Concrete-filled Steel Tubular Short Columns Research Articles

Local buckling of steel plates in concrete-filled steel tubular columns at elevated temperatures

Local buckling remarkably reduces the strength of steel plates in rectangular thin-walled concrete-filled steel tubular (CFST) columns at ambient temperature. This effect is more remarkable at elevated temperature. However, there have been very limited experimental and numerical investigations on the local and post-local buckling behavior of steel plates in CFST columns at elevated temperatures. This paper presents numerical studies on the local and post-local buckling behavior of thin steel plates under stress gradients in rectangular CFST columns at elevated temperatures. For this purpose, finite element models are developed, accounting for geometric and material nonlinearities at elevated temperatures. The initial geometric imperfections and residual stresses presented in steel plates are considered. Based on the finite element results, new formulas are proposed for determining the initial local buckling stress and post-local buckling strength of clamped steel plates under in-plane stress gradients at elevated temperatures. Moreover, new effective width formulas are developed for clamped steel plates at elevated temperatures. The proposed formulas are compared with existing ones with a good agreement. The effective width formulas developed are used in the calculations of the ultimate axial loads of rectangular CFST short columns exposed to fire and the results obtained are compared well with the finite element solutions provided by other researchers. The initial local buckling and effective width formulas can be implemented in numerical techniques to account for local buckling effects on the responses of rectangular thin-walled CFST columns at elevated temperatures.

Axial compression behavior of circular recycled concrete-filled steel tubular short columns reinforced by silica fume and steel fiber

This paper presents an experimental work for short circular steel tube columns filled with normal concrete (NAC), recycled aggregate concrete (RAC), and RAC with silica fume and steel fiber. Ten specimens were tested under axial compression to research the effect of silica fume and steel fiber volume percentage on the behavior of recycled aggregate concrete-filled steel tube columns (RACFST). The failure modes, ultimate loads and axial load- strain relationships are presented. The test results indicate that silica fume and steel fiber would not change the failure mode of the RACFST column, but can increase the mechanical performances of the RACFST column because of the filling effect and pozzolanic action of silica fume and the confinement effect of steel fiber. The ultimate load, ductility and energy dissipation capacity of RACFST columns can exceed that of corresponding natural aggregate concrete-filled steel tube (NACFST) column. Design formulas EC4 for the load capacity NACFST and RACFST columns are proposed, and the predictions agree well with the experimental results from this study.

Finite element analysis of large diameter high strength octagonal CFST short columns

Octagonal masonry columns have been broadly used in outstanding architectural heritage due to their aesthetical appearance and to support high loads. Nowadays, concrete-filled steel tubular (CFST) columns have been utilised to carry such high load. However, a recent trend in the construction of the CFST columns is to use the high strength concrete (HSC) to increase the usable floor spaces in different buildings due to the global limited land areas. Therefore, this paper combines the architectural demand of the octagonal column shape with the advantages of the CFST columns by investigating numerically, by means of finite element (FE) modelling, the octagonal CFST short columns. Accordingly, validated FE models for the octagonal CFST short columns are employed to perform parametric studies to widen the available knowledge about their behaviour. The paper is mainly devoted for investigating large diameter columns using extensive series of diameter-to-thickness (D/t) ratios ranging from 40 to 200, most of which filled with HSC up to 100MPa. The ultimate strengths, based on available experiments in literature and current FE investigation, are compared with the existing design models. This comparison indicates that the existing design models are conservative and their accuracy is affected by the D/t ratios. Therefore, a new design model is suggested based on the existing provisions by taking the D/t ratios of the columns into account. This suggested design model is validated by using the existing experimental results and is found to give excellent results. The important factors that affect the strength and behaviour of the octagonal CFST short columns filled with HSCs are additionally discussed in detail, with new conclusions added to literature for the first time.

Round-ended rectangular concrete-filled steel tubular short columns: FE investigation under axial compression

Concrete-filled steel tubular (CFST) columns with round-ends own the same advantages of typical CFST columns, besides their aesthetical appearance. The smoothness of the cross-section gives the effectiveness to resist running water impact when they are used as piers. Despite these advantages, there are limited researches on the behaviour of round-ended CFST columns. The paper investigates the behaviour of round-ended rectangular CFST (RRCFST) columns. Three-dimensional finite element (FE) models for RRCFST columns are developed using the ABAQUS software. The novelty of this FE model is the consideration of the confinement in the round-ended concrete. The existing experimental behaviour has been captured properly, compared with other previously suggested FE models. After the validation of FE models, a parametric study is generated taking into account wider parameters than those previously considered by other researchers. The results show two different axial load-strain responses based on the B/t ratios of the cross-sections. RRCFST columns with small B/t ratios are found to fail in a ductile manner with large axial strains. The failure of the columns with relatively high B/t ratios has been found to occur suddenly with a rapid reduction in the strength after reaching the ultimate load. The numerical results indicate that the brittle failure is associated with the columns formed from outer slender steel cross-sections. The FE strengths are compared with the available design model which was formulated based on limited research results. This design model is found to predict the strengths unconservatively. A new design model, providing better estimates, has been suggested at the end.

Behaviour and design of hexagonal concrete-filled steel tubular short columns under axial compression

Concrete-filled steel tubular (CFST) columns have frequently been utilised in the construction of mid-rise and high-rise buildings as they offer smaller cross-sectional size to load carrying capacity ratio than ordinary reinforced concrete or steel solutions. The steel tube component of CFST columns can be shaped into different forms to further increase its strength and this article focuses on hexagonal CFST short columns in compression. Firstly, the literature is revised and it was found that the available experiments on the hexagonal columns cover relatively limited hexagonal dimensions and material properties. Additionally, existing design models were observed to be inaccurate for certain diameter-to-thickness (D/t) ratios of the columns. Accordingly, this paper intends to widen the available pool of data and proposes a new design model to design hexagonal CFST short columns in compression. This is made herein through comprehensive finite element (FE) models by using Abaqus software, carefully validated against experimental results and subsequent parametric studies covering a wide range of hexagonal dimensions of regular cross-section (circular-like). The effect of various D/t ratios, material steel grades and concrete compressive strengths (fc′) on both the behaviour and strength of the hexagonal CFST short columns is investigated. Based on observations made and conclusions drawn upon analysing numerical data generated, a new design model is presented which provides better strengths compared with available design models and with accurate predictions for the full range of D/t ratios.

Size effect of circular concrete-filled steel tubular short columns subjected to axial compression

In this paper, thirty-six short columns with different diameters (150mm ≤ d ≤ 460mm) and steel ratios (4.0% ≤ α ≤ 10.0%) were tested to failure to investigate the size effect of circular concrete-filled steel tubular short columns subjected to axial compression. Size effects on the peak axial stress, peak axial strain, composite elastic modulus, and ductility coefficient were studied. The experimental results showed that the peak axial stress, peak axial strain and ductility coefficient of the specimens tended to decrease with the increase in the column diameter. The values of the composite elastic modulus remained almost constant when the diameter of the specimens increased, indicating that size effect on the composite elastic modulus was not obvious. Meanwhile, size effect on the peak axial stress was influenced by the steel ratio in the range of 4–10%. Furthermore, the size effect tended to decrease as the steel ratio increased. By comparing with the current codes, a reduction coefficient was introduced to consider the size effect of concrete core. Based on the reduction coefficient, the size effect of the concrete core inside the steel tube is found to be weaker compared with that of the unconfined concrete columns because of the confinement effect.

Behaviour of axially loaded circular concrete-filled bimetallic stainless-carbon steel tubular short columns

Abstract Circular concrete-filled bimetallic steel tubular (CFBST) columns are considered as a new type of structural composite members. An experimental investigation has recently been conducted on the performance of these concentrically-loaded circular CFBST stub columns. However, the fundamental response of these columns under axial compression has not been investigated numerically. Therefore, finite element (FE) analysis of axially loaded circular CFBST stub columns is discussed in this paper. An existing concrete constitutive model with the confinement mechanism is modified for the current CFBST columns. The nonlinear stress-strain relationship of stainless steel is utilised in the FE analysis. The current FE model accounts for the influences of initial imperfections, geometric and material nonlinearities. The ultimate strengths and load-strain responses predicted from the analysis are validated against the available test results and observations in literature. The comparisons indicate that the FE model accurately estimates both the ultimate strengths and load-strain characteristics of the concentrically-loaded circular CFBST stub columns. The developed model is then utilised to investigate the effects of the geometric configurations and material properties on the load-strain characteristics, ultimate capacity, ductility and steel contribution ratio of circular CFBST stub columns. The existing design recommendations for conventional circular concrete-filled steel tubular columns are then checked for computing the peak load of the circular CFBST stub columns, and the best strength predictor with the least deviation compared with the experimental values is recommended at the end for design.

Confined concrete model of circular, elliptical and octagonal CFST short columns

The confined concrete stress-strain curves utilised in computational models of concrete-filled steel tubular (CFST) columns can have a significant influence on the accuracy of the predicted behaviour. A generic model is proposed for predicting the stress-strain behaviour of confined concrete in short circular, elliptical and octagonal CFST columns subjected to axial compression. The finite element (FE) analysis is carried out to simulate the concrete confining pressure in short circular, elliptical and octagonal CFST columns. The concrete confining pressure relies on the geometric and material parameters of CFST columns. The post-peak behaviour of the concrete stressstrain curve is determined using independent existing experimental results. The strength reduction factor is derived for predicting the descending part of the confined concrete behaviour. The fibre element model is developed for the analysis of circular, elliptical and octagonal CFST short columns under axial loading. The FE model and fibre element model accounting for the proposed concrete confined model is verified by comparing the computed results with experimental results. The ultimate axial strengths and complete axial load-strain curves obtained from the FE model and fibre element model agree reasonably well with experimental results. Parametric studies have been carried out to examine the effects of important parameters on the compressive behaviour of short circular, elliptical and octagonal CFST columns. The design model proposed by Liang and Fragomeni (2009) for short circular, elliptical and octagonal CFST columns is validated by comparing the predicted results with experimental results.

Constitutive model for confined ultra-high strength concrete in steel tube

With the development of material science and production technology, ultra-high strength concrete (UHSC) with uniaxial compressive strength up to 200MPa has been made available commercially and used for concrete filled steel tubular (CFST) columns in high-rise buildings. Finite element analysis is a necessary tool to analyze CFST columns, but its accuracy depends on the generic constitutive model for the confined UHSC in the steel tube. This paper proposes a new constitutive model for confined UHSC based on (1) a yield criterion which is a function of hydrostatic pressure and lode angle, (2) a non-associated flow rule with a dilation angle that is a function of the confining pressure and the equivalent plastic strain, and (3) a hardening/softening rule which is dependent on the confining pressure and the equivalent plastic strain. The parameters of the proposed constitutive model are calibrated by a series of uniaxial compression, biaxial compression and triaxial compression tests of UHSC specimens. The constitutive model is then implemented in ABAQUS and verified by the test results of short CFST columns. Comparison of the test and predicted results in terms of compression load versus axial strain and lateral strain curves demonstrates that the proposed model can predict accurately the maximum resistance of the stub CFST columns as well as the interactive behavior between the steel tube and the confined concrete core.

Analytical buckling of slender circular concrete-filled steel tubular columns with compliant interfaces

This paper presents an efficient mathematical model for studying the global buckling behavior of concrete-filled steel tubular (CFST) columns with compliant interfaces. The present mathematical model is used to evaluate exact critical buckling loads and modes of CFST columns for the first time. The results prove that the presence of finite interface compliance may significantly reduce the critical buckling load of CFST columns. A good agreement between analytical and experimental buckling loads of circular CFST columns is obtained if at least one among longitudinal and radial interfacial stiffnesses is high. The design methods compared in the paper give conservative results in comparison with the experimental results and analytical results for almost perfectly bonded layers. The parametric study reveals that critical buckling loads of CFST columns are very much affected by the diameter-to-depth ratio and concrete elastic modulus. Moreover, a material nonlinearity has a pronounced effect for short CFST columns, and a negligible effect for slender ones.

The correction coefficient of size effect on the axial compression strength of circular concrete-filled steel tubular short columns

In this paper, 413 experimental results from circular concrete-filled steel tube short columns, which were subjected to formulae under the design specifications of CECS 28:201219, AIJ (2008)20, ANSI /AISC 360-0521 and EC4 (2004)22 are presented. Generally, when the specimen diameter increases, the ratio of experimental values to calculated values decreases; this illustrates the influence of existent size effect on the compression strength of concrete-filled steel tube columns. The correction coefficients of size effect obtained from the data fitting were subjected to the formulae. Subsequently, the experimental values to calculated values ratio did not change with size of the specimen. Having considered 1·0 as the critical value of the confinement index, the data were divided into two groups. When the correction coefficients of the two groups using the same formula were compared, it was seen that the smaller the confinement index was, the larger the size effect would be. When the results obtained from different formulae were compared, the size effect on the formulae using the confinement index was found to be smaller than that on the formulae with a superposition form of capacity.

Nonlinear analysis of axially loaded circular concrete-filled stainless steel tubular short columns

Experiments show that the ultimate compressive strength of stainless steel is much higher than its tensile strength. The full-range two-stage constitutive model for stainless steels assumes that stainless steels follow the same stress–strain behavior in compression and tension, which may underestimate the compressive strength of stainless steel tubes. This paper presents a fiber element model incorporating the recently developed full-range three-stage stress–strain relationships based on experimentally observed behavior for stainless steels for the nonlinear analysis of circular concrete-filled stainless steel tubular (CFSST) short columns under axial compression. The fiber element model accounts for the concrete confinement effects provided by the stainless steel tube. Comparisons of computer solutions with experimental results published in the literature are made to examine the accuracy of the fiber element model and material constitutive models for stainless steels. Parametric studies are conducted to study the effects of various parameters on the behavior of circular CFSST short columns. A design model based on Liang and Fragomeni's design formula is proposed for circular CFSST short columns and validated against results obtained by experiments, fiber element analyses, ACI-318 codes and Eurocode 4. The fiber element model incorporating the three-stage stress–strain relationships for stainless steels is shown to simulate well the axial load–strain behavior of circular CFSST short columns. The proposed design model gives good predictions of the experimental and numerical ultimate axial loads of CFSST columns. It appears that ACI-318 codes and Eurocode 4 significantly underestimate the ultimate axial strengths of CFSST short columns.

Performance and calculations of recycled aggregate concrete-filled steel tubular (RACFST) short columns under axial compression

This paper describes a series of tests on steel tubular short columns of circular section filled with normal concrete (NA) and recycled aggregate concrete (RAC). Twenty-two specimens, including 11 small diameter of RAC-filled steel tubular (RACFST) columns named CA series and 11 larger diameter of RACFST columns called CB series, were tested to research the influence of variations in confinement index and replacement ratio of recycled coarse aggregate (RCA), from 0% to 100% with 10% increasing. The test results show that both types of RACFST columns and normal concrete-filled steel tubular (CFST) columns failed due to oblique shear pressure damage with a drum-like. Comparisons are made with predicted bearing capacities of RACFST columns using the existing theories, such as Unified Strengthen theory (I), Confinement theory (II) and Superposition theory (III). The equation for axial compression stress-strain curves of the whole process are proposed in this investigation for RACFST short columns, which can be directly used in theoretical and numerical analysis as well as practical engineering design and evaluation of RACFST structures.

Compressive strength of circular concrete-filled double skin tubular short columns

This paper focuses on the compressive strength of the concrete-filled double skin steel tubular (CFDST) short columns. Columns with external and internal circular carbon steel tubes are merely considered. First, this paper summarises previously developed formulas for predicting the compressive strength of the CFDST columns, along with the formula recently suggested by Yu et al. (2013) for solid and hollow circular concrete-filled tubular (CFST) columns. The various formulas for predicting compressive strength are then compared with test results. Test results are then organised and evaluated according to the relevant test specimen parameter; the diameter-to-thickness ratio (D/te). It is found that the available tests do not cover the full range of the D/te ratio. Hence, numerical nonlinear simulations, based on the finite element (FE) method using the software package ABAQUS/Standard, are constructed to compensate for the shortage in the available results. Through comparison with test and FE results, a new design formula is suggested. Such formula is shown to be more accurate than available formulas for estimating the compressive strength of the CFDST short columns. This recommended design model requires relatively less calculation efforts, and provides less scattered predictions than those using the current design rules. At the end, an illustrative example for the calculation of the compressive strength of the CFDST columns using the currently proposed formula is provided.

Unified Solution of the Ultimate Bearing Capacity for Stiffened and Thin-Walled Square CFST Short Columns under Axial Compression

Directed against the stiffened and thin-walled square concrete-filled steel tubular (CFST) short columns, the paper simplifies the square CFST columns into the circle ones by introducing the strength reduction coefficient of concrete and the equivalent constraint reduction coefficient. On the basis of the unified strength theory, the united solution of the ultimate bearing capacity for square CFST columns is deduced after considering the influence of strength-differential in tension and compression and intermediate principal stress. Some influencing factors are also analyzed in the paper. Through comparing the results of proposed formula with that of experiments of relevant literatures，the rationality of proposed formula is proved. The solution may provide theoretical references for the study of the ultimate bearing capacity for stiffened and thin-walled square CFST short columns.

Behaviour of circular concrete-filled lean duplex stainless steel tubular short columns

Lean duplex stainless steel material (EN 1.4162) has recently gained significant attention for its higher structural performance and corrosion resistance compared to the austenitic type. Circular lean duplex stainless steel tubes filled with concrete are innovative composite columns which have not been studied experimentally or numerically. This paper presents the fundamental behaviour of circular concrete-filled lean duplex stainless steel tubular (CFSST) short columns under axial compression. Three dimensional finite element (FE) models for CFSST columns subjected to axial compression are developed using the FE package ABAQUS. The lean duplex stainless steel material is modelled using the two-stage constitutive laws while the concrete is simulated using accurate concrete confinement models. The FE models are verified by comparisons with existing experimental results on hollow stainless steel columns, concrete-filled steel tubular columns and CFSST columns. Parametric studies are undertaken to investigate the effects of concrete compressive strength and diameter-to-thickness (D/t) ratio on the behaviour of CFSST columns. The results show that the ultimate axial strength of circular CFSST columns increases with increasing the concrete compressive strength but decreases with an increase in the D/t ratio. Circular CFSST columns with different D/t ratios exhibit the same initial stiffness. The lean duplex stainless steel tubes cannot provide good confinement on the concrete when D/t ratio is large. The ultimate axial strengths of CFSST columns predicted by the FE models are also compared with those calculated by the Eurocode 4, ACI code, the continuous strength method (CSM) by Lam and Gardner and Liang and Fragomeni's design formulas. The comparative study shows that Eurocode 4 and the CSM give good estimates of the ultimate axial strengths of CFSST columns with D/t<40 but overestimates the strengths of columns with D/t≥40. The ACI code gives too conservative estimates of the ultimate loads of CFSST columns as it does not consider the concrete confinement effects. Finally, it was found that the modified Liang and Fragomeni's design formulas yield the best predictions of the ultimate axial strengths of CFSST columns over the entire range of D/t ratios.

Study of Post-Fire Bearing Capacity of Self-Stress Ceramsite Concrete Filled Steel Tubes

Mechanical behavior and bearing capacity of self stress ceramsite concrete filled steel tubes (noted as SCCST) after exposure to fire are experimentally studied. Effect of the parameters, such as value of furnace temperatures, self stress on the post-fire bearing capacity and ductility of the specimens after exposure to fire were especially discussed. The experimental results show that all the specimens of SCCST have higher post-fire bearing capacity and better ductility, and there was no obvious descending segment in the load-strain curves of the most specimens after exposure to fire. It was concluded that the furnace temperature subjected to the specimens and the initial self stress produced in the specimens of the composite short columns has some effect on the axial bearing capacity of ceramsite concrete-filled steel tubular short columns after exposure to fire, and the volume of expansion agent of 10% has most great effect on post-fire bearing capacity of SCCST after exposure to fire.

Experimental Research on the Performance of Micro-expansion and Self-compacting Concrete-Filled Steel Tubular Short Column under Axial Compression

Five specimens had been tested under axial compression, whose deformation and stress performance is mainly focused on both theory and experiment. The working mechanism had been analyzed on experimental results. Furthermore, some testing phenomenon was explained based on damage mechanics theory. In addition, the calculating equation for ultimate load-carrying capacity, which is reduced by the theory of hooping strengthening, has been proved to be suited and safe for this kind of structure. The research results offer the theoretical and experimental references for engineering practice of concrete-filled steel tubular structure and design index of core concrete.

The Study on Ductility of Thin-Walled Square Concrete-Filled Steel Tubular Short Columns Configured with Binding Rebars

To better solve the problems of local buckling and stability of thin-walled concrete-filled steel tubular short columns and improve the ability of specimens’ overall deformation, long or short binding rebars were set up and anchored on the inner surfaces of the steel tubes; meanwhile the static load test was performed. The results show that the main factors affecting the bearing capacity are steel pipe wall thickness, concrete strength, binding rebars in turn and the ductility is concrete strength, binding rebars, wall thickness. Long or short rebars also significantly improve the deformation properties of specimens. Taking the common specimens’ average ductility factor of 2.30 as a benchmark, the average factor of specimens configured with short rebars is 2.85, increased by 24% and configured with the long is 3.34, increased by 45%. Deformation properties of the specimens substantially increase and security of the structure enhances after setting up rebars.

Behavior of CFST short column and beam with initial concrete imperfection: Experiments

Gap between steel tube and concrete core could be recognized as a type of initial concrete imperfection in concrete-filled steel tubular (CFST) structure. This paper studies the influence of the gap on the compressive and flexural behavior of CFST members. A total of 21 specimens, including 14 short columns subjected to axial compressive loading and 7 beams subjected to bending, were tested. The main parameters of these tests were the gap type (circumferential and spherical-cap) and gap ratio. The influence of gap on the failure mode, ultimate strength and flexural stiffness of the CFST specimens were investigated.