Behavior of concrete-encased concrete-filled steel tube columns under diverse loading conditions: A review of current trends and future prospects

Abstract The concept of concrete-encased concrete-filled steel tube (CE-CFST) columns was conceived during the quest to enhance the safety of structures in high seismic regions. Over the past two decades, researchers have extensively investigated the performance of CE-CFST columns under a range of loading conditions using experimental, analytical, and numerical methods. Previous research has shown that integrating outer RC and encasing steel tubes into concrete-encased concrete-filled steel tube columns boosts their structural performance under a wide diversity of loading conditions. This paper aims to provide a comprehensive analysis of various models proposed by researchers in different regions and summarize all results that have been breakthroughs in the last decade. Researchers revealed that this type of structure only uses less than 85% of its flexural capacity. There are still significant research gaps in depicting these structures’ behavior under diverse loading conditions, both analytical, numerical and experimental. The authors show the highlights of CE-CFST columns’ performance and give a detailed review of the corresponding reinforced concrete (RC) columns and concrete-filled steel tube (CFST) columns. The highlights for further researches on CE-CFST are provided for elaborating other mechanical properties. To sum up, the CE-CFST composite columns provide resilient and sustainability in structure during engineering applications.


Introduction
Concrete-encased concrete-filled steel tube columns are novel composite columns often employed in high earthquake-prone areas notably in high-rise and long-span structures (J.Zhou et al. 2020;D.-Y. Ma et al. 2018).These composite columns are composed of internal part called concrete-filled steel tubular components encircled by reinforced concrete (RC) part.Modern structures are widely constructed using a hybrid of steel and reinforced concrete materials to form composite materials owing to their excellent characteristics.Since 1995, CE-CFST columns have been used in China to boost the sustainability of steel-reinforced concrete composite structures (L.H. Han, Li, and Bjorhovde 2014).Compared CE-CFST column to conventional reinforced concrete columns offer superior stiffness, strength, durability, fire resistant and other mechanical performance owing to its structural components (Cai et al., 2020a).CE-CFST columns can be made with cross-sectional shapes in square RC-circular CFST, square RC-square CFST, and RC-CFST sections that are circular, as depicted in Figure Figure 1.To construct most of China's high-rise structures, the square RCcircular CFST part type are often used.The utilization of columns with square outer RC and circular steel tube to construct beam-column junctions has been shown an improvement in structural integrity of buildings (Nguyen et al., 2019;Morino, 2003;Guo et al. 2020).While, the circular RCcircular CFST kind of section is typically considered as a superior option for bridge piers due to the circular stirrups in the circular RC, which may give more confinement to the whole section, resulting in greater strength and ductility.
Worldwide engineers strive to make and keep up with civil infrastructure systems that can withstand natural disasters and climate change (Alatshan et al., 2020;Ban & Shi, 2018;Gunawardena et al., 2019).The recommended approaches towards meeting this goal are to utilize the materials and design structures with sustainable structural and functional behavior to sustain the construction loads during their lifecycle (L.H. Han and An 2014;W. Xu, Han, and Li 2016).The codes such as the Chinese code GB50936-2014, the European code EC 4, the American code AISC 360-16, the Australian code AS/NZS 2327, the Canadian code CAN/CSA S16-01, and the Japanese code AIJ 2008 have been used to create reliable codification guides for predicting the strength and performance of CE-CFST columns under various load conditions (Wang et al., 2022).A review of existing literatures (J.Wang et al. 2021;Hou et al., 2019;L. H. Han, Hou, and Xu 2018) illustrated that mechanical features and durability of CE-CFST columns have received substantial attention.Nevertheless, there are limited studies emphasizing the relationship between CE-CFST columns and steel beams and its construction is more complex than the regular RC columns.Incorporating steel and reinforced concrete materials result in a structure with remarkable mechanical features, most notably a high load-carrying capacity and a high level of earthquake-resistant (K.Zhou and Han 2020;Wang et al., 2020a;K. Zhou and Han 2018b).The CE-CFST columns have another significant advantage, which has shown that not only does the restraint of RC delay the local buckling of the steel materials, but that the compressive strength of concrete is also strengthened by the confining effect of steel materials (Anas et al., 2022a;Feng et al., 2021).

Figure 1. Most common used cross-sections of CE-CFST columns.
As a result of the growing interest in CE-CFST columns researches in recent years, the construction sectors have seen tremendous growth (K.Wang, Zhu, Yang, Yan, Xu, et al. 2021;J. Zhou et al. 2020;Rahmani et al., 2019).Some earthquake-prone regions of China, the box section CE-CFST columns were already utilized in conjunction with the CFST columns (Ellobody & Young, 2011).The utilization of CE-CFST columns in diverse structures has come up with a noteworthy solution.To enhance the functionality CE-CFST column, the outer reinforced concrete material has been replaced by of Engineered Cementitious Composite (ECC) to form ECC-encased CFST columns, its performance under axial compression load was carried out by (An, Han, and Roeder 2014;Feng et al., 2021).Due to the outer engineered cementitious composite material, columns possess better durability than the CFST and RC member (An, Han, and Roeder 2014;Qian et al., 2016).When CE-CFST box columns are utilized in bridge arches or piers, they are often loaded under axial compression or a combination of axial compression and bending moment, depending on the application (Cai, Pan, and Lu 2018;JChen et al., 2019).CE-CFST columns provide a maximum loadcarrying capacity and ductility owing to the embedded steel tube.Moreover, they have shown that reinforced concrete provides a constraint from the outside that prevents the steel tube from buckling outward, thus enhancing corrosion and fire resistance (Cai, Pan, and Li 2018).Further findings obtained after fire exposure also showed the gap in this research area of the bond performance between the interior CFST and exterior RC (J.Zhou, Li, and Guo 2020).
Researchers and engineers are concerned about the safety of structural components attributable to Russia's invasion of Ukraine (Anas et al., 2021).To ascertain the composite columns' loadcarrying mechanisms under high intensive loadings that may lead to inordinate or progressive failure of the structures, Anas et al. (2022b) investigated the behaviour of composite structural members concrete-filled double-skin steel tubes and concrete-filled double steel tubular columns subjected to impulsive loadings.Several researchers noticed the wide use of CE-CFST structures from the practical application as CE-CFST columns, CE-CFST box arch ribs, as depicted in Figure 2 (2) and CE-CFST piers as shown in Figure 2 (2).Furthermore, the Conceptual response of CE-CFST columns exposed to cyclic lateral force was performed (Qian et al., 2016;Anas et al. 2022).Structural characteristics of CE-CFST box stub columns under axial compression were conducted (Chen et al., 2019).The present study aims to offer an overview of the performance of CE-CFST columns subjected to various loading scenarios.Design ideas for practical applications and better code coverage are also highlighted in this evaluation.Numerous scholars have extensively examined the performance of CE-CFST structures, such as bond efficiency, static loading, cyclic loading, and prediction model and confinement effects.This study highlighted a wide range of research gaps in its behavior performance worthy of further investigation.

Bond-behavior between reinforced concrete and steel tube
According to most studies on the mechanical features of CE-CFST columns, the bond's strength and structural integrity are the most important factors to consider when predicting the behavior of steel-concrete columns because of their contribution to the ultimate strength of columns (Li et al., 2020b).Contact behavior between steel and reinforced concrete should be intelligently evaluated to ensure the service condition, carrying capacity, and reliability (X.H. Guo et al. 2020).Another substantial benefit of CE-CFST columns is evidence that the restraining of RC prevents local buckling of steel materials, and thus the confining effect of the steel materials boosts the compressive strength of concrete (Feng et al., 2021).Numerical modeling of concrete and steel interaction in CE-CFST column is considered in two ways based on the structure of CE-CFST column (Han et al., 2016).According to L. H. Han and An (2014), the outer concrete is confined by a stirrup and the core concrete is confined by its steel tube as depicted in Figure Figure 3 that there is interaction stress between steel tube-core concrete and outer concrete-steel tube.Based on the above analysis, the interaction between CE-CFST compression components is investigated, where P 1 is the steel tube-core concrete interaction stress, P 2 is the steel tube-outer concrete interaction stress, and P 3 is the stirrup's confined stress on the outer concrete.
By referring to the recent study, the Mohr-Coulomb friction model and the hard contact methods adopted by Menezes et al. (2019) during an analysis of the mechanical properties of the CE-CFST column in the tangential and normal directions for meshes discretization.Ponsubbiah et al. (2020) studied the bond interference of steel tubes and concrete.A similar study reported by Wu et al. (2020) to ensure the co-working of two materials push-out test can reveal the bond behavior.The influence of load transfer among these materials indicated that the mechanical features of the composite member were predominantly monitored by the bond strength between the two components (Han et al., 2016).The CE-CFST members under combined compression and torsion were analyzed by Li et al. (2018).A verified FEM explores CE-CFST's fullrange behavior and simplified equations are introduced to validate torsional capacity of CE-CFST.From Figure Figure 4, it can be seen that the interaction stress between the outermost RC and innermost CFST in a square concrete-encased CFST can be based on finite element analysis.FEM reveals interaction stress at the CFST's extremities but none in the core.The boundary condition creates a distinct end-to-center stress distribution, while endplates limit RC and CFST torsion (M.Xu et al. 2018).

CE-CFST columns to structural members' connection
The effectiveness of CE-CFST columns to beam connection should be assessed to ensure joint and load transfer ductility capacity.Many researchers conducted an experimental and analytical study on CE-CFST columns to beam joint connection (Pawar et al., 2019;Şermet et al., 2021;J. Li et al. 2018).Numerical model to steel beam column 3-D joint connection to establish its seismic performance has been introduced by Li et al. (2021).There were several issues to consider assessing the performance, such as non-linearity of materials, interactions between various components, confinement effect, and damage (Zhao et al., 2019), the results revealed that the embedded CFST part strengthens reinforcement to the panel zone of the connection joint.The performance of CE-CFST column and beam joint connection in fire and after fire exposure has been presented by Alam & Anas (2021) and Zhou & Han (2018a).The findings revealed two kinds of failure: failure during fire exposure and post-fire loading.This joint could retain high post-fire residual strength without extra fire protection.It can be seen from Figure 5(a) that the numerous study focused on connections and beam-tocolumn sub-assembly phases of composite structures (K.Zhou and Han 2020).To ensure the robustness of the structural system and fast construction in different connection detail, Feng et al. (2021) and Mou et al. (2021) evaluated the earthquake resistance of CE-CFST columns to beam connection as depicted in Figure 5(b).The transmission mechanism of vertical load through the shaped beam to large dimensional CFST columns has been assessed by B. Li et al. (2021).As shown in Figure 5(c), on-site application of CE-CFST columns to beam joint has been elaborated.The mechanical performance and design of CE-CFST column linked to partially encased composite beam has been explored by Wang et al. (2021).The research used the bolting technique for column beam connecting as depicted in Figure 5(d) which illustrate 2D connection joint between CE-CFST column and beam.Khateeb et al. (2020) attempted a novel experimental approach to examine the effectiveness of new CFST circular column-to-foundation connections subjected to cyclic loading.The results revealed that the approach has significant flexibility to sustain inelastic deformation cycles subjected to moderate extreme seismic loading.

CE-CFST column base
Column base, also known as the column-to-foundation connection, is the most crucial structural component to consider during the design of the CE-CFST structure, and it must be strong and stiff enough to transfer load from the superstructure.Stiffness and strength are critical properties for column bases when using CFST columns (L.H. Han, Hou, and Xu 2018).There are three main kinds of column bases that have been explored recently: a base plate, an embedded, and a concreteencased column base.Based on the analytical study (Wang et al., 2020b), the findings on the seismic performance of a hexagonal CE-CFST column base indicated that the outer RC components and the axial load level possess a substantial impact on the seismic efficiency of the column base.CE-CFST column base are moderately complex to be used in construction than other types of column bases (W.Xu, Han, and Li 2016).As a result, some performance behavior of composite columns has been explored to date to assess the uniaxial compression performance of rectangular CFST columns with various internal construction features (Hongying Dong et al., 2018;Lee, Park, and Choi 2019).With the help of finite element analysis, the hexagonal CE-CFST column base's earthquake resistance was attempted to be studied (L.H. Han, Hou, and Xu 2018).The bases for these columns, which serve as the link between the superstructure and the foundation, were crucial in establishing how adequately the bases for these composite columns performed.Finally, findings revealed that CE-CFST column base possesses a high degree of stiffness and strength.

Behavior under axial compression loading
To further strengthen the interaction between RC and steel tubes when exposed to ultimate compressive stresses, horizontal tie bars are welded to the inner surface of steel tubes.It also enhances mechanical performance by placing steel reinforcement cages inside CFST columns.This increases the columns' maximum bearing capacity and ductility (Hongying Dong et al., 2018).The peak loads, together with load eccentricity ratios of 0.5 and 1.5, were 19.3% and 74.43% greater than the corresponding RC columns (An, Han, and Zhao 2013).The results obtained by utilizing FEM revealed that its ultimate strength and stiffness are more crucial than that of corresponding RC box or CFST built-up column.Material and stability failure are the two failure modes observed (An, Han, and Zhao 2014).As depicted in Table 1, compression loading of a CE-CFST column were assessed by various researchers to further facilitate understanding on the performance of CE-CFST columns through experimentally and analytically studies.An et al. (2015) examined the CE-CFST box stub columns under axial compression.The structural behavior of columns under axial compression has been investigated (Chen et al., 2019).FEM simulations were used to evaluate the interaction characteristics of RC and steel tubing, which were taken into account.The ultimate strain of the columns ranged from 0.02 to 0.0025.The findings indicated that longitudinal stiffeners positioned inside CFST columns effectively distribute axial forces and boost the structure's stiffness, bearing capacity, and ductility.
Meanwhile, the axial compression of circular CE-CFST stub columns was studied experimentally and analytically (Y.J. Li et al. 2016).Inner CFST enhances composite column ductility over RC columns.As the ratio of steel tube diameter to CE-CFST tube diameter was raised from 0.32 to 0.44 and 0.52, ultimate compressive strength rose by 34% and 124%, respectively.External concrete reached its maximum strength, whereas exterior RC and interior CFST loads are 97% and 96.3% of their ultimate strengths.The behavior of CE-CFST stub columns under axial compression using FEA was conducted by Han et al. (2014).As the whole diameter of the steel tube of CFST to the sectional width of CE-CFST section increment, a lower strength difference was obtained among core CFST and CFST column.The numerical study analysis on the square CE-CFST stub columns after fire exposure has been conducted (Xiang, Pan, and Wang 2018).After exposing this column to fire, the parametric study revealed that axially compressive strength and axial stiffness significantly decreased the decrement caused by fire exposure duration and sectional core area.It is also noted that even though Lee et al. (2019) The composite columns with high-strength circular steel tubes were exposed to eccentric and concentric axial compression loads to determine their stiffness, strength, ductility, and failure modes.Due to the high-strength circular steel tube and confined infill concrete's better resistance, less ductility was observed even after concrete crushing.Herein, L.-H.Han, Ma, and Zhou (2018) described that each component of composite materials has its function to fulfill in the structural and functional performance of the structures.

Behavior under tensional loading
In some scenarios, tension and flexural members are sometimes needed under tensile or during wind and seismic loading.Wang et al. ( 2019) Developed an experimental and numerical study on the load transfer mechanism of CFSTs with encased built-up angles under concentric tensional

Intensity parameters Major observation/ conclusion
Ref.

Compression loading
To perform experimental and analytical study on the circular CFST stub columns subjected to axial compression.
The ratio of the steel tube diameter to the column diameter and the concrete strength.
Due to the interior CFST, CE-CFST columns are somewhat more ductile than RC columns.(Li et al. 2016) To conduct numerical study analysis of square CE-CFST stub columns after fire exposure.
Steel tube area ratio, longitudinal bar ratio, compressive strength of concrete, yield strength of steel tube.
Axially compressive strength and axial stiffness decrease significantly (Xiang et al., 2018) Tensional loading To conduct study on Loadtransfer mechanism of CFST with encased built-up angles under concentric tension.
The diameter-to-thickness ratio, connecting type, and configuration of encasedangles.
The numerical research demonstrates that the encased-angles' strength could be used at about 85% of its yield strength when the critical area was yield.(Xu et al. 2019) Behaviour and design of demountable CFST columncolumn connections under tension.
The CFST reduction ratio was lower than the reference hollow steel tubes, implying higher tensile member durability.(Li et al. 2018a) loading.The capacity of the encased-angles could be used for almost 85% of their yield strength when the critical zone was yielded.Rahmani et al. (2019) carried out the flexural behavior of highstrength prestressed CE-CFST sections to combine steel tubes and prestressed strands.The experimental investigation on the behavior and design of demountable CFST column-column joints subjected to the tension was carried out.This research aimed to analyze the tensile resistance of the nonlinear FEM with the aforementioned joints by establishing the failure mode and analyzing various parameters to improve its workability (D.Li et al. 2018).The results demonstrated that the ultimate tensile strength of the joint was adequate for its intended purpose.In addition, it is suitable for use on-site during construction.According to this study, demountable connections for CFST columns fulfill robustness requirements.
For instance, Han et al. ( 2017) carried out experimental and theoretical studies on circular CFST under a hybrid tensional loading and chloride corrosion.Notably, J. Chen, Wang, and Jin (2017) studied the behavior of CFSTs with reinforcing bars or angles subjected to axial tensional loading.The results revealed that the bolt and flange joints between angles had no influence on the tensile strength of the steel tube, but its impact has been observed on elastic tensile stiffness.An et al. ( 2014) experimentally examined the flexural performance of CE-CFST specimens.Thin-walled steel tubes have been utilized in CE-CFSTs to get to full plastic strength with no local buckling before they attain the final state flexural capacity.Chen et al. (2020) experimentally and analytically approaches have been used to examine the flexural performance of CE-CFST box specimens.It was revealed that there is a uniform distribution of the flexural failure mechanism and the flexural cracks in the pure bending zone.It was found that the RC parts load the load-bearing capacity when the CFST parts reached 82% of their ultimate strength at ultimate curvature.

Behavior under cyclic loading
Numerous researches have been carried out to evaluate the mechanical behavior of CE-CFST columns subjected cyclic loading.An experimental study was carried out to better understand the hysteretic performance of prestressed composite joints with CE-CFST Columns (K.Wang & Luo, 2019).The results possessed various kinds of mixed-mode failure such as shear failure, flexural failure at panel zone, and beam ends.However, to explore an experiment on hexagonal CE-CFST columns subjected to an axial compression loading and a cyclic bending moment (D.Y. Ma, Han, Ji, et al. 2018).Table 2 shows a CE-CFST column under cyclic loading.According to (Nasery, Hüsem, Okur, and Altunişik 2020a;Nasery, Hüsem, Okur, and Altunişik 2020b), modal parameters before and after damage were retrieved using frequency and time domain techniques with quasi-static cyclic testing.With damage, natural frequencies lessen.Pre-and post-cycling load test findings indicated that no mode shape conformity.And, 26.57% and 36.54%have the most divergent natural frequencies.Based on these studies (Hüsem et al., 2018;Nasery, Hüsem, Okur, Altunışık, et al. 2020), ambient vibration tests were used to extract natural frequency, mode shape, and damping ratio from the dynamic features of column-beam connection joints in concrete-encased composite structures.Numerical and experimental results agree well, and enhanced frequency domain decomposition is used in experiments to obtain modal parameters (F.Fei-yu Liao et al., 2014).
Therefore, Fei-yu Liao et al. (2014) highlighted the effects of various factors on the joint's seismic behavior.The comparable damping coefficients of composite connection with RC beams and steel beams are 0.09-0.126and 0.13-0.212,respectively.The seismic behavior of CE-CFST column joints is superior to that of RC joints, making them more suitable in the seismic zone (Cai et al., 2018a).The functionality of CE-CFST columns exposed to cyclic lateral loading has been assessed by Qian et al. (2016).The curvature ductility coefficient of CE-CFST column seemed to be 15% higher than that of reinforced concrete columns, inferring that they outperformed under cyclic loading.Unfortunately, for the non-prestressed joint specimen obtained only flexural failure at beam ends (Yuan et al., 2018).When prestressing levels rose by 28%, shear deformations in panel zones at yielding and peak loads dropped by 22% and 43%.When axial compressive ratios rose almost two times, shear deflection dropped by 25% and 51%.
Moreover, a study of composite frames with CE-CFST columns based on numerical models of the specimens were used to ascertain the whole hysteretic process (Wang et al., 2017).For a computational analysis study of nonlinear analysis of composite frames made of rectangular CFST beam-columns and steel framing under static and dynamic force has been conducted by Tort et al. (2010).The experimental together with computational findings were well correlated.This mixed FEM offers effective approach for monitoring detailed local constitutive reactions in terms of evaluating whole 3D structures.CE-CFST box members are employed in arches and piers under axial compression or a combination of axial compression with bending.Due to their superior bending strength and stiffness, bridge girders are seldom exposed to pure bending.Furthermore, Ma et al. ( 2018) Conducted an experimental test on hexagonal CE-CFST columns subjected to cyclic loading.The failure mechanisms and bending behavior of box members were examined (Han et al., 2015); during the analysis of the specimen, the variation of steel tube diameter from 0.0749 to 0.1022 m and height from 0.84 to 1.26 m was taken into consideration, the result obtained on the performance was compared with that of RC box members.Nonlinear FEA for hysteretic performance of ECC-encased CFST columns has been conducted (Cai et al., 2020).Moreover, the ECC component boosted the composite column's post-peak ductility and energy dissipation capability.

Behavior under combined compression and torsion
Construction industry has recently incorporated CE-CFST columns because of their small crosssections and can typically utilized in high-rise or long-span piers.CE-CFST columns under hybrid compression and torsion force are incompletely defined.The CE-CFST columns' compression and torsion performance was evaluated (Qing Xin Ren et al., 2017).The specimens tested under these conditions showed the typical failure mechanism of diagonal cracks in the exterior concrete The joint stiffness, strength, ductility and energy dissipation capacity.
The connection details significantly affected the joint strength, stiffness, energy dissipation capacity and ductility (Li et al., 2020b) oriented 45 degree to the member axis.The amount of axial load also influenced stiffness and strength.Even though less than a 30% reduction in the CFST ratio for both column cross-sections diminished the rigidity index and strength index.These two indices rise dramatically when compared to CFST or RC specimens.As illustrated in Table 3, compression and torsional loads were incorporated in the analysis of the CE-CFST members.With the help of analytical and parametric studies conducted by Han et al. (2018).The torsion moment versus rotational angle curve and failure mode was in good agreement by comparing the results of the FEM simulations with the experimental results.To assess the performance of CE-CFST columns subjected to combined compression and bending have been reported (An and Han 2014).FEA model parameters were utilized for parametric analysis.The results indicated that composite columns may be damaged by external concrete failure or instability collapse.The failure pattern of columns with a slenderness ratio lesser than 60 may still be considered an exterior concrete failure mode.

Behavior subjected under long-term sustained loading
In recent years, CE-CFST columns attracted engineers attention in tall building and long-span structures.Based on an experimental study (Li et al. 2019), the efficiency of the columns subjected to combined preload and long-term sustained force was studied.The influence of various parameters was considered: loading age, sectional CFST ratio, and preload ratio.When preload and long-term sustained load is added together, the capacity of the specimen is reduced by 8.7%.After realizing that the CE-CFST column's service life is often subjected to long-term sustained loading (D.Y. Ma, Han, Li, et al. 2018), the experimental and analytical behavior of CE-CFST stub columns subjected to long-term sustained loading compared its performance with that of corresponding CFST and RC counterpart.The influence of various parameters was taken into consideration during this study.After long-term loading, the column's strength decreased by 12.4%.The initial axial load sustained by the outer and interior concrete decreased by 23.5% and 5.6%, respectively, during long-term loading.CE-CFST creep deformation was greater than CFST and less than RC.Experimental and FEA results possess good agreement to track the behavior of CE-CFST columns.FEA model and time-dependent were utilized to assess the behavior of columns under long-term sustainable loading (D.Ma, Han, Li, et al. 2018).

Behavior under seismic loading
To maintain good strength and appropriate earthquake response, the study conducted by Chen et al. ( 2014) looked into how a new through-beam connection among the CFST column and the RC worked during and after a big earthquake.The results showed that the joints exhibited considerable axial compressive capacity and ductility.For clarity, the seismic behavior of joints consisting of a CE-CFST column and an RC beam with a steel ring was investigated using numerical modeling

Intensity parameters Major observation/ conclusion
Ref.

Conducted experimental investigation CE-CFST columns under combined compression and torsion.
The test parameters included the cross sectional shape, the axial load level, the crosssectional area of the inner CFST and the specimen type.
The experimental results behaved in a ductile manner owing to the existence of the inner CFST component.
(Qing Xin Ren et al., 2017) Conducted analytical behaviour on CE-CFST columns under combined compression and torsion.
The material strengths, arrangement of rebars, steel ratio of inner CFST component and CFST ratio.
CFST ratio was found to have the most crucial effect on the functionality of CE-CFST under combined compression and torsion.(Li et al. 2018c) and experimental analysis (Ponsubbiah et al., 2020).In earthquakes, columns with a square casing and a circular steel ring with three-ring layers and one concentric ring operate well and give the optimum confinement effect.Exposed CFST column bases with embedded steel are being looked at to see how they react to an earthquake (Qiao et al., 2019).The findings revealed that the new column bases' strength, stiffness, and the energy loss of embedded steel rebar are significantly larger than typical exposed CFST column bases.In addition, Due to the variation in base plate forms, the seismic response of column bases varies considerably.In contrast, Hou et al. (2019) studied CE-CFST box section beams and columns subjected to lateral impact.Impact energy altered the impact period and residual mid-span deformation of the beam-column section, and specimens failed under a shear flexural pattern.The CE-CFST column's RC component resists impact loading and restricts the CFST components from severe damage.
The study on earthquake resistance behavior of a precast CE-CFST composite wall with twin steel tube joints is assessed in Zhou et al. (2020).Precast CE-CFST walls with two identical steel tube joints over existing precast concrete shear walls had great assembly reliability and energy loss capacity equivalent to cast-in-place walls.At the same time, the seismic behavior of CE-CFST column to steel beam connections with distinct joint details was investigated (W.Li, Xu, and Qian 2020).Herein, Xu et al.'s (2016) research work on the seismic behavior of hexagonal CFST Shear studs facilitates the column's base ductility coefficient rose from 4.95 to 7.03, as its elastic stiffness and ultimate strength improved by 2.6% and 8.9%, respectively.When the axial load stage raised from 0.28 to 0.56, the elastic stiffness and the maximum strength were raised by 19.3% and 13.5%, respectively.High-rise buildings and members of CFST construction were tested for their ability to withstand earthquakes (L.Han and Li 2014).Numerous studies have been conducted to evaluate the performance of CE-CFST columns as illustrated in Table 3, an analysis of earthquake hazards in spatial frames made of steel beams coupled to L-shaped CFST columns (Zhang et al., 2018).This composite frame failed the strong-column-weak-beam.With the help of five specimens, diameter-to-thickness ratios and internal reinforcement over time have been investigated.The diameter-to-thickness ratio significantly influenced local buckling but did not affect the eventual limit condition defined by tube fracture (Brown et al., 2015).

Behavior under fire
It is crucial to perform a comprehensive analysis of the structural behavior to fire exposure (Zhou and Han, 2018b).Experimental and numerical studies explored CE-CFST columns carrying fullrange firing.Heat transfer in the CE-CFST column was modeled using ABAQUS 2D-FEA.Due to the exterior reinforced concrete's insulating effect, the columns have better fire resistance and postfire residual capacity than CFST columns (Raju & Sasikumar, 2020;Romero et al., 2020).An experimental and computational investigation on the fire resistance of steel-concrete composite structures was undertaken.The latest research in this area and the advancement of the design guidelines were reported.Advanced materials were examined for improving the fire behavior of CFST columns and slim-floor beams (On, Columns, and Fire 2014;Zhou & Han, 2016a;S. Hou, Han, and Song).The earthquake-resistant behavior of CFST columns exposed to fire (Li et al., 2020b).Two essential factors to consider are the coupling effect of fire exposure and sustained axial loads.A validated FEA model was created and assessed in the simulation of CFST columns subjected to fire exposure (Romero et al., 2015).The performance of the CE-CFST column subjected to a fullrange fire has been explored by Zhou et al. (2019).The investigation is comprised of four phases: room temperature loading, standard fire exposure with force, cooling, and post-fire force up to failure.Predicted and experimental results agree well, although several components may be improved.Finally, essential modeling features were sensitively analyzed.It was decided to perform experimentation on the functioning of the columns following exposure to fire, which included both heating and cooling phases (Zhou & Han, 2016b).

Behavior under shear
In the high seismic zone, this kind of composite column is favorable to be used when shear force is subjected to beam-column joint and there is shear capacity reduction (Chen et al., 2015).
Therefore, to resolve this issue by ensuring the continuity of prestressed composite connection linked to CE-CFST columns, Wang et al. (2021) proposed the study to investigate the shear capacity of joints of CE-CFST columns.A proposed formula can be utilized to compute the shear capacity.Deduced outcomes revealed were less than those obtained using FEA, which means that prestressed composite connections with CE-CFST columns are safe and could be used in engineering practices.Analyzed CE-CFST specimens with circular cross-sections in response to impacts at low lateral velocity (Hu, Han, and Hou 2018).Meanwhile, encased CFST in the CE-CFST column brought significant characteristics compared to the corresponding RC.Mainly, focusing on the shear mechanism of the composite column of CE-CFST members subjected to low-velocity impact has been examined (Hou et al. 2021).The results revealed that shear failure dominantly appears on the outer RC components through experimental and analytical studies.In contrast, the failure mode observed on the core CFST was made of the flexural pattern.
More importantly, a novel on-site assembly connection between the steel beam and CE-CFST column was introduced (Wang et al., 2020b).It was revealed that during engineering practice, there is a limited contribution to enhancing the performance behavior of the connection, which was obtained through raising the strength of stirrup and steel reinforcement.The applied load and shear transfer within steel-encased CFST columns with shear connections were investigated through numerical and experimental studies (F.Xu et al. 2021).The effectiveness of a CE-CFST exposed to a low-velocity impact was examined experimentally and numerically.Results demonstrated that, in contrast to the outer RC components, which showed primarily shear failure, the core CFST exhibited deformations in a flexural pattern.During this investigation, the effect of the examined factors on the impact energy and middle span deformations of the specimens was taken into account (C.C. Hou et al. 2021;Anas, Alam, et al., 2022).Because of its considerable shear resistance, the CFST section might substantially attenuate the shear failure of the outer RC section upon impact.At the same time, the exterior RC sections protect the CFST section and reduce its flexural deflection.This composite effect makes the CE-CFST structure favorable to resist impact loading.

Engineered cementitious composite and fiber-reinforced polymer
To date, engineered cementitious composite (ECC) has become popular owing to its distinct character of numerous cracking and strain hardening (Cai et al., 2020b).Substitution of the outer reinforced concrete in CE-CFST column with ECC produces ECC-encased CFST columns, According to micromechanics and fracture mechanics theories, ECC materials are developed (Cai et al., 2018b).Engineered cementitious composite materials are fiber-reinforced cementitious materials capable of withstanding severe tensile strains and exceptional crack-control capabilities, by referring to Figure Figure 6.While using ECC in concrete, the tensile strain capacity is between 2% and 7%, where the crack width is still controlled below 60 μm.Cai et al. (2018) attempted to reveal the physical and mechanical properties of ECC-encased CFST columns exposed to eccentric loading to enhance the durability and performance under fire.Cai et al (2018) conducted an experimental investigation to determine the characteristics of ECC-encased CFST columns subjected to axial loading.It was revealed that the ECC-encased CFST column have remarkable composite effects and consistent deformation performance among materials used (Cai et al., 2020a).It was analyzed by considering different constitutive models when studying ECC-encased CFST columns (Cai, Pan, and Li 2018).However, the research did not investigate its failure mechanisms.Thus, the eccentric ratio has a substantial impact on the failure process of the column.
Engineered cementitious composite and Fiber-Reinforced Polymer (FRP) were introduced to replace outer RC in order to improve the engineering properties of this composite material (Cai et al., 2018a).Fiber-Reinforced Polymer tube-concrete-encased steel composite columns are innovative material made of FRP, encased steel section, and concrete-filled among materials (Ren et al., 2020).By preventing huge repercussions from the conceivable blast events (Anas, Shariq, Alam, et al., 2022) explored the behaviour of square and circular RC columns with conventional and seismic lateral reinforcements under blast loading and the influence of carbon-fiberreinforced-polymer wrapping on the blast performance of the columns.The strengthened doubleconfined square column with inner circular stirrups performed well against blast loading (Tahzeeb, Alam, and Muddassir 2022).After that, Scholar (2019) conducted eccentric loading and encasement on ECC-encased CFST columns, modeled columns under varied eccentric loads as depicted in Figure Figure 7.The results of the tests revealed that increasing eccentricity affects load-bearing capacity.Comparing the load-carrying capability of CFST columns with ECC to RC, ECC outperformed by 11.39%, 12.7%, and 14.8% for eccentricities of 50 mm, 100 mm, and 150 mm.A study was conducted to examine the hysteretic functionality of ECC-encased CFST columns (Cai et al., 2018a;Cai, Pan, and Lu 2018).It was revealed that columns exhibited good ductility behavior by comparing their cumulative energy dissipation revealed to be twice that of corresponding CE-CFST columns (Bingqing Dong et al., 2021).Furthermore, enhancing ECC-encased CFST columns is possible by raising the steel tube diameter and stirrup ratio (Cai et al., 2020c).Therefore, the application of engineered cementitious composite and fiber reinforced polymer rely on extensive development in the construction industry.

Numerical studies and modeling techniques of CE-CFST columns
In the numerical study of the CE-CFST column, finite element modal analysis of specimens is one of the approaches which is mostly used with the help of various software such as ABAQUS, ANSYS, etc. by considering the material properties and boundary conditions, loading, and connection joints (Hüsem et al., 2018).Most of the material properties considered are modulus of elasticity, grade of steel, Poisson's ratio, compressive strength of concrete and weight per unit volume.The finite element model (FEM) is exhaustively used in structural studies to analyze, design, and predict the performance of CE-CFST columns.This method has been successfully used for CE-CFST (An, Han, and Zhao 2014;J. Zhou, Li, and Guo 2020).Updated FEM is employed with the goal of lessening the estimated uncertainties between numerical and experimental dynamics features.Further comprised of numerical finite element analysis and analytical calculations, non-destructive ambient vibration tests are frequently used to test dynamic without external forces.Apart from the finite element model approach in numerical modal analysis of the performance of CE-CFST column Eigen vectors method can also be used to get natural frequencies and mode shapes (K.Zhou and Han 2019).
Load-carrying capacity of CFST columns are considered to be utmost because of their excellent compression resistance in concrete and tension resistance in steel, CFST columns are deemed effective and reliable due to their ability to dramatically enhance the strength of the structure.Based on experimental investigation conducted (Abramski, 2018) based on practical method of determining loading carrying capacity of CFST column.To deduce the approach the main key parameters was taken into consideration are the column slenderness, the tube thickness, the bond strength between a steel tube and a concrete core.Within considering performance of CFST under various loading condition, based on Li et al. (2020a), coupling effect of fire exposure and sustained axial stress were studied for CFST columns.The results revealed that coupling effects caused analogous residual deformation, residual stress, hysterical curves, and ultimate strengths will lead to fail of structure during and after fire exposure.The load can be applied on CFST column through the concrete core or through the entire cross-section (Li et al., 2020).CFST columns have high strength and ductility behavior, and they can sustain heavy loads with high performance.

Merits of utilizing CE-CFST columns
(i) Under compressional loading, compressive strength of concrete in CE-CFST column is strengthened by the confining effect of steel materials; (ii) Through numerous literature it was revealed that the restraint of RC delay the local buckling of the steel materials in CE-CFST structures; (iii) CE-CFST structures possess high robustness of structural system compare to the corresponding RC or CFST structure; (iv) CE-CFST are considered to be stronger and stiffer enough to transfer load from superstructure to substructure; (v) This composite column can be applied in various structures such as building, bridge, workshops, stations, etc. owing to it has shown excellent performance in lifespan of structure.In this way, the application of CE-CFST columns has attracted the interest of various construction stakeholders.

Demerits of utilizing CE-CFST columns
In reference to previous research, the raised problem is of how the exterior concrete is brittle material and easy to be crushed in the lifecycle of CE-CFST columns has emerged as a significant problem, particularly for the column exposed to harsh environments.To avoid this problem, the outer concrete of CE-CFST was replaced with engineered cementitious composite (ECC) to produce ECC-encased CFST columns.
(ii) When compared with CFST column or RC column, the construction process of CE-CFST columns is more complex in terms of time, materials and skills required.

Conclusions
The majority of the research on CE-CFST columns' implications in engineering practices has risen tremendously.Various studies have been published on CE-CFST columns to examine its performance under diversified loading condition.This study's drawback is that it is based on Englishbased papers.Nonetheless, this review paper illustrated a systematic literature review of CE-CFST columns used for civil engineering structures and has covered a broad range of relevant research publications and findings on CE-CFST columns' behavior under different loading conditions such as static performance, cyclic performance, prediction model and bond performance of CE-CFST columns.The main conclusion can be summarized as follows: (1) Performance under static loads (i) When diameter-to-thickness ratio decreased, CE-CFST columns can better withstand axial loads and have more ductility and by comparing the structural characteristics of CE-CFST columns with the CFST columns revealed the larger stiffness, extra fire-resistant, improved lifespan, and effective joint with RC beam.
(ii) The bonding effect is better for CE-CFST columns.The interaction among materials in the CE-CFST column is first based on the connection between external concrete and steel tube, which influences its complicated functionality.The second is between reinforcement bars and exterior concrete.height-to-width ratio and eccentricity are used to evaluate column behavior.
(2) Performance under cyclic loads (i) Based on the study findings undertaken on CE-CFST frames, it has revealed that they hold up effectively under cyclic loads.The seismic behavior of CE-CFST columns shows better behavior with more than 15% ductility curvature than conventional reinforced concrete columns.Furthermore, it has been demonstrated that the inner steel tube will significantly improve ductility and flexural strength.
(ii) The change in axial compressive deformation during cooling is 36 times more significant than that during heating, indicating that the cooling deformation should be carefully addressed when evaluating the functionality of a CE-CFST column exposed to fire.
(3) Prediction model and bond behavior Due to the fact that CE-CFST column strength is reliant on material strength and their interaction, several parameters such as diameter-to-thickness ratio, slenderness ratio, strength index, and rigidity index may be used to predict load-carrying capacities and performance behavior.Numerous prediction model has been released to predict failure mechanisms for the composite column.For column base, failure mechanisms were detected in different tests, involving failure of the bottom part and collapse of the CFST section; the predictive models used were satisfactory, but they failed to tackle the real-time behavior of the bonds between steel and concrete and their effect to the overall performance of the CE-CFST columns.Numerous authors have established finite element (FE) models to depict the stress-strain relationships, axial load capacity, and buckling of CE-CFST columns.

Recommendations for future prospects
The information presented in this article provides an insight into the research progress made in the domain of CE-CFST column research and can facilitate researchers and practitioners in identifying fundamental influences from authors, publications, institutions, references, and research concerns.A thorough evaluation of these advancements to CE-CFST research will empower various industry professionals to seek further lifelong benefits studies, thereby bridging the gap in understanding the concept of CE-CFST practice in the construction industry.Based on the above discussion, the following recommendation can be derived: (i) To fully comprehend the performance of CE-CFST structures and enable their applications, more studies are required on the behavior of CE-CFST columns, including creep and shrinkage behavior, impact behavior, fire performance, durability, and construction constraints; (ii) Based on current research status, it was revealed that there are still a limited number of studies related to CE-CFST box columns with CFST components inserted together in corners and web walls; (iii) FRP application in CE-CFST columns attracts researchers' attention due to its excellent corrosion protection, high strength, stiffness, and ductility.Unfortunately, based on the research conducted by (Ren et al., 2020), it was identified that more research related to the performance of FRP tube concrete-encased steel columns is still needed; (iv) Nearly majority of the investigations completed recently and discussed here are based on the seismic behavior of columns and member behavior.However, it was shown that there are still a few limited studies on the CE-CFST column connection.Additionally, the FEA model and analytical method may be simply extended to column bases with a variety of confined concrete parametric curves and CFST sections; (v) As a new construction material in engineering field, there is a limitation to the design specifications, codes and guidelines for the CE-CFST columns; (vi) To further facilitate an understanding of the functionality of CE-CFST structures and facilitate their applications, there is still a need for further research by considering creep and shrinkage behavior, impact behavior, fire effectiveness, robustness, and construction constraints; (vii) Future researchers are required to study the effectiveness operation of Composite joints consisting of CE-CFST columns and pre-stressed concrete-encased steel beams; (viii) The various literature review indicates a knowledge gap on the analysis-oriented model of complex mechanical properties of the interaction comprising RC, encased steel, and FRP tubes; You are free to: Share -copy and redistribute the material in any medium or format.Adapt -remix, transform, and build upon the material for any purpose, even commercially.The licensor cannot revoke these freedoms as long as you follow the license terms.
Under the following terms: Attribution -You must give appropriate credit, provide a link to the license, and indicate if changes were made.You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.

No additional restrictions
You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits.

Figure
Figure 3. Interaction model between steel and concrete (Han and An 2014).

Figure
Figure 4. Interaction stress between the outside RC and inner CFST based on FEM (Li, Han, and Hou 2018).

Figure
Figure 5. CE-CFST columns to beam joint; (a) columns to steel beam connection; (b) columns to RC beam connection; (c) Structure constructed using CE-CFST column to RC beam connection; (d) Two dimensional CE-CFST column to beam connection (An, Han, and Zhao 2013).

Figure
Figure 7.Comparison between typical failure mode of CE-CFST columns and ECC-encased CFST column; (a) For CE-CFST column; (b)For ECC-encased CFST column based on the study done byCai et al. (2020).
and dialog with, expert editors and editorial boards • Retention of full copyright of your article • Guaranteed legacy preservation of your article • Discounts and waivers for authors in developing regions Submit your manuscript to a Cogent OA journal at www.CogentOA.com