Concrete-encased CFST Research Articles

Acoustic Emission characteristics and damage evolution of Concrete-Encased CFST columns under compressive load

Concrete-Encased Concrete Filled Steel Tubular (CE-CFST) is usually served as a compression member in engineering structures due to its high performance. It is crucial to reveal its compressive failure mechanism and the damage evolution law. This study used Acoustic Emission (AE) technology to monitor the compressive failure behavior of seven groups of CE-CFST columns with different diameter-width ratio, slenderness ratio and eccentricity, and then the AE signal characteristics and structural damage evolution law were discussed. Results show that the curve of AE characteristics can be used for effectively identifying the damage stages of CE-CFST structures. The failure process can be divided into six stages: initial compaction, stable growth of micro cracks, unstable propagation of macro cracks, collapsing of the outer RC structure, bulging of the core CFST structure and overall failure. The AE characteristic parameters, RA-AF value and b value are closely related to stress and crack of structure, and can be used for early warning of structural failure during the stage of unstable crack propagation. Particularly, the failure of outer concrete can be judged as b value drops to the minimum. The current study is of great significance for understanding the damage evolution process and achieving damage assessment of CE-CFST structures.

Experimental and numerical investigations of bolted assembled joints to concrete encased CFST columns with different connection details

Concrete encased CFST (CECFST) composite components have prominent advantages over traditional CFST components such as better durability, high stiffness and strength, etc. However, the encased concrete outside caused certain difficulty in connecting to the steel beam. In this paper, two types of bolted assembled joints between CECFST column and steel beam were proposed and tested under cyclic loads, in which the end plate and cover plate joints were included. Typical failure modes of the joints were analyzed and characterized. In comparison, the CECFST column in end plate joints exhibited more severe failure and higher strength than that in cover plate joints. Seismic performance including hysteretic curves and energy dissipation were also discussed. In addition, numerical modelling of the joint was established considering the complicated material models and interactions in the joint core, of which the accuracy was validated against the test results. Further, parametric analyses were performed on the cover plate joints with CECFST columns through the numerical approach. It demonstrated that the axial ratio, thickness and strength of cover plate, diameter of bolt, strength and thickness of steel tube significantly affected the stiffness and strength of the joint. The experimental and numerical results revealed that the proposed two types of joints with CECFST column performed relatively favorable seismic behavior and could be employed in engineering practice.

Compressive behavior of concrete-encased CFST column with initial stress

Concrete-encased CFST (CE-CFST) member refers to a hybrid structure consisting of outer reinforced concrete and inner CFST components. This paper studies the compressive behavior of CE-CFST stub columns with initial stress on steel tube. Nine compression tests on CE-CFST stub columns were completed. The influences of initial stress degree and eccentricity on the failure mode, load-displacement behavior and ultimate strength were investigated. The crushed concretes with locally buckled longitudinal rebars were observed at the middle height of specimens. The ultimate strength of CE-CFST stub column slightly decreases as initial stress degree increasing. The maximum decreasing range on the ultimate compressive strength was 9.3% as the initial stress degree increasing in range of 0–0.5. A finite element model of CE-CFST stub column was developed and verified against test results. The parameters study and full-range compression process analysis were performed using the verified finite element model. The strength contribution provided by the inner CFST and outer RC components and interactions between concretes and steel tube were discussed. The influence mechanism of initial stress on the compressive behavior of CE-CFST stud column was revealed by the test and finite element results. The constraint effect on the core concrete provided by steel tube decreased due to the presence of initial stress, which reduced the strength contribution of the CFST component to the CE-CFST column. A simplified method was proposed to predict the influence coefficient of initial stress on the ultimate strength of the CE-CFST column based on the test and finite element results.

Axial Compressive Behavior of Concrete-Encased CFST Stub Columns with High-Level Two-Stage Initial Stresses

Axial Compressive Behavior of Concrete-Encased CFST Stub Columns with High-Level Two-Stage Initial Stresses

Ultimate bearing capacity of UHPC-encased CFST medium-long columns under axial compression

To study the effect of slenderness ratio on the ultimate bearing capacity of UHPC (ultra-high performance concrete)-encased concrete-filled steel tubular (UC-CFST) columns, a total of nine specimens under axial compression were tested. A finite element model, considering the effects of material nonlinearity and interaction between concrete and steel tube, was developed. A full-range analysis was carried out on the typical load–deformation response of the UC-CFST medium-long columns. The experimental and numerical analysis results showed that with the increase in slenderness ratio, the deformation capacity of the UC-CFST columns increased, the ultimate bearing capacity decreased, and the failure mode changed from exceedance of compressive strength to loss of stability. The internal forces and stress distributions at the mid-height section of the columns at different stages were also investigated. Finally, the applicability of various methods for calculating the ultimate bearing capacity of ordinary concrete-encased CFST (OC-CFST) columns to UC-CFST columns was discussed, including the methods recommended by Chinese standard T/CECS 188-2019, European standard Eurocode 4, Australian/New Zealand joint standard AS/NZS 5100.6 : 2017 and American standard ANSI/AISC 360-16. It was found that the method recommended in AS/NZS 5100.6 : 2017 was suitable for calculating the ultimate bearing capacity of UC-CFST medium-long columns.

Seismic Behaviors and Resilience of Concrete-encased Concrete-Filled Steel Tubular Columns with Debonded High-Strength Rebars: Experiment and Assessment

ABSTRACT A total of six cantilever square concrete-encased CFST columns were fabricated for testing under both lateral load and constant compressive load. The debonded high-strength PSB 930 steel rebars were used as the longitudinal rebar to provide resilience for specimen columns. Experimental results indicated that the specimens with debonded PSB longitudinal rebars had smaller residual deformation and smaller yield sectional stiffness compared with the specimen with normal steel rebars because the yielding progress of the longitudinal rebar was delayed. Based on the parametric analytical results, some models are proposed to assess the yield and post-yield sectional stiffness and the residual drift.

Experimental study of UHPC-encased CFST stub columns under axial compression

A novel composite structural member, UHPC-encased concrete-filled steel tubular (UC-CFST) column, is proposed in this study to address the shortcomings of the ordinary concrete-encased CFST (OC-CFST) columns. A total of 22 specimens subjected to axial compression loading were tested, including nine UC-CFST stub columns, nine OC-CFST stub columns, and four CFST stub columns. The experimental results showed that the encasing UHPC of UC-CFST columns was less damaged than the encasing ordinary concrete of OC-CFST columns and did not spall when the UHPC was crushed. The UC-CFST stub columns reached their ultimate load-bearing capacity when the encasing UHPC attained the peak compressive strain, which was much greater than that of the OC-CFST stub columns. Compared to the OC-CFST stub columns, the ultimate load-bearing capacity of UC-CFST stub columns improved markedly by 75%–289% and the compressive stiffness increased by 22%–49%, while the ductility decreased by about 50% on average. Furthermore, increasing the area ratio of the encased CFST in the UC-CFST stub columns was unfavorable for the compressive stiffness and load-ultimate bearing capacity. Finally, by comparing the different calculation methods for ultimate load-bearing capacity of OC-CFST columns recommended by Chinese standard CECS 188, American standard AISC 360, Japanese standard AIJ-SRC, and European standard Eurocode 4, it was found that the calculation methods applicable to OC-CFST columns would underestimate the ultimate load-bearing capacity of UC-CFST columns to varying degrees. Thus, a modified calculation formula based on the CECS 188 standard was proposed for accurate prediction of the ultimate load-bearing capacity of the UC-CFST stub columns.

Parametric study on prestressed concrete-encased CFST subjected to bending using nonlinear finite element modeling

Parametric study on prestressed concrete-encased CFST subjected to bending using nonlinear finite element modeling

Performance of concrete-encased CFST subjected to low-velocity impact: shear resistance analysis

The performance of concrete-encased concrete-filled steel tube (concrete-encased CFST) subjected to low-velocity impact is investigated through experimental and numerical studies in this paper. A total of 40 concrete-encased CFST specimens, 20 square and 20 circular ones, are tested under a drop hammer apparatus. It shows that while the outer RC components have a shear-dominated failure mode, the core CFST deforms in a flexural pattern. The influence of tested parameters on the impact force and midspan deflection of the specimens is summarized. A finite element analysis (FEA) model is then established to investigate the shear mechanism of concrete-encased CFST under impact. A comparative study between a concrete-encased CFST member and an RC member shows that the core CFST component could effectively mitigate the shear failure of the outer RC section under impact due to its high shear resistance; meanwhile, the outer RC component could well protect the core CFST and reduce its flexural deformation. Such composite effects make the concrete-encased CFST a desirable impact-resisting structure. Subsequently, a parametric study on the impact velocity shows that the debonding between outer RC and core concrete can be more prominent for the impact with a relatively higher velocity, and the use of shear studs is suggested for members under such circumstances.

Flexural performance of concrete-encased CFST box members

The flexural behavior of concrete-encased CFST box members is presented in this paper. Twelve bending tests were conducted to evaluate the influence of section size, steel tube diameter and bending direction on the performance of the concrete-encased CFST box beams. The test results were also discussed, including the observed failure patterns and developments of concrete cracks during the whole loading procedure. A numerical model was then developed to investigate the flexural behavior of the composite beams. The study of the full-range analysis, load transfer mechanism and stress distribution was carried out. Then the parametric studies were performed based on the FEA modeling. Finally, a simplified method for predicting the stiffness and flexural strength of the concrete-encased CFST box beams was presented.

Seismic behavior of composite shear walls incorporating high-strength materials and CFST boundary elements

An innovative shear wall was proposed, which was composed of high-strength concrete and steel rebars, as well as concrete-encased CFST columns embedded at boundary elements. To study the cyclic and resilient behavior of the proposed wall, four walls with shear span ratios of 2.2 were designed and tested under quasi-static cyclic loads. The test parameters were the type of longitudinal bars at boundary elements, the presence of steel fibers, and the axial compression ratio. As test results showed, within the limit of axial compression ratio, the proposed shear walls had small residual deformation until a 2% loading drift ratio; after that, the hysteresis curve became full and the energy dissipation capacity increased, indicating the desirable collapse resistance. Specifically, the use of ultra-high strength longitudinal bars at boundary elements significantly decreased the residual deformation and improved the reparability. In addition, steel fibers effectively enhanced the deformation capacity of the proposed walls. Finally, given the confined effects of CFST columns and stirrups on concrete, a prediction model for the lateral load-bearing capacity was established. The comparison between predicted and test results from this study and existing research verified the good accuracy of this model.

Behavior and analytical investigation of assembled connection between steel beam and concrete encased CFST column

Concrete encased CFST (CECFST) column is a composite member consisted of outer encased concrete and inner CFST. Currently, a cast-in-place connection between a CECFST column and a reinforcement concrete beam is generally employed in engineering practice due to the outer encased concrete hinders the application of assembled connection, and the steel beam is usually connected to the CECFST column by on-site welding. In this paper, a novel on-site assembled connection between a CECFST column and a steel beam was proposed. In considering various material models and complex contacts, a FE model for this type of joint was established and verified. Further, parametric analyses of the assembled connections to concrete encased CFST columns were conducted to analyze the structural behavior. More importantly, an extended component model was developed to cover the feature of CECFST column focusing on the initial stiffness and moment resistance of the assembled connection. The load transfer mechanism for bolt in tension and beam flange in compression were considered in the modelling for evaluating the stiffness and strength of column wall in tension and in compression, respectively. The numerical results from the parametric analyses coincided well with the predicted results. The numerical and analytical results declared that this novel assembled connection between the CECFST column and steel beam performed favorable structural behavior and could be employed in assembled structures with CECFST columns.

Seismic performance assessment of a precast concrete-encased CFST composite wall with twin steel tube connections

A new type of Precast Concrete-encased high-strength Concrete-filled Steel tube (PCCS) composite shear wall with twin steel tube connections is proposed. Six two-storey PCCS wall specimens were prepared and tested under reversed quasi-static cyclic loading. The axial load ratio, the out-of-plane load eccentricity and the connection of the outer steel tubes are considered in this investigation of the assembly integrity and seismic performance of the PCCS walls. The experimental results show that the PCCS walls exhibit good assembly integrity and satisfactory seismic performance in terms of hysteresis behaviour, ductility, stiffness degradation and energy dissipation capacity. The use of bolts to radially fasten the inner and outer steel tubes can constrain the twin steel tubes. The out-of-plane load eccentricity significantly influences the cyclic behaviour. The specimens with large load eccentricity still have good deformation and ductility capabilities, which can be attributed to the ability of the embedded high-strength concrete-filled steel tube columns to improve the out-of-plane flexural capacity of a PCCS wall. Finally, the PCCS walls are compared with other typical precast concrete walls and cast-in-place concrete-encased concrete-filled steel tube composite walls in terms of ductility capacity, and a calculation model for shear bearing capacity is proposed and evaluated.

Flexural Performance of High-strength Prestressed Concrete-encased Concrete-filled Steel Tube Sections

The sections composed of concrete and steel, which include concrete-encased concrete-filled tubes, generally have defects due to the low tensile strength of concrete. Therefore, an appropriate method was used for the combination of concrete-filled tubes (CFT) and prestressing strands which is encased in concrete. The conventional design guidelines are commonly developed for materials with normal strength thus further investigation is required to be conducted for sections with high-strength materials. In order to develop the design process, high-strength concrete and steel have been utilized in this study to examine the effects of steel and concrete strengths on the core concrete confinement, sectional size and flexural behavior of high-strength prestressed concrete-encased CFST (HS-PCE-CFST) beams. Hence, a total of thirteen HS-PCE-CFST beams were modeled via ABAQUS finite element software. The main variables include the steel tube yield strength, compressive cylinder strength of the core and outer concrete and the steel tube diameter to section width ratio. Furthermore, experimental results were employed to verify the finite element model. The bending moment, ductility, flexural stiffness and failure mode of beams are also examined. The results confirm that among the compressive strength of the outer and core concrete and the steel tube yield strength, change in the outer concrete compressive strength has a greater effect on the change of flexural parameters, also increasing the ratio of steel tube diameter to section width causes a minor increase in the ultimate bending moment and serviceability level flexural stiffness, but a major escalation in the initial flexural stiffness.

Structural behaviour of concrete-encased CFST box stub columns under axial compression

The axial performance of a type of concrete-encased CFST box stub column, which consists of the outer reinforced concrete (RC) box column and six embedded CFST components, is investigated in this study. A total of eight axial compression tests on the concrete-encased CSFT box specimens under axial compression were carried out. A finite element analysis (FEA) model was developed to analyze the structural behavior of the composite columns. The effects of material nonlinearity and interaction properties between the concrete and the steel tubes on the simulation were considered. The verified FEA model was then used to conduct a full-range analysis on the load-deformation responses. The results of typical failure modes, internal load and stress distributions and the contact stresses between the concrete and the steel tubes were also discussed. Parametric studies were conducted to investigate the effects of geometric and material properties on the compressive behavior of the concrete-encased CSFT box members. Finally, a simplified model was proposed to predict the ultimate strength and the initial stiffness of the concrete-encased CFST box stub columns under axial compression.

Long-term experimental behavior of concrete-encased CFST with preload on the inner CFST

Concrete-encased concrete-filled steel tube members (concrete-encased CFST) are innovative composite structural elements which are having gradually increasing utilization in high-rise buildings and bridge structures. In practice, the inner CFST component is generally erected first and thus subjected to constructional load before the outer reinforced concrete (RC) component is formed. Afterward, the whole composite cross-section is under long-term sustained load in the service stage. This paper studies the structural behaviour of such composite columns under combined preload and long-term sustained load through a series of experimental tests. Concrete-encased CFST columns under different loading conditions, together with their CFST counterparts, are tested and compared. The main parameters include preload ratio, loading age and sectional configuration. Influence of these parameters on the structural behaviour in terms of failure mode, bearing capacity, stiffness and ductility are evaluated. Finally, a simplified design method is recommended to account for the influences of combined preload and long-term sustained load on the ultimate strength of the concrete-encased CFST columns, where the combined effects can be reflected by a corresponding set of coefficients.

Modelling the behaviour of concrete-encased concrete-filled steel tube (CFST) columns subjected to full-range fire

This paper presents an advanced finite element analysis (FEA) model to predict the fire behaviour of concrete-encased concrete-filled steel tube (CSFT) columns. The sequentially coupled thermal-stress analysis method provided in the software package ABAQUS was used. Full-range fire simulation given in this paper included four phases, i.e. loading (to a certain load level) at ambient temperature, standard fire exposure (heating) with the load applied, cooling down phase and postfire loading up to final failure. Calibrated empirical models for steel tube, unconfined and confined concrete (confined by steel tube and by reinforcement, respectively) over the four phases were chosen separately and applied to the FEA model. Numerical results from the model were compared with previously reported results of the experiments on the concrete-encased CFST columns, in terms of ultimate strength at ambient temperature, temperature field, failure modes, fire resistance, axial deformation versus time relationships, load versus axial deformation relationships and postfire residual strength. It is found that acceptable agreement was reached between the predictions and experimental observations, although some aspects could be further improved. Sensitivity analyses on several identified parameters of the modelling were conducted. Recommendations on simulating the full range fire behaviour of concrete-encased CFST columns were proposed based on the numerical study presented in this paper. In addition, the calculated internal force of the tested specimens was extracted and analysed, which confirmed the design concept of the composite action of concrete-encased CFST columns in fire.

Experimental study on the hysteretic behavior of ECC-encased CFST columns

Concrete-encased concrete-filled steel tube (concrete-encased CFST) columns, which is a conjunction of normal steel reinforced concrete (RC) and CFST columns, have been widely used in structural engineering. However, the outer RC component tends to crush in the early stage due to the significantly different mechanical performance of outer concrete and inner CFST. In this paper, it is proposed to substitute outer concrete with engineered cementitious composite (ECC) to form ECC-encased CFST columns. This paper presents an experimental study on the hysteretic behavior of ECC-encased CFST columns. Eleven specimens, including seven ECC-encased CFST columns and four concrete-encased CFST columns were tested under cyclic loading. According to the test results, ECC-encased CFST columns exhibited stable ductile behavior and the cumulative energy dissipation is about twice as much as that of concrete-encased CFST columns with same geometry. Also, the increase of steel tube diameter and stirrup ratio have positive effects on the hysteretic behavior of ECC- encased CFST columns.

Concrete-encased CFST columns under combined compression and torsion: Analytical behaviour

This paper presents the analytical behaviour of concrete-encased concrete filled steel tubular members (concrete-encased CFST members for short) under the combined effects of compression and torsion, which is a typical loading condition for structural members such as bridge piers under earthquake. A finite element model (FEM) is established to account for the complex material nonlinearity and interaction, accuracy of which is verified by a set of test data. Full range analysis of the composite members under combined compression and torsion is then presented. Typical failure modes are investigated, whilst the behaviour of RC and CFST components in the composite members are compared with those of individual RC members and CFST members under the same loading condition. Parametric analysis is carried out as well to evaluate the influence of significant factors, including the material strengths, arrangement of rebars, steel ratio of inner CFST component and CFST ratio. Torsion-compression relations of concrete-encased CFST are investigated. A simplified calculation method is validated using the simulation results in order to predict the torsional capacity of axially loaded concrete-encased CFST.

Concrete-encased CFST columns under combined compression and torsion: Experimental investigation

Concrete-encased concrete filled steel tube (concrete-encased CFST) columns have been increasingly used in high-rise buildings and bridges in China in recent years. In practice, these composite columns may be subjected to combined compression and torsion under earthquake loads when they are adopted as columns in high-rise buildings or piers of viaducts. The corresponding behaviour of such composite columns thus needs to be thoroughly investigated to fill this knowledge gap. Results of a series of tests on concrete-encased CFST columns under combined compression and torsion are presented in this paper. A total of 26 tests were conducted on three types of specimens, i.e., concrete-encased CFST columns, hollow reinforced concrete (RC) columns without inner CFST components and conventional CFST columns. Based on the test results, effects of cross-sectional shape, axial load level and cross-sectional area of the inner CFST on the failure mode and the torque-rotation angle relation are investigated. Design formulae are then proposed which can reasonably predict the torsional strength of concrete-encased CFST columns.