Concrete-encased Concrete-filled Steel Tube Research Articles

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

The new bolt-welded joint in concrete-encased Concrete-Filled Steel Tube (CFST) without cutting flange demonstrates great mechanical properties and durability, which is also easily constructed at high altitudes or in long-span arch bridges. It is crucial to study the mechanical properties of composite member made of the bolt-welded joint and outer concrete. In this study, a parametric analysis on bearing behavior of bolt-welded joints in concreted-encased CFST under eccentric compression was explored, based on experiments and a validated finite element model. Furthermore, a simplified calculation formula for the eccentric bearing capacity of the bolt-welded joint in concreted-encased CFST was proposed. The results indicated that the bearing capacity increased with the increase of steel tube strength, core concrete strength, reinforcement strength, outer concrete strength, steel ratio, and reinforcement ratio. Increasing the steel ratio and the steel tube diameter-section ratio can enhance the ductility of the member. A rise in stirrup spacing had a detrimental impact on both bearing capacity and ductility. An excessive amount of eccentricity diminished the bearing capacity of the member and alter its failure mode. The proposed simplified calculation formula for eccentric bearing capacity was in strong agreement with both experimental and numerical results.

Experimental study on exterior precast concrete beam-to-CECFST column joints with wet connections under cyclic loading

Concrete-encased concrete-filled steel tube (CECFST) structural system has gained popularity in recent years due to its several advantages over conventional structures. Additionally, precast technology has become an effective way of improving construction efficiency and environmental performance. Therefore, it is essential to investigate the structural behaviour of precast concrete (PC) beam-to-CECFST column joints to ensure their optimal performance in applications. In this regard, this paper presents an experimental study of 4 precast exterior joints subjected to lateral cyclic loading. The study investigated the effect of replacing normal concrete with steel fibre-reinforced concrete at the joint region and the presence of axial load on PC composite joints. Based on the test results, cyclic load-carrying resistance, failure modes and crack patterns of the PC specimens were recorded and analysed. In addition, a Digital Image Correlation (DIC) system was adopted to show the crack evolution of joint specimens and calculate shear strains of concrete encasement at the joint region. Further comparisons were conducted in terms of ductility, energy dissipation, stiffness and strength degradation to investigate the joint performance. Finally, existing American and European design code provisions were modified and used to predict joint shear strength. Their predictions were reasonably close to the test results.

Investigation of axial-flexural behaviour of concrete-encased concrete-filled steel tube slender beam–columns

In recent years, concrete-encased concrete-filled steel tube (CECFST) columns have gained popularity in structural engineering. Research studies have shown that this novel type of composite columns exhibits superior strength, ductility, stiffness, and resistance to fire and corrosion property when compared to alternative types of steel-concrete composite columns. Considering the increasing use of slender columns in practice, this study investigated structural behaviour of CECFST slender beam–columns subjected to combined compression and bending. In this paper, six pin-ended CECFST slender specimens with different section configurations, load eccentricities and slenderness ratios were tested under eccentric load. Key test results consisted of buckling mode analysis, load–deformation history, moment–curvature relationship and strain response at various locations. Results generally showed that P-δ effect deteriorated structural behaviour of CECFST slender beam–columns and this unfavourable effect should be addressed properly in the appropriate design code for composite columns. In this regard, design approaches in European code and American specification were evaluated in aspects of N–M interaction curve, moment magnification factor and effective flexural stiffness (EI)eff. It was identified that both codes tend to overestimate (EI)eff, resulting in an unconservative second-order analysis. Moreover, a parametric analysis was conducted which showed (EI)eff correlates negatively to eccentricity and slenderness, but positively to steel tube size and concrete strength. However, these relationships were not reflected in the predicted (EI)eff according to European code and American specification.

Flexural behavior of square concrete-encased concrete-filled double-skin steel tube members

This study presents a comprehensive investigation involving a series of experimental tests and finite element model (FEM) simulations conducted on a novel composite member subjected to bending, namely a concrete-encased concrete-filled double-skin steel tube (CECFDST) member. Comparative tests of composite performance were first conducted for a total of six specimens, including two CECFDST members, two hollow reinforced concrete (HRC) members, and two concrete-filled double-skin steel tube (CFDST) members. These tests aimed to explore the failure modes and performance of the CECFDST members under bending. The findings revealed that the CECFDST specimens exhibited good stiffness and bearing behavior, showcasing good cooperation between the components. Notably, they achieved a bearing capacity 8.1% higher than the combined value of the HRC and CFDST specimens. Then, an FEM was developed to analyze the flexural performance of CECFDST members, including the stress and constraint mechanisms, and its accuracy was verified. Furthermore, a systematic parametric analysis was carried out using the verified FEM to identify the key parameters influencing the bending performance and the composite effect. Finally, a simplified calculation method was developed to predict the flexural capacity of CECFDST members.

Experimental and numerical investigation on precast CECFST column-to-column connections subjected to axial and eccentric compression

Concrete-encased concrete-filled steel tube (CECFST) is a new type of composite member featuring superior structural performance and high construction productivity. This form of structural members has been increasingly used as columns in high-rise buildings and as piers in long-span bridges. Recently, to promote the application of CECFST columns in countries short of labour, e.g., Singapore, North America and Europe, an advanced precast CECFST column-to-column connection with rapid construction and excellent tensile performance was developed by the authors. However, behaviour of the developed precast connection subjected to axial and eccentric compression has not been studied so far. In this paper, an experimental programme including five connection tests was conducted to investigate compression performance of the developed precast connection. The test results showed that compression resistance of the precast connection could achieve that of CECFST columns, indicating that it is feasible to connect CECFST columns by the newly proposed precast connection to enhance construction productivity. Furthermore, an extensive finite element (FE) modelling programme was carried out to generate more numerical data by conducting parametric studies. The experimental and numerical results showed that connection configuration plays a pivotal role in load-carrying capacity of the precast connection. Based on EN 1994-1-1, one simplified strength prediction model was proposed for the proposed precast CECFST column-to-column connection. Compared with experimental and numerical results, the predicted strength was conservative and consistent, indicating that the proposed approach can be used in the design of the precast connection.

Behaviors of eccentrically loaded ECC-encased CFST columns after fire exposure

This paper intends to investigate the mechanical behaviors of eccentrically loaded engineered cementitious composite (ECC)-encased concrete filled steel tube (CFST) columns after fire exposure. Totally seven ECC-encased CFST columns and one concrete-encased CFST column were designed and tested with the consideration of different parameters, i.e., heating-up time, eccentricity ratio, stirrup spacing and longitudinal reinforcement ratio. It was found that the severe concrete spalling was noticed for concrete-encased CFST column, while the integrity of ECC-encased CFST column was well maintained under identical fire-exposed circumstances. After fire exposure for 45 min, the eccentric load carrying capacity and flexural stiffness of ECC-encased CFST column are 57.8 % and 63.5% higher than that of concrete-encased CFST column, respectively. A theoretical model was proposed to predict the residual eccentric load strength of ECC-encased CFST columns after fire exposure, and the maximum error of the calculation result was within 12 %.

Analysis and calculation method for concrete-encased CFST columns under eccentric compression

This paper presents a computational method for concrete-encased concrete-filled steel tube (CFST) columns under eccentric compression, which adopts the form of equivalent rectangular stress integrated by material constitutive curve. When establishing finite element model via ABAQUS, confinement provided by stirrup was considered to modify the constitutive law of encased concrete, so that the numerical modeling was in good agreement with tests. By analyzing the simulation results, the applicability of basic assumptions, such as plane section assumption, was discussed, and a database of 207 models with various parameters was created. The developed method took into account the influences of geometric nonlinearities of section shape, and its accuracy was verified against various collected tests and the database. It turned out that the formula presented was applicable to a wide range and had sufficient precision, while the predictions of N-M interaction curves of concrete-encased CFST columns in EC4, AISC and CECS were conservative due to the ignorance of the exact integration about irregular section-shape.

Testing, modelling and design of concentrically-loaded concrete-encased concrete-filled steel tube slender column

Concrete-encased concrete-filled steel tube (CECFST), an innovative type of concrete-steel composite structure, has been increasingly employed in mega construction projects due to its outstanding structural performance. However, although intermediate and sometimes slender columns have become commonplace in high-rise buildings and large-scale bridges, the research on member behaviour of CECFST slender columns barely exists in the current literature. In this paper, a comprehensive experimental and numerical modelling programme was conducted to study buckling resistance and structural behaviour of CECFST slender columns. Pin-ended concentric load column tests were carried out on four slender columns, together with a fixed-ended stub column as a reference specimen. Key test results of all specimens, such as failure modes and effects of slenderness on strength, ductility and stiffness, showed that buckling failure and P-δ effect degraded the performance indices of CECFST slender columns. Afterwards, a finite element (FE) modelling programme was performed, where a range of member imperfections was incorporated and validated against test results. Parametric studies were then conducted to supplement the test data bank, from which over sixty FE results were generated. The test and FE failure loads obtained were utilised to examine the applicability of current design methods in the European code, American specification and Australian standard. The buckling curve 'b' set out in the European code was found to result in the most accurate and consistent buckling resistance predictions, while the American specification and Australian standard generally yielded over-conservative and unsafe results, respectively.

Structural mechanism-based intelligent capacity prediction methods for concrete-encased CFST columns

Intelligent capacity prediction using machine learning (ML) approach has been gradually employed in structural design. Due to the presence of complicated interactions and multiple factors that strongly influence the capacity of concrete-encased concrete-filled steel tube (CFST), implementing ML methods for such members is expected to be more challenging and requires great care. However, most current ML approaches rely heavily on data laws and ignore the underlying physical mechanisms. The physical mechanisms of proposed models may thus fail due to over-fitting, especially when the training data is not broad enough to reflect the actual laws. In this study, an innovative framework is proposed for combining the data-driven models with physical mechanisms to estimate the axial compression capacity of concrete-encased CFST, in which the mechanism verification is integrated into the optimization of model hyperparameters. A refined finite element analysis (FEA) model is established and validated, based on which a comprehensive database consisting of 143 experimental and 1560 simulated samples is constructed. Six single ML algorithms and two ensemble methods are adopted and compared, with their underlying mechanisms further revealed by the shapley additive explanation (SHAP) approach. The results reveal that all models developed through proposed modeling method demonstrate overall good performance and reflect structural mechanisms to a fair extent, where the XGBoost model outperforms others in terms of accuracy and dispersion. Besides, inputting combined parameters instead of basic ones is found to effectively solve the multi-factor problems using ML approaches.

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

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

Axial compressive performance of square concrete-encased concrete-filled double-skin steel tube stub columns

This paper explored the axial compressive performance of the concrete-encased concrete-filled double-skin steel tube (CECFDST) column, which comprised inner concentric steel tubes, sandwiched concrete, reinforcement cage and outer concrete. The characteristic stirrup value (λv) and nominal confinement factor (ξ) were chosen as the primary variables to investigate the confinement effects of the CECFDST. Firstly, the failure mode, ultimate strength, axial displacement, and strain responses of 10 specimens were tested and analyzed. The results showed that the combined confinement effects on the behavior of CECFDST were significant. The nominal confinement factor would impact the constraint on the sandwiched concrete and the ultimate strength of the specimens. Increasing the characteristic stirrup value could improve the ductility of the specimens. Secondly, a finite element model (FEM) was established and validated using test results. The full-ranged analysis of the typical FEM was presented, and contact stresses between the steel tube and concrete were quantified. After that, a systematic parametric analysis was conducted to explore the impacts of the primary influential factors related to the confinement effects. At the peak load, the RC component reached the corresponding ultimate strength, and the strength of the CFDST component was in the rising stage. In addition, the influence of contact stress between the RC and CFDST components on the ultimate strength was negligible. The strength reduction index of the square CECFDST stub column was proposed according to the parametric study. Finally, based on the superposition principle, a simplified design method was derived to predict the ultimate strength for the square CECFDST stub column. The simplified design formulas produced satisfactory predictions based on the test and numerical results.

Effects of support type and geometric shape of steel tube on concrete-encased concrete-filled steel tube beam under low velocity impact

Concrete-encased concrete-filled steel tube (CEFST) beams prepared by placing steel tube in RC beam is a beam type that has widespread use. The current study examines the effects of steel tube geometry and support conditions on the behavior of CEFST beams under low velocity impact loading. First, CEFST beam specimens with different lengths and different tube section geometries (circular and square) were produced in the laboratory. These four test specimens with two different support conditions were exposed to low velocity impact load tests. Finite element models of the test specimens were created using Abaqus\\Explicit software. In the next stage, 4 finite element models were verified by the experimentally obtained damage conditions, maximum displacements, and support reaction forces. In the final stage, as parametric study 12 finite element models were prepared in addition to 4 verified models and analyzed under low velocity impact loading. In addition to experimental values, Von Mises stresses, Equivalent plastic strain, Friction energy, and Damage energy values were also obtained by the numerical study and thus the effects of steel tube geometry and support conditions were comparatively examined. The obtained results showed that CEFST beams with a square tube section exhibited better performance under a sudden load such as impact compared to the beams with a circular tube section. Moreover, both numerical and experimental results pointed out that the beam length beyond support has a positive impact on the performance of beam under impact loading.

Axial compressive behaviors of engineered cementitious composites-encased concrete filled steel tube columns after fire exposure

Engineered cementitious composite (ECC)-encased concrete filled steel tube (CFST) column, which is composed of outer steel reinforced ECC and inner CFST components, has broad application prospects in high-rise buildings due to its higher strength and ductility than the concrete-encased CFST column counterpart. This paper investigates the fire performance of ECC-encased CFST columns under and after fire exposures. Totally thirteen ECC-encased CFST columns and two concrete-encased CFST columns were prepared and tested considering different parameters, i.e., heating-up time, stirrup spacing and fire sides. It was found that the outer ECC component provides better fire protection to the inner CFST than the conventional concrete. Besides, the axial compressive strength and ductility of the ECC-encased CFST column were 32.2 % and 10.3 % higher than those of concrete-encased CFST columns after fire exposure. A finite element (FE) model and design equations were also proposed and verified by the experimental results, which could be applied to simulate and predict the compressive behavior of ECC-encased CFST columns after fire exposure.

Experimental and analytical investigation on precast concrete-encased concrete-filled steel tube column-to-column dry connections under axial tension

Precast technologies featuring rapid construction and reduced on-site manpower have been gaining widespread adoption in the construction industry, particularly in reinforced concrete (RC) structures and composite steel–concrete structures over the past few decades, due to rising labour cost. However, to date, no workable precast connections have been developed for concrete-encased concrete-filled steel tube (CECFST) columns, which are increasingly used in ultra high-rise buildings and large-span bridges owing to its superior structural performance and excellent fire resistance. In this regard, to promote applications of CECFST columns in countries with increasing labour cost, this study develops two innovative precast CECFST column-to-column dry connections (PCDCs), through which the construction of CECFST structures would minimise heavy on-site work, such as welding or grouting of joints, formwork, and propping systems. Ten laboratory tests including four component tests and six column-to-column connection tests, were conducted to investigate tensile performance of the developed PCDCs. Key test results of all the ten specimens, such as load versus deformation relationships and failure modes, showed that the PCDCs had excellent tensile resistance and ductility. Furthermore, this paper carried out a series of theoretical analyses to quantify the ductility and tensile strength of the PCDCs through proposed load transfer mechanisms and component-based models. The design method of the PCDCs was presented by a simple flowchart and a worked example. With excellent structural performance, the PCDCs developed in this paper demonstrated huge potential to replace conventional cast-in-place (CIP) connections of CECFST columns.

Derivation of a normalised failure surface for square CECFST columns embedded with circular steel tube: A unified approach

Concrete-Encased Concrete-Filled Steel Tube (CECFST) columns are a new type of composite columns that possess great strength, stiffness, ductility, corrosion and fire resistance. To fully realise their benefits, an accurate design approach of calculating the strength of these members is necessary. In this paper, the plastic stress distribution (PSD) and load contour (LC) methods in the design code of predicting the cross-section strength of CECFST columns is first evaluated through numerical and published experimental data. Considering the shortcomings of conventional design methods, a unified design approach is then developed. Compared to the conventional PSD method, the developed method can yield conservative resistance predictions for uniaxial eccentric loaded (UEL) CECFST columns. In addition, instead of using linear moment interaction curves to predict the strength of biaxial eccentric loaded (BEL) CECFST columns, elliptical moment interaction curves are adopted in the proposed design approach. Most importantly, a normalised 3D axial-load–biaxial-moment (n–my–mz) interaction surface can be generated through the proposed method, which only requires determining 6 critical points on the interaction surface. Unlike the conventional design approach, in which the UEL and BEL composite columns need to be respectively designed through PSD and LC methods, a unified design procedure can be achieved through the proposed approach. Besides, the confinement effect from both the steel tube and the stirrups can also be considered in the proposed approach and much higher accuracy and consistency can be achieved in predicting the cross-section strength of CECFST columns.

Experimental investigation of CIP and PC beam-to-CECFST column joints with simple connection details under lateral cyclic loading

Due to the excellent structural performance of Concrete-encased concrete-filled steel tube (CECFST) columns, they have been applied in real practice for several years. In addition, precast technology can be utilised in the construction of CECFST composite structures to further improve productivity. To investigate the behaviour of precast concrete (PC) and cast-in-place (CIP) beam-to-CECFST column joints, a total of 6 joint specimens, including 4 PC and 2 CIP specimens, were thus experimentally studied under lateral cyclic loading. In this paper, the main studied parameters included connection details, construction methods and existence of column axial load. The ultimate strength, failure mode, crack patterns and hysteretic performance of each specimen were analysed in detail. According to the test results, the performance of CIP and PC joint specimens were discussed in the aspect of energy dissipation, ductility and damping coefficients, strength and rigidity degradations. It was found that the PC specimens exhibited comparable cyclic behaviour to the CIP specimens in regards to the maximum strength, hysteretic performance and failure mode. Moreover, based on the measured shear strains, contributions of joint shear resistance from steel tube, inner and outer concrete were quantitatively investigated. Thereafter, the development of shear stress in the outer concrete was compared with the nominal concrete shear strengths from American and European design provisions.

Experimental study and analytical evaluation on seismic performance of CECFST moment resisting frame with bolted connections

Concrete encased concrete-filled steel tube (CECFST) column possessed better durability and fire performance compared with ordinary concrete-filled steel tube (CFST) column. In engineering practice, the CECFST column is usually connected to steel beam using welded joints. It would be better to adopt bolted joints for fast construction and to avoid welding fracture under seismic loads. However, limited research has been performed on bolted CECFST joints. In this paper, an experimental program is conducted on bolted CECFST column joints. Typical failure modes were characterized and the moment-rotation behavior was discussed. An analytical model was established by Opensees, in which the moment-rotation model was calculated based on the component method and the shear behavior of joint panel was also accounted. The analytical method was verified by the test results and could be applied to design the bolted joints in engineering practice. Afterwards, a prototype CECFST moment resisting frame was numerically analyzed to detect its seismic performance under frequently-occurred (FOE), design-based (DBE) and maximum considered earthquakes (MCE). Subsequently, the fragility of the CECFST moment resisting frame was also explored, which verified that the CECFST moment resisting frame had high robustness and superior anti-collapse capacity. The test and analytical results confirmed that the CECFST moment resisting frame using bolted joints performed satisfactory seismic behavior and could be employed in engineering practice.

Axial Compression Performance and Residual Strength Calculation of Concrete-Encased CFST Composite Columns Exposure to High Temperature

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

Effects of Support Type and Geometric Shape of Concrete-Encased Concrete-Filled Steel Tube Beam Under Impact Loading

Effects of Support Type and Geometric Shape of Concrete-Encased Concrete-Filled Steel Tube Beam Under Impact Loading

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

The existing method to improve the coordination performance of the inner and outer parts of concrete-encased concrete-filled steel tube (CFST) composite columns by increasing the volume-stirrup ratio causes difficulties in construction due to over-dense stirrups. Thus, this paper proposes an open polygonal composite stirrup with high strength and high ductility CRB600H reinforced rebar, and seventeen specimens were constructed, and their axial compressive performance was tested. The main parameters considered were the volume-stirrup ratio, the steel tube size, the stirrup type and the stirrup strength. The test results indicated: For the specimens restrained by open octagonal composite stirrups, compared with the specimen of 0.5% volume-stirrup ratio, the compressive bearing capacity increased by 14.6%, 15.7% and 21.5% for volume-stirrup ratio of 0.73%, 1.07% and 1.61%, respectively. For the specimens restrained by open composite rectangle stirrups, compared with the specimen of 0.79% volume-stirrup ratio, the compressive bearing capacity increased by 7.5%, 6.1%, and -1.4% for volume-stirrup ratio of 1.12%, 1.58% and 2.24%, respectively. The restraint ability and the bearing capacity of the octagonal composite stirrup are better than other stirrup types. The specimens equipped with open polygonal composite stirrup not only had a higher ductility than those with the traditional closed-loop stirrup, but they also had a higher axial bearing capacity than those with an HPB300 strength grades stirrup. Therefore, the open composite stirrup can be used in practical engineering. A new calculation method was proposed based on the stress-strain models for confined concrete under different restrain conditions, and the predicted value was close to the experimental value.