High Strength Concrete Encased Steel Research Articles

Experimental and theoretical study on flexural behavior of high strength concrete encased steel beams with steel fibers

In this study, the flexural behavior of high strength concrete encased steel beams with steel fibers (HSCSBSF) under bending is investigated. The experiments of four HSCSBSF including one HSCSBSF without steel fiber (0% volume ratio of steel fiber) are carried out to investigate the effect of the volume ratio of steel fiber on the flexural behavior of HSCSBSF. All specimens fail by the yielding of reinforcement in tension zone and the crushing of concrete in compression zone. Due to the positive effect of steel fiber on the internal bonding strength of concrete, HSCSBSF with higher volume ratio of steel fiber shows higher cracking load. The bearing capacity (bending capacity) of HSCSBSF increases by 34.3% when the volume ratio of steel fiber varies from 0% to 2%. Compared with HSCSBSF with 0% volume ratio of steel fiber, the flexural stiffness of HSCSBSF with higher volume ratio of steel fiber is obviously improved and the ductility coefficient (the ratio of the displacement at failure load to the displacement at the yield load) of HSCSBSF with 2% volume ratio of steel fiber increases by 25%. Based on the material test of concrete, a value for the influence coefficient of steel fiber on the tensile strength of concrete is suggested. A model for predicting the bearing capacity of HSCSBSF is then derived from theoretical analysis of HSCSBSF. A good agreement between the proposed model and various test data is made.

Prediction of fire resistance of concrete encased steel composite columns using artificial neural network

Concrete encased steel (CES) columns, also known as steel reinforced concrete (SRC) composite columns, exhibit superior fire resistance due to concrete acting as a protection layer for the embedded steel section. While modern design codes have provided design guides for the fire resistance of CES columns, they are only applicable to those made of normal strength concrete. For high strength CES columns, advanced analysis is needed to capture the brittleness of high strength concrete at elevated temperature. In this paper, two methods, namely the artificial neural network (ANN) and the analytical equations, are proposed to predict the fire resistance of axially-loaded CES columns made of high strength concrete. To train the ANN, a finite difference model is developed to compute the temperature field in CES columns and it is used to establish a database containing 15,200 specimens. The cross-sectional dimensions and materials grades of the specimens are carefully selected to cover a wide range of values including those commonly adopted in real-life applications. The inputs of the ANN are identified through an extensive parametric analysis. The selected ANN consists of 7 inputs, 3 outputs and 2 hidden layers and achieves a high determination coefficient R2 value of 0.999. For practical implementation, analytical equations are also derived and achieve high R2 values above 0.953. The predictive power of the ANN and the analytical equations are examined against the observations obtained from actual fire tests, showing reasonable accuracy of prediction. Both methods are simple, of high accuracy and have implicitly accounted for temperature-dependent material degradation, and hence do not require input of temperature-dependent material properties and advanced analysis software.

Fire performance of composite columns made of high strength steel and concrete

The use of high strength steel and concrete materials for composite column construction is partly impeded by the lack of research on their fire performance. In EN1994-1-1, the highest concrete grade allowed for designing composite columns is capped at C50. This paper investigates the behaviour of concrete encased steel (CES) composite columns made of C120 concrete and S500/690 steel section under ISO834 standard fire through a series of experimental and numerical analyses. Fire tests were carried out on five high strength CES column specimens subject to varying eccentricity of loading to study their behaviours in fire. Polypropylene fibres were added into the high strength concrete mix to prevent the explosive spalling so that explicit modelling of cover spalling phenomenon can be omitted in the numerical analysis. A unified method is proposed to determine the transient strain of high strength concrete at elevated temperatures, allowing the stress-strain curves in EN1992-1-2 to be extended to high strength concrete up to C120. A numerical model, which can capture the strain reversal of concrete caused by the flexural deformation of columns at elevated temperatures, is proposed to enable a good prediction of axial displacement responses of CES columns in comparison with the test results. Finally, based on the validated numerical model and parametric studies, a new tabulated data method is proposed for the fire resistance design of CES columns with concrete class up to C90 concrete and steel section grade up to S550.

Experimental investigation on fire resistance of high-strength concrete encased steel composite columns

This paper presents the experimental findings on fire resistance of concrete encased steel (CES) composite columns made of C120 concrete and S500/S690 high-strength steel section. Eight full-scale CES columns were tested under concentrated load and heated under the ISO834 fire until failure. The fire performance of each CES column specimen was evaluated by analysing the fire resistance time, axial displacement-time curve, temperature-time curve, rotational angle of end support, post-fire conditions of the concrete surface and failure mode. The experimental study showed that the addition of polypropylene fibre was effective in minimizing explosive concrete spalling in high-strength CES columns to achieve comparable fire resistance time as those of normal-strength CES columns. A database containing 40 CES specimens was established to assess the accuracy of the current design methods in predicting the fire resistance of CES columns. The limitations of each design method were discussed based on whether they can provide safe and accurate predictions of the fire resistance of tested CES column specimens. Finally, recommendations were provided to modify the current methods for fire resistant design of CES columns made of high-strength concrete.

Investigation on axial load-shorting behaviour of high strength concrete encased steel composite section

Experimental work on steel H-section fully encased by high strength concrete showed the occurrence of premature failure due to concrete cover spalling. This failure mode limits the utilization of full material strength and hence impedes the attainment of anticipated load-carrying capacity. This paper proposes an analytical model to predict the full range axial load-displacement behaviour of concrete encased steel composite section made from concrete grade up to C130 and steel section grade up to S690. The analytical model captures the compression behaviour of concrete encased steel composite section at the constitutive level including premature cover spalling, concrete confinement from stirrups and steel section. Concrete fracture energy is used to estimate the cover spalling strain, and a confinement model is proposed to evaluate the confining pressure provided by the steel section. Good agreement is observed between the analytical predictions and test results in terms of initial stiffness, peak load, residual strength, and the full range of load-displacement response. Based on the findings, recommendations are put forward for practical design of high strength concrete encased steel composite columns.

Axial-moment interaction of high strength concrete encased steel composite columns: Experimental investigation

This paper investigates the failure mechanism and structural behavior of high strength Concrete Encased Steel (CES) composite columns subject to compression and bending. Twelve composite column specimens with C90 concrete and S500 steel section were tested under concentric compression, eccentric compression and four-point bending. All specimens had the same geometry and were reinforced with the same steel section and same amount of reinforcing bars. In some specimens, micro steel fiber was added into concrete mix with 0.5% or 1.0% dosage to delay or prevent the potential premature cover spalling. The load-carrying capacity, load-deformation response, post-peak ductility, flexural stiffness, as well as damage pattern are comprehensively analyzed. Comparison is made between the experimental results and analytical predictions using modern design codes such as EN 1994-1-1 and AISC 360–16. From the four-point bending test, the flexural stiffness obtained from the test seems to be smaller than the codes' predicted values, indicating that modification is needed to achieve better prediction of structural response of composite beams at the service load level. As for the axial force (N)-bending moment (M) strength interaction diagram, it is found AISC 360–16 gives overall more conservative prediction than EN 1994-1-1 method for high strength CES columns with C90 concrete and S500 steel. However, both methods overestimate the axial compression capacity although they can predict their flexural capacity well.

Design and testing of concrete encased steel composite beam-columns with C90 concrete and S690 steel section

This paper investigated the structural performance of Concrete Encased Steel (CES) beam-columns made of C90 concrete and S690 welded steel section. Seven large-scale beam-column specimens including four steel reinforced concrete encased specimens and three fiber-reinforced concrete encased specimens were tested under axial compression and end moments. The end moment was applied by a compression load acting at an eccentricity. The test results in terms of failure mode, load-carrying capacity, deformation capability and second-order moment were presented. In addition, the effects of load eccentricity, steel fiber content, steel contribution ratio, and stirrup configuration on the column strength and ductility were analyzed. The existing design methods suggested in EN 1994-1-1 and AISC 360-16 were examined in constructing the axial force-moment interaction diagram and in predicting the effective flexural stiffness for second order analysis. A fiber element analysis method was developed and the predicted results were verified against the load displacement response and the failure loads from the tests. The analysis method was then used to generate additional data for beam-columns of various design parameters. Finally, an analytical method was proposed based on regression analysis to estimate the short-term effective flexural stiffness for the design of high strength CES beam-columns.

Experimental and numerical investigation of high-strength concrete encased steel columns with rectangular-spiral stirrups

In the present study, seismic performance of High-strength Concrete Encased Steel columns with Rectangular - spiral Stirrups (HCESRS) is investigated. The composite specimen was tested under reverse cyclic loading and numerically simulated by the finite element method. Using ABAQUS software, numerical analysis of the specimen was performed, including hysteresis curve, skeleton curve, stress comparison and energy dissipation. Experimental results were compared with the numerical simulation to verify the accuracy of the numerical analysis. The influence of parameter changes such as shear span ratio, axial compression, steel content of I-shape and steel yield strength of HCESRS column were investigated. It was found that as the shear span ratio increases, the ultimate bearing capacity of the member decreases and the deformation capacity increases. Due to the brittle failure characteristics of high strength concrete, the limit value of axial compression ratio of HCESRS column was systematically studied, and effect of axial compression ratio on the ductility for HCESRS column has been investigated. This numerical analysis reliably evaluated and analyzed the seismic response of the HCESRS column.

Axial-moment interaction of high strength concrete encased steel composite columns: Design recommendation

This paper investigates the axial force-moment (N-M) interaction behaviour of high strength concrete encased steel (CES) composite columns. A numerical method is developed to simulate the nonlinear inelastic behaviour of CES composite beam-columns, which includes concrete cover spalling, concrete confinement effect, buckling of longitudinal reinforcing bars, and stain-hardening of the encased steel section. The numerical model is implemented in a self-developed computer program to generate the load-deflection response, moment-curvature relation, as well as the N-M strength interaction diagrams. The accuracy of the proposed analysis method is validated by comparison with the experimental results report in the companion paper and the test data available in literatures. To further understand the resistance of CES columns consisting of high strength concrete and high strength steel, parametric study is performed to evaluate the effect of concrete compressive strength, steel yield strength, load eccentricity, as well as steel area ratio. Finally, a simplified method is proposed to construct the N-M strength interaction curve of CES composite columns. The proposed method can be used to design CES beam columns for concrete grade up to C100 and steel grade up to S960.

Assessment of high-strength concrete encased steel composite columns subject to axial compression

This paper investigates the axial load capacity of concrete encased steel composite stub columns with high strength concrete and steel materials. A total of 14 column specimens with varying material strengths and different steel section shapes were tested under concentric compression load. The test results revealed that the design methods in EN 1994-1-1 and JGJ 138-2016 overestimate the axial load capacity of high strength concrete encased steel columns. Adding a small percentage of steel fiber in the high strength concrete was found to improve the compression resistance of the composite section. A new test database consisting of 51 partially encased composite sections and 82 fully encased composite sections was established, covering a wide range of section geometric and material grades. Parametric study was then carried out to assess current design methods in predicting the axial capacity of such composite columns with respect to material strengths, steel contribution ratio, reinforcement ratio, section slenderness ratio, confined concrete area ratio, and concrete confinement efficiency. The effectiveness of concrete confinement in partially encased composite column was evaluated and a simplified method was proposed to compute the enhanced concrete strength based on regression analysis. For fully concrete encased composite columns, a concrete strength reduction factor was proposed to be used with EN 1994-1-1 to predict the compression resistance. Design recommendation was made considering the early cover spalling of high strength concrete and the material compatibility between steel and concrete in composite column design.

Behavior of high strength concrete encased steel composite stub columns with C130 concrete and S690 steel

This paper presents an experimental program that studies the structural behaviour of high strength Concrete Encased Steel (CES) composite columns. The structural performance under compression, including the damage pattern, load-carrying capacity, post-peak ductility, and load-displacement response is experimentally investigated. A total of 14 specimens were tested under concentric compression. The parameters studied in this program include concrete compressive strength, steel yield strength, stirrup spacing, incorporation of steel fiber, as well as the shape of the structural steel section. To evaluate the material compatibility between high strength concrete and high strength steel, two concrete grades (C90, C130) and two steel grades (S500, S690) were used to prepare the test specimens. In addition, 0.5% volume fraction of steel fiber was added in concrete mix to minimize the inherent brittleness of high strength concrete. The comparison between test results and analytical predictions reveals the inability of existing design codes to estimate high strength CES columns, unless steel fiber and dense reinforcement are used in combination. The effect of material strength, steel fibers, volumetric ratios of hoop reinforcement, and shape of steel section on both strength and ductility of CES columns was assessed through a comprehensive parametric study. The analysis of test results demonstrates that steel contribution ratio plays a dominant role in the ductility, whereas increasing hoop reinforcement ratio and adding steel fiber has negligible effect. Finally, a simplified formula is proposed to evaluate ductility of high strength CES columns.

Experimental study on high strength concrete encased steel composite short columns

This paper investigates on the axial compressive behavior of high strength Concrete Encased Steel (CES) composite short columns. Six column specimens made of high strength concrete were tested under monotonic pure compression. The structural performance was investigated including failure pattern, load-carrying capacity, load-deformation response, and post peak ductility. The major parameters affecting the ultimate strength of such composite columns are studied including concrete strength, spacing of transverse reinforcement bars, and the inclusion of steel fibers in the high strength concrete. Based on the experimental work, it is found that the failure mode of CES columns with high concrete strength C120 is characterized by concrete cover spalling, which triggers a sudden drop of compression capacity once the peak resistance is reached. With the decrease of transverse reinforcement spacing, the ultimate load capacity of high strength CES columns can be enhanced due to better concrete confinement but the failure is still brittle with abrupt unloading from the peak load. The test results also indicated that the provision of 0.5% volume of steel fiber is able to prevent the concrete cover spalling, and hence the load-carrying capacity can be enhanced. Finally, the test results together with additional test data collected from the literature are used for comparison with codes’ prediction using the EN 1994-1-1 and JGJ 138-2016. The code methods give excellent prediction of CES columns with normal strength steel and concrete grade up to C90, but they over-estimate the maximum resistance of CES columns with concrete strength higher than 90 MPa. To extend the current design methods for CES members with higher strength concrete, two analytical methods were proposed which shows good correlation with test results, thus enabling the design of CES members with higher strength of concrete.

Experimental behavior of VHSC encased composite stub column under compression and end moment

This paper investigates the structural behavior of very high strength concrete encased steel composite columns via combined experimental and analytical study. The experimental programme examines stub composite columns under pure compression and eccentric compression. The experimental results show that the high strength encased concrete composite column exhibits brittle post peak behavior and low ductility but has acceptable compressive resistance. The high strength concrete encased composite column subjected to early spalling and initial flexural cracking due to its brittle nature that may degrade the stiffness and ultimate resistance. The analytical study compares the current code methods (ACI 318, Eurocode 4, AISC 360 and Chinese JGJ 138) in predicting the compressive resistance of the high strength concrete encased composite columns to verify the accuracy. The plastic design resistance may not be fully achieved. A database including the concrete encased composite column under concentered and eccentric compression is established to verify the predictions using the proposed elastic, elastoplastic and plastic methods. Image-oriented intelligent recognition tool-based fiber element method is programmed to predict the load resistances. It is found that the plastic method can give an accurate prediction of the load resistance for the encased composite column using normal strength concrete (20-60 MPa) while the elastoplastic method provides reasonably conservative predictions for the encased composite column using high strength concrete (60-120 MPa).

Buckling behaviour of high strength concrete encased steel composite columns

This paper provides an insight into the buckling behavior of high strength concrete encased steel (CES) columns through a comprehensive investigation including experimental, numerical and analytical analyses. Three long CES column specimens made of high strength concrete C100 and S355 H-section were tested under axial compression. The maximum test loads obtained from these tests were compared with the buckling resistance predicted by EN 1994-1-1, AISC 360-10 and ACI 318-08. Nonlinear finite element analyses were performed to predict the buckling resistance and trace the load displacement behaviour of these columns. In the finite element model, the column initial imperfections were carefully chosen to predict the maximum resistance and the load-displacement response, and compared with the equivalent imperfection values stipulated in the modern design codes. In order to examine the validity and limitation of the current design approaches in predicting the buckling resistance of CES columns made of high strength concrete, a statistical study was undertaken based on the established database covering a wider range of material strength and geometric configurations. In the statistical study, the buckling resistance and effective flexural stiffness obtained from the tests were compared with the codes' predictions from EN 1994-1-1, AISC 360-10 and ACI 318-08. Finally, the reliability of the current design methods is assessed by correlating their accuracy with respect to the variation of material strength, column slenderness ratio, load eccentricity and steel contribution ratio.

Shear performance of prestressed ultra high strength concrete encased steel beams

Due to the high compressive strength and durability properties of ultra high strength concrete, prestressed ultra high strength concrete beam was used extensively in bridge engineering, but it possessed obvious brittle behavior. Encasing structural steel into it was a good way for alleviating the problem of brittleness. The purpose of this study was to investigate shear performance of prestressed ultra high strength concrete encased steel beams. A total of fifteen prestressed ultra high strength concrete encased steel beams and seven prestressed ultra high strength concrete beams were tested to shear failure under simply supported three-point loading conditions. The primary variables of this investigation included the presence or not of structural steel, shear span-depth ratio, degree of prestress, ratio of stirrup and thickness of web. The shear performance was evaluated based on cracking pattern, load–deflection behavior and shear ductility. Test results showed that prestressed ultra high strength concrete encased steel beams had higher residual shear capacity and post-cracking stiffness as well as by far better shear ductility than prestressed ultra high strength concrete beams. In addition, influence of experimental parameters on the shear performance of prestressed ultra high strength concrete encased steel beams and prestressed ultra high strength concrete beams also was discussed and compared, respectively.