• Steel and Composite Structures
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• 제47권5호
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• pp.645-661
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• 2023

Concrete-filled steel tubular columns with double inner steel tubes (CFST-DIST) are a novel type of composite members developed from conventional concrete-filled steel tubular (CFST) columns. This paper investigates the structural performance of circular CFST-DIST stub columns using nonlinear finite element (FE) analysis. A numerical model was developed and verified against existing experimental test results. The validated model was then used to compare circular CFST-DIST stub columns' behavior with their concrete-filled double skin steel tubular (CFDST) and CFST counterparts. A parametric study was performed to ascertain the effects of geometric and material properties on the axial performance of CFST-DISTs. The FE results and the available test data were used to assess the accuracy of the European and American design regulations in predicting the axial compressive capacity of circular CFST-DIST stub columns. Finally, a new design model was recommended for estimating the compressive capacity of CFST-DISTs. Results clarified that circular CFST-DIST columns had the advantages of their CFST counterparts but with better ductility and strength-to-weight ratio. Besides, the investigated design codes led to conservative predictions of the compressive capacity of circular CFST-DIST columns.

• Steel and Composite Structures
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• 제39권2호
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• pp.189-200
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• 2021

The compressive strength of circular concrete filled steel tubular (C-CFST) stubs strengthened with carbon fiber reinforced polymer (CFRP) is studied theoretically. According to previous experimental results, the failure process and mechanism of circular CFRP-concrete filled steel tubular (C-CFRP-CFST) stubs is analyzed, and the loading process is divided into 3 stages, i.e., elastic stage, elasto-plastic stage and failure stage. Based on continuum mechanics, the theoretical model of C-CFRP-CFST stubs under axial compression is established based on the assumptions that steel tube and concrete are both in three-dimensional stress state and CFRP is in uniaxial tensile stress state. Equations for calculating the yield strength and the ultimate strength of C-CFRP-CFST stubs are deduced. Theoretical predictions from the presented equations are compared with existing experimental results. There are a total of 49 tested specimens, including 15 ones for comparison of yield strength and 44 ones for comparison of ultimate strength. It is found that the predicted results of most specimens are within an error limit of 10%. Finally, simplified equations for calculating both yield strength and ultimate strength of C-CFRP-CFST stubs are proposed.

• Steel and Composite Structures
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• 제19권1호
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• pp.111-129
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• 2015

This paper first presents an experimental study of twelve specimens for their creep performance, including nine concrete-filled steel tubular (CFST) columns and three plain concrete columns, subjected to three levels of sustained axial loads for 1710 days. Then, the creep strain curves are predicted from the existing creep models including the ACI 209 model, the MC 78 model, and the MC 90 model, and further a fitted creep model is obtained by experimental data. Finally, the creep effects of a CFST arch bridge are analyzed to compare the accuracy of the existing creep models. The experimental results show that the creep strains in CFST specimens are far less than in the plain concrete specimens and still increase after two years. The ACI 209 model outperforms the MC 78 model and the MC 90 model when predicting the creep behavior of the CFST specimens. Analysis results indicate that the creep effects in the CFST arch bridge are significant. The deflections and stresses calculated by the ACI 209 model are the closest to the fitted model in the three existing models, demonstrating that the ACI 209 model can be used for creep analysis of CFST arch bridges and can meet the engineering accuracy requirement when lack of experimental data.

• Steel and Composite Structures
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• 제33권4호
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• pp.615-627
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• 2019

This paper aims to study the compressive behavior of circular hollow and concrete-filled steel tubular stub columns under simulated marine atmospheric corrosion. The specimens after salt spray corrosion were tested under axial compressive load. Steel grade and corrosion level were mainly considered in the study. The mechanical behavior of circular CFST specimens is compared with that of the corresponding hollow ones. Design methods for circular hollow and concrete-filled steel tubular stub columns are modified to consider the effect of marine atmospheric corrosion. The results show that linear fitting curves could be used to present the relationship between corrosion rate and the mechanical properties of steel after simulated marine atmospheric corrosion. The ultimate strength of hollow steel tubular and CFST columns decrease with the increase of corrosion rate while the ultimate displacement of those are hardly affected by corrosion rate. Increasing corrosion rate would change the failure of CFST stub column from ductile failure to brittle failure. Corrosion rate would decrease the ductility indexes of CFST columns, rather than those of hollow steel tubular columns. The confinement factor ${\xi}$ of CFST columns decreases with the increase of corrosion rate while the ratio between test value and nominal value shows an opposite trend. With considering marine atmospheric corrosion, the predicted axial strength of hollow steel tubular and CFST columns by Chinese standard agree well with the tested values while the predictions by Japanese standard seem conservative.

• Steel and Composite Structures
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• 제13권6호
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• pp.587-606
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• 2012

Concrete-filled-steel-tubular (CFST) columns have been well proven to improve effectively the strength, stiffness and ductility of concrete members. However, the central part of concrete in CFST columns is not fully utilised under uni-axial compression, bending and torsion. It has small contribution to both flexural and torsion strength, while it can be replaced effectively by steel with smaller area to give similar load-carrying capacity. Also, the confining pressure in CFST columns builds up slowly because the initial elastic dilation of concrete is small before micro-crackings of concrete are developed. From these observations, it is convinced that the central concrete can be effectively replaced by another hollow steel tube with smaller area to form double-skinned concrete-filled-steel-tubular (CFDST) columns. In this study, a series of uni-axial compression tests were carried out on CFDST and CFST columns with and without external steel rings. From the test results, it was observed that on average that the stiffness and elastic strength of CFDST columns are about 25.8% and 33.4% respectively larger than CFST columns with similar equivalent area. The averaged axial load-carrying capacity of CFDST columns is 7.8% higher than CFST columns. Lastly, a theoretical model that takes into account the confining effects of steel tube and external rings for predicting the uni-axial load-carrying capacity of CFDST columns is developed.

• Steel and Composite Structures
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• 제9권1호
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• pp.19-38
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• 2009

The present paper provides test data to evaluate the seismic performance of recycled aggregate concrete (RAC) filled steel square hollow section (SHS) beam-columns. Fifteen specimens, including 12 RAC filled steel tubular (RACFST) columns and 3 reference conventional concrete filled steel tubular (CFST) columns, were tested under reversed cyclic flexural loading while subjected to constant axially compressive load. The test parameters include: (1) axial load level (n), from 0.05 to 0.47; and (2) recycled coarse aggregate replacement ratio (r), from 0 to 50%. It was found that, generally, the seismic performance of RACFST columns was similar to that of the reference conventional CFST columns, and RACFST columns exhibited high levels of bearing capacity and ductility. Comparisons are made with predicted RACFST beam-column bearing capacities and flexural stiffness using current design codes. A theoretical model for conventional CFST beam-columns is employed in this paper for square RACFST beam-columns. The predicted load versus deformation hysteretic curves are found to exhibit satisfactory agreement with test results.

• Steel and Composite Structures
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• 제49권5호
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• pp.533-546
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• 2023

Cost-effective solutions provided by composite construction are gaining popularity which, in turn, promotes the appearance on the market of new types of composite sections that allow not only to take advantage of the synergy of steel and concrete working together at room temperature, but also to improve their behaviour at high temperatures. When combined with high performance materials, significant load-bearing capacities can be achieved even with reduced cross-sectional dimensions. Steel-reinforced concrete-filled steel tubular (SR-CFST) columns are one of these innovative composite sections, where an open steel profile is embedded into a CFST section. Besides the renowned benefits of these typologies at room temperature, the fire protection offered by the surrounding concrete to the inner steel profile, gives them an enhanced fire performance which delays its loss of mechanical capacity in a fire scenario. The experimental evidence on the fire behaviour of SR-CFST columns is still scarce, particularly when combined with high performance materials. However, it is being much needed for the development of specific design provisions that consider the use of the inner steel profile in CFST columns. In this work, a new experimental program on the thermo-mechanical behaviour of SR-CFST columns is presented to extend the available experimental database. Ten SR-CFST stub columns, with circular and square geometries, combining high strength steel and concrete were tested. It was seen that the circular specimens reached higher failure times than the square columns, with the failure time increasing both when high strength steel was used at the embedded steel profile and high strength concrete was used as infill. Finally, different proposals for the reduction coefficients of high performance materials were assessed in the prediction of the cross-sectional fire resistance of the SR-CFST columns.

• Steel and Composite Structures
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• 제18권1호
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• pp.71-90
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• 2015

Axial compression tests have been carried out on 18 rectangular concrete-filled cold-formed steel tubular (CFST) columns with the aim of investigating the axial behaviour of rectangular CFST columns under different loading methods (steel loaded-first and full-section loaded methods). The influence of different loading methods on the ultimate strength of the specimens was compared and the development of Poisson's Ratio as it responds to an increasing load was reported and analysed. Then, the relationship between the constraining factor and the strength index, and the relationship between the constraining factor and ductility index of the specimens, were both discussed. Furthermore, the test results of the full-section loaded specimens were compared with five international code predicted values, and an equation was derived to predict the axial carrying capacity for rectangular CFST columns with a steel loaded-first loading method.

• Steel and Composite Structures
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• 제48권1호
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• pp.89-102
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• 2023

This study investigates the eccentric performance of concrete-filled steel tubular (CFST) stub columns strengthened with steel tube and sandwiched concrete (STSC) jackets. It was revealed that the STSC jacketing method effectively weakened the cracking of concrete in CFST columns on the convex side and the crash on the concave side. Substantial increases in the eccentric bearing capacities were demonstrated after strengthening. A numerical study was further conducted. The decrease in diameter-to-thickness ratio and increase in strength of outer tube contributed to increase in peak load of all components, whereas the increase in sandwiched concrete strength resulted in load increase on itself and had negligible effects on other components. The parametric study showed the effect of inner concrete strength on columns' bearing capacity was magnified after strengthening, whereas that of inner tube thickness was reduced. Within the parameters investigated, high-strength concrete and high-strength steel can be applied without the concern of early abrupt failure of inner low-strength concrete or steel tube.

• Steel and Composite Structures
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• 제25권4호
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• pp.443-456
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• 2017

Many authors have established the usefulness of concrete filled steel tubular (CFST) sections as compression members while few have proved their utility as flexural members. To explore their prospective as part of CFST frame structures, two types of connections using extended end plate and seat angle are proposed for exterior joints of CFST beams and CFST columns. To investigate the performance and failure modes of the proposed bolted connections subjected to static loads, an experimental program has been executed involving ten specimens of exterior beam-to-column joints subjected to monotonically increasing load applied at the tip of beam, the performance is appraised in terms of load deformation behaviour of joints. The test parameters varied are the beam section type, type and diameter of bolts. To validate the experimental behaviour of the proposed connections in CFST beam-column joints, finite element analysis for the applied load has been performed using software ATENA-3D and the results of the proposed models are compared with experimental results. The experimental results obtained agree that the proposed CFST beam-column connections perform in a semi-rigid and partial strength mode as per specification of EC3.