Concrete-filled Steel Tube Stub Columns Research Articles

Gap defect and freeze-thaw cycles effects on mechanical characteristics of concrete-filled steel tube stub columns with experimental and theoretical investigation

As concrete-filled steel tube (CFST) structures become more prevalent in engineering, their durability garners significant attention. Mechanical characteristics of concrete-filled steel tube stub columns under the combined influence of gap defect and freeze-thaw cycles is presented with experimental and theoretical investigation in the paper. Twenty-six concrete-filled steel tube stub columns were designed to study the combined effects on failure mechanisms, axial compression-strain response, strength, and ductility. The experimental results indicate that as the number of freeze-thaw cycles and gap ratio increase, the deformation of the specimen increases and the deformation time advances. The specimen's strength and ductility are decreased by the combined effect of these factors. In the most severe cases, the specimen's overall strength and ductility are reduced by 29.4 % and 44.0 %, respectively. Subsequently, a finite element numerical model is established to parametrically analyze the concrete-filled steel tube stub column. Results show that the degree of restraint in the concrete-filled steel tube significantly influences the reduction in strength and ductility of the specimens. The stronger the restraining effect, the less the degradation of strength and ductility caused by the combined impact of the two factors. Therefore, it is crucial to effectively restrain the concrete within the concrete-filled steel tube to ensure structural safety and durability.

Experimental and numerical analysis of L-shaped concrete-filled steel tube stub columns

This paper reports experimental and numerical studies on the cross-sectional compression resistance and behaviour of a novel L-shaped concrete-filled steel tube system. The novel composite system is proposed to eliminate concave corners and enhance the confinement of the outer steel tube to the concrete infill. A testing programme was firstly conducted, including nine tensile steel coupon tests, six compressive concrete cube tests and ten stub column tests. Upon testing, all the ten L-shaped concrete-filled steel tube stub column specimens were found to fail by local buckling. Finite-element models were then developed in the finite-element analysis software ABAQUS and validated against test results. The validated models were then employed to perform parametric studies to obtain an additional set of 48 numerical data. Test and numerical results were used to perform a design analysis, in which the suitability of the design rules in the European code EN 1994–1-1, the American specification ANSI/AISC 360 and the Chinese code GB 50936 for the developed L-shaped concrete-filled steel tube stub column system was assessed. The analysis results reveal that both EN 1994–1-1 and GB 50936 yield unsafe compression resistance predictions for the studied L-shaped concrete-filled steel tube stub columns, owing to the improper consideration of the contributions of the outer steel tube and the concrete infill to the overall capacity, while ANSI/AISC 360 offers safe design with high accuracy. The research findings are expected to contribute to the safe and wide use of the proposed novel L-shaped concrete-filled steel tube system in structural engineering and also to the further development of the current international design standards.

Evolution of Confinement Stress in Axially Loaded Concrete-Filled Steel Tube Stub Columns: Study on Enhancing Urban Building Efficiency

In the context of green building and sustainable urban development, understanding the mechanical behavior of structural components like concrete-filled steel tube (CFST) columns is crucial due to their improved load-bearing capacity, energy efficiency, and optimized material usage, which enhance structural resilience and sustainability. This research addresses the complex development of confining stress and its impact on the concrete core (CC) behavior within these columns, which are essential for urban infrastructure. Through extensive numerical studies, this study proposes a model to estimate the confining stress in axially loaded CFST short columns. Study findings reveal that the confinement effectiveness is influenced by variables such as compressive strength (CS) of the concrete, cross-sectional shape, and depth-to-wall thickness percentage. Additionally, the confinement is also significantly affected by the yield strain of steel εy/εc to the peak strain of unconfined concrete εc. A three-dimensional finite element model (FEM) was built for the simulation of the columns’ nonlinear behavior and was rigorously validated against experimental data. This model aids in the design and implementation of more efficient and resilient urban structures, supporting the principles of sustainable construction. The study underscores the importance of structural integrity in sustainable urban development and provides valuable insights for improving the design of green building materials.

Axial local bearing behavior of circular reinforced concrete filled steel tube stub columns

In this work, the axial local compression of circular reinforced concrete-filled steel tube (CRCFT) stub columns is studied. Sixteen specimens, including one circular reinforced concrete stub column (CRCC) specimen and fifteen CRCFT specimens, were prepared, considering different variables, such as the local compression area ratio β, sectional steel ratio α, volume stirrup ratio ρv, longitudinal reinforcement ratio ρ, and concrete strength fc. The axial local compression of specimens were experimentally investigated in terms of the damage process, failure mode, load-displacement relationship, ultimate bearing capacity, stiffness, deformation capacity, and strain characteristic. The finite element model of the CRCFT was also developed and validated. The parametric analysis of the local compression area ratio β, sectional steel ratio α, and volume stirrup ratio ρv facilitated the further investigation of the axial local compression behavior of CRCFT. The results showed steel tube buckling and concrete shear failure exhibited by CRCFT specimens. With the increasing local compression area ratio β, the ultimate bearing capacity reduced by 3.6–47.9 %, whereas the local compression strength improvement coefficient βcl increased by 11.8–273.9 %. With the increase in section steel ratio α and volume stirrup ratio ρv, the ultimate bearing capacity, stiffness, and deformation capacity were improved. Longitudinal reinforcement ratio ρ and the concrete strength fc also influenced the stiffness of CRCFT specimens. A simplified calculation method for the ultimate bearing capacity of CRCFT specimens was also presented.

Compressive axial load performance of GFRP–confined recycled aggregate concrete–filled steel tube stub columns

This study examined the axial compressive properties of glass–fiber reinforced plastic (GFRP)–confined recycled aggregate concrete–filled steel tube (RACFST) stub columns. The differences in the failure modes, load–displacement curves, and load–strain curves of the composite columns with different parameters were analyzed, including the number of GFRP layers, steel tube thickness, recycled aggregate substitution rate, and recycled aggregate concrete compressive strength. The results revealed a bulging failure mode in the middle of the GFRP–confined RACFST stub columns. With the increasing substitution rate of recycled aggregate, the bulging deformation of the specimen increased. This eventually resulted in a reduction in the stiffness and bearing capacity but an improvement in the ductility at a later stage. The early stiffness and ultimate bearing capacity of the specimen can be increased by thickening the steel tube or strengthening the recycled aggregate concrete. However, the ductility is greatly reduced in the later stage. Based on the axial compression test of GFRP–confined RACFST stub columns, the axial compression bearing capacity formula was derived. Compared with the existing formula, the formula presented in this paper has greater accuracy. In addition, a finite element (FE) model of the composite column was established to analyze the stress development during the compression process.

Eccentric compression behaviour of normal and ultra-high performance concrete-filled stainless steel tube stub columns

To investigate the mechanical properties of normal concrete-filled stainless steel tubes (CFSST) and ultra-high performance concrete-filled stainless steel tubes (UFSST) under eccentric compression, 6 axial compression specimens and 18 eccentric compression specimens were designed. The effect of different concrete strengths, eccentricity and diameter-to-thickness ratios (D/t) on the damage modes, load-displacement curves, load-strain curves, and lateral deflection distribution curves of the specimens were investigated. The test results showed that stainless steel tubes exhibited different degrees of buckling after failure. The load-displacement curve of CFSST presents a slight upward trend after the elastic-plastic phase, whereas that of the UFSST enters a slowly decreasing phase. And the increase in eccentricity leads to a decrease in the load bearing capacity, ductility, and initial stiffness of the specimen. As the D/t decreases, the load carrying capacity of the specimen increases. Additionally, parametric analysis was conducted on CFSST and UFSST specimens, including the eccentricity, concrete strength, stainless steel yield strength, and D/t. Compared with the design codes of EC 4, AISC 360, and GB/T 51446, the ultimate axial force-bending moment (Nu-Mu) curve was inaccurate. Thus, a prediction model for the Nu-Mu curve of CFSST and UFSST stub columns was proposed, and the predicted results of the proposed model were in good agreement with the experimental results.

Axial compression behavior and reliable design approach of rectangular dune sand concrete-filled steel tube stub columns

Concrete-filled steel tube (CFST) columns, incorporating various types of concrete, are increasingly prevalent in engineering applications. This study develops a finite element (FE) model and a design methodology for rectangular dune sand-concrete-filled steel tube (DS-CFST) columns. Investigating the composite behavior of rectangular DS-CFST columns and the effects of different DS replacement ratios, the results indicate a reduction in the ultimate bearing capacity compared to traditional CFST columns, attributed to diminished confinement effects, particularly at higher DS replacement ratios. Analysis of different design factors, including concrete and steel strengths, steel ratio, and aspect ratio, assesses their impacts on axial load-bearing capacity and composite behavior. This analysis leads to the establishment of a design formula, validated through experiments and FE analysis. The formula's efficacy and conciseness are evaluated against current design codes, with a reliability study assessing sensitivity to fluctuations. The proposed formula demonstrates accurate predictions of the ultimate load-bearing capacity.

Damage analysis of axially compressed concrete-filled steel tube stub columns based on acoustic emission

Concrete-filled steel tube (CFST) widely applied in engineering structures due to its superior behavior requires monitoring and assessment for damage level. Here, the acoustic emission (AE) technique was used to monitor the fracture process of concrete core in CFST stub columns subjected to axial compression. Test and analyzed results show that the damage process of CFST specimens can be divided into four stages: compaction, elastic-plastic stage, strengthening and secondary strengthening. In the elastic-plastic stage, the evolutionary features of the AE event rate, cumulative energy, Ib-value and crack classification are capable of providing an early warning for cracked concrete core. Full crack propagation can be identified by the rapid increase in the proportion of shear cracks near the inflection point of load, which is impermissible in engineering structures. According to the analyses of the AE event rate and signal intensity in the elastic-plastic stage, the confinement of steel tube with thicker wall thickness or higher strength is delayed, which implies that this confinement is suggested to be triggered early. It is indicated that the AE technique has the potential to monitor and evaluate the damage process of CFST stub columns under axial compression, which can provide additional insight into the failure mechanism and assist in the scheme of repairs.

Axial compression behavior of coal gangue coarse aggregate concrete-filled steel tube stub columns

To investigate the load-strain curves and failure modes of spontaneous-combustion coal gangue coarse aggregate (SCGCA) concrete-filled steel tube (CFST) stub columns under axial compression, 36 SCGCA CFST specimens and 18 comparative SCGCA concrete specimens were firstly tested with various SCGCA replacement ratios, concrete strengths and steel tube thicknesses. The results reveal that the bearing capacity of SCGCA CFST significantly decreases with increasing SCGCA replacement ratio, while corresponding strain increases. The addition of SCGCA can activate confining stress offered by steel tube in advance. The hoop strain of SCGCA CFST specimens increases with increasing SCGCA replacement ratio under same axial strain, resulting in enhanced confining stress. An analysis-oriented model of SCGCA CFST stub columns was then established on basis of test results, and an extended parameter analysis was further conducted using the model. Finally, the feasibility of design methods in existing design codes for evaluating bearing capacity of SCGCA CFST stub columns under axial compression was discussed.

Experimental study on axial compression properties of concrete filled steel tubular stub columns in varying ambient temperature

To better understand the mechanical property issues of in-service concrete-filled steel tube (CFST) bridges susceptible to natural environmental temperatures, 13 CFST stub columns were assessed under axial compression with different ambient temperatures and steel ratios, with the strength of the core concrete as the main parameters. The influence of ambient temperature on the peak strain and joint elastic modulus of the CFST stubs was investigated. The influence mechanism of steel content and core concrete strength on the residual toughness of the CFST at different ambient temperatures was revealed. In this study, we proposed a modified constitutive equation for CFST, taking into account the ambient temperature. We demonstrated that the peak strain, residual toughness ratio, and elastic modulus of the CFST stubs significantly decreased when the temperature decreased. The calculated values of the modified constitutive equation for CFST, taking into account the ambient temperature, were in good agreement with the test results and could be used to predict the trends in stress properties of the CFST stubs at different temperatures. The results could serve as a critical reference for the design and calculation of CFST structures in very cold regions.

Compression behavior of CFST columns under combined influence of gap and freeze-thaw

Owing to factors related to construction quality and environment, concrete-filled steel tube (CFST) structures that have been in use for many years in regions experiencing seasonal freeze–thaw cycles are susceptible to the combined effects of a spherical-cap gap and freeze–thaw cycling. Therefore, in this study, a combination of experiments and the finite element method is used to investigate the effects of these two factors on the axial compression behavior of CFST stub columns. The experimental results show that the main failure mode of the specimen is local buckling at the end or the middle and inward depression of the steel tube on the existing gap side. The degradation of mechanical parameters, such as the ultimate bearing capacity and ductility of the specimens, gradually increased with increasing gap ratio and number of freeze–thaw cycles. In the numerical simulation section, the change in the stress response to freeze–thaw cycling on the gap side is analysed. The impact of the constraint effect coefficient on the ultimate bearing capacity degradation of the specimens is also investigated. Finally, a formula for calculating the ultimate capacity of CFST stub columns affected by the two factors is proposed, and the formula for calculating the ultimate bearing capacity in the code is improved. The results of this study can provide a reference for durability prediction and structural safety evaluation of CFST structures with a spherical-cap gap in cold regions.

Circular recycled aggregate concrete-filled stainless steel tube stub columns after exposure to fire: Experiments, simulations, and design

This paper presents experimental and numerical investigations into the post-fire cross-section compressive behaviour and resistances of circular recycled aggregate concrete-filled stainless steel tube (RACFSST) stub columns. Twelve stub column specimens, fabricated from concretes with three recycled coarse aggregate replacement ratios (0%, 35% and 70%), were tested after exposure to the ISO-834 standard fire for 0 min (i.e. at ambient temperature), 15 min, 30 min and 45 min. The experimental investigation included heating and cooling of specimens, cylinder tests and post-fire tensile coupon tests and stub column tests. The test results, including load–end shortening curves, failure loads and failure modes, were fully reported and the initial compressive stiffnesses and confinement effect were discussed. The numerical investigation was subsequently conducted, where thermal and mechanical finite element models were developed and validated against the test results and then used to perform parametric studies to generate further numerical data. Given the absence of design codes for stainless steel–recycled aggregate concrete composite structures after exposure to fire, the relevant design rules for circular natural aggregate concrete-filled carbon steel tube stub columns at ambient temperature, as set out in the European code, Australia/New Zealand standard and American specification, were assessed, using post-fire material properties, for their applicability to circular RACFSST stub columns after exposure to fire, based on the test and numerical data. The assessment results generally revealed that the European code and the Australian/New Zealand standard led to an acceptable level of design accuracy and consistency, while the American specification resulted in rather conservative and scattered post-fire cross-section compression resistance predictions.

Behavior of concrete-filled double steel tube stub columns under partial compression

This paper reports the investigation on the behavior of stub columns having two nested steel tubes filled with concrete, abbreviated as CFDST, consisting of core concrete, inner and outer steel section and sandwich concrete. A total of twenty-seven stub columns were fabricated and tested. The experimental parameters included the wall thickness of the outer steel tube, strength grade of the concrete, ratio of the partial compressive area and the cross-sectional shape of the bearing plate. The typical failure modes, load-displacement relationships, and stress-strain relationships were studied. The results showed that outward bulging of the upper end of the outer steel tube occurred for the CFDST columns, and the degree and extent of bulging increased with the ratio of the partial compressive area. The bearing capacity of the columns loaded on the partial compressive area increased with the wall thickness of the outer steel tube and concrete strength. When the wall thickness of the outer steel tube increased, the partial bearing capacity increased by 25.3% on average. Reduction of the ratio of the partial compressive area improved the compressive bearing capacity. However, the shape of the bearing plate had a minor effect of the bearing capacity. Based on the experimental results, the FEM model was developed for analyzing the bearing capacity of the CFST and CFDST columns. The results of the analyses were found consistent with the experimental results.

Post-fire behaviour and resistances of square recycled aggregate concrete-filled stainless steel tube stub columns

Experimental and numerical studies on the cross-section compressive behaviour and residual resistances of square recycled aggregate concrete-filled stainless steel tube (RACFSST) stub columns after exposure to fire are reported in this paper. An experimental programme was firstly carried out on twelve stub column specimens with three recycled coarse aggregate replacement ratios (0%, 35% and 70%) after exposure to the ISO-834 standard fire for 0 min (i.e. at ambient temperature), 15 min, 30 min and 45 min. The test results, including load–end shortening curves, failure loads and failure modes, were presented, with the initial compressive stiffness and confinement effect analysed. The experimental programme was followed by a numerical modelling programme, where thermal and mechanical finite element models were developed and validated against the test results and afterwards used to conduct parametric studies to generate additional numerical data over a wide range of cross-section dimensions. Based on the test and numerical data, the relevant design rules for square natural aggregate concrete-filled carbon steel tube stub columns at ambient temperature, as specified in the European code, Australia/New Zealand standard and American specification, were evaluated, using post-fire material properties, for their applicability to square RACFSST stub columns after exposure to fire. The evaluation results generally revealed that the European code and Australian/New Zealand standard led to a good level of design accuracy, while the American specification resulted in slightly conservative post-fire cross-section compression resistance predictions.

Behavior of seawater sea sand concrete-filled plastic-lined steel tube stub columns under axial compression

This study presents the experimental results of seawater sea sand concrete (SSC)-filled plastic-lined steel tube (SSC-PLST) stub columns subjected to axial compression. The concrete strength is set as the main test parameter, and the full-range behavior of SSC-PLST specimens is recorded in terms of failure mode, load-deformation response, load-carrying capacity, and law of stress-strain development. The experimental results show that all the SSC-PLST stub columns behave in a ductile manner, which is similar to the conventional concrete-filled steel tube (CFST) members. During testing, obvious axial deformation accompanied with outward local buckling in the plastic-lined steel tube is observed. The composite actions between the infilled SSC and plastic-lined steel tube are demonstrated through the analysis of load-carrying capacity and strain development. As the concrete strength increases, the SSC-PLST columns tend to have an increasing load-carrying capacity, but the corresponding ductility shows a downward trend. Finally, the ultimate load-carrying capacity of SSC-PLST stub columns is predicted using the calculation methods that are already available for conventional CFST members, and the efficiency is evaluated.

A unified formulation for axial compression of steel tube-confined concrete and concrete-filled steel tube stub columns

Steel-confined concrete (STCC) columns and concrete-filled steel tube (CFST) columns have different load boundary conditions. In STCC columns, the steel tube transmits minimal axial load. Existing design practices employ different formulas to predict the axial load-bearing capacity of STCCs and CFSTs based on these varying load boundary conditions. However, there has always been a practical and challenging demand for researchers and engineers to develop a unified design formula that accommodates composite columns with different sections and load boundary conditions. This demand aligns with the contemporary design concept of continuous structural design optimization. This paper proposes an analytical solution for the elastic deformation of circular STCCs under axial compression, considering the specific load boundary conditions in circular STCC columns. This solution is then extended to encompass polygonal sections, resulting in a unified formula for determining the strength of STCC columns. Additionally, the influence of load boundary conditions on the confined effect of the core concrete is quantified by reformulating and simplifying the proposed formula. These adjustments are informed by the formulation for the axial load capacity of CFST columns proposed by Yu Min et al. Finally, a simple and convenient unified formulation for calculating the axial compressive strength of both STCC columns and CFST columns is proposed for practical engineering applications.

Behavior of CFRP strengthened concrete-filled steel tubes with spherical-cap gap under axial compression

Carbon fiber-reinforced polymer (CFRP) strengthening can be considered as an effect approach to compensate the unfavorable influence caused by the spherical-cap gap on concrete-filled steel tube (CFST) structures. In the present study, the influence of CFRP wrapped on the mechanical behavior of CFST with spherical-cap gap (CFST-G) is examined. Sixteen CFST stub columns are experimentally tested under axial compression loads, considering the gap ratio and the number of CFRP layers (nCFRP) as the main parameters for analysis. The test results show that the bearing capacity decreases with increasing gap ratio, and the wrapped CFRP effectively improves the axial bearing capacity of CFST-G. Notably, the strengthening effect increases as nCFRP increases. Meanwhile, finite element (FE) models are established and verified against the test results. The proposed FE model is then used to conduct behavior analysis, and the working mechanism of CFST-G with CFRP strengthened is revealed. Parametric studies are also carried out by the FE model. The results show that, under the same gap ratio, the strength index decreases with increasing steel strength, steel ratio, and nCFRP, whereas it increases as the concrete strength increases. Based on the test and numerical analysis results, a simplified formula to calculate the bearing capacity of CFRP-strengthened CFST-G is proposed, providing a basis and reference for practical engineering and design codes.

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.

Analytical and experimental investigation on axial compression capacity of carbon fiber-reinforced plastic-confined concrete-filled double-skin steel tube stub columns

Concrete-filled double-skin steel tubes (CFDSTs) are widely used in civil engineering. During the service life of CFDSTs, their bearing capacity can be easily weakened by the corrosion of outer steel tubes. This study investigated the interaction mechanism and reinforcement effects of carbon fiber-reinforced plastic (CFRP)-confined CFDSTs under axial compression. Typical failure modes, axial load-shortening curves, and axial load-strain behaviors of CFDST and CFRP-confined CFDST (CFRP-CFDST) specimens were obtained. The test results showed that the ultimate bearing capacity of stub columns with concrete strength of C40 and C60 increased by 19.78% and 15.12%, respectively, when wrapped with two layers of CFRP sheets, and by 41.60% and 37.99%, respectively, when wrapped with four layers of CFRP sheets. In addition, the FE modeling of CFRP-CFDSTs was established and validated by experimental results. On this basis, the parametric analysis was conducted. To avoid local buckling and yield failure of the inner steel tubes before the outer steel tubes, a calculation formula was proposed for predicting the minimum wall thickness of the inner tube. Furthermore, the prediction formulas for the bearing capacity of CFDSTs and CFRP-CFDSTs were proposed.

Eccentric compressive behavior of square concrete-filled stainless steel tube (CFSST) stub columns

To study the eccentric compressive behavior and mechanism of square concrete-filled stainless steel tube (CFSST) stub columns, a total of 12 specimens with different steel ratio and load eccentricity under eccentric load was investigated. The results demonstrated that the failure modes of all square CFSST stub columns were the outward local buckling of the stainless steel tube at the compression side and two adjacent sides. The load-longitudinal deformation curve of specimens exhibited three stages, including elastic stage, elastic-plastic stage, and descending stage. The longitudinal strain distribution in the cross section at the mid-height of the specimens generally satisfied the plane-section assumption during the loading process. Increasing the steel ratio of specimens significantly improved the ultimate bearing capacity. Meanwhile, the ultimate bearing capacity of the specimens decreased with the load eccentricity, and the second-order effect became more pronounced with the increase in eccentricity. Finally, based on the existing specification, this study proposed a prediction formula for the eccentric bearing capacity of square CFSST stub columns by modifying the eccentric reduction coefficient accompanied by adopting the method of equivalent cross section. The formula was verified using an established database of eccentric compression CFST stub columns in the existing literatures.