• Earthquakes and Structures
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• v.3 no.6
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• pp.889-910
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• 2012

Because of the heavy demand of confining steel to restore the column ductility in seismic regions, it is more efficient to confine these columns by hollow steel tube to form concrete-filled-steel-tube (CFST) column. Compared with transverse reinforcing steel, steel tube provides a stronger and more uniform confining pressure to the concrete core, and reduces the steel congestion problem for better concrete placing quality. However, a major shortcoming of CFST columns is the imperfect steel-concrete interface bonding occurred at the elastic stage as steel dilates more than concrete in compression. This adversely affects the confining effect and decrease the elastic modulus. To resolve the problem, it is proposed in this study to use external steel confinement in the forms of rings and ties to restrict the dilation of steel tube. For verification, a series of uni-axial compression test was performed on some CFST columns with external steel rings and ties. From the results, it was found that: (1) Both rings and ties improved the stiffness of the CFST columns and (2) the rings improve significantly the axial strength of the CFST columns while the ties did not improve the axial strength. Lastly, a theoretical model for predicting the axial strength of confined CFST columns will be developed.

• Steel and Composite Structures
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• v.17 no.3
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• pp.271-285
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• 2014

This paper aims to examine the behavior of slender reinforced concrete columns confined with external glass fiber reinforced polymers (GFRP) sheets under eccentric loads. The experimental work conducted in this paper is an extension to previous work by the author concerning the behavior of eccentrically loaded short columns strengthened with GFRP wrapping. In this study, nine reinforced concrete columns divided into three groups were casted and tested. Three eccentricity ratios corresponding to e/t = 0, 0.10, and 0.50 in one direction of the column were tested in each group. The first group was the control one without confinement with slenderness ratio equal 20. The second group was the same as the first group but fully wrapped with one layer of GFRP laminates. The third group was also fully wrapped with one layer of GFRP laminates but having slenderness ratio equal 15. The experimental results of another two groups from the previous work were used in this study to investigate the difference between short and slender columns. The first was control one with slenderness ratio equal 10 and the second was fully wrapped and having the same slenderness ratio. All specimens were loaded until failure. The ultimate load, axial deformation, strain in steel bars, and failure mechanisms of each specimen were generated and analyzed. The results show that GFRP laminates confining system is less effective with slender columns compared with short one, but this solution is still applied and it can be efficiently utilized especially for slender columns with low eccentric ratio.

• Steel and Composite Structures
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• v.25 no.5
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• pp.603-615
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• 2017

This experimental research presents the seismic performance of steel reinforced high-strength concrete (SRHC) short columns. Eleven SRHC column specimens were tested under simulated earthquake loading conditions, including six short column specimens and five normal column specimens. The parameters studied included the axial load level, stirrup details and shear span ratio. The failure modes, critical region length, energy dissipation capacity and deformation capacity, stiffness and strength degradation and shear displacement of SRHC short columns were analyzed in detail. The effects of the parameters on seismic performance were discussed. The test results showed that SRHC short columns exhibited shear-flexure failure characteristics. The critical region length of SRHC short columns could be taken as the whole column height, regardless of axial load level. In comparison to SRHC normal columns, SRHC short columns had weaker energy dissipation capacity and deformation capacity, and experienced faster stiffness degradation and strength degradation. The decrease in energy dissipation and deformation capacity due to the decreasing shear span ratio was more serious when the axial load level was higher. However, SRHC short columns confined by multiple stirrups might possess good seismic behavior with enough deformation capacity (ultimate drift ratio ${\geq}2.5%$), even though a relative large axial load ratio (= 0.38) and relative small structural steel ratio (= 3.58%) were used, and were suitable to be used in tall buildings in earthquake regions.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2006.03a
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• pp.269-275
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• 2006

In the design of bridge piers in seismic area, the ductility requirement is one of the most important design criteria. In order to enhance the seismic performance of RC columns, it is necessary to make the ductility of columns larger by covering RC columns with steel tubes or confining RC columns by arranging transverse reinforcement such as hoop ties closely. Concrete encased composite columns can be utilized for bridge piers especially in seismic area. In this paper, finite element analyses are performed to study the nonlinear behavior of concrete encased composite columns with single core steel or multiple steel elements under static and quasi-static loads. The cross-sections of these specimens ate composed of concrete-encased H-shaped structural steel columns and a concrete-encased circular tube with partial in-filled concrete. Test parameters were the amount of the transverse reinforcement, encased steel member, and loading axis. Through the comparison between FE analyses and test results, adequate material models for confined concrete and unconfined concrete ate investigated. After getting the proper analysis models for composite columns, several parameters are considered to suggest design considerations on the details of composite piers.

• Computers and Concrete
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• v.8 no.1
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• pp.23-41
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• 2011

The behaviour of concrete confined by transverse reinforcement is a classical topic. Numerous studies have been conducted to establish the stress-strain relationships for concrete under various confining reinforcement arrangements. Many empirical and semi-empirical formulas exist. Simplified analytical models have also been proposed to evaluate the increase in the strength and ductility of confined concrete. However, relatively few studies have been conducted to utilise advanced computational models for a realistic simulation of the behaviour of concrete confined by transverse reinforcement. As a matter of fact, high fidelity simulations using the latest numerical solvers in conjunction with advanced material constitutive models can be a powerful means to investigating the mechanisms underlying the confining effects of different reinforcement schemes. This paper presents a study on the use of high fidelity finite element models for the investigation of the behaviour of concrete confined by stirrups, as well as the interpretation of the numerical results. The development of the models is described in detail, and the essential modelling considerations are discussed. The models are then validated by simulating representative experimental studies on short columns with different confining reinforcement schemes. The development and distribution of the confining stress and the subsequent increase in the axial strength are examined. The models are shown to be capable of reproducing the behaviour of the confined concrete realistically, paving a way for systematic parametric studies and investigation into complicated confinement, load combination, and dynamic loading situations.

• Computers and Concrete
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• v.12 no.4
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• pp.499-518
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• 2013

Fiber reinforced polymer (FRP) jackets have been widely used as an effective tool for the strengthening and rehabilitation of concrete structures, especially damaged concrete columns. Therefore, a clear understanding of the compressive behavior of FRP-confined concrete is essential. The objective of this paper is to develop a simple efficient method for predicting the compressive strength, the axial strain at the peak stress, and the stress-strain relationship of FRP-confined concrete. In this method, a compressive strength model is established based on Jefferson's failure surface. With the proposed strength model, the strength of FRP-confined concrete can be estimated more precisely. The axial strain at the peak stress is then evaluated using a damage-based formula. Finally, a modified stress-strain relationship is derived based on Lam and Teng's model. The validity of the proposed compressive strength and strain models and the modified stress-strain relationship is verified with a wide range of experimental results collected from the research literature and obtained from the self-conducted test. It can be concluded that, as a competitive alternative, the proposed method can be used to predict the compressive behavior of FRP-confined concrete with reasonable accuracy.

• Steel and Composite Structures
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• v.51 no.6
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• pp.587-600
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• 2024

In order to investigate the seismic performance of regionally confined concrete circular column with trapezoid stirrups (TRCCC) under high axial compression ratio, the confinement mechanism of regionally confined concrete was analyzed. Three regionally confined concrete circular columns with trapezoid stirrups were designed, and low cyclic loading tests were conducted at three different axial compression ratios (0.9, 1.1, 1.25) to study the failure mode, hysteresis curve, skeleton curve, deformation capacity, stiffness degradation and energy dissipation capacity of the specimens. The results indicate that the form of regional confinement concrete provides more uniform confinement to the normal confinement, and the confinement efficiency at the edges is 1.4 times that of normal confined concrete. The ductility coefficients of the specimens were all greater than 3 under high axial compression ratios, and the stiffness and horizontal bearing capacity increased with the increase of axial compression ratio. Therefore, it is recommended that the code of design specifications can appropriately relax the axial compression ratio limit for TRCCC. Finally, the spacing between stirrups of TRCCC was analyzed using ABAQUS software. The results showed that as the spacing between the stirrups decreased, the cracking load and peak load of TRCCC increased continuously, but the rate of increase decreases.

• Structural Monitoring and Maintenance
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• v.1 no.3
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• pp.273-292
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• 2014

The effectiveness of a retrofitting method for concrete columns with particular weaknesses is experimentally evaluated and presented in this paper. Structural deficiencies namely the inadequacy of transverse reinforcement and short length of lap splices are very common in columns found in structures built prior to the 1960s and 1970s. Recent earthquakes worldwide have caused severe damages and collapses of these structures. Nevertheless, the importance of improving the load transfer capacity between the deficiently lap-spliced bars is usually underestimated during the strengthening procedures applied in old buildings, though critical for the safety of the residents' lives. Thus, the seismic performance of the enhanced columns is frequently overestimated. The retrofitting approach presented herein involves reinforced concrete jacketing of the column sub-assemblages and welding of the lap-spliced bars to prevent the splice failure and conform to the provisions of modern design Codes. The cyclic lateral loading response of poorly confined original column specimens with insufficient lap splices and the seismic behavior of the retrofitted columns are compared. Test results clearly demonstrate that the retrofitting procedure followed is an effective way of significantly improving the seismic performance of substandard columns found in old buildings.

• Computers and Concrete
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• v.12 no.4
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• pp.413-429
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• 2013

The primary aim of this study is to develop a three dimensional finite element (FE) model to predict the axial stress-strain relationship and ultimate strength of the FRP-wrapped UHPC columns by comparing experimental results. The reliability of four selected confinement models and three design codes such as ACI-440, CSA-S806-02, and ISIS CANADA is also evaluated in terms of agreement with the experimental results. Totally 6 unconfined and 36 different types of the FRP-wrapped UHPC columns are tested under monotonic axial compression. The values of ultimate strengths of FRP-wrapped UHPC columns obtained from the experimental results are compared and verified with finite element (FE) analysis results and the design codes mentioned above. The concrete damage plasticity model (CDPM) in Abaqus is utilized to represent the confined behavior of the UHPC. The results indicate that agreement between the test results and the non-linear FE analysis results is highly satisfactory. The CSA-S806-02 design code is considered more reliable than the ACI-440 and the ISIS CANADA design codes to calculate the ultimate strength of the FRP-wrapped UHPC columns. None of the selected confinement models that are developed for FRP-wrapped low and normal strength concrete columns can safely predict the ultimate strength of FRP-wrapped UHPC columns.

• Steel and Composite Structures
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• v.20 no.3
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• pp.571-597
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• 2016

A new unified design formula for calculating the composite compressive strength of the axially loaded circular concrete filled double steel tubular (CFDST) short and slender columns is presented in this paper. The formula is obtained from the analytic solution by using the limit equilibrium theory, the cylinder theory and the "Unified theory" under axial compression. Furthermore, the stability factor of CFDST slender columns is derived on the basis of the Perry-Robertson formula. This paper also reports the results of experiments and finite element analysis carried out on concrete filled double steel tubular columns, where the tested specimens include short and slender columns with different steel ratio and yield strength of inner tube; a new constitutive model for the concrete confined by both the outer and inner steel tube is proposed and incorporated in the finite element model developed. The comparisons among the finite element results, experimental results, and theoretical predictions show a good agreement in predicting the behavior and strength of the concrete filled steel tubular (CFST) columns with or without inner steel tubes. An important characteristic of the new formulas is that they provide a unified formulation for both the plain CFST and CFDST columns relating to the compressive strength or the stability bearing capacity and a set of design parameters.