Concrete Corbels Research Articles

Behavior of wet precast beam column connections under progressive collapse scenario: an experimental study

Progressive collapse denotes a failure of a major portion of a structure that has been initiated by failure of a relatively small part of the structure such as failure of any vertical load carrying element (typically columns). Failure of large part of any structure will results into substantial loss of human lives and natural resources. Therefore, it is important to prevent progressive collapse which is also known as disproportionate collapse. Nowadays, there is an increasing trend toward construction of buildings using precast concrete. In precast concrete construction, all the components of structures are produced in controlled environment and they are being transported to the site. At site such individual components are connected appropriately. Connections are the most critical elements of any precast structure, because in past major collapse of precast structure took place because of connection failure. In this study, behavior of three different 1/3rd scaled wet precast beam column connections under progressive collapse scenario are studied and its performance is compared with monolithic connection. Precast connections are constructed by adopting different connection detailing at the junction by considering reinforced concrete corbel for two specimens and steel billet for one specimen. Performance of specimen is evaluated on the basis of ultimate load carrying capacity, maximum deflection and deflection measured along the span of the beam. From the results, it is observed that load carrying capacity and ductility of precast connections considered in this study are more than that of monolithic connections.

Bond strength of glass fiber-reinforced polymer bars in concrete after exposure to elevated temperatures

This paper presents the test results of an experimental study to investigate the effect of elevated temperatures on the bond properties of glass fiber-reinforced polymer bars in concrete. For this purpose, a total of 39 pullout specimens with glass fiber-reinforced polymer bars were cast for bond strength tests. In addition to the laboratory temperature, specimens were exposed to heating regimes of 100, 200, 300 and 350℃ for a period of 1, 2 or 3 h. The test results are presented in terms of bond strength, bond–slip relationship, and mode of failure. All specimens failed by shearing of the concrete corbels surrounding the bars and almost no damage was seen in the glass fiber-reinforced polymer bars. The results showed that the bond strength decreased as the temperature or exposure period increased. Reductions of about 20% of the original bond strength were recorded after exposure to 100 and 200℃ for 3 h. Significant reductions of about 50% in the bond strength were recorded after exposure to 350℃ for periods of 2 and 3 h. A modification to the ACI (American Concrete Institute) and CEB-FIP (Comité Euro-International du Béton-Fédération Internationale de la Précontrainte) equations was developed to consider the effect of elevated temperatures and it showed good agreement with test results.

Numerical analysis of reinforced high strength concrete corbels

This paper deals with comparisons of numerical results of reinforced high strength concrete corbels and experimental results obtained from the literature. The application of precast structures and high strength concrete have increased in the last years and taking into account the possibility of brittle failures using this solution, specific investigations are demanded to some structural details, as for example, corbels. It should be mentioned that brittle failures and complex regions are difficult to be analyzed by means of computational resources and for that reason it is argued if the available packages software are able to correctly describe the behavior of high strength concrete, specifically for corbels. In this way, the numerical simulations described in this paper have been conducted using the Finite element method. A parametric study was then carried out using the validated model to investigate the effect of the shear span-to-effective depth ratio, the main reinforcement rate and the compressive strength of concrete in the failure load and the contribution of secondary reinforcements. The main conclusions are: the numerical and experimental results showed a very close agreement, not only in the cracking and failure modes, but also in the load–deflection and load-reinforcement strain responses; there is a strong linear correlation of the failure load and shear span-to-effective depth ratio, the main reinforcement rate and the compressive strength of concrete; the horizontal secondary reinforcement improves the load capacity and could be considered in the design models; and the vertical secondary reinforcement only affects in the cracking distribution and ductility of the corbels.

Effective Strengths of Concrete Struts in Strut-Tie Models of Reinforced Concrete Corbels

스트럿-타이 모델 방법은 철근콘크리트 코벨의 극한강도 해석 및 설계에 매우 효과적이다. 그러나 스트럿-타이 모델 방법을 이용한 철근콘크리트 코벨의 정확한 해석 및 안전한 설계를 위해서는 스트럿의 유효강도를 정확하게 결정하여야 한다. 이 연구에서는 여러 연구문헌에서 제안된 철근콘크리트 코벨의 대표적인 세 종류의 스트럿-타이 모델을 활용하기 위하여 철근콘크리트 코벨의 기하학적 형상, 수직 및 수평 하중의 조합 비, 그리고 휨철근 및 수평전단철근 비 등 주요 설계변수들의 영향을 정확하게 반영할 수 있는 스트럿 유효강도 식을 개발, 제안하였다. 현행 여러 설계기준서의 스트럿 유효강도 식과 이 연구에서 제안한 유효강도 식을 이용하여 파괴실험이 수행된 243개 철근콘크리트 코벨의 극한강도를 평가하였으며, 그 결과의 비교분석을 통해 이 연구에서 제안한 스트럿 유효강도 식의 적합성을 평가하였다. The strut-tie model approach has proven to be effective in the ultimate analysis and design of structural concrete with disturbed regions. For the reliable analysis and safe design of the structural concrete, however, the effective strengths of concrete struts must be determined accurately. In this study, the equations of the effective strengths of concrete struts, which are useful for the three types of determinate and indeterminate strut-tie models of reinforced concrete corbels, were proposed. The effects of shear span-to-effective depth ratio, the vertical-to-horizontal force ratio, and flexural and horizontal shear reinforcement ratios were reflected in the development of the proposed equations. To examine the appropriateness of the proposed and existing equations, the ultimate strengths of 243 reinforced concrete corbels tested to failure were evaluated by using the three types of corbel strut-tie models.

An Indeterminate Strut-Tie Model and Load Distribution Ratio for Reinforced Concrete Corbels

철근콘크리트 코벨의 파괴거동은 코벨의 전단경간비 및 주인장 철근비를 포함한 여러 변수들로 인해 복잡하다. 이 논문에서는 철근콘크리트 코벨의 강도 및 거동 특성을 논리적이고 합리적인 방법으로 반영하여 전단경간비가 1.0 이하인 철근콘크리트 코벨을 설계할 수 있는 단순한 형태의 1차 부정정 트러스 구조의 스트럿-타이 모델을 제안하였다. 또한 부정정 스트럿-타이 모델을 정정 트러스 구조의 스트럿-타이 모델로 변환시켜 현행 스트럿-타이 모델 설계기준에 의한 코벨의 설계를 가능하게 하는 하중분배율을 제안하였다. 하중분배율 결정 시 스트럿과 타이의 재료적 비선형 거동을 고려할 수 있는 부정정 스트럿-타이 모델의 비탄성 구조해석을 통해 철근콘크리트 코벨의 강도 및 거동을 지배하는 전단경간비, 주인장 철근비, 수직하중에 대한 수평하중의 비, 그리고 콘크리트의 압축강도 등의 주요설계변수들의 영향을 반영하였다. 이 연구에서 제안한 부정정 스트럿-타이 모델 및 하중분배율의 적합성을 검증하기 위해 파괴실험이 수행된 다수의 철근콘크리트 코벨의 극한강도를 평가하였으며, 그 결과를 ACI 318-11의 전통적인 설계기준 및 정정 트러스 구조의 스트럿-타이 모델을 기반으로 하는 현행 주요 설계기준의 스트럿-타이모델 방법에 의한 평가결과와 비교분석하였다. The ultimate behavior of reinforced concrete corbel is complicated due to the primary design variables including the shear span-to-effective depth ratio a/d, flexural reinforcement ratio, load condition, and material properties. In this study, a simple indeterminate strut-tie model reflecting all characteristics of the ultimate strength and complicated structural behavior is proposed for the design of the reinforced concrete corbels with shear span-to-effective depth ratio of <TEX>$a/d{\leq}1$</TEX>. A load distribution ratio, defined as the fraction of applied load transferred by horizontal truss mechanism, is also proposed to help structural designers perform the design of reinforced concrete corbels by using the strut-tie model approaches of current design codes. For the development of the load distribution ratio, numerous material nonlinear finite element analyses of the proposed indeterminate strut-tie model were conducted by changing primary design variables. The ultimate strengths of reinforced concrete corbels tested to failure were evaluated by incorporating the proposed strut-tie model and load distribution ratio into the ACI 318-11's strut-tie model method. The validity of the proposed model and load distribution ratio was examined by comparing the strength analysis results with those by the ACI 318-11's conventional design method and strut-tie model methods of current design codes.

PRIMARY AND SECONDARY REINFORCEMENTS IN REINFORCED CONCRETE CORBELS

The study is concerned with normal-strength concrete corbels. 30 such corbels were studied by finite element modelling and the variables considered include ratios of primary and secondary reinforcement, type of applied loading, vertical or horizontal. Finite element modelling with a software package LUSAS was used to analyse four series of corbels namely PV series (primary reinforcement with vertical loading), SV series (secondary reinforcement with vertical loading), PH series (primary reinforcement with horizontal loading) and SH series (secondary reinforcement with horizontal loading). The results indicate that corbels with neither primary reinforcement nor secondary reinforcement fail suddenly. In the case of PV series and SV series, corbels increase in ratio of primary and secondary reinforcement generally resulted in enhancement of strength and ductility when subjected to only vertical loading. This increase is significant up to 0.4% in the case of primary reinforcement and 0.3% in the case of secondary reinforcements. No noticeable change in ultimate load or ductility was observed for corbels in PH series and SH series.

Response of Concrete Corbels Reinforced with Internal Steel Rebars and External Composite Sheets: Experimental Testing and Finite Element Modeling

This paper aims to investigate the structural response of concrete corbels reinforced internally with steel rebars and externally with carbon fiber-reinforced polymer (CFRP) composite sheets. Nine specimens were constructed and tested. Test parameters included the amount of internal longitudinal steel rebars and configuration of the external composite sheets. Nine two-dimensional finite-element (FE) models were developed, assuming a perfect bond between the CFRP and concrete. Three additional FE models were developed in which an interfacial bond stress-slip model was adopted between the diagonal CFRP reinforcement and the concrete. The external CFRP composite reinforcement resulted in up to 40% increase in the load capacity. The contribution of the external CFRP reinforcement to the load capacity decreased with an increased amount of internal steel rebars. The addition of primary longitudinal CFRP sheets in a direction parallel to the primary steel rebars reduced the steel strains and increased the yield and ultimate loads. The inclusion of secondary CFRP longitudinal reinforcement at the midheight of the corbels did not result in additional strength gain. The diagonal CFRP reinforcement restricted growth and widening of the shear cracks, and hence, increased the gain in the load capacity. The numerical load capacity was in the range of 10% error band. The numerical crack pattern, deflection response, and steel and CFRP strain responses were in good agreement with those measured experimentally. The integration of the interfacial bond stress-slip model in the FE analysis between the diagonal CFRP reinforcement and concrete resulted in more conservative/accurate predictions for the structural response.

Subassemblage cyclic loading test of RC frame with buckling restrained braces in zigzag configuration

SUMMARYA new buckling restrained braced frame system is proposed for reinforced concrete building structures, which is featured by the zigzag configuration of the braces and the corresponding connection details. The connection details tend to separate the vertical and horizontal components of force imposed by the braces to be resisted by independent structural components to make the behavior of the connection easier to estimate and control. The performance of the brace connection details was evaluated through cyclic load testing on 1/2‐scale subassemblies of the proposed system, each of which consisted of a reinforced concrete part and a set of buckling restrained braces. To simplify the test control, the specimens were rotated 90° in the test and were loaded by two displacement controlled actuators. The test results show that the normal and the shear resistance of the gusset plate connection are essentially independent of each other. However, the rotation of the gusset plate with respect to the beam‐to‐column joint may result in nonuniform force distribution of the anchor bolts, the primary resistance for tensile force. At the same time, such rotation may also subject the concrete corbels, the primary shear resistance, to unfavorable tensile force. In addition, it is also confirmed that the buckling restrained braces performed well in the proposed system. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Reinforced concrete corbels strengthened with carbon fiber reinforced plastics

A total of nine reinforced concrete corbels were tested, in this study. Six were externally strengthened with carbon fiber reinforced plastics (CFRP), in the horizontal direction. The cross-sectional area of CFRP and the shear span-to-effective depth ratios are the parameters considered, in this study. Test results indicate that the higher the cross-sectional area of CFRP, the higher is the shear strength of the corbels, and the lower the shear span-to-effective depth ratios, the higher is the shear strength of corbels. The shear strength predicted by the design provisions in section 11.8 of the ACI Code, the strut-and-tie model in Appendix A of the ACI Code, and the softened strut-and-tie (SST) model were compared with the test results. The comparisons show that both the strut-and-tie model in Appendix A of the ACI Code, and the SST model can accurately predict the shear strength of reinforced concrete corbels, strengthened with CFRP.

Macro-mechanical strut and tie model for analysis of fibrous high-strength concrete corbels

Reinforced concrete (RC) corbels becoming a frequent attribute in the building construction with the increasing use of pre-cast high-strength concrete (HSC). The use of fibrous high-strength concrete (FHSC), increases corbel ductility and thus defines the mode of failure of the corbels, depending on the fiber parameters. In this paper a macro-mechanical strut and tie model is proposed for analysis of fibrous high-strength concrete corbels. In this model the fibers can be used as a replacement of horizontal stirrups, due to increasing of shear friction of (FHSC). The analytical macro-mechanical model takes into consideration the effect of fiber volume, fiber length, and fiber diameter, random distribution of fibers, fiber HSC interface and shear span-to-depth ratio, respectively. This model is compared with available experimental results found in literature and a good agreement is obtained. The parametric study is performed to examine for different parameters affecting the analysis of (FHSC) corbels using the proposed macro-mechanical strut and tie model.

Shear capacity of reinforced concrete corbels using mechanism analysis

A mechanism analysis is developed to predict the shear capacity of reinforced concrete corbels. Based on shear failure observed in experimental tests, kinematically admissible failure mechanisms are idealised as an assemblage of two rigid blocks separated by a failure plane of displacement discontinuity. Shear capacity predictions obtained from the developed mechanism analysis are in better agreement with corbel test results of a comprehensive database compiled from the available literature than other existing models for corbels. The developed mechanism model shows that the shear capacity of corbels generally decreases with the increase of shear span-to-depth ratio, increases with the increase of main longitudinal reinforcement up to a certain limit beyond which it remains constant, and decreases with the increase of horizontal applied loads. It also demonstrates that the smaller the shear span-to-overall depth ratio of corbels, the more effective the horizontal shear reinforcement.

ANN Model for Predicting Ultimate Shear Strength of Reinforced Concrete Corbels

The artificial neural network (ANN) model was developed using previous experimental data on Reinforced concrete (RC) corbels to simulate the behavior of RC corbels. The neural network model has six input parameters representing the concrete compressive strength ( ), shear span (a), effective depth (d), corbel width (b), area of main reinforcement (As), area of secondary reinforcement (Ah), one output parameter representing the ultimate shear load (Vu). A back propagation neural network (BPNN) with the log-sigmoid activation function is adopted due to its accuracy of prediction. The ANN model is constructed using the experimental data from the literature. The ANN predicted ultimate shear load which compared with those calculated by ACI318-08 code Formula and Russo model. The neural network model is to predict the shear load of RC corbel more accurate than the ACI318-08 code Formula, and Russo model. Through the parametric studies using the ANN model, the effects of various parameters such as ( , a, d, b, As, and Ah) on the behavior of RC corbel were shown. The results reveal that the proposed network model captures the RC corbel underlying shear behavior very well.

Secondary Reinforcement in Concrete Corbel

This paper is concerned with finite element analysis of corbels. Analyses were carried out on 254 × 405 × 254-mm reinforced concrete corbels. In this study 24 reinforced concrete corbels were considered. Variables considered in this investigation are: number, position, and different arrangement of secondary reinforcement. The results show that arrangement of the secondary reinforcement affects both strength and ductility of corbels.

Influence of Steel Fibers and Headed Bars on the Serviceability of High-Strength Concrete Corbels

Vertical loading tests are reported for six double-sided, high-strength concrete corbel specimens. The primary variables of the investigation were the percentage of steel fibers and the anchorage method of the main tension tie. The test results indicated that performance in terms of load-carrying capacities, stiffness, ductility, and crack width was improved, as the steel fibers were added and the percentage of steel fibers was increased. The corbel specimens with headed bars used as the main tension-tie reinforcement showed superior load-carrying capacities, stiffness, and ductility compared with the corbel specimens in which the main tension ties were anchored by welding to the transverse bars. From the test results, it is expected that the load-carrying capacities, serviceability, and durability of high-strength concrete corbels would be improved by using steel fibers and headed bars. Experimental results presented were also compared with various prediction models.

Strengthening of reinforced concrete corbels with GFRP overlays

AbstractThe strength and post-peak performance of reinforced concrete corbels, strengthened with epoxy bonded glass fiber reinforced polymer (GFRP) overlays, were experimentally investigated. The test variables were the corbel shear span to depth ratio, corbel main reinforcement ratio, and the number and orientation of the GFRP fibers. In total, 24 normal strength concrete, one-third scale, corbel specimens, without hoop reinforcement, were tested to failure under quasi-static gravity loading. Test results revealed that GFRP overlays can easily be used for the enhancement of corbel load bearing capacity, depending on the fiber orientation. The main reinforcement ratio and the number of GFRP plies were found to be the two main variables affecting the level of strength gain in the corbel specimens.

FE analysis of reinforced concrete corbels with enhanced continuum models

The paper presents quasi-static FE-simulations of the behaviour of short reinforced concrete corbels with and without shear reinforcement. Concrete was modelled with the aid of three various constitutive continuum models: an isotropic elasto-plastic, an isotropic damage and an anisotropic smeared crack approach. To ensure the mesh-independent numerical results and to describe strain localization, a characteristic length of micro-structure was introduced by means of a non-local theory. To simulate the behaviour of reinforcement, an associated elasto-plastic constitutive model with isotropic hardening was assumed. The effects of a characteristic length, bond between concrete and reinforcement, tensile and compressive fracture energy and shear reinforcement on strain localization were carefully investigated. The numerical results were compared with corresponding laboratory tests.

Influence of cracked concrete models on the nonlinear analysis of High Strength Steel Fibre Reinforced Concrete corbels

In the present study, new material constitutive relationships for High Strength Steel Fibre Reinforced Concrete (HSSFRC) have been formulated by means of a regression analysis of many experimental data (available in literature) by using SPSS (Statistical Package for the Social Sciences) program. These proposed material constitutive relationships have been employed for formulating the material finite element models to study the behaviour of HSSFRC corbels. The corbel is considered in a plane stress state and an eight-node serendipity element has been employed to represent the concrete, a strain hardening plasticity approach is used to model the compression behaviour, the yield condition is formulated in terms of the first two stress invariants, a tensile strength criterion is used to predict crack initiation, two tensile stress degradation functions are used to model the tension stiffening. Five different cracked shear moduli have been compared in this paper. For plain high strength reinforced concrete corbels, the first tension stiffening ( TS 1) model with cracked shear modulus model ( G ¯ 4 ) and the second tension stiffening ( TS 2) with model ( G ¯ 2 ) give good results compared with experimental results. For HSSFRC corbels, the second tension stiffening ( TS 2) model with cracked shear modulus models ( G ¯ 2 ) and ( G ¯ 3 ) give good results compared with experimental results. The numerical results of the proposed material models and corbels behaviour compared with published experimental results showed good agreement.

Modelling and numerical simulation of high strength fibre reinforced concrete corbels

In the present study, new constitutive models for high strength steel fibre reinforced concrete (HSSFRC) have been formulated by means of a regression analysis of many experimental data (from literature) by using SPSS-statistical program. This proposed constitutive models have been employed for formulating the material finite element models to study the behaviour of HSSFRC corbels. The corbel is considered in a plane stress state and eight-node Serendipity element has been employed to represent the concrete, a strain hardening plasticity approach is used to model the compression behaviour, the yield condition is formulated in terms of the first two stress invariants, a tensile strength criterion is used to predict crack initiation. The numerical results of proposed material models and corbels behaviour compared with published experimental results showed good agreement.

강섬유 및 헤디드 바를 활용한 고강도 콘크리트 내민받침의 성능 향상

Technology Research Center, Samsung C&T Corporation, Seoul 137-956, KoreaABSTRACT High-strength concrete corbels with varying percentage of steel fibers and two different anchorage types (weldingto transverse bar, headed) for the main tension tie were constructed and tested. The results showed that performance in terms ofload carrying capacities, stiffness, ductility, and crack width was improved, as the percentage of steel fibers was increased. In addi-tion, the corbel specimens in which headed bars were used as the main tension tie reinforcements showed superior load carryingcapacities, stiffness, and ductility compared to the corbel specimens in which the main tension ties were anchored by welding tothe transverse bars. From the test results, it is expected that load carrying capacities, durability, and constructibility of high strengthconcrete corbels would be improved by using steel fibers and headed bars. Experimental results presented in this paper were alsocompared with various prediction models proposed by researchers and presented in codes. The truss model proposed by Fattuhiprovides fairly good predictions for fiber reinforced high-strength concrete corbels.Keywords : corbels, steel fibers, headed bars, high strength concrete

Shear strength of reinforced concrete corbels

In this study, 15 reinforced concrete corbels with shear span-to-depth ratio greater than unity were tested. Test results indicate that the shear strength of corbels increases with increase in compressive strength of concrete, parameter of main reinforcement and parameter of vertical stirrups. The shear strength of corbels also increases with decrease in shear span-to-depth ratio. An analytical model for determining the shear strength of reinforced concrete corbels is also proposed in this paper. Both the shear strengths predicted by the proposed model and the approach of the ACI code are compared with available test results. The comparison shows that the proposed model can predict more accurately the shear strength of reinforced concrete corbels than the approach of the ACI code.