Circular Concrete-filled Steel Tubular Stub Research Articles

Experimental and numerical investigation into locally corroded circular concrete-filled steel tubular stub columns strengthened by CFRP

Concrete-filled steel tube (CFST) is vulnerable to corrosion in service, especially local corrosion. Nevertheless, limited work has been conducted on the behaviors and strengthening methods of locally corroded CFSTs. This paper presents experimental and numerical investigations into the behavior of locally corroded CFST stub columns and locally corroded CFST stub columns strengthened by carbon fiber reinforced polymer (CFRP). Tests were conducted on thirteen specimens with local corrosion of steel tubes simulated by means of artificial notches, and the notch length and number of CFRP layers were chosen as test parameters. The test results revealed that the performance of the specimens strengthened by CFRP was significantly improved, compared with that of the unstrengthened specimens. The maximum compression resistance increment of the CFRP-strengthened specimens was up to 58.8%. Subsequently, finite element (FE) models were developed and verified by the experimental results. Based on the FE models, parametric analyzes were conducted to investigate the axial behavior of locally corroded circular CFST stub columns strengthened by CFRP. The parametric analysis results indicated that CFRP can compensate for the loss of confinement effect of steel tube caused by local corrosion. In addition, when the hoop force provided by CFRP is equal to that provided by the yield stress of steel tube, the compression resistance of locally corroded CFST stub columns strengthened by CFRP is even slightly higher than that of CFST stub columns without corrosion.

Behavior of concrete-filled circular steel tubular stub columns exposed to corrosion and freeze–thaw cycles

This work experimentally and numerically studies the behavior of concrete-filled circular steel tubular stub columns experiencing corrosion and freeze–thaw cycles. The considered parameters are the corrosion rate, freeze–thaw cycles, and the number of alternations of the corrosion and freeze–thaw cycles. The failure modes and axial load–displacement curves are measured and discussed. The experimental results show that all the columns exhibit shear failure, and the core concrete suffers severe damage as the corrosion rate increases. The residual bearing capacity and ductility coefficient decrease with an increase in the corrosion rate and freeze–thaw cycles. A finite element (FE) model is also developed using the material thickness and strength reduction analysis method and validated using the experiments in this study. Further, a parametric analysis is conducted to identify the influences of the critical parameters on the load–bearing capacity of the columns. The finite element model results demonstrate that exposure to the alternations of corrosion and freeze–thaw cycles significantly influences the residual bearing capacity of the columns: the higher the number of alternations of corrosion and freeze–thaw cycles, the lower the residual bearing capacity of the columns. Finally, a simplified method is proposed for predicting the ultimate bearing capacity of concrete-filled circular steel tubular stub columns exposed to corrosion and freeze–thaw cycles.

A new design equation to estimate the axial compressive strength of circular concrete-filled steel tubular stub columns

This paper proposes a new design equation for circular concrete-filled steel tubular (CCFST) stub columns. A database consisting of 394 CCFST stub column tests are first compiled. Finite element (FE) analysis is then performed to generate 174 new results to cover the gaps in the database. Based on these, the correlations between the strength enhancement index and the tube diameter-to-thickness ratio, section slenderness, and relative strength factor are investigated. The experimental and FE ultimate loads are used to assess the accuracy of the existing codified strength predictions. Overall, it is found that the European Code (EC 4) provides more accurate strength predictions than the American (AISC 360-16) and Japanese (AIJ) codes, but is unconservative for high-strength CCFST short columns. A new unified design equation by incorporating the effect of ξ is then proposed. The strengths predicted by the proposed equation are comparably accurate as those predicted by EC 4. The proposed equation allows a fast and convenient estimation of the axial compressive strength of CCFST stub columns, with no need to calculate or classify the section slenderness of CCFST stub columns. Hence, it is recommended for practical strength estimation.

Tests of circular concrete-filled steel tubular stub columns with artificial notches representing local corrosions

Concrete-filled steel tubular (CFST) columns have been used in civil structures due to their affordability, high strength, superior ductility, and demonstrated fire resistance. However, there are remaining issues regarding the performances of the CFST columns with the locally corroded steel tubes, hindering the widespread acceptance of this type of members in practice. This paper focused on the circular CFST stub columns in which the steel tubes are locally corroded. To develop the critical data needed for understanding the behavior of such stub columns, a group of thirty specimens were designed, constructed and tested. Based on the recommendations from prior research, the local corrosions in the steel tubes of the specimens were represented by the artificial notches. The test parameters varied among the specimens included the notch orientations, lengths, depths, and widths. The test results revealed the influences of these parameters on different aspects of the responses of the stub columns with the corroded steel tubes. Computer models were also developed for the specimens and validated by the test results. Based on the test results and the results from parametric analyses using the validated computer models, a practical model to compute the peak resistance of a CFST stub column with the locally corroded steel tube was proposed and verified.

Axial compressive behaviour of circular concrete-filled steel tubular stub columns with an inner bamboo culm

Circular concrete-filled steel tubular columns (CFST) with an inner bamboo culm, by replacing the internal steel tube of concrete-filled double skin steel tubular (CFDST) columns with a structural bamboo culm, promote the use of natural materials with lower cost and lighter weight. This type of composite column is expected to integrate the advantages of all the three kinds of materials. Twenty-two circular stub columns including 12 CFST with an inner bamboo culm, 3 CFST columns, 4 CFDST columns and 3 hollow CFST columns were tested under axial compression. The test parameters include the bamboo culm thickness, the bamboo culm diameter and the diameter to thickness ratio of the external steel tubes. Test results indicated that the columns with an inner bamboo culm had excellent ductility. Furthermore, the columns with an inner bamboo culm showed strain-hardening behaviour after an initial decline in its axial load-displacement curve, but the inner bamboo culm contributed mildly to the ultimate load. Finally, a unified formula was proposed to predict the load-carrying capacities of all the four types of composite columns.

Compressive behavior of circular concrete-filled steel tubular columns under freeze-thaw cycles

In this paper, the compressive behavior of circular concrete-filled steel tubular stub columns under freeze-thaw cycles was studied. The influence of different concrete grades and number of freeze-thaw cycles were both investigated through experimental study. The results show that with the increase of the number of freeze-thaw cycles, the axial strength of specimens decreases regardless of concrete grade. The axial strength deterioration of the CFST specimens under different freeze-thaw cycles increases with the decrease of concrete strength, so does the reduction of enhancement factor. The ultimate displacement and the ductility indexes of CFST specimens are barely affected by the freeze-thaw cycles. Based on the test results, a reduction factor ksr for quantifying the strength reduction of CFST and a factor kcsr for considering the strength degradation of core concrete are proposed to predict the ultimate compressive strength of CFST stub column under freeze-thaw cycles.

Tests on axial strength of circle CFST stub columns under marine atmosphere in cold region

This paper aims to study the compressive behavior of circular concrete-filled steel tubular stub columns under marine atmosphere in cold region. The specimens after salt spray corrosion and freeze-thaw cycles were tested under axial compressive load where steel grade, corrosion rate and freeze-thaw cycles were considered. The experimental phenomena and results are discussed in detail. Current design methods for circular concrete-filled steel tubular stub columns were modified to consider the effect of atmospheric corrosion and freeze-thaw cycles. The results show that the corrosion rate, the cycle number and steel strength have little effect on the failure pattern of circular CFST stub columns, neither on the failure pattern of core concrete. The ultimate strength of specimens is reduced approximately linearly with the increase of corrosion rate and freeze-thaw cycles regardless of steel strength. Strength reductions of the specimens using Q235 steel are smaller than those of the counterparts using Q345 steel. The ultimate displacement and ductility indexes of the specimens are decreased by corrosion rate and freeze-thaw cycle. The enhancement factor φ tends to increase with the increase of test period which indicates that confinement effect contributes to resist the strength reduction due to corrosion and freeze-thaw cycle. Various design standards combined with the reduction factor proposed based on experimental results are adopted to predict the ultimate strength of CFST stub columns under atmospheric corrosion and freeze-thaw cycle.

Comparative study of stirrup-confined circular concrete-filled steel tubular stub columns under axial loading

This paper presents a comparative study of circular concrete-filled steel tubular stub columns with three different stirrup confinement types: bidirectional stirrups, loop stirrups and orthogonal stirrups. Axial compression tests have been carried out aiming at investigating the effects of the stirrup form and volume-stirrup ratio on the mechanical behavior of the stirrup-confined circular CFT stub columns, and ABAQUS was used to carry out the 3D numerical modelling. Radial stress of the core concrete and the composite action among the steel tube, stirrups and the core concrete have been investigated. It is found that the confinement provided by stirrups on core concrete strongly outperforms that provided by steel tube, steel sections or steel reinforcement. Furthermore, a simplified approach was developed to predict the ultimate bearing capacity of stirrup-confined circular CFT stub columns, which agreed well with the experimental and numerical results.

Finite element study to address the axial capacity of the circular concrete-filled steel tubular stub columns

This paper presents finite element analysis using the software ABAQUS to the circular CFST stub columns. The produced FE model is verified by comparing results of the FE simulation, failure load, and load-axial strain curves, to the experimental results. Then a parametric study has been done to examine the effect of the steel tube yield strength and size. A method is then proposed to calculate the confined concrete compressive strength. The proposed model was verified against 382 previously tested data. It has been found that both the FE and the numerical model are in excellent agreement with the experimental data. The numerical model proposed in this paper compared to the models proposed by leading design codes and previously proposed models by other researchers. It has been found that the current model predicts the ultimate capacity of these columns with excellent accuracy while the compared-with numerical models were generally providing conservative predictions.

Elasto-Plastic Analysis of Concrete-Filled Circular Steel Tubular Stub Columns after Exposed to High Temperatures

Based on the research achievements of the mechanical properties of concrete at uniaxial compression and steel at uniaxial tension after exposed to high temperatures, the axisymmetric- triaxial-compressive stress-strain relationships of concrete and multiaxial stress-strain relationships of steel after exposed to high temperatures was suggested. Based on continuum mechanics, the mechanical model of concentric cylinders of circular steel tube with concrete core of entire section loaded after exposed to high temperature was determined. By applying Elasto-Plastic Analysis Method, theoretical calculation formulas for composite elastic modulus and composite stress-strain relationships of concrete-filled circular steel tubular (CFST) stub columns were proposed and a FORTRAN program was developed and the load-axial strain relationships of CFST stub columns after exposed to high temperatures were analyzed. The analysis results were in reasonable agreement with the experiment ones from references.

Behavior of concrete and concrete-filled circular steel tubular stub columns at constant high temperatures

Based on reanalyzing test results of uniaxial compressive behavior of concrete at constant high temperatures in China, with the compressive cube strength of concrete from 20 to 80 MPa, unified formulas for uniaxial compressive strength, elastic modulus, strain at peak uniaxial compression and mathematical expression for unaxial compressive stress-strain relations for the concrete at constant high temperatures were studied. Furthermore, the axial stress-axial strain relations between laterally confined concrete under axial compression and multiaxial stress-strain relations for steel at constant high temperatures were studied. Finally, based on continuum mechanics, the mechanics model for concentric cylinders of circular steel tube with concrete core of entire section loaded at constant high temperatures was established. Applying elasto-plastic analysis method, a FORTRAN program was developed, and the concrete-filled circular steel tubular (CFST) stub columns at constant high temperatures were analyzed. The analysis results are in agreement with the experiment ones from references.