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 study on behavior of UHPC-filled circular stainless steel tube stub columns under eccentric compression

Ultra-high performance concrete (UHPC) filled stainless steel tube (UFSST) is a newly proposed composite structure specifically designed to withstand harsh natural environments. To study the behavior of the UFSST column, 3 axial compression and 9 eccentric compression UFSST stub columns were tested respectively. And a comprehensive parametric analysis was conducted by employing a refine finite element model validated by the test data. The results show that the thickness (t) of stainless steel tubes is a key parameter affecting the eccentric compression performance of UFSST stub column. The increase in t can significantly improve the performance of UFSST stub columns, including the ultimate bearing capacity (Nu), ultimate bending moment (Mu), and ductility (DI). Finally, a modified computational model for the bearing capacity of UFST and UFSST stub columns under eccentric compression was proposed and showed a higher prediction accuracy. For UFST and UFSST stub columns, the prediction model yielded mean values of 0.99 and 1.00 for the ratio of predicted to experimental results, with standard deviations of 0.06 and 0.08, respectively.

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.

Behavior of UHPC filled high-strength square steel tubular stub columns under combined flexural and axial loads

This study experimentally and numerically investigated the behaviors of UHPC-filled high-strength square steel tube stub columns with compact, noncompact and slender sections under combined flexural and axial loads. Eighteen tests were conducted, addressing the gap of limited tests on concrete-filled square steel tubular stub columns with noncompact or slender sections. Additionally, finite element analyses were developed and benchmarked for parametric studies. The results of tests and parametric studies show that (1) steel tube outward buckling and concrete crushing occurred at 1/4–1/2L of the compression zone; (2) the load-axial displacement curves organized into three phases: elastic, elastic-plastic, and recession phase; (3) the compressive strength increased with increasing tube thickness, concrete strength, and steel yield strength. The compressive strength decreased with increasing eccentric ratio and slenderness coefficient. Test and numerical results are utilized to assess the suitability of current design specifications for predicting compressive strength. The evaluation showed that neither GB 50936–2014 nor Eurocode 4 provided reasonable predictions of strength, while AISC 360–16 provided conservative predictions.

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.

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.

Experimental and numerical study of stirrup‐confined concrete‐filled L‐shaped steel tube stub column under axial compression

AbstractIn this paper, three concrete‐filled L‐shaped steel tube (CFLST) stub columns subjected to axial compression were tested, with one normal CFLST stub column and two stirrup‐confined specimens. The investigation focused on the effect of the equivalent stirrup ratio on ultimate bearing capacity, axial compression stiffness, ductility factor, and strain ratio of the columns. 3D finite element (FE) models of the specimens were constructed and verified against the tests results and data from literature Parametric analysis was carried out to analyze the influence of different section length–width ratios, steel ratios, material strength grades, and equivalent stirrup ratios on the ultimate bearing capacity. Additionally, the confinement effect of stirrups on the core concrete was analyzed. By generalizing the stress nephogram of the core concrete at the ultimate state and considering the equilibrium of forces, a formula was proposed for calculating the ultimate bearing capacity of the stirrup‐confined CFLST stub column under axial compression using the superposition method. The accuracy of the proposed formula was demonstrated by comparing its results with experimental results, FE model results, and those proposed by other scholars.

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.

Theoretical model for spontaneous combustion gangue coarse aggregate concrete filled steel tube stub columns

Spontaneous combustion gangue coarse aggregate concrete filled steel tube (SCGCA-CFST) provides a possibility for the structural application of spontaneous combustion gangue coarse aggregate (SCGCA) concrete. To establish an accurate stress-strain model for SCGCA-CFST, 12 SCGCA-CFST specimens and 18 SCGCA concrete specimens were tested under monotonic axial compression with steel tube thickness and SCGCA replacement ratio (0%, 50%, 100%) as variables. The experimental results demonstrated that as the SCGCA replacement ratio increased, the elastic modulus of SCGCA concrete decreased by 6.65%–11.65%, the cylinder compressive strength decreased by 11%–21.95%, and the axial strain at peak axial stress increased by 7.1%–14.27%. Furthermore, the crack extension forms and dilation properties of the core concrete in SCGCA-CFST differed significantly from those of normal coarse aggregate concrete filled steel tube (NCA-CFST). The incorporation of SCGCA reduces the effectiveness of steel tube confinement, which causes the existing stress-strain models of NCA-CFST overestimate the enhancement effect of steel tube confinement on the strength and deformability of core SCGCA concrete. Based on this, the analysis-oriented stress-strain model for NCA-CFST was modified to be applicable for SCGCA-CFST by incorporating the influence of aggregate characteristics, and the accuracy of that was verified.

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.

Prediction of ultimate bearing capacity of concrete filled steel tube stub columns via machine learning

Prediction of ultimate bearing capacity of concrete filled steel tube stub columns via machine learning

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.

An improved constitutive model for steel tube-confined ultra-high-strength concrete

This study aims to accurately simulate the compressive behavior of ultra-high-strength concrete (UHSC) under various confinement conditions through conventional triaxial and equi-biaxial compressive tests. The triaxial compressive tests revealed that the stress and dilation angle of the concrete varies with changes in axial plastic strain and confining pressure, which is not well-considered in existing mechanical models for UHSC material. To address this limitation, an improved constitutive model based on the concrete plastic damage (CDP) model is proposed in this paper. The improved model incorporates the confining pressure in the flow rule and the hardening/softening rule, in addition to the axial plastic strain. The tensile and compressive meridians of the yield surface are calibrated based on the triaxial and biaxial compressive test results. Furthermore, the study proposes formulas for expressing the stress–strain relation and the lateral-to-axial plastic strain relation of UHSC under different confining pressures. These formulas help generate the hardening/softening rule and the dilation angle evolution of the flow rule, respectively. To verify the proposed constitutive model, the study simulated tests on UHSC-filled steel tube (UHSCFST) stub columns under compression using the finite element method. The simulated results of the peak bearing capacity of the tested UHSCFST stub column specimens are satisfactory, and the interaction between the steel tube and the concrete is reasonably predicted. The proposed improved constitutive model has the potential to accurately simulate the compressive behavior of UHSC under different confinement conditions, which can aid in the design of composite columns made with UHSC materials.