• Steel and Composite Structures
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• v.29 no.6
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• pp.689-701
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• 2018

Corrosion of steel reinforcement is a principal cause of deterioration of RC columns. Making these corrosion-damaged columns conform to new safety regulations and functions is a tremendous technological challenge. This study presented an experimental investigation on steel-concrete jacketed corrosion-damaged RC columns. The influences of steel jacket thickness and concrete strength on the enhancement performance of the strengthened specimens were investigated. The results showed that the use of steel-concrete jacketing is efficient since the stub strengthened columns behaved in a more ductile manner. Moreover, the ultimate strength of the corrosion-damaged RC columns is increased by an average of 5.3 times, and the ductility is also significantly improved by the strengthening method. The bearing capacity of the strengthening columns increases with the steel tube thickness increasing, and the strengthening concrete strength has a positive impact on both bearing capacity, whereas a negative influence on the ductility. Subsequently, a numerical model was developed to predict the behavior of the retrofitted columns. The model takes into account corrosion-damage of steel rebar and confining enhancement supplied by the steel tube. Comparative results with the experimental results indicated that the developed numerical model is an effective simulation. Based on extensive verified numerical studies, a design equation was proposed and found to predict well the ultimate eccentric strength of the strengthened columns.

• Steel and Composite Structures
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• v.39 no.2
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• pp.201-213
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• 2021

Currently, there is a lack of research in the design approach to avoid column wall failure in the concrete filled double skin steel tubular (CFDST) column-beam connections. In this paper, a finite element model has been developed and verified by available experimental data to analyze the failure mechanism of CFDST column-beam connections. Various finite element models with different column hollow ratios (χ) were established. The simulation result revealed that with increasing χ the failure mode gradually changed from yielding of end plate, to local failure of the column wall. Detailed parametric analyses were performed to study the failure mechanism of column wall for the CFDST column-beam connection, in which the strength of sandwiched concrete and steel tube and thickness of steel tube were incorporated. An analytical model was proposed to predict the moment resistance of the assembled connection considering the failure of column wall. The simulation results indicate that the proposed analytical model can provided a conservative prediction of the moment resistance. Finally, an upper bound value of χ was recommend to avoid column wall failure for CFDST column-beam connections.

• International journal of steel structures
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• v.18 no.5
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• pp.1577-1588
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• 2018

As the excellent mechanical performance and easy construction of concrete filled steel tubes (CFST) composite structure, it has the potential to be used to strengthen RC pier columns. Therefore, tests were conducted on 2 reinforcement concrete (RC) stub columns and 9 RC columns strengthened with circular CFST under axial loading. The test results show that the circular CFST strengthening method is effective since the mean bearing capacity of the RC columns is increased at least 3.69 times and the ductility index is significantly improved more than 30%. One of the reasons for enhancement is obvious confinement provided by steel tube besides the additional bearing capacity supplied by the strengthening materials. From the analysis of the enhancement ratio, the strengthening structure has at least an extra 20% amplification except for taking full advantage of the strength of the strengthening material. Through the analysis of confining stress provided by steel tube and the stress-strain relationship of confined concrete, it is found that the strength of the core concrete can be increased by 21-33% and the ultimate strain can be enhanced to beyond $15,000{\mu}{\varepsilon}$.

• Steel and Composite Structures
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• v.44 no.1
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• pp.119-139
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• 2022

This paper aims to use the artificial intelligence approach to develop a new model for predicting the ultimate axial strength of the circular concrete-filled steel tubular (CFST) stub columns. For this, the results of 314 experimentally tested circular CFST stub columns were employed in the generation of the design model. Since the influence of the column diameter, steel tube thickness, concrete compressive strength, steel tube yield strength, and column length on the ultimate axial strengths of columns were investigated in these experimental studies, here, in the development of the design model, these variables were taken into account as input parameters. The model was developed using the backpropagation algorithm named Bayesian Regularization. The accuracy, reliability, and consistency of the developed model were evaluated statistically, and also the design formulae given in the codes (EC4, ACI, AS, AIJ, and AISC) and the previous empirical formulations proposed by other researchers were used for the validation and comparison purposes. Based on this evaluation, it can be expressed that the developed design model has a strong and reliable prediction performance with a considerably high coefficient of determination (R-squared) value of 0.9994 and a low average percent error of 4.61. Besides, the sensitivity of the developed model was also monitored in terms of dimensional properties of columns and mechanical characteristics of materials. As a consequence, it can be stated that for the design of the ultimate axial capacity of the circular CFST stub columns, a novel artificial intelligence-based design model with a good and robust prediction performance was proposed herein.

• Steel and Composite Structures
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• v.43 no.5
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• pp.625-637
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• 2022

Axial compression capacity (P_u) is a significant yet complex parameter of concrete-filled steel tube (CFST) columns. This study offers a novel ensemble tool, adaptive neuro-fuzzy inference system (ANFIS) supervised by equilibrium optimization (EO), for accurately predicting this parameter. Moreover, grey wolf optimization (GWO) and Harris hawk optimizer (HHO) are considered as comparative supervisors. The used data is taken from earlier literature provided by finite element analysis. ANFIS is trained by several population sizes of the EO, GWO, and HHO to detect the best configurations. At a glance, the results showed the competency of such ensembles for learning and reproducing the P_u behavior. In details, respective mean absolute errors along with correlation values of 4.1809% and 0.99564, 10.5947% and 0.98006, and 4.8947% and 0.99462 obtained for the EO-ANFIS, GWO-ANFIS, and HHO-ANFIS, respectively, indicated that the proposed EO-ANFIS can analyze and predict the behavior of CFST columns with the highest accuracy. Considering both time and accuracy, the EO provides the most efficient optimization of ANFIS and can be a nice substitute for experimental approaches.

• Steel and Composite Structures
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• v.41 no.5
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• pp.649-661
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• 2021

This paper presents experimental and analytical studies to understand the behavior of crumb rubber concrete (CRC)-filled fiber reinforced polymer (FRP) and steel tube double skin column (DSC) and beam (DSB) members under cyclic loading. The main test variable was the percentage of rubber which ranged from 0 to 40%. For column members, different heights corresponding to different aspect ratios were examined to understand the to understand the effect of DSCs' slenderness on the cyclic response of the columns. the. The behavior of the specimens in terms of failure mode, strain development, energy dissipation, load-displacement response were presented and compared. The ability of the current provisions of the Australian codes to predict the capacity of such double skin members was also evaluated based on the test results. This study concluded that the reduction in the concrete strength was more severe at the material level compared to structural level. Also, as the load changed from axial compression in columns to pure moment in beams the negative effect of rubber percentage on the strength became less significant.

• Steel and Composite Structures
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• v.47 no.2
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• pp.289-296
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• 2023

The failure of a thin-walled tube was studied in this paper based on three failure models. Both proportional and non-proportional loading paths were applied. Proportional loading consisted of combined tension-torsion. Cyclic non-proportional loading was also applied. It was a circular out-of-phase axial-shear stress loading path. The third loading path was a combination of a constant internal pressure and a bending moment. The failure models under study were equivalent plastic strain, modified Mohr-Coulomb (Bai-Wierzbicki) and Tearing parameter models. The elasto-plastic analysis was conducted using J2 criterion and nonlinear kinematic hardening. The return mapping algorithm was employed to numerically solve the plastic flow relations. The effects of the hydrostatic stress on the plastic flow and the stress triaxiality parameter on the failure were discussed. Each failure model under study was utilized to predict failure. The failure loads obtained from each model were compared with each other. The equivalent plastic strain model was independent from the stress triaxiality parameter, and it predicted the highest failure load in the bending problem. The modified Mohr-Coulomb failure model predicted the lowest failure load for the range of the stress triaxiality parameter and Lode's angle.

• Progress in Superconductivity and Cryogenics
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• v.10 no.1
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• pp.24-27
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• 2008

For the development of superconducting fault current limiters(SFCLs) having large current capacity, we fabricated an SFCL element that consists of Bi-2212 superconductor and Cu-Ni alloy tubes. First, Ag was plated on the surface of the Bi-2212 for the enhancement of soldering process. On the Ag-plated Bi-2212 tube, a Cu-Ni alloy tube was soldered using optimized solders and soldering conditions. The BSCCO/Cu-Ni composite was processed mechanically to have a helical shape for the improvement of the SFCL characteristics. The total current path of the SFCL element was 1330 mm long with 12 turns, and had critical current of 340 A at 77 K. Finally, we carried out the fault test using the fabricated SFCL element. It showed successful current limiting performance under the fault condition of 50 $V_{rms}$ and 5.5 kA. From the results, the rated voltage of the SFCL element was decided to be 0.4 V/cm, and the power capacity was 12 kVA at 77 K. The fabrication process of the SFCL and the fault test results will be presented.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.3A
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• pp.265-275
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• 2010

Concrete filled steel tube and concrete encased steel tube columns are expected to have confined effects of concrete by steel and reinforced effects of local buckling by concrete. On the basis of confined state concrete models of previous researches, stress-strain and load-displacement relations of RC, CFT and CET columns are analyzed by steel ratio. After comparing analysis results with experimental results, Modified stress-strain relations are derived through evaluation the influence upon confined effects of concrete in each cases. Also, the modified stress-strain models are carried out to be compared with specified strength of various countries.

• Computers and Concrete
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• v.33 no.1
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• pp.41-53
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• 2024

Under constant and cyclic axial compression, square composite short columns reinforced with Self Compacting Concrete (SCC) added with scrap rubber infilled inside steel tubes and with different types of concrete were cast and tested. The test is carried out to find the effectiveness of utilizing an aggregate manufactured from industrial waste and to address the problems associated with the need for alternative reinforcements along with waste management. The main testing parameters are the type of concrete, the effect of fiber inclusion, and the significance of rubber-infilled steel tubes. The failure modes of the columns and axial load-displacement curves of the steel tube-reinforced columns were all thoroughly investigated. According to the test results, all specimens failed due to compression failure with a longitudinal crack along the loading axis. The fiber-reinforced column specimens demonstrated improved ductility and energy absorption. In comparison to the normal-weight concrete columns, the lightweight concrete columns significantly improved the axial load-carrying capacity. The addition of basalt fiber to the columns significantly increased the yield stress and ultimate stress to 9.21%. The corresponding displacement at yield load and ultimate load was reduced to 10.36% and 28.79%, respectively. The precision of volumetric information regarding the obtained crack quantification, aggregates, and the fiber in concrete is studied in detail through image processing using MATLAB environment.