Circular Concrete-filled Steel Tube Stub Research Articles

Behavior of eccentrically loaded circular concrete-filled steel tube stub columns with localized corrosion

In this paper, 12 concrete-filled steel tube (CFST) stub columns with localized corrosion were tested under eccentric load. The effects of the depth ratio, size and location of localized corrosion, and loading eccentricity on the eccentric compression behavior of circular concrete-filled steel tube stub columns were investigated. A finite element model of circular CFST stub columns with localized corrosion under eccentric load was developed and then parameter analysis was carried out. Based on the experimental and numerical results, it was found that the load bearing capacity decreases with increasing the depth ratio and size of localized corrosion. The localized corrosion at the mid-height caused more significant capacity deterioration than that at the column ends. In addition, the localized corrosion with γ = 0° and 180° has more adverse effects on the capacity of eccentrically loaded CFST columns than that with γ = 90°. Moreover, the localized corrosion with γ = 0° (at compression side) is more harmful than that γ = 180° (at tension side) for the specimens with compression-controlled failure, and vice versa for the specimens with tension-controlled failure. Finally, a design model of eccentrically loaded CFST column with localized corrosion was proposed based on an established equivalent specimen. The results suggested that the proposed model predicted the load bearing capacities with reasonable accuracy.

Axial compressive behavior of circular concrete-filled steel tube stub columns with steel slag coarse aggregate

Steel slag, an industrial waste by-product, has been used as an alternative sustainable aggregate material in construction applications to attempt to manage solid waste. In this context, this work conducts axial compression tests on 5 groups of 10 circular steel slag concrete-filled steel tube (SSCFST) stub columns to study the feasibility of using steel slag coarse aggregate in concrete-filled steel tubes. The failure mode, load–strain curve, axial bearing capacity, initial axial stiffness, critical strain, and lateral deformation coefficient of the columns are analyzed. Further, the effects of the steel slag replacement ratio and cross-section dimension on the axial compressive properties of the specimens are quantified. These test results are employed to verify the reliability of a finite element model of the circular SSCFST stub columns established using Abaqus software. Then, a parametric analysis is performed to investigate the factors influencing the axial compressive performance of circular SSCFST stub columns. Moreover, the prediction accuracy of the existing design methods for the axial bearing capacity of circular SSCFST stub columns is evaluated. The results show that the replacement ratio of steel slag influences the axial compressive properties of concrete-filled steel tubes negligibly: the axial bearing capacity and initial axial stiffness of the columns decrease only by 1.4% and 2.4% respectively, at a steel slag replacement ratio of 100%. Increasing the cross-section diameter from 160 mm to 180 and 200 mm improves the axial bearing capacity of the columns by 25.6% and 54.6% respectively and reduces the critical strain by 12.4% and 21.7% respectively. The confinement factor is still critical to affecting the axial compressive performance of circular SSCFST stub columns. Finally, the formulas proposed in standards GB 50936–2014 and AIJ-2008 can be employed to calculate the axial bearing capacity of SSCFST stub columns.

Static strength of CFRP-strengthened preloaded circular concrete-filled steel tube stub columns, Part I: Experimental test

In the process of strengthening circular concrete-filled steel tube (CFST) stub columns in servicing period with carbon fiber reinforced polymer (CFRP), the CFST stub columns are generally preloaded. This study concerns the efficiency of CFRP in strengthening preloaded CFST stub columns. Thirteen specimens are designed and tested under axial compression, including two un-strengthened CFST specimens, four circular CFRP-strengthened concrete-filled steel tube (CFRP-CFST) specimens without preloading and seven preloaded CFRP-CFST specimens. The failure process, the failure mechanism, and the static strengths of the specimens, including yield strength and ultimate strength, are analyzed based on the experimental results. Improvement of the static strengths of the CFRP-strengthened specimens is discussed. The effect of preloading and CFRP layers on the static strength is investigated. The results show that preloading has certain effect on the yield strength. However, its effect on the ultimate strength is very little and it can be ignored. Increasing CFRP layers can improve the strengths of the preloaded CFST specimens effectively.

Static strength of CFRP-strengthened preloaded circular concrete-filled steel tube stub column columns – Part II: Theoretical and numerical analysis

Carbon fiber reinforced polymer (CFRP) has high strength and is lightweight, and it can be used to strengthen circular concrete-filled steel tubes (CFSTs). For CFSTs in service, they are usually subjected to preloading. By considering the strain lag of CFRP, a theoretical model for analyzing CFRP-strengthened preloaded CFST stub columns is presented, and corresponding equations are derived for calculating the yield strength and the ultimate strength. Using the death and birth technique of CFRP element, finite element (FE) modeling is also presented to simulate the failure process of a CFRP-strengthened preloaded CFST stub column. The reliability and the accuracy of both theoretical equations and the FE model are evaluated by comparing the predicted results with experimental results reported in Part I. Finally, the CFRP-strengthening efficiency for the yield strength and the ultimate strength is assessed through the parametric study on 48 models. The effects of CFRP layers and preloading percentage on the strength of CFRP-strengthened preloaded CFST stub columns are also discussed.

Elastoplastic Analysis of Circular Steel Tube of CFT Stub Columns under Axial Compression.

Composite action between the components of the concrete-filled steel tube (CFT) is complex and it is difficult to accurately obtain the experimental relationship between the steel tube and the core concrete of CFT columns. The triaxially stressed core concrete has been studied by hydrostatic test in past research, while little research has been focused on the mechanical behavior of steel tube of CFT columns. It is difficult to obtain the experimental constitutive relationship of the steel tube of CFT columns to reflect the real-time influence of biaxial stress state and local buckling of steel plate on the steel tube. To clarify the mechanical behavior of the steel tube of CFT columns, this paper proposed an elastoplastic analytical method considering biaxial stress state and local buckling of steel tube to obtain the stress-strain curve of the steel tube. This method applied the Hook's law and the plasticity theory to interpret the information conveyed by the measured vertical and hoop strain histories of the steel tube. To verify its effectiveness, 11 circular concrete-filled steel tube stub columns were fabricated and tested under axial compression. Superposition results of the axial load-strain of steel tube and core concrete were compared against the experimental curves. The widely used Sakino-Sun model of the confined concrete was adopted to calculate the axial load-strain curve of the core concrete. Satisfactory agreements between the calculated and experimental results confirmed the rationality of the proposed method in tracing the constitutive relation of the biaxially stressed steel tube even after the occurrence of the local buckling. The obtained stress-strain relationship is critical for establishment of mathematical constitutive model and finite element model of steel tube.

Axial compressive behavior of circular concrete-filled steel tube stub columns prepared with spontaneous-combustion coal gangue aggregate

The compressive behavior of circular concrete-filled steel tube (CFST) stub columns prepared with spontaneous-combustion coal gangue aggregate (SCGA) was investigated to study the effect of SCGA substitution level by experimental test and finite element simulation. The test variables included SCGA replacement ratios (0%, 50% and 100%) and steel ratios (8.3%, 11.6% and 14.2%). The study concerns the failure mode, ultimate compressive strength, elastic stiffness, ductility of the specimens, and the confinement effect of the steel tube. The test results showed that the failure mode of specimens prepared with SCGA was similar to that of reference CFSTs; compressive strength, elastic stiffness and ductility of the specimens were reduced by 2.93–4.81% (8.25–10.18%), −16.17–8.24% (3.01–22.16%) and 7.78–10.61% (11.86–16.34%) respectively with the increasing replacement ratio of SCGA from 0% to 50% (100%). A finite element model was proposed based on ABAQUS software and benchmarked against the test data from this study. The numerical results indicated that the SCGA replacement ratio and the confinement effect were the major factors of the compressive behavior of CFSTs. After comparing the test data with the existing design codes, modified design equations considering replacement ratio and confinement effects was proposed to predict the ultimate compressive strength of the specimens prepared with SCGA. Accuracy of the derived modified equation was evaluated, with the mean value of 0.984 and a standard deviation of 0.076. The study provides an experimental basis of the utilization of SCGA in CFST stub columns.

Compatibility Optimal Design of Axially Loaded Circular Concrete-Filled Steel Tube Stub Columns.

Numerous studies have been carried out on the axially loaded circular concrete-filled steel tube (CCFST) stub columns. However, to date, no clear evaluation criterion for the compatibility of its design parameters has been established. In the present study, the compatibility of the design parameters (concrete compressive strength , steel yield strength , diameter D and thickness of steel tube t) of axially loaded CCFST stub columns was quantitatively investigated in terms of the contribution of the composite actions to the axial bearing capacity of the columns. The composite ratio was proposed as an indicator to represent the effectiveness of the composite actions. A numerical framework of the determination of was established, making use of a series of existing widely recognized constitutive models of structural steel and concrete. Some modifications were carried out on these models to ensure the numerical stability of the presented analysis. Moreover, the rationality of the combined use of these models was verified. The analytical results show that excessive or very small ratio should be avoided in design. Meanwhile, the combined use of low-strength steel and high-strength concrete should be avoided. A table of optimal ratios corresponding to different material strength matches was provided for designers. Finally, an optimization of the design parameters using the proposed method and the existing design specification was performed.

Stress paths of confined concrete in axially loaded circular concrete-filled steel tube stub columns

Previous study has suggested that the stress paths of confined concrete in concrete-filled steel tube (CFT) stub columns influence the compressive strengths of the concrete, which, however, is based on the test of concrete-loaded CFT columns. In this paper, to fully understand the compressive strength of confined concrete in CFT column, the stress paths of confined concrete in the whole-section-loaded CFT columns are investigated. A detailed parametric study is conducted on the stress paths and their corresponding effects on compressive strength. Test results suggest that the lateral stress of the concrete in a whole-section-loaded CFT column generally occurred in a later stage and less significant than that in a concrete-loaded CFT column. The compressive strength of confined concrete in the whole-section-loaded CFT column with a small confinement coefficient is path-dependent, and the confinement effect of the concrete is generally weaker than that in concrete-loaded CFT column. With the increase of the confinement coefficient, the compressive strengths of confined concrete in whole-section-loaded CFT columns become path-independent. Based on the test results, a compressive strength model incorporating the stress path effect is developed for confined concrete in both whole-section-loaded and the concrete-loaded CFT columns. Good agreement between the experimental results and the ones predicted by the model is found, indicating a high performance of the proposed model.