Concrete-filled Square Steel Tube Research Articles

Axial compressive behavior of pre-damaged concrete-filled square steel tube columns repaired with section circularization and CFRP composites

This study focused on repair treatments for the effective reuse of damaged concrete-filled square steel tube (CFSST) columns with carbon fiber-reinforced polymer (CFRP) composites. Forty specimens were designed to investigate the effects of pre-damage levels, section circularization, and the number of CFRP layers on the compressive behavior of the repaired CFSST columns. Test results indicated that the method of section circularization and CFRP wrapping effectively restored the degraded initial axial stiffness of the pre-damaged CFSST columns and increased the ultimate bearing capacity of the pre-damaged CFSST columns by 49–180 % compared to their undamaged condition. Considering the influence of the pre-damage level and the contribution of section circularization, the ultimate bearing capacity calculation model was suggested for predicting the compressive behavior of the repaired CFSST columns. The experimental results and collected test data have well verified the accuracy and applicability of the calculation model.

Mechanical properties of CFSST with steel reinforcement cage under biaxial eccentric compression

To study the biaxial eccentric compression mechanical properties of high-strength concrete-filled square steel tube (CFSST) columns (high-strength materials are used for both steel tube and concrete) with steel reinforcement cage, twelve high-strength CFSST columns with the same cross-sectional dimensions were designed and fabricated with the form of the steel reinforcement cage, eccentricity, and the steel tube width-to-thickness ratio as the main parameters. The constraint enhancement mechanism of the concrete by the steel reinforcement cage and the effect of different parameters on the biaxial eccentric ultimate bearing capacity of column specimens was further analyzed by combining with ABAQUS simulation. The results show that the bearing capacity of the specimens decreases sharply with the increase of eccentricity and steel tube width-to-thickness ratio; the steel reinforcement cage can improve the bearing capacity and ductility of the specimen but has little effect on the stiffness; the arrangement of double orthogonal horizontal steel reinforcement can enhance the constraint effect of the steel tube in the middle of the cross-section edge length on the concrete of the specimens within a certain height range and weaken the constraint effect of the corner within the same height range; the enhancement effect of the steel reinforcement cage on the biaxial eccentric ultimate bearing capacity of the column specimens gradually decreases with the increase of eccentricity; when the eccentricity is constant, the effect of the steel reinforcement cage on the improvement of the biaxial eccentric ultimate bearing capacity of the column specimens with different width-to-thickness ratios is basically the same.

Behavior of Concrete-Filled U-Shaped Steel Beam to CFSST Column Connections

Two new types of connection between concrete-filled U-shaped steel (CFUS) beams and concrete-filled square steel tube (CFSST) columns were presented in this study, including rebar-sleeve with internal diaphragm connection and rebar-through with internal diaphragm connection. Based on the experiments of the rebar-plate with internal diaphragm connections between CFUS beams and CFSST columns under cyclic loading, the nonlinear finite element models of the tested specimens were developed and validated by comparing them with the experimental results. The numerical results were in agreement with the experimental results in terms of failure modes, stress distribution, and load-displacement skeleton curves. Based on the FEA results, the mechanical behavior of the two new types of connection were comprehensively discussed and compared. Furthermore, this parametric study was conducted for the rebar-sleeve with internal diaphragm connection to investigate the effect of specific parameters on the capacity of the connection. The parameters included: The thickness of U-shaped steel, the ratio of longitudinal reinforcement in the concrete slab, the strength of concrete in the beam, the strength of the U-shaped steel, and the thickness of the internal diaphragm. The results indicate that the thickness of the U-shaped steel (tb), the ratio of the longitudinal reinforcement in concrete slab (ρ), and the strength of the U-shaped steel have significant effects on the loading capacity of the connection—the loading capacity increases by about 20% when tb increases from 6 mm to 8 mm, increases by about 45% when ρ increases from 1.5% to 4.8% under negative P, and increases by about 20% when the steel yield strength (fy) increases from 235 Mpa to 420 Mpa.

Effect of bidirectional load on seismic performance of jacketed external diaphragm joints of high-strength concrete-filled high-strength steel tube frame

As one of the solutions for promoting green building and building industrialization, the application of high-strength concrete-filled high-strength steel tube (HCFHST) can further improve the structural performance of concrete-filled square steel tube (CFST). The beam-column joints, being a major stress component of the frame structure, influence the overall structure's performance during earthquake activity. In the external diaphragm joint of the frame, the stiffness of the steel tube at the connection between the external diaphragm and the CFST column is insufficient; aiming for this, a new type of jacketed external diaphragm joint (JED joint) connected between HCFHST column and steel beam were proposed in this paper. The ABAQUS was used to establish an analysis model to simulate the seismic performance of the joint accurately, and 5 kinds of bidirectional load conditions corresponding to the actual seismic action were designed to carry out numerical simulation for the evaluation of the seismic behavior of the joints of 17 specimens (4 groups), revealing the influence law of bidirectional load and providing design suggestions. The results showed that the hysteresis curve of the JED joint was full, the ductility coefficient was between 2.41 and 4.11, the equivalent viscous damping coefficient was between 0.48 and 0.6, and the deformation and energy consumption capacity in the joint domain reached the ductility requirements specified in GB 50011 and FEMA-350. According to AISC-LRFD and EC3, JED joints were determined to be semi-rigid. The seismic performance of the JED joint under bidirectional load was superior to that under unidirectional load. For the seismic performance of space joints, the most unfavorable load angle joint is characterized by bidirectional synchronous seismic action in the direction of 45°, edge joints are characterized by bidirectional asynchronous seismic action, and middle joints are manifested as constant load and unidirectional seismic joint action.

Optimum reinforcement ratio of axially loaded reinforced concrete-filled square steel tube column

For the purpose of clarifying the influencing mechanisms of the longitudinal reinforcement on loading performances of Reinforced Concrete-Filled Square Steel Tube (R-CFSST) short columns under axial load, experimental and numerical studies were performed. Setting the longitudinal reinforcement ratio as parameter, ten R-CFSSTs with five different ratios and three Concrete-Filled Square Steel Tube (CFSST) specimens for comparison were fabricated and tested. Based on the test, the influencing law of longitudinal reinforcement on failure, loading behavior, stiffness, peak load, composite efficiency, ductility and stress state of steel tube of R-CFSSTs were theoretically investigated. A novel material model and simulation method for concrete core was proposed and verified against the test, and thereafter 32 model specimens with four wall-thicknesses of steel tube coupling with eight reinforcement ratios were simulated. The strength prediction equations from ANSI/AISC A360–16, Eurocode 4 and AS/NZS 5100.6 and GB 50936–2014 were studied, modified and compared each other, and based on the test and simulation results, novel equations were proposed and validated against the experiment. The results demonstrate followings: longitudinal reinforcement significantly impacts the behavior of R-CFSSTs as the steel tube does, and thus it is recommended to configure reinforcements in CFSST for the improved performances; proper ratios of longitudinal reinforcement enable the R-CFSSTs to acquire better balance between performance and construction cost, and a range for the optimum ratios is recommended between 1.0% and 3.2%, regardless of wall-thickness of steel tube; the proposed equations are recommended for more accurate and stable prediction of load bearing capacity of R-CFSSTs.

Axial compressive behavior of steel fiber reinforced concrete-filled square steel tube stub columns

This study aims to assess the influence of steel fiber on the mechanical property of concrete-filled square steel tube (CFSST) stub columns. A total of 36 specimens, with different concrete strength grades (C = 40 MPa, 50 MPa or 60 MPa), various steel tube thicknesses (t = 3 mm, 3.75 mm or 4.5 mm) and diverse steel fiber volume contents (Vf = 0%, 0.6%, 1.2% or 1.8%), are conducted under axial compressive loading. Failure modes, ultimate loads, load-displacement behavior and relative load-strain behavior are discussed. Moreover, a finite element (FE) model is proposed to investigate the reinforcing mechanism of steel fibers in CFSST columns. The test results show that the ultimate bearing capacity of CFSST columns slightly increases with the increasing of steel fiber volume content, but the energy dissipation capacity and ductility are visibly improved. However excessive steel fibers (exceeding 1.2% in this study) may cause a negative effect. FE analysis demonstrates that steel fibers with a moderate amount not only postpone the failure of concrete core, but also improve the carrying loads of concrete core at ultimate state. Finally, a design formula is put forward for the ultimate bearing capacity of CFSST stub columns.

Seismic performance of TFOSBs bolted endplate connection to CFSST with repaired threaded holes

This paper evaluates the seismic behavior of short thread-fixed one-sided bolts (TFOSBs) bolted connection of an I-section beam to concrete-filled square steel tube (CFSST) with repaired threaded holes by enlarging threaded holes through cyclic tests. The parameters investigated were endplate thickness, inner stiffener type, thread anchored length and bolt installation sequence. The test results are presented in terms of strength, rigidity, ductility and energy dissipation capacity. Based on the experimental results: 1) the failure modes of the connections with repaired threaded holes by enlarging threaded holes were similar to the failure modes of the connections having intact hole threads; 2) the connections with repaired threaded holes exhibited 1.8% ∼ 23.8% higher ductility and 3.9% ∼ 46.6% higher energy dissipation capacity compared with the connections having intact hole threads, and the ultimate bearing capacity of those two types of joints were comparable; and 3) the bolt installation sequence has a negligible influence on the failure modes and seismic behavior of connections with repaired threaded holes. In addition, the existing calculation methods of yield moment capacity for TFOSBs bolted endplate connections under various failure modes were still suitable for connections with repaired threaded holes, and the theoretical values and test results were well-matched with errors no greater than 15%.

Study on axial compression bearing capacity of novel concrete-filled square steel tube columns

The way that strengthening the bond effect between the concrete and the inner steel by welding technology can effectively improve the axial bearing performance of the concrete-filled square steel tube (CFSST) column. However, this method requires high welding quality. In order to simplify the construction and reduce the difficulty of construction, two kinds of novel section forms of CFSST column without welding have been proposed. One is CFSST column reinforced with rhombic stirrups (specimens in SSSC group), the other is double-skin CFSST column (rhombic inside & square outside) (specimens in SSC group). A total of 21 specimens were fabricated for axial compression test, including 10 specimens in SSSC group, 7 specimens in SSC group, 1 CFSST column reinforced with inclined I-steel, 1 CFSST column reinforced with I-steel, 1 CFSST column reinforced with circle steel tube, and 1 CFSST column. The axial load-displacement curves of the specimens were obtained, and the failure modes and the influence parameters of specimens with different sections were analyzed and compared. The results show that the failure modes can be divided into local buckling failure and instability failure during plastic loading according to the aspect ratio. The specimens in SSC group have the highest average peak load, and the best residual bearing capacity, and the most eximious plastic strengthening characteristics in the later stage than other specimens. The decreasing rate of unit peak load of SSSC group was the slowest when the steel content was increased, which improves the bearing capacity overall. Increasing the core concrete area (inner steel tube side length) can enhance the bearing capacity of the specimens with longitudinally continuous build-in steel (reinforced with steel tube). While, it is not economical to improve the peak load of the specimens with large section (side length >200 mm) by increasing the thickness of the outer steel tube when the build-in steel is longitudinally discontinuous (reinforced with rhombic stirrups). Based on the result analyses, the calculation formulas considering the stability of axial compressive bearing capacity of specimens in SSSC and SSC group have been derived by superposition method. And the calculated results are in good agreement with the test results.

Cyclic behavior of concrete-filled square tubular columns with stiffened plastic hinge region

The seismic behavior of concrete-filled square steel tube (CFST) columns is greatly influenced by the local-buckling of steel tubes within the plastic hinge region. This paper focuses on a novel stiffener that strengthens the potential plastic hinge regions to enhance the local-buckling resistance of steel tube, hence improving the overall seismic performance of the CFST columns. To verify its effectiveness and investigate the hysteretic behavior of the stiffened CFST (referred to as SCFST) columns, three CFST and three counterpart SCFST columns were fabricated and tested under cyclic lateral load while subjected to constant axial load. Furthermore, flexural capacities calculated using representative design equations in current codes as well as a newly proposed method were compared with the experimental results. Calculation method for the effective strengthening length of SCFST was presented. The test and analytical results showed that: (1) this stiffeners could effectively delay the local buckling of steel tube and change its local buckling modes; (2) the stiffener significantly enhanced the lateral load capacity of SCFST columns with improvement of 14%, 15%, and 38% comparing with the counterpart CFST columns; (3) the thinner the steel tubes, the more significant the effect of the stiffener on the ultimate lateral capacities; (4) the design equations prescribed in current codes tended to underestimate the ultimate flexural strength of SCFST columns with a discrepancy between 22% and 60%; (5) the proposed calculation method taking account of the effect of the stiffener, could more accurately predict the flexural strength of SCFST columns than current design equations with an improved discrepancy ranging between 7% and 32%.

Seismic behavior and design of repairable precast RC beam–concrete-filled square steel tube column joints with energy-dissipating bolts

The dry jointed precast frame structure has the potential in less reliance on wet concrete, low cost and repairability after earthquake. The seismic behavior of the novel precast dry connections with different beam end arrangements was studied in this work. For the precast specimens, the concrete-filled square steel tube (CFSST) column and RC beam were prefabricated, and field assembly was carried out by energy-dissipating (ED) bolts and unbonded prestressed tendons. Design equations were formulated according to the strong-column–weak-beam concept and the capacity design philosophy. The viability of the repair method of the precast internal beam–column connections after earthquake was explored. The joints were repaired by replacing the ED bolts and loaded again. The seismic indicators, such as ductility and energy dissipation capacity, were determined and compared. The results showed that the excellent structural characteristics and practical design method could ensure low damage to the joints except for the ED bolt yield. The new-type precast beam–column joints exhibited good hysteretic performance with high ductility factors greater than 5. After repair, the failure mode of the joints was controlled by the plastic hinge on the beam. The repaired joints had good strength stability, and the ultimate bearing capacity was the same as before repair. The ductility of the repaired joints was 13.29%–32.14% higher than before. The energy dissipation capacity could reach more than 1.2 times that of the cast-in-place concrete joints, which proved the feasibility of the repair method.

Experimental Study on CFRP-Confined Circularized Concrete-Filled Square Steel Tube Short Columns

This study investigates the suitability of the circularization technique for strengthening square concrete-filled square steel tube (CFSST) short columns. A total of 16 specimens were tested under axial compression. The main parameters under investigation were concrete strength, the thickness of arc cement mortar layer components (CAM), and the layers of carbon fiber-reinforced polymer (CFRP) sheets. Test results indicated that the failure mode of CFRP-confined circularized CFSST (C-C-CFSST) columns was similar to that of CFRP-confined concrete columns. The CFRP-confined circularized strengthening method can increase confinement efficacy and reduce the stress concentration at the corners of CFSST columns. Three existing CFRP-confined concrete stress-strain models were evaluated using the test results. The predictions of the Lam and Teng stress-strain model agree well with the test data.

Research on shear strength of CFSST column and H-section with beam composite joint of unequal depth

Four concrete-filled square steel tube (CFSST) columns and H-sections with beam composite joints of unequal depth were designed based on existing specifications and manufactured at ⅓ scale. A failure test of the specimens was carried out under lateral cyclic loading. Different height ratios between the left and right beam, ζ, (ζ = 0.52, 0.39, 0.26, 0.13) were tested to analyze the failure mode, hysteresis characteristics, ductility, strain, and deformation of the specimens and the damage characteristics of the structure. An equation was established to determine the shear bearing capacity of the CFSST column and H-section. The results show that shear failure in the joint core area is the dominant failure mode of the specimen. The greater the beam ratio, the smaller the yield displacement of the specimen is, and the earlier the joint core area reaches the yield strain. The damage value obtained from Chen's model is in good agreement with that obtained from the test and accurately describes the damage evolution and damage limit of the specimen from “basically intact” to “completely damaged”. The shear resistance of the joint core area is primarily borne by the steel tube web, the main concrete compression strut in the core area, and the restrained compression strut. The theoretical values and the experimental values of the shear bearing capacity of the CFSST column and H-section are in good agreement, demonstrating the accuracy of the proposed theoretical equation.

Development and testing of precast concrete-filled square steel tube column-to-RC beam connections under cyclic loading

Compared with the cast-in-place construction technique, the precast structure has advantages such as high industrialization, good quality, and environmental protection. Previous studies and investigations have shown that fabricated structures require secure connection technology, which could solve critical seismic problems associated with structures. The complex reinforcement layout in joint zone and the inefficient connection processes erode the economic value of the fabricated structure. This paper presents a novel precast concrete-filled square steel tube (CFSST) column-to-RC beam connection with energy dissipating (ED) bolts and prestressing tendons. In total, eight precast specimens with different axial compression ratios, ED bolt diameters, steel tube thicknesses, and beam end arrangement were tested under cyclic loading. The results show that the structural drift ratio obtained is up to 5.75%. The hysteresis curves of all specimens are plump. Even the specimen with the smallest diameter ED bolts can reach twice the energy dissipation capacity of the cast-in-place frame RC joint. The average ductility coefficients of all models are greater than 5. The precast CFSST column has good integrity and less damage after the earthquake. The final failure modes of the fabricated connections are governed by the plastic hinges at the beam. Besides, a calculation method for the flexural bearing capacity of the novel joint is developed and compared with the test results, which verified the feasibility of the model. The proposed precast connections exhibit superior resilience against earthquakes and can be used in active seismic regions.

Tensile behavior and load transfer mechanism of concrete-filled square steel tube exterior-diaphragm connections stiffened with PBL

An innovative concrete-filled square steel tube (CFSST) exterior-diaphragm connection stiffened with perfobond ribs (PBL) is proposed to overcome the limits of CFSST exterior-diaphragm connections. Different CFSST connections, including one ordinary CFSST connection, one CFSST exterior-diaphragm connection, two CFSST exterior-diaphragm connections stiffened with PBL, and two CFSST connections stiffened with PBL, were tested under tensile loads. The tensile behavior, including failure modes, initial stiffness, yield load, peak load, and deformation of different connections, was analyzed and compared based on the test results. By analyzing the stress distribution in critical sections of branch plates or exterior diaphragms and the contribution of different parts to the total tensile load in critical sections of joint regions at different loading stages using the finite element method, the load transfer mechanism of different connections was analyzed and compared quantitatively. The results showed that the PBL transferred a considerable percentage of the tensile load from the branch plates or exterior diaphragms to the core concrete, which resulted in a more uniform stress distribution in the branch plates and exterior diaphragms and caused the core concrete to have a direct function in the load transfer in joint regions; thus, the load transfer and deformation condition of connections were improved. The innovative CFSST exterior-diaphragm connections stiffened with PBL had excellent tensile behavior with the largest initial stiffness, yield load, and peak load, the smallest joint region deformation, and the most uniform stress distribution, thus failed due to the yielding of branch plates.

Mechanical performance of concrete-filled square steel tube stiffened with PBL subjected to eccentric compressive loads: Experimental study and numerical simulation

Abstract This paper describes an experimental study on the flexural and eccentric compressive behavior of concrete-filled square steel tube (CFSST) beam-columns stiffened with PBL stiffeners. Three types of CFSST beam-columns were fabricated and tested under the flexural moment and eccentric compressive load. Among the three types of columns, the two types of columns were strengthened with the PBL and steel plate stiffeners; the other type of columns was not strengthened. In addition, extensive finite element analyses (FEA) were carried out to evaluate the stress distribution. Flexural test results show that failure modes of specimens without stiffeners were different from specimens with the PBL or steel plate stiffeners. The results also demonstrate the distribution of cracks in the concrete was affected by stiffeners and the columns with the PBL stiffeners have the most uniformed crack distributions. The capacity improved by the PBL stiffeners was insignificant. However, the flexural stiffness of the specimen stiffened with the PBL stiffeners was about 20% larger than the unstiffened specimen. Eccentric compressive load test results show the capacity of specimen with the PBL stiffeners was 6.95% larger than the unstiffened specimen. The eccentric compressive capacity decreased with the increase of the eccentricity ratio or slenderness ratio. FEA results show that stiffeners can effectively improve the mechanical behavior of CFSST specimens under bending and eccentric compressive loads.

Analysis of Concrete-Filled Square Steel Tube Short Columns under Eccentric Loading

The concrete-filled square steel tube (CFSST) columns have been widely applied in structural engineering. Although many constitutive models have been proposed to describe CFSST short columns under concentric loading, the applicability of the existing concentric model in the analysis of CFSST short columns under eccentric loading has not been properly verified. In this paper, the eccentric behaviors of CFSST short columns were investigated with the software of ABAQUS/standard. It was found that the contact stress between steel tube and inner concrete was seriously affected by eccentric ratios, indicating that the confinement effect of steel tube on inner concrete was different under concentric and eccentric loading. In this paper, a new stress-strain model of inner concrete which considered the influence of eccentricity was developed and verified with existing experimental results. It was found that the proposed stress-strain model was more accurate in simulating the eccentric behaviors of CFSST short columns.

Damage Detection of Concrete-Filled Square Steel Tube (CFSST) Column Joints under Cyclic Loading Using Piezoceramic Transducers.

Concrete-filled square steel tube column (CFSSTC) joints are the most important parts of concrete-filled steel tube frame structures. It is of great significance to study the damage of CFSSTC joints under the seismic loads. In this paper, embedded piezoceramic transducers are used to monitor the damage of core concrete of CFSSTC joints under cyclic loading and surface-bonded piezoceramic disks are used to monitor the debonding damage of the steel tube and core concrete of two specimens. The damages of the joints under different loading levels and different loading cycles are evaluated by the received signal of the piezoceramic transducers. The experimental results show that the amplitude of the signal attenuates obviously with the appearance of damage in the joints, and the degree of attenuation increases with the development of the damage. The monitoring results from piezoceramic transducers are basically consistent with the hysteresis loops and skeleton curves of the CFSSTC joints during the cyclic loading. The effectiveness of the piezoceramic transducers are verified by the experimental results in structural health monitoring of the CFSSTC joint under cyclic loading.

Finite Element Analysis on Mechanical Performance of Middle Long CFST Column with Inner I-Shaped CFRP Profile under Axial Loading

The mechanical performance of middle long concrete-filled square steel tube (CFST) column with I-shaped CFRP profile inside was studied in this paper. The numerical model of middle long CFST column with I-shaped CFRP profile inside was established by ABAQUS finite element software on the basis of the reasonable constitutive relationship model of material. The whole process curve of load deformation was analyzed, which can be divided into four stages: elastic stage, elastic-plastic rise stage, elastic-plastic decline stage and rebound stage. The model damaged because of buckling in the mid-span. The effect of steel strength and concrete strength, CFRP profile ratio and thickness of steel tube on mechanical performance of middle long column was studied. The bearing capacity of model increased with the increase of steel strength. With the increase of the concrete strength, the ultimate bearing capacity increased, but the ductility decreased. The ultimate bearing capability increased with the increase the CFRP profile ratio and steel tube thickness.

Mechanical behavior of concrete-filled square steel tube with FRP-confined concrete core subjected to axial compression

A novel column consisting of steel, concrete and fiber-reinforced polymer (FRP) is presented and assessed through experimental study. The sectional form of steel–concrete–FRP–concrete (SCFC) columns has a square steel tube as the outer layer and a circular filament-wound FRP tube as the inner layer, and concrete is filled between these two layers and within the FRP tube. Thus the column can be regarded as a concrete-filled square steel tube (CFST) with a FRP-confined concrete core (FCCC). In comparison to existing composite column sections, several advantages are achieved such as ease of connection to beams, the stronger confinement of core concrete, ductile response, and residual load capacity. 18 SCFC stub columns are tested with different concrete strengths, FRP tube thickness and steel tube thickness in comparison to four traditional composite columns. The distinctive behaviors of SCFC columns, including significant post-yield stiffness, ductile failure, and stable residual bearing capacity are found. Test results show that the strengths of concrete, FRP, and steel are effectively utilized at different stages, including the initial linear stage, the secondary linear stage (hardening stage), and the post-peak stage. The influences of key design parameters on the mechanical characteristics of initial compressive stiffness, peak strain, residual load-bearing capacity, and ductility are also clarified.

Finite Element Analysis on Concrete-Filled Square Steel Tube Short Columns with Inner CFRP Profiles under Axial Compression

The new composite structure concrete-filled square steel tube (CFST) column with inner CFRP profiles is proposed. A finite element model is presented to investigate the mechanical behavior of CFST short columns with internal CFRP profiles subjected to axial compression using ABAQUS based on reasonable constitutive models of materials. In a addition, the content of CFRP profiles and width thickness ratio of steel tube’ effect are considered on mechanical properties of the column. Based on the model, the whole stage of axial compression of the short columns, failure mode and the stress mechanism of the ultimate bearing capacity state are calculated and analyzed.