• Structural Engineering and Mechanics
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• v.61 no.5
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• pp.625-635
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• 2017

Sets of nonlinear formulations together with an energy-based damage index (DI) are proposed to model the behavior and quantify the damage of the confined and unconfined concretes under monotonic and cyclic loading. The proposed formulations and DI can be employed in numerical simulations to determine the stresses and the damages to the fibers or the layers within the sections of reinforced concrete (RC) components. To verify the proposed formulations, an adaptive finite element computer program was generated to simulate the RC structures subjected to monotonic and cyclic loading. By comparing the simulated and the experimental test results, on both the full-scale structural members and concrete cylindrical samples, the proposed uniaxial behavior modeling formulations for confined and unconfined concretes under monotonic and cyclic loading, based on an iterative process, were accordingly adjusted, and then validated. The proposed formulations have strong mathematical structures and can readily be adapted to achieve a higher degree of precision by improving the relevant coefficients based on more precise tests. To apply the proposed DI, the stress-strain data of concrete elements is required. It can easily be calculated by using the proposed nonlinear constitutive laws for confined and unconfined concretes in this paper.

• KCI Concrete Journal
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• v.13 no.1
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• pp.69-78
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• 2001

The inelastic behavior of a reinforced concrete columns is influenced by a number of factors : 1) level of axial load, 2) tie spacing, 3) volumetric ratio of lateral steel, 4) concrete strength, 5) distribution of longitudinal steel, 6) strength of lateral steel, 7) cover thickness, 8) configuration of lateral steel, 9) strain gradient, 10) strain rate, 11) the effectively confined concrete core area, and 12) amount of longitudinal steel. A new constitutive model of a confined concrete is suggested in order to investigate the nonlinear behavior of the reinforced concrete columns under concentric loading. The developed constitutive model for the confined concrete takes into account the effects of effectively confined area as well as the horizontal and longitudinal distributions of the confining pressures. None of the existing models incorporated these two main effects at the same time. A total of different six constitutive models for the behavior of the confined concrete under concentric compression were compared with the sixty-one test results reported by different researchers. The superiority of the developed model in its accuracy is demonstrated by evaluating the error function, which compares the weighted averages for the sum of squared relative differences in peak compressive strength and corresponding strain, stress at strain equal to 0.015, and total area under stress-strain curve up to strain equal to 0.015.

• Computers and Concrete
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• v.10 no.4
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• pp.331-347
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• 2012

This paper describes the varying material model, the analysis method and the software development for reinforced concrete circular columns confined by spiral or hoop transverse steel reinforcement and subjected to eccentric loading. The widely used Mander model of concentric loading is adapted here to eccentric loading by developing an auto-adjustable stress-strain curve based on the eccentricity of the axial load or the size of the compression zone to generate more accurate interaction diagrams. The prediction of the ultimate unconfined capacity is straight forward. On the other hand, the prediction of the actual ultimate capacity of confined concrete columns requires specialized nonlinear analysis. This nonlinear procedure is programmed using C-Sharp to build efficient software that can be used for design, analysis, extreme event evaluation and forensic engineering. The software is equipped with an elegant graphics interface that assimilates input data, detail drawings, capacity diagrams and demand point mapping in a single sheet. Options for preliminary design, section and reinforcement selection are seamlessly integrated as well. Improvements to KDOT Bridge Design Manual using this software with reference to AASHTO LRFD are made.

• Steel and Composite Structures
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• v.38 no.2
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• pp.165-176
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• 2021

Existing research on confined concrete filled steel tubular (CCFT) columns has been mainly focused on static or cyclic loading. In this paper, square section CCFT and CFT columns were tested under both static and impact loading, using a 10,000 kN capacity compression test machine and a drop weight testing equipment. Research parameters included bonded and unbonded fiber reinforced polymer (FRP) wraps, with carbon, basalt and glass FRPs (or CFRP, BFRP, and GFRP), respectively. Time history curves for impact force and steel strain observed are discussed in detail. Experimental results show that the failure modes of specimens under impact testing were characterized by local buckling of the steel tube and cracking at the corners, for both CCFT and CFT columns, similar to those under static loading. For both static and impact loading, the FRP wraps could improve the behavior and increase the loading capacity. To analyze the dynamic behavior of the composite columns, a finite element, FE, model was established in LS-DYNA. A simplified method that is compared favorably with test results is also proposed to predict the impact load capacity of square CCFT columns.

• 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.

• Geomechanics and Engineering
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• v.32 no.1
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• pp.97-110
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• 2023

The coefficient of lateral earth pressure at rest in loose sand during virgin loading, K₀ , and during unloading, K'₀ , have been determined from laterally confined load-unload tests. The tests included measurement of lateral pressure with null pressure gauges, side wall friction with newly designed friction meters and applied pressure and base pressure with load cells. The importance of accounting for side-wall friction when evaluating the distribution of vertical pressure over the height of the soil specimen was demonstrated. Relatively uniform friction was observed during loading, but this was not the case during unloading unless friction reduction measures were employed. While the measured value of K₀ was found to be close to, if slightly higher than the value commonly estimated on the basis of friction angle, φ', the ratio of K'₀ to K₀ was found to reasonably fit an expression of the form K'₀/K₀ = 1 + C·log(OCR), with C equal to 1 in the present tests.

• Structural Engineering and Mechanics
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• v.16 no.1
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• pp.63-81
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• 2003

This paper describes experimental and theoretical investigations on the behaviour of thin walled composite (TWC) filled columns under eccentric loading conditions. Details of the experimental investigation including description of the test columns, testing arrangements, failure modes, strain characteristics, load-deformation responses and effects of various geometric and material parameters are presented. The current paper also introduces the use and effect of lightweight Volcanic Pumice Concrete (VPC) in TWC columns. Analytical models for the design of columns under eccentric loading conditions have been developed taking into consideration the effect of confined concrete. The performance of design equations is validated through experimental results. The proposed design models are found to produce better results compared with available design procedures and Code based formulations. A computer program is developed to generate the interaction diagrams based on the proposed design equations that can be used for design purposes.

• International Journal of Concrete Structures and Materials
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• v.11 no.1
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• pp.135-149
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• 2017

The prediction of the actual ultimate capacity of confined concrete columns requires partial confinement utilization under eccentric loading. This is attributed to the reduction in compression zone compared to columns under pure axial compression. Modern codes and standards are introducing the need to perform extreme event analysis under static loads. There has been a number of studies that focused on the analysis and testing of concentric columns. On the other hand, the augmentation of compressive strength due to partial confinement has not been treated before. The higher eccentricity causes smaller confined concrete region in compression yielding smaller increase in strength of concrete. Accordingly, the ultimate eccentric confined strength is gradually reduced from the fully confined value $f_{cc}$ (at zero eccentricity) to the unconfined value $f^{\prime}_c$ (at infinite eccentricity) as a function of the ratio of compression area to total area of each eccentricity. This approach is used to implement an adaptive Mander model for analyzing eccentrically loaded columns. Generalization of the 3D moment of area approach is implemented based on proportional loading, fiber model and the secant stiffness approach, in an incremental-iterative numerical procedure to achieve the equilibrium path of $P-{\varepsilon}$ and $M-{\varphi}$ response up to failure. This numerical analysis is adapted to assess the confining effect in rectangular columns confined with conventional lateral steel. This analysis is validated against experimental data found in the literature showing good correlation to the partial confinement model while rendering the full confinement treatment unsafe.

• Proceedings of the Korea Concrete Institute Conference
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• 2006.05a
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• pp.62-65
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• 2006

This study investigated the behavior of R/C column confined with headed crossties subjected to eccentric loading. The 16 specimens are designed to have adequate confinement steel, determined by ACI seismic design. The variables studied in this research test are eccentricity to depth ratios, spacing of lateral steel and the anchor type of end of crossties. From the test results, all columns showed similar behavior up to the peak load but those columns laterally confined with head presented more ductile behavior after the peak load. The comparisons indicate that the flexural behavior of confined-concrete columns can be computed resonable accurately by P-M interaction curve.

• Structural Engineering and Mechanics
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• v.84 no.1
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• pp.101-111
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• 2022

It is widely known that axially loaded fiber-reinforced polymer (FRP) confined concrete presents significant and enhanced mechanical properties with reference to the unconfined concrete. Therefore, to predict the mechanical behavior of FRP-confined concrete two quantities-peak strength and ultimate strain are required. Despite the significant advances, the determination of the ultimate strain of FRP-confined concrete is one of the most challenging problems to be resolved. This is often attributed to our persistence in desiring the conventional methods as the sole technique to examine this phenomenon and the complex nature of the ultimate strain of FRP-confined concrete. To bridge the research gap, this study adopted two machine learning (ML) techniques-artificial neural network (ANN) and Gaussian process regression (GPR)-to analyze observations obtained from 627 datasets of FRP-confined concrete circular and non-circular sections under axial loading test. Besides, the techniques are also used to predict the ultimate strain of FRP-confined concrete. Seven parameters namely width/diameter of the specimens, corner radius ratio, the strength of concrete, FRP elastic modulus, FRP thickness, FRP tensile rupture strain, and the axial strain of unconfined concrete-are the input parameters used to predict the ultimate strain of FRP-confined concrete. The results of the current study highlight the merit of using AI techniques in structural engineering applications given their extraordinary ability to comprehend multidimensional phenomena of FRP-confined concrete structures with ease, low computational cost, and high performance over the existing empirical models.