Axial compressive behavior of concrete-encased CFST stub columns with open composite stirrups

Concrete-encased concrete-filled steel tube (CFST) composite columns provide high bearing capacity, good seismic performance and an easier connection with arbitrary angle beams, which are widely used in high-rise buildings. Considering the high frequency of building fires, experimental research investigated the axial compressive behavior of the composite columns’ exposure to high temperature in this paper. Fourteen specimens after exposure to high temperatures with different parameters, including the heating temperature, steel tube diameter and concrete cover thickness, were fabricated to test under axial compressive loading. The failure pattern, load-displacement curve, bearing capacity, initial stiffness, deformation performance and damage rule of the specimens were discussed. The test results showed obvious differences in damage of specimens subjected to various high temperatures. The failure of the specimens began with the spalling and crushing of the concrete at the edge and ends in a lantern shape. The load-displacement curves of the specimens were significantly affected by high temperature, while the influence the of steel tube diameter and concrete cover thickness was relatively weak. A method of calculating axially loaded capacity for the composite column exposure to high temperature is proposed considering the effects of the main parameters of heating temperature and steel tube position, and the calculated results are in good agreement with the experimental results.

In order to study the bearing capacity of high-performance concrete-filled high-strength steel tube (HCHST) composite stub columns subjected to eccentrical load, 22 HCHST composite stub columns, 4 concrete-filled steel tube (CFST) composite stub columns and 8 high-performance concrete-filled steel tube (HCST) composite stub columns were designed with the cubic compressive strength of concrete (fcu), the yield strength of steel tube (fy), the thickness of tube wall (t), the eccentricity (e) and slenderness ratio (λ) as the main parameters. Considering the nonlinear constitutive model of concrete and simplified constitutive model of steel, the finite element (FE) model of HCHST composite stub columns was established by ABAQUS software. By comparison with the existing test results, the rationality of the constitutive model and boundary conditions was verified. The variation of ultimate bearing capacity and the typical failure modes of HCHST composite stub columns under different parameters was analyzed. The results show that the specimens exhibit obvious bulge outward at the end of the steel tube and shear failure at the end of concrete. High-performance concrete (HPC) can significantly improve the ultimate eccentrical compression bearing capacity of composite columns, and high-strength steel tubes have better restraint effect on HPC. With the increasing of t, the ultimate eccentrical compression bearing capacity and the load-holding capacity of HCHST columns increases gradually, while the ultimate eccentrical compression bearing capacity decreases gradually with the increasing of λ and e. By introducing the reduction coefficient of eccentricity (φ1) and the reduction coefficient of slenderness ratio (φ2), the calculation formula of the eccentric bearing capacity of HCHST columns is proposed by statistical regression, which can lay a foundation for the application of HCHST columns in practical engineering.

The square steel tube–timber–concrete composite L-shaped columns are lighter in weight due to the inclusion of wood and exhibit superior seismic performance. This combination not only reduces transportation and labor costs but also enhances earthquake resistance. The wood contributes lightness and flexibility, the steel provides strength, and the concrete offers excellent compressive performance, thereby achieving an optimized design for performance. To investigate the axial compression performance of square steel tube–timber–concrete composite L-shaped short columns, axial compression tests were conducted on eight groups of L-shaped columns. The study examined ultimate load, failure modes, load–displacement relationships, initial stiffness, ductility, and bearing capacity improvement factors under different slenderness ratios, steel tube wall thicknesses, and wood content rates. The results show that the mechanical performance of the composite columns is excellent. Local buckling of the steel tube is the primary failure mode, with ‘bulging bands’ forming at the middle and ends. When the wood content reaches 25%, the synergy between the steel tube, concrete, and wood is optimal, significantly enhancing ductility and bearing capacity. The ductility of the specimen increased by 31.1%, and the bearing capacity increased by 4.14%. The bearing capacity increases with the steel tube wall thickness but decreases with increasing slenderness ratio. Additionally, based on the Mander principle and considering the partitioned constraint effects of concrete, a simplified calculation method for the axial compressive bearing capacity was proposed using the superposition principle. This method was validated to match well with the test results and can provide a reference for the design and application of these composite L-shaped columns.

Examining mechanical behavior of steel-bamboo composite I-section column under long-term loading

Experimental and numerical study on the eccentric compressive performance of RAC-encased RACFST composite columns

Experimental study on the axial compression of GFRP-waste stone powder alkali-activated concrete-steel tube composite stub columns

Based on the axial compression test of 5 ultra-high performance concrete (UHPC) columns confined by high-strength stirrups and 4 UHPC columns confined by normal-strength stirrups, the bearing capacity, failure mode, steel strain and stress-strain curve for the confined UHPC columns were studied, and the influences of volume stirrup ratio, strength, spacing and configuration of stirrup on the axial compression behavior of confined UHPC were analyzed by combining ductility and toughness index. The results show that all the specimens have ductility damages, and the damage degree of UHPC columns confined by high-strength stirrups is lighter. The confined UHPC columns with high volume stirrup ratio, closely spaced, high-strength and complex ties configuration have good confinement efficiency, bearing capacity and deformation performance, which also leads to an ideal axial compression behavior. The effect of volume stirrup ratio on the axial compression behavior of specimens is greater than that of stirrup strength. Among the three factors affecting the volume stirrup ratio, such as spacing, configuration and diameter of stirrup, the stirrup spacing contributes most to the improvement of confinement performance, followed by the configuration and diameter of stirrup. The UHPC confined by high-strength stirrups presents better axial compression performance and residual bearing capacity than the normal-stirrups. The micro-bending of the longitudinal bars accelerates the cover spalling, while the high-strength stirrups with close space effectively delays the buckling of longitudinal bars, and significantly improves the overall performance of the confined UHPC. The micro-bending of longitudinal bars weakens the confine effect of the high-strength stirrups on the core UHPC, and the combination of high-strength longitudinal bars and stirrups is suggested. Based on the test data, the formulas to determine the bearing capacity of UHPC columns confined by stirrups are drawn.

Eccentric compression capacity of bolt-welded joints in concrete-encased CFST

This study explored the axial compression behavior of elliptical concrete-filled steel tubes with encased steel considering spherical-cap gap (GSECFST) composite short columns. We designed 25 composite column specimens by varying the steel tube yield strength (fty), steel skeleton yield strength (fsy), concrete cubic compression strength (fcu), steel tube thickness (t), steel skeleton sectional area (As), the long and short half-axis ratio (a/b), the gap ratio (Xsg), and the slenderness ratio (λ). Based on the nonlinear constitutive models of the materials and the nonlinear contact effect among materials, the ABAQUS 6.20 finite element software established the refined finite element models of these composite short columns. Also, the rationality of the finite element modeling with a spherical-cap gap was verified by comparing it with the existing experimental results. The influence regularity of various parameters on the load (N)-displacement (Δ) curves, bearing capacity, initial stiffness, and ductility of the composite short columns was obtained. In addition, the failure modes, N-Δ process, sectional strain distribution, and gap feature index of the constraint partition model for GSECFST axial compression short columns were revealed. The results showed a weakened interaction between the elliptical steel tube and concrete. Also, the axial compression bearing capacity, initial stiffness, and core concrete ductility were reduced because of the spherical-cap gap. As fty, fsy, fc, and Asy increased, the axial bearing capacity, initial stiffness, and ductility of GSECFST composite short columns improved significantly but decreased with increasing of a/b, Xsg, and λ. When the gap ratio of the spherical crown was less than 4%, the outer steel tubes in the mid-span area of the GSECFST composite short columns buckled in the direction of the elliptical short axis under axial compression, and the concrete expanded outward and crushed. The failures were similar to those of the specimens without the spherical-cap gap. Based on the sectional constraint partition model, we propose the calculation formula of axial compression bearing capacity for GSECFST composite short columns. Consequently, this study is a reference for the elastic-plastic analysis of frame systems with similar composite columns.

Performance of concrete-encased CFST stub columns under axial compression

Ultra-high toughness cementitious composites （UHTCC） are fiber-reinforced， cement-based materials known for their exceptional toughness， crack resistance， and tensile strain hardening. These properties ensure that UHTCC resists cracking under normal conditions and effectively prevents the intrusion of chloride ions and water， thereby enhancing structural durability and energy dissipation. As a result， UHTCC holds significant potential for use in critical projects and key infrastructure. This study introduces a UHTCC-encased circular steel tube composite column， offering advantages such as lightweight construction， ease of assembly， high bearing capacity， and superior durability. The axial compressive performance and steel tube buckling behavior of the proposed composite column， which incorporates both stud and PBL shear connectors， were investigated using a combined approach of theoretical and finite element analysis.A detailed finite element model of the composite column was developed and validated against experimental data， with an error range of 1.2%–2.3%. The finite element parameter analysis showed that as the number of PBLs increased from 4 to 12， the bearing capacity of the composite column with width-to-thickness ratios of 25 and 150 increased by 15% and 10%， respectively. Finally， theoretical calculations were performed to determine the steel tube’s critical buckling load， from which the composite column’s axial compressive capacity was derived.

This paper studies the behavior of concrete-encased concrete-filled steel tube (CFST) members under axial tension. Experimental results of thirteen specimens were reported. The variables in the test were the diameter of the steel tube, bond condition between the inner CFST and outer reinforced concrete (RC) components. Push-out tests were conducted to investigate the bond performance between the inner CFST and outer RC components. A finite element model (FEM) was then developed to perform further analysis on the tensile behavior of the concrete-encased CFST member. The internal load distribution between the inner CFST and outer RC components, the interaction between the core concrete and steel tube, and the interaction between the outer concrete and steel tube were investigated. The core concrete and steel tube worked together well, and the tensile strength of the steel tube was enhanced because of the “composite effect,” while the outer RC component was separated from the inner CFST and thus work...

In order to improve the mechanical properties of prefabricated monolithic composite columns, high-performance fiber-reinforced cement composite (HPFRCC) material was used to prefabricate mold shells and form a composite column with post-cast ordinary concrete. Axial compression tests were conducted on five HPFRCC-prefabricated shell composite columns and one RC prefabricated shell composite column to study the mechanical performances of prefabricated monolithic composite columns. The influences of the volume stirrup ratio, the longitudinal reinforcement ratio, and the shell material on axial compression performance were also studied. The results showed that using HPFRCC-prefabricated shells could improve the deformation performance of the composite column. The compressive strain corresponding to the yielding load of the HPFRCC-prefabricated formwork column was 23.85% higher than that of the RC-prefabricated shell composite column, and the compressive strain corresponding to the peak load increased by 26.72%. The longitudinal reinforcement ratio slightly affected the axial compression bearing capacity of the prefabricated shell composite column. Compared with ECCT−01, the peak load of ECCT−04 increased by 0.6%, and the peak load of ECCT−05 increased by 1.4%. As the volume stirrup ratio increased, the deformation performance of the HPFRCC-prefabricated composite column improved. Compared with ECCT−01, the compressive strain corresponding to the yield load and peak load of ECCT−02 increased by 24.21% and 7.33%, respectively. The compressive strain values corresponding to the yield load and peak load of ECCT−03 decreased by 25.58% and 24.01%, respectively.

SummaryTo investigate the seismic behavior of high‐strength concrete‐encased concrete‐filled steel tube (HSC‐encased CFST) composite column, eight specimens with a high axial compression ratio were tested under lateral cyclic loading and constant axial compressive loading in this paper. Key points on the load–displacement skeleton curve of the composite columns were obtained based on the experimental results. The test parameters include the axial compression ratio, volume stirrup ratio, stirrup form, and the diameter‐width ratio. All specimens failed in the flexural mode, with the crushing of compressive concrete and losing of vertical load‐carrying capacity. The ductility and deformation capacity decrease with the axial compression ratio increasing, and enhancing the volume stirrup ratio within 1.33–1.63% can provide an effective confinement of composite column. The ultimate strength of specimen with the octagonal stirrup is 23.8% higher than that with well stirrup, and the ductility and load‐carrying capacity are enhanced due to the increase of diameter‐width ratio. A degrading trilinear restoring force model of HSC‐encased CFST composite column with a high compression ratio is proposed. The comparisons made between calculated results and test results show such a high agreement, which indicates the proposed restoring force model is applicable to such HSC‐encased CFST composite column. It provides a theoretical basement for further research on the nonlinear dynamic analysis of such CFST composite column.

The new type of steel–concrete composite member, i.e. the hollow steel-reinforced concrete (HSRC) composite column, is proposed to improve the mechanical properties of hollow reinforced concrete (RC) members. In this study, the ABAQUS software is used to establish a finite element model (FEM) and analyze the axial compression performance of the HSRC composite stub column. The reliability of the FEM is verified by the experimental data. Then, the full range analysis on the load versus deformation relation of the typical specimen is presented, and the contact stresses on the interface between the outer RC and the inner steel tube are analyzed. Based on these analyses, the material and geometric parameters that affect the axial compression performance are investigated to determine the key factors influencing the contact stress. Finally, the contribution of the contact stress to the axial bearing capacity is quantified, and a simplified design method for the axial bearing capacity of the HSRC composite stub column is proposed. The results show that the FEM based on the ABAQUS software in which material nonlinearity is considered, can effectively predict the axial compression performance of the proposed the HSRC composite stub columns. The maximum difference between predicted and experimental data is 18.4%. The results also show that the contact stress has a significant effect on the axial bearing capacity of the HSRC composite stub columns, which increases with the increase of the steel ratio and the decrease of the hollow ratio. In addition, it was found that the axial compression bearing capacity of HSRC composite stub columns can be effectively predicted by introducing an enhancement coefficient of concrete strength, which is influenced by the hollow ratio, the steel ratio, the yield strength of the steel tube, and strength of the concrete.