Effect Of Concrete Confinement Research Articles

Finite element analysis for axial compressive behavior of CFT columns assembled with rectangular wave-shaped ribs

This is an analytical study on the axial compression behavior of newly developed Concrete-Filled Steel Tube (CFT) columns assembled with rectangular wave-shaped ribs for thin-walled design. Based on previous experimental results, a Finite Element (FE) method suited for CFT columns was developed. Next, the stress distribution and principal stress of concrete were analyzed, as were the stress distribution and local buckling of steel tubes. Based on the results, a parameter study was conducted and used as basic data for the design model. This CFT column effectively suppresses local buckling compared to conventional CFT columns, ultimately improving the concrete confinement effect. Additionally, the compressive strength of concrete increased due to the improved confinement effect, which differed depending on the rib aspect ratio. Thus, a design model incorporating the rib aspect ratio as an independent variable was proposed. The model was validated through comparison between the experimental results and FE model data and was found to accurately predict the behavior.

Confinement effects within the seismic design of reinforced concrete frames: reliability assessment and comparison

The current Italian code “NTC2018” and Eurocode “EC8” provide specific design requirements in terms of flexural overstrength factors for reinforced concrete (RC) buildings in seismic areas. This study proposes to improve the design methodology by including explicitly the confinement effects within the seismic design of a new regular RC frame due to their influence on the capacity design principles (at structural and member level). Specifically, the seismic performance of a RC frame designed according to codes is compared, in reliability terms, with the one of the same frame designed including the concrete confinement effects. Moreover, a comparison between three constitutive models for confined concrete is performed. Selecting L’Aquila (Italy) as reference site, the two design methodologies combined with the three confinement models are numerically implemented. Scaling 30 non-frequent natural ground motions to increasing seismic intensity measures, according to the site seismic hazard, non-linear incremental dynamic analyses (IDAs) have been developed. Assuming the interstory drift indices as engineering demand parameters, the peak responses have been fitted through log-normal distributions to define IDA curves. Subsequently, appropriate limit state thresholds have been adopted to define the seismic fragility curves. Finally, through the convolution integral and Poisson model, the seismic reliability curves have been derived showing the importance of the proposed design methodology. In fact, for the frame under investigation, the seismic performance strongly improves in terms of ductility respecting all the reliability objective levels if the confinement effects are explicitly considered in the seismic design. Furthermore, the different confinement models provide similar results when the proposed seismic design is adopted, implying a reduction of the model uncertainty.

Experimental Investigation and Numerical Analysis of the Axial Load Capacity of Circular Concrete-Filled Tubular Columns

This paper focuses on the experimental investigation of the axial load capacity of CFT (concrete-filled steel tube) columns under actual construction conditions during building reconstruction. A total of four samples were loaded up to failure. The varied parameters were the column length and absence/presence of the concrete infill within the steel tube. Further, the analysis is extended to developing a numerical model in the finite element-based software ABAQUS version 6.9. This numerical model includes material and geometrical nonlinearities and was validated with the experimental results. The contribution of the concrete core to the column capacity and the concrete core confinement effect are discussed. Finally, the column capacity was calculated according to several design codes: the Eurocode 4 with and without considering the confinement effect, American specifications, Australian standards, the American Institute of Steel Construction, and the Architectural Institute of Japan. The Eurocode 4 considering the confinement effect provides the closest results to those obtained in the tests.

Axial load behavior of CFT columns assembled with rectangular wave-shaped ribs

To solve defects caused by welding, a rib assembled CFT column was developed and the central compression performance, including the ultimate compressive strength, was evaluated through actual experiments. The results indicated that the internal ribs of this CFT column effectively suppressed local buckling, thereby enhancing the concrete confinement effect, and ultimately increasing the ultimate compressive strength. This effect was maximized when the rib specifications met a certain value, and it enabled the CFT column to achieve an equivalent level of axial stiffness and ductility compared to conventional CFT columns. Despite its assembly from multiple segments, the CFT column demonstrated high integrity, proving that the cross-sectional area of the steel tube could be applied according to current codes. The difference in concrete compressive strength due to the concrete confinement effect, which depends on the aspect ratio of the rib, was confirmed to be due to the effective confined area ratio (ke). Therefore, a design model for this CFT column was proposed, and as a part of the preliminary review process, limiting conditions for the aspect ratio of the ribs were also proposed.

Behavior and design of concrete filled stainless steel tubular columns under concentric and eccentric compressive loading

Concrete Filled Stainless Steel Tubular (CFSST) column is a novel type of composite columns that features excellent structural performance, corrosion resistance as well as aesthetic appearance. Owing to the differential mechanical properties between stainless steel and carbon steel, extending the current design codes of conventional CFST columns to CFSST columns may cause unreliable predictions. To this end, this paper summarizes the current state-of-art of CFSST columns and compiles the database of CFSST columns subjected to concentric compressive loading and eccentric compressive loading. Based on the analysis of current studies, the applicability of current international design codes is comprehensively evaluated, and the numerical simulation is also performed through three-dimensional Finite Element (FE) modeling and two-dimensional Fiber Section (FS) modeling technique, where the concrete confinement effect is properly incorporated. Parametric study is then conducted using the validated FS model to investigate the influence of concrete compressive strength, steel yield strength and section slenderness ratio on load-carrying capacity. Finally, a new approach is proposed to construct the axial force-bending moment interaction diagram of CFSST columns, which is simple to use and generates enhanced prediction accuracy as compared with the current design method.

Finite element modeling of square high-strength concrete short columns longitudinally reinforced with steel equal-angles under axial loading

Conventional steel bars in Reinforced Concrete (RC) square columns are utilized as longitudinal reinforcement and steel ties are utilized as transverse reinforcement for concrete confinement. The concrete confinement in RC columns with square section is less than that with circular section. Recently, Steel-Equal Angle (SEA) as longitudinal reinforcement has been investigated for square RC columns. The RC columns with SEA as longitudinal reinforcement exhibit higher strength and ductility than the RC columns with conventional steel bars as longitudinal reinforcement. However, experimental and computational investigations have been very limited on RC columns with SEA as longitudinal reinforcement. Existing studies investigated only the effects of lateral tie spacing on the performance of RC columns reinforced with SEA sections. However, the effects of the other important column parameters, such as the width and thickness of SEA sections, yield stress of SEA sections, and the concrete compressive strength, on the axial performance of RC columns reinforced with SEA sections have not been fully investigated yet. This study develops finite element (FE) models for quantifying the behavior of axially loaded square RC short concrete columns reinforced with SEA. The inelastic buckling of SEA cross-sections and the effects of concrete confinement are considered in the simulation. The accuracy of the FE analysis is verified using available test results. The influences of material and geometric properties on the compressive behavior of square RC short concrete columns incorporating SEA are studied. The results show that the model accurately estimates the behavior of square SEA reinforced concrete short columns. Moreover, the width of the SEA sections, the concrete compressive strength, and the lateral tie spacing have the most significant effects on the performance of SEA reinforced concrete columns subjected to axial loads.

A double-cylinder model for bond analysis of deformed bars with eccentric concrete confinement

Bonding between deformed bar and concrete is critical for the load capacity of reinforced concrete structure. In practical engineering, the deformed bar is always eccentrically embedded in a concrete member instead of concentrically installed in it. In this paper, a double-cylinder model, which considers the additional concrete confinement outside the conventional cylinder of concentric embedment case as an extension of cylinder diameter, is first proposed to model overall concrete confinement effect on the deformed bar for the eccentric embedment case. Thereafter, combining the eccentric concrete confinement with other main influencing factors on the bond, such as friction coefficient, rib height and rib spacing, the ultimate bond strength is predicted for the concrete splitting failure mode. By comparing with the prediction bond strengths and experimental results from previous tests, a series of confinement adjustment factors based on the concentric cases are confirmed for various eccentric situations. As a result, a formula of the concrete confinement transition between the eccentric and concentric cases is obtained based on the least square method. Besides, parametric studies are conducted to study the effects of cover thickness, reinforcing bar diameter, concrete tensile strength on the bond strength between deformed bar and concrete.

Probabilistic Calibration of Compressive Stress–Strain Models for FRP-Confined Concrete in Square Cross Section Members

The confinement effect of concrete by fiber-reinforced polymer (FRP) jackets may significantly enhance the axial compressive performance of concrete, although the results are variable. Therefore, rational calibration of deterministic models for compressive strength and the associated strain and stress–strain (SS) curves is required based on available experimental databases and selected probabilistic models. To improve the accuracy of the probabilistic model, second branch classifications for the stress–strain curve of FRP-confined concrete are identified within a series combination of uncertainties with computerized classification algorithms before probabilistic calibrations. The Bayesian theorem and Markov chain Monte Carlo (MCMC) approaches were used to update a probabilistic model that includes the critical variables that have been established in previous research. Furthermore, eight representative deterministic strength enhancement models, three strain enhancement models, and four stress–strain models were chosen for evaluation using credible intervals (CIs) and confidence levels (CLs) at different strain levels. Different types of FRPs were also analyzed individually to ensure the validity and reliability of the findings. The suggested probabilistic models can predict the properties of ultimate axial stress and related strain, thus offering an effective method for calibrating the confidence level and computational correctness of deterministic models previously published in the literature.

Tensile resistance of novel interlocked angle connectors used in steel-concrete-steel sandwich structures

The mechanical performances of steel-concrete-steel (SCS) sandwich structures were generally governed by the interfacial bonding between steel and concrete provided by shear connectors. This paper aims to propose a new interlocked angle connector (IAC) for sandwich structures to enhance the steel-concrete interfacial bonding performances, and its tensile behaviors were also experimentally and analytically studied in detail. There were 14 specimens with different parameters being designed and tested, and the different confinement effects of concrete around IACs were also considered in this study. The experiments revealed three failure modes of IACs embedded in concrete, i.e., concrete breakout failure, tensile fracture failure of the steel angle and concrete direct shear failure. The evident influences of confinement effect of concrete on the failure modes and tensile resistances of IACs were also observed. In addition, the tensile resistance of IACs was found to be significantly improved with the increasing of embedded depth of IACs and thicknesses of steel sheet. Analytical models were proposed for predicting the tensile resistances of IACs, and the predicted values showed a good match with the test results. Moreover, the tensile resistances of IACs were compared with those of existing semi-direct-link connectors, and the proposed IACs provided 55.9% higher tensile resistances than the existing semi-direct-link connectors. Thus, the steel-concrete interfacial bonding performances of SCS structures with IACs would be significantly enhanced. In subsequent studies, the mechanical performances of IACs under the combined actions of tension and shear should be concerned.

Study on confinement mechanism of core concrete in steel tubular-corrugated steel plate confined concrete columns

This paper investigated the confinement mechanism of core concrete in steel tubular-corrugated steel plate confined concrete (S-CSC) column. Fourteen S-CSC columns and one CFST column were tested under axial compression, and the corresponding failure modes and load-displacement curves were observed. The strain changes of corrugated steel plate at different positions and square steel tubular were measured. Combined with the test results, the influences of some parameters including the section size, the steel tubular width, the wave height of corrugated steel plate, the thickness of corrugated steel plate and etc. on the strength index of core concrete and the ductility index of the specimens were analyzed. And the confinement mechanism of the core concrete in S-CSC column was studied. It was found that the confinement coefficient ξ which considered the influence of material strength and section size on the confinement effect of core concrete can reflect the confinement mechanism of core concrete effectively. Based on L.-H. Han's model, a constitutive relation for core concrete in S-CSC column by modifying key parameters was proposed and implemented in finite element simulation to predict the mechanical behaviors of axially loaded S-CSC column from the results of the tests. Comparing the test results of S-CSC columns which were described in this paper and the FE results, the satisfactory agreement was observed. Finally, it is verified that the size effect has no effect on the strength index of core concrete in S-CSC column basically.

Interaction diagram for RC column strengthened by steel angles and strips

This paper presents an analytical model to construct the interaction diagrams (normal force and moment) for the RC column strengthened using the steel jacket technique. The proposed model is defined using the strain distribution block by determining the location of the neutral axis in the concrete section. The proposed analytical formulation is verified by experimental results performed by previous researches and numerical models using the nonlinear program ANSYS. The factors affecting the capacity of the strengthened column are taken into consideration, such as the amount of loads resisted by the steel cage, steel strips spacing, and the effect of concrete confinement. The results of the proposed model are in good agreement with the results from the experimental and numerical work used in verification. A practical design formula has been presented for strengthened columns.

Lateral response of double skin tubular column to steel beam composite frames

Concrete filled double-skin steel tubular (CFDST) column comprises two inner and outer steel tubes with infill concrete between tubes. CFDST columns are used in many structural systems such as offshore structures and high rise buildings. The aim of this research is to examine the performance of composite frames composed of CFDST columns and steel beam under the influence of lateral loading. The frames were modeled and analyzed utilizing ANSYS finite element (FE) software. The linear and nonlinear behavior of steel and concrete materials and confinement effects of inner and outer steel tubes on the infill concrete were considered in the analysis. Three key parameters were considered in the present study. They are the axial load and slenderness ratios of CFDST column as well as linear stiffness ratio of the beam–column. The effects of these parameters on the behavior of the composite frames were evaluated comparatively. Load-deformation responses were achieved for various cases of the investigation. The verification of the developed FE model was evaluated by considering the analysis results with the experimental data existing in the literature. The findings attained from the FE modeling were in consonance with the experimental results. Besides, it was observed that the above parameters had a substantial influence on the load-displacement relationship and the performance of the studied composite frames.

Numerical investigations of circular double-skin steel tubular slender beam-columns filled with ultra-high-strength concrete

The use of ultra-high-strength concrete in double-skin concrete-filled steel tubular (DCFST) composite columns has received considerable attention in recent years. However, no numerical study on the behavior of eccentrically loaded circular DCFST slender beam-columns made of ultra-high-strength concrete has been reported. This paper proposes computational modeling procedures for accurately predicting the axial load–deflection responses and axial load-moment interaction diagrams of circular DCFST slender beam-columns filled with ultra-high-strength concrete when subjected to the combined action of bending and axial compression. The computational procedures based on the fiber element methodology incorporate the effects of concrete confinement, second-order and initial geometric imperfection on the behavior of DCFST slender beam-columns. The enhanced King-Werner’s algorithm is implemented in the fiber-based procedures to solve the nonlinear moment equilibrium equation. The accuracy of the present model is verified with experimental results published in the literature and finite element analysis results. The numerical results reveal that the confined concrete model can capture the response of ultra-high-strength concrete with a sudden drop after reaching the peak load and the ductility in the softening branch. The significance of several influencing factors on the structural behavior is investigated in the comprehensive parametric study including 154 specimens with a wide range of column geometric and material properties. The design of circular DCFST slender beam-columns with ultra-high-strength concrete in accordance with Eurocode 4 is discussed. The artificial neural network (ANN) model is developed to propose new empirical formulas for evaluating the member resistance of DCFST slender beam-columns. The proposed equations give accurate and reliable predictions of the column’s resistance and can be employed in practical design.

Reinforcement corrosion of circular concrete columns under sustained load

This paper presents the results of an experimental study of the axial capacity of circular spirally-reinforced concrete columns that were subjected to accelerated corrosion while sustaining a constant compressive service load. Two sets of columns of five columns each were designed with different spiral pitch to investigate the effect of concrete confinement on the corrosion propagation. Columns with non-confining spiral reinforcement had only their longitudinal reinforcing steel corroded, whereas in the other set of specimens, the spiral reinforcement was corroded and isolated from the vertical reinforcing steel. The cracking initiation, pattern, and widening of the concrete cover were monitored for a period of 4-10 months, and relations obtained between the crack widening and propagation and the elastic shortening of the column and steel mass loss were established. After the accelerated corrosion period, eight columns were tested to failure under either concentric or eccentric load; the vertical deformation, cracking pattern and failure type were monitored and recorded. The failure of specimens with non-confining spiral reinforcement was characterized by sudden crushing of the concrete and longitudinal reinforcement buckling, with a reduction in load capacity of 30-40%. Columns with confinement reinforcement failed in a ductile manner by progressive spalling of the concrete cover, maintaining their load capacity in spite of the corroded spiral reinforcement.

BEHAVIOR OF STEEL PLATE-CONCRETE COMPOSITE SHEAR WALL UNDER CYCLIC LOADING

Steel-Concrete composite shear wall has become popular recently as it compensates for the disadvantages of concrete and steel plate shear walls and combine the advantage of both. However, there is no detail study that identifies the most critical parameters. This study aims at investigation of steel plate-concrete composite shear wall behavior under cyclic loading with variables such as concrete strength, grade of steel plate, total number of tie constraints and thickness of steel plate. ABAQUS/Standard is used for numerical modeling in this study. As the concrete strength decreases from 86.1Mpa to 45Mpa, the load capacity declined by 11.76% and higher stiffness was recorded in specimen with higher grade of concrete. The ductility factor is inversely proportional to grade of concrete from 86.1Mpa to 60Mpa which increases from 4.26 to 4.68 and the ductility factor of specimen with 45Mpa strength is recorded as 3.81. The energy dissipation capacity is directly proportional to the grade of concrete used. Using high grade steel plate increases the lateral load capacity significantly and exhibited more ductile behavior. Specimen with S355 steel grade exhibited 14.01% increment of the average load capacity while the specimen with S245 steel grade has shown reduction by 9.21%. Similarly, the ductility factor and energy dissipation capacity of specimen with variable grade of steel are directly proportional. Reduction of tie constraints has no significant effect on the behavior in this study due to high confinement effect of concrete by surrounding steel plate. Specimens with thicker steel plate exhibited good energy dissipation capacity.

Nonlinear finite element analysis of reinforced concrete columns: Evaluation of different modeling approaches for considering stirrup confinement effects

AbstractThe present study aimed to introduce an accurate modeling strategy for the three‐dimensional (3D) nonlinear finite element analysis of reinforced concrete (RC) columns considering the effects of the stirrup confinement. For this purpose, concrete columns with different geometric characteristics and various reinforcement details whose experimental test data are available, were analyzed using the finite element method. The concrete damaged plasticity (CDP) was used to model the nonlinear behavior of the concrete material in the compression and the tension. The ideal elastic–plastic constitutive law was utilized for modeling the behavior of longitudinal and transverse rebars. Through comparing the experimental data and numerical results, the effect of various parameters, such as the type of stress–strain relationships used for modeling the behavior of confined concrete and the type of element used to model the stirrups, were evaluated. Additionally, the sensitivity of numerical results against the CDP model parameters (i.e., the viscosity parameter, the flow potential eccentricity, the dilation angle, the ratio of biaxial compressive strength to its uniaxial compressive strength, and the ratio of the second stress invariant on the tensile meridian) were investigated. The results show that the presence and modeling of stirrups in the RC columns alone are not enough for capturing the effects of concrete confinement. The use of beam elements instead of truss elements for modeling the stirrups as well as the use of Vallenas stress–strain relations for modeling the uniaxial behavior confined concrete, leads to numerical results that are more close to the experimental data.

Structural Performance of Concrete-Filled GFRP Tube Slender Columns in Ambient and Standard Fire Conditions

Abstract Concrete-filled glass fiber-reinforced polymer (GFRP) tube (CFFT) columns benefit from significant concrete confinement effects that enhance its structural performance compared to conventi...

Numerical simulation and data-driven analysis on the flexural performance of steel reinforced concrete composite members

The flexural stiffness and flexural capacity of composite member are decisive indexes for the serviceability limit state and ultimate limit state design. To deepen the understanding of the flexural behavior of Steel Reinforced Concrete (SRC) composite members, a computationally efficient numerical technique based on fiber section analysis is employed with discussion of concrete confinement effect and steel section placement location. A dataset of SRC flexural members from published literature, covering a wide spectrum of geometrical and material properties, was built and compared with numerical predictions. The comparison signifies that the proposed numerical model is able to reproduce the moment-deflection relation of SRC flexural members, giving close estimation of the flexural stiffness, flexural capacity and flexural ductility. Parametric study is then carried out using the validated numerical model by varying concrete class, steel grade, steel section size and other influential parameters. Based on the numerically generated database, Machine Learning (ML) analysis is performed through an Artificial Neutral Network (ANN) algorithm. Compared with current design codes, the ML technique efficiently predicts the flexural capacity and flexural stiffness with high level of accuracy and robustness. As a consequence, it can be employed to intelligently estimate the flexural characteristics of SRC flexural members.

Effect of active and passive concrete confinement on the bond stress-slip response of steel bars in tension

This paper summarizes the results of an experimental investigation undertaken to evaluate the effect of different types of concrete confinement on the bond strength and the local bond stress-slip behavior of steel bars in tension. Thirty-two pullout specimens were tested. The investigated parameters included type and level of confinement, and ratio of concrete cover to steel bar diameter (c/db). Two categories covering four different types of confinement were evaluated: (1) active confinement applied as external transverse pressure and (2) passive confinement using: (i) conventional transverse steel ties, (ii) carbon fiber reinforced polymer (CFRP) jackets, (iii) a combination of CFRP jackets and transverse steel ties, or (iv) a combination of CFRP jackets and CFRP anchors. The specimens developed three distinct modes of bond failure: splitting mode associated with splitting of the concrete cover, pullout mode associated with pullout of the steel bar from the concrete, and a combination of pullout and splitting modes. Confining the concrete with any of the confinement methods used increased significantly the local bond strength, altered the failure mode from splitting to pullout or mixed splitting/pullout, and improved the ductility of local bond stress-slip response when compared to unconfined concrete. These improvements were highly dependent on the type and level of confinement used as well as the ratio of concrete cover to bar diameter c/db. The failure modes, the maximum local bond strength, and the local bond stress-slip behavior corresponding to the different types of confinement were compared and discussed vis-à-vis unconfined concrete. The intrinsic shapes of the experimentally measured local bond stress-slip responses were compared with the prediction of available analytical bond constitutive relationships.

Formulation and calibration of two-dimensional constitutive models for composite structures based on panel tests

Steel Plate Concrete (SC) composite members have been widely adopted because of its cost-efficiency and enhanced structural behavior. While researchers have attempted to study its in-plane shear behavior in the past twenty years, very limited number of large-scale pure shear tests were performed due to the challenge of experimental set-up and the availability of facilities. In this paper, a series of uniaxial loading tests and two full-scale pure shear panel tests of SC members were reported, on which the “mechanics-based Membrane Model of SC elements (MM-SC)” is developed. The MM-SC model is based on the fixed-angle crack formulation and the smeared-crack formulation, in which the experimental-based uniaxial constitutive models are implemented, considering the local buckling of faceplate, the tension stiffening of steel plate, the strength degradation of cracked concrete and the confinement effect of concrete. The proposed MM-SC model is subsequently incorporated into the object-oriented software OpenSEES. The simulation results of proposed model well predict the SC test observations in terms of critical branch points and structural behaviors, including initial stiffness, cracking strength, post-crack stiffness, yield strength, maximum strength, and failure modes.