tube area ratio, longitudinal bar ratio, compressive strength of concrete, yield strength of steel tube, and sectional core area ratio. Results from parametric studies show that axial compressive strength and axial stiffness of square concrete-encased CFST stub columns after fire decrease significantly. The decline of axial stiffness is higher than that of axial compression ultimate bearing capacity. Fire exposure time and sectional core area ratio have significant influence on the reduced level of axial compressive strength and axial stiffness of square concrete-encased CFST stub columns after fire exposure. Steel tube area ratio and reinforcement ratio have some influence on the reduced level. Compressive strength of concrete and yield strength of steel tube have limited influence on the reduced level.

Parametric Study on Mixed Torsional Behavior of U‐Shaped Thin‐Walled RC Girders

Nowadays, U‐shaped thin‐walled concrete girders have been widely applied in the urban construction of rail viaducts in China as well as worldwide. However, the mixed torsional behaviors of these structures are not well understood. In this paper, the mixed torsional behaviors of the U‐shaped thin‐walled RC girders are theoretically analyzed, and a method predicting failure modes and ultimate torques is proposed. Nonlinear FE models based on ABAQUS to simulate the mixed torsional behaviors are built and calibrated with the test results. Parametric studies considering three crucial parameters (boundary condition, span length‐section height ratio, and ratio of longitudinal bars to stirrups) are conducted based on both the above suggested calculating method and the FE modeling. The calculated and the simulated results agree well with each other and with the test results. It is found that the failure modes of the U‐shaped thin‐walled RC girders under torsion are influenced by all the three parameters. Three kinds of failure modes are observed: flexural failures dominated by warping moment, shear failures caused by warping torque and circulatory torque, and flexural‐shear failures in the cases where flexural failure and shear failure appear almost at the same time.

Numerical study on the cyclic response of AFRP reinforced columns with externally unbonded energy dissipaters

Fiber-reinforced polymer bars are gaining increased use in the construction market as a replacement to steel reinforcement for concrete structures. However their use as compression members is limited due to their mechanical properties, typically low flexural rigidity, so that local buckling may occur relatively at low stress levels. Their premature rupture in reinforced concrete (RC) columns could be eliminated by enhancing the columns’ response elastically and concentrating damage to the externally installed, replaceable post-tensioned structural fuses. This paper is part of a larger numerical and experimental investigation to develop and validate design methods for a rocking aramid fiber reinforced polymer (AFRP) reinforced concrete column. The numerical study, implemented using the Extreme Loading for Structures software, consists of a total of thirteen columns. The columns were subjected to axial load followed by cyclic lateral load up to failure to examine the effectiveness of the rocking connection and overall response. The effect of key variables such as longitudinal bar ratio, structural fuse sizes and tie spacing was investigated. Ductility and energy dissipation gains of the rocking AFRP RC columns were compared to that of the conventionally reinforced steel column, and expected performance of the rocking connection was achieved.

Performance of concrete-encased CFST box stub columns under axial compression

A finite element analysis (FEA) model is developed to predict the full range response of concrete-encased CFST box stub columns under axial compression. In the model, concrete across the composite section is divided into four regions, i.e. the outer unconfined concrete outside the stirrup, the outer concrete in the web walls, the outer confined concrete in the corners, and the core concrete in the steel tubes. Different material models are used in each region. The analytical results are compared to past experiments, and generally good agreement between the predicted and measured results is obtained. Load-axial strain, loading distribution between the inner CFST and outer RC components and interface stresses between steel and concrete are analyzed. The influence of the web wall slenderness is also investigated. Parameter studies investigate the influence of concrete and steel strength, steel ratio of CFST, longitudinal bar ratio, stirrup spacing and ratio of the diameter of CFST to the sectional width on the ultimate load. A simplified model is proposed to predict the ultimate load of the concrete-encased CFST box stub columns under axial compression.

Behaviour of concrete-encased CFST columns under combined compression and bending

The performance of concrete-encased CFST column under combined compression and bending is studied in this paper. A finite element analysis (FEA) model is developed to analyse the behaviour of the composite column, and generally good agreement is achieved between the measured and predicted results in terms of the failure mode, the load-deformation relation and the ultimate load. Typical failure modes, full-range response of load-lateral deflection relation, loading distributions of the inner CFST and the outer RC components, the contact stress between the steel tube and the concrete of the composite columns are analysed. The influence of slenderness ratio and loading paths on the composite columns are also investigated. Influence of parameters, such as the strength of concrete and steel, steel ratio of CFST, longitudinal bar ratio and diameter of CFST on the sectional capacity of the concrete-encased CSFT columns is analysed based on the FEA model. A simplified model is proposed to calculate the sectional capacity of concrete-encased CFST columns under combined compression and bending.

Research on Deformation Index Limits of RC Shear Walls Based on Material Damage

This paper divides the seismic performance of shear wall into five levels: integrity, slight damage, slight ~ moderate damage and serious damage which are defined based on material damage. And physical and mechanical description of shear walls in each performance level is given. The displacement angle is selected as the seismic performance index limit. The numerical analysis of 524 pieces of shear walls has been made to discuss the influence on seismic deformation index limits of component according to axial compression ratio, flexure shear ratio, the nominal shear stress level, the hoop characteristic value and the reinforcement ratio of longitudinal bars. With mathematical statistic method, the calculation formula for deformation index limits of components is obtained, which can be used as the basis in the performance-based seismic evaluation of shear wall structures.

프리캐스트 콘크리트와 현장타설 콘크리트 복합 보의 전단강도

최근 다른 압축강도로 타설된 프리캐스트 콘크리트(PC)와 현장타설 콘크리트(CIP)의 복합 부재의 사용이 증가하고 있지만 현행 기준에는 서로 다른 강도로 복합화된 부재의 전단강도에 대한 설계 기준이 없다. 그래서 이번 연구에서 서로 다른 압축강도(24 MPa, 60 MPa)로 분리 타설된 보의 전단강도 실험을 수행하여 복합 부재의 전단강도에 대해 알아보았다. 변수로는 단면형상, 휨철근비, 그리고 전단경간비를 고려하였다. 실험 결과 값과 현행 전단 기준식과 단면적비로 계산한 유효 콘크리트 강도를 이용한 예측 값을 비교하였다. 실험 결과를 분석해보면 철근비가 낮고 압축대에 60 MPa가 사용된 실험체들에 대해 설계 기준식을 과대평가하였다. 실험 결과를 기준으로 PC와 CIP 복합부재의 전단설계 기준을 제안하였다.

RC 사각단면 기둥의 전단거동특성과 축방향철근비를 고려한 초기전단강도

횡하중을 받는 RC 기둥의 전단강도는 기둥의 변위연성도가 증가함에 따라 감소하는 것으로 알려져 있다. 연성도의 증가에 따른 전단강도의 감소율은 초기전단강도에 따라 크게 좌우되므로 이를 합리적으로 예측하기 위해서는 초기전단강도의 평가가 매우 중요하다. 기둥의 전단거동은 단면모양, 형상비, 축력, 축방향철근비, 연성도 등 다양한 요인에 의하여 영향을 받아 복잡하다. 본 연구에서는 형상비, 단면의 중공비, 축방향철근비, 중공 및 중실단면을 변수로 하는 시험체를 제작하여 실험적 연구를 수행하여 전단거동특성을 살펴보았다. 또한, 축방향철근이 전단강도에 미치는 영향을 분석하여 형상비와 축력을 고려한 기존의 초기전단평가식을 보완하였으며, 그 타당성을 검증하였다. It is well known that the shear strength of an RC column subjected to a lateral force decreases with the increase of the displacement ductility of column. This decreasing rate of shear strength is quite dependent on the initial shear strength. Therefore, the evaluation of the initial shear strength is important to predict the shear strength with reasonable accuracy. The shear behavior is complex because many parameters, such as the sectional shape, aspect ratio, axial force, longitudinal bars and ductility, are mutually interactive. In this study, the initial shear strength has been investigated by experiments varying parameters such as the aspect ratios, void ratios, ratio of longitudinal bars and sectional types. A new empirical equation for the initial shear strength, considering the ratio of the longitudinal bars, has been proposed and its validity has been assessed.

Application of ANN to evaluate effective parameters affecting failure load and displacement of RC buildings

Abstract. This study investigated the efficiency of an artificial neural network (ANN) in predicting and determining failure load and failure displacement of multi story reinforced concrete (RC) buildings. The study modeled a RC building with four stories and three bays, with a load bearing system composed of columns and beams. Non-linear static pushover analysis of the key parameters in change defined in Turkish Earthquake Code (TEC-2007) for columns and beams was carried out and the capacity curves, failure loads and displacements were obtained. Totally 720 RC buildings were analyzed according to the change intervals of the parameters chosen. The input parameters were selected as longitudinal bar ratio (ρl) of columns, transverse reinforcement ratio (Asw/sc), axial load level (N/No), column and beam cross section, strength of concrete (fc) and the compression bar ratio (ρ'/ρ) on the beam supports. Data from the nonlinear analysis were assessed with ANN in terms of failure load and failure displacement. For all outputs, ANN was trained and tested using of 11 back-propagation methods. All of the ANN models were found to perform well for both failure loads and displacements. The analyses also indicated that a considerable portion of existing RC building stock in Turkey may not meet the safety standards of the Turkish Earthquake Code (TEC-2007).