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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.

Concrete-filled steel tube (CFST) is a composite structure consisting of a steel tube filled with concrete. The steel tube and its concrete core work together to carry external loads applied to the structure. Due to the interaction between the two kinds of materials, i.e., the confinement effect of steel tube on the core concrete, CFST structures can effectively delay/avoid the local buckling of the steel tube and the brittle failure of the concrete core. The superior structural properties of CFST include high strength and stiffness, good ductility, excellent seismic and fire performances, and easy construction. Therefore, CFST structures have been extensively used in large-scale constructions including industrial plants, subway stations, transmission towers, high-rise buildings, and bridges, etc. CFST structures have also offered substantial benefits on architectural appearance and engineering economy. The confinement effect in CFST throughout its service-life essentially contributes to the structural advantages, whilst it has, at the same time, brought about enormous complexities in the corresponding load-resisting mechanisms. Firstly, an in-depth understanding and accurate characterization of the confinement effect, and the development of some feasible constitutive models for the core concrete that facilitate the analysis and design of major infrastructures, becomes the key issue to be addressed. The salient loading factors in the service-life of CFST structures need to be considered in the constitutive models, such as long-term sustained load, low-cycle cyclic load and full-range fire. Secondly, the establishment of damage mechanism analysis and strength prediction models for CFST structures considering long-term sustained load, complex loading conditions, earthquake and fire becomes another key issue that needs to be resolved for the safety design of major CFST constructions. Life-cycle based damage mechanism and analytical theory of concrete-filled steel tubular structures have been investigated and established based on nearly 20 years of persistent and systematic study. The design methods for the ultimate loading capacity of CFST structures were further proposed. The major research outcomes can be summarized in the following three aspects: (1) The interaction between the steel tube and its core concrete as well as the passive confinement mechanism in CFST were revealed. A general constitutive model for the confined core concrete under various stress/action conditions was established with the “confinement factor” as an essential variable. The considered actions include long-term sustained stress, cyclic load and elevated temperature. (2) Theoretical models were established to account for the damage mechanism of CFST structures under long-term sustained load and complex loading conditions. The ultimate strength of CFST members under various loading conditions was analysed, including compression, tension, bending, shear, torsion, and the combinations of different loads and load paths. The creep and shrinkage of concrete as well as the deformation of CFST under long-term sustained load were accurately simulated. (3) Theoretical models were established to simulate the damage mechanism of CFST under earthquake and full-range fire. The hysteretic properties and ductility of CFST members under low-cycle cyclic loading were revealed. The model for calculating the fire resistance of CFST columns was proposed, in which the coupling effects of loads and full-range fire were taken into account. This paper briefly reviews the development of CFST structures and outlines the life-cycle based analytical theory for CFST structures. Finally, the potential development trends of the CFST structures are discussed.

Reusing structural elements is an effective way to prompt sustainable development with reduced energy consumption and gas emission. In current practice, steel members can be easily deconstructed and reused whilst the recycling of structural materials in steel-concrete composite construction has been found challenging. With the increasing material consumption in composite construction due to their well-recognized structural benefits, it is essential to explore the demountability of such structures. Limited previous research indicates that there is a lack of understanding or mature design specification for demountable composite connections. This paper thus presents an innovative design of demountable K-joints with concrete-filled steel tubular (CFST) chords and circular hollow section (CHS) braces connected using blind bolts. A detailed finite element analysis (FEA) modelling was established and validated against reported test data on both bolted and welded CFST connections. The model was then used to evaluate the possible failure modes, full-range load-deformation relationships, stress distributions and material interactions for the innovative connections. Behaviour of the demountable CFST K-joints was compared with that of traditional welded ones. Finally, simplified methods for the strength prediction of demountable CFST K-joints were proposed based on theoretical derivation and parametric analysis on the effects of various factors.

This paper presents a finite element analysis (FEA) model for the behaviour of square concrete filled double skin steel tube (CFDST) under local bearing force. Feasibility of the developed FEA model was validated extensively against available experimental results. The predicted results obtained from the FEA modelling were found to match well with those observed from the tests in terms of failure patterns, load versus deformation curves and bearing capacities. The full-range load-transferring mechanism of square CFDST under local bearing force was then analyzed using the FEA model. Comprehensive parametric studies were also carried out to investigate the effect of various parameters on the bearing capacity of square CFDST subjected to local bearing force. The evaluated parameters mainly included the hollow ratio of CFDST, geometric configuration of the bearing member (BM) and the CFDST, the width to thickness ratio of steel tubes, and the strengths of steel and concrete materials. Based on the parametric analysis results, simplified design formulae were proposed for calculating the bearing capacity of square CFDST under local bearing force. Validated through the reported experimental data, the proposed formulae were featured with a reasonable accuracy and could be adopted in the practical design of square CFDST members subjected to local bearing force.

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.

This paper presents a numerical investigation on concrete-filled steel tubular (CFST) beam-columns with square sections subjected to combined sustained load and corrosion. A general numerical method is used for the simulation of the time dependent behaviour of the core concrete, while the wall-thickness loss of the outer steel tube induced by corrosion is considered in this model simultaneously. The established finite element analysis (FEA) modelling is verified with test data and used for further investigations of the load-deformation relationship, load-transfer mechanism, stress and strain development, failure modes and ultimate states on CFST beam-columns when suffering combined sustained load and corrosion. Parametric studies are conducted to reveal the effects of key parameters, based on which simplified design method is suggested for square CFST beam-columns under combined sustained load and corrosion.

This paper reviews the previous studies on bond behaviour of concrete-filled steel tubes (CFST) and recycled aggregate concrete-filled steel tubes (RAC-FST). The bond behaviour between the steel tube and the recycled aggregate concrete (RAC) of RAC-FST is experimentally investigated then. A series of push-out tests on 56 RAC-FST specimens are conducted with parameters of cross-section type, cross-sectional dimension, strength of RAC, replacement rate of recycled coarse aggregate and interface treatment (i.e. oilin, polishing). Mechanism of the effects of chemical adhesion, micro-interlocking and macro-interlocking on the bond strength is discussed and investigated based on the testing results. The analytical results show that the cross-section type and cross-sectional dimension are two main parameters that influence the bond strength between the steel tube and the RAC core. It can be concluded that, in general, the bond behavior of the RAC-FST is similar to that of the steel-concrete surface in concrete-filled steel tubes (CFST). Finally, empirical formulas are proposed for the calculations of the bond strength between the core concrete and the outer steel tubes.

This paper presents an experimental study on concrete-filled steel tubular (CFST) beam-columns with square sections under combined sustained load and chloride corrosion. A total of 11 specimens (including 7 CFST members and 4 reference hollow steel tubes designed for comparison) were tested under 120-day sustained load in accelerated chloride corrosion environment and then loaded to failure. The main test parameters include load ratio (ratio between the long-term sustained load and the ultimate capacity of the specimen under short-term loading) and corrosion depth. The failure modes, full-range load-displacement relationship, load-strain relationship and ductility were measured and compared. Based on the experimental observation, mechanism of CFST beam-column members under the combined effects of sustained load and chloride corrosion was discussed and compared to the reference hollow steel tubes. Traditional design methods were modified for the consideration of combined sustained load and corrosion, which were calibrated and evaluated against the test results.

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.

When transmitting highly corrosive offshore hydrocarbons, one economical approach is lining internally a thin corrosion resistance alloy (CRA) layer within an ordinary carbon steel pipe. Most commonly the liner is mechanically bonded to the outer pipe by mechanical expansion. The lined pipe is expected to make optimize usage of the two types of materials, providing significant corrosion and structural resistance. This study investigates full history interaction behaviors between the outer pipe and liner and the structural behaviors of the lined pipe under axial compression, through finite element analysis (FEA). Comprehensive finite element (FE) models are developed, where the manufacturing process, the non-linear material properties of stainless steel and the interactions between the constituent components are considered. The numerically determined failure modes, load-deformation histories and ultimate strengths are compared with those from tests. The interaction stress and axial loading histories of the specimen under axial compressions are studied, where the internal content pressure and external hydrostatic pressures are considered simultaneously. Confinement factor is suggested to ensure the structural reliability of the liner in engineering practice. Parametric studies are conducted to study the effects of the outer pipe strength, liner strength, outer pipe thickness and liner thickness on the structural behaviors of the axially loaded lined pipes.

The concrete-encased concrete filled steel tubular (CFST) column to RC beam joint is composed of the concrete-encased concrete filled steel tubular (CFST) column and the reinforced concrete (RC) beam. Concrete-encased CFST column consists of a CFST component and an outer reinforced concrete (RC) component. This paper presents an experimental investigation of the seismic performance of this kind of composite joints. Thirteen joints were tested under a constant axial load on the column and a cyclic vertical load on the beam. The test parameters include the joint types, the connection types, the axial load level, the joint core volumetric stirrup ratio, the beam longitudinal rebar ratio and the sectional dimension of the beam. Four types of typical failure modes are found, and the corresponding hysteretic relationships are analyzed based on the test results. The seismic performance of the composite joint is also evaluated by several indexes, such as rigidity degradation, strength degradation, ductility, and energy dissipation. Finally, a comparison with other typical joints is conducted to exhibit the seismic performance of the concrete-encased CFST column.

One type of submarine composite pipeline structure, with carbon steel-concrete-stainless steel (CCS) double-skin tube (DST), was introduced in this paper. This composite pipeline was expected to make optimal use of the three types of the materials, and provide significant structural and internal corrosion resistance. This study investigated the compressive and flexural behavior of the composite pipeline under internal content pressure and external hydrostatic pressure through finite element analysis (FEA). Finite element models were developed, where non-linear material properties of stainless steel and composite actions between constituent parts were considered. The models were verified through the comparisons between the numerically and experimentally determined results, in terms of load-deformation histories, failure modes and ultimate strength. Structural behaviors of the composite pipeline under pressures were compared with those without content and hydrostatic pressure. Parametric studies were carried out to investigate the effects of the outer carbon steel strength, inner stainless steel strength, concrete strength and hollow ratio on the compressive and flexural behaviors of the composite pipelines subjected to pressures.

This paper presents experimental and numerical study on the behaviour of ultra-high strength steel (UHSS) hollow tubes under low velocity lateral impact loading. A total of six specimens were tested under lateral impact loading. The finite element (FE) model was established with the consideration of the strain rate effect and validated against test results. The parametric study showed that UHSS hollow tubes had the similar failure pattern when compared to that of normal strength steel members. The mid-span deflection and the residual deformation decreased significantly when the UHSS tube was applied.

Latest development in material and manufacturing technologies make it possible to increase the yield strength of steel to more than 1200 MPa. These steel grades, suitable for structures prone to extreme loadings, give potentials for considerable weight reduction and a cost-effective way to produce energy-efficient members. Having analysed the impact behaviour of corrugated members recently, this paper now investigates the performance of hybrid corrugated (HC) sections consisting of mild-steel corrugated plates and ultra-high strength (UHS) steel tubes. Three different types of trapezoidal corrugated plates are considered so as to examine the effect of geometrical corrugation parameters on the performance of entire member. Along with this, in order to study the contribution of UHS tubes in the performance of HC members under impact loading, their performance is also examined separately. In combined lateral impact and axial loading scenario, the tube is initially compressed axially; thus an indenter perpendicularly impacts the specimen at mid-span. An advanced numerical model in which nonlinear strain-rate dependent material behaviour of steel is taken into account is also developed by using ABAQUS/Explicit. Thus, the residual deformations, time histories, global lateral deflection, and energy absorptions are discussed and compared with those of experimental results.

Concrete-encasedconcrete-filled steel tube (CFST) column consists of a CFST component and an outer reinforced concrete (RC) encasement. This paper numerically investigates the behaviour of concrete-encased CFST columns subjected to long-term sustained loading. A finite element analysis (FEA) model is established which incorporates the time-dependent behaviour of concrete under sustained loading using the viscoelastic model. The FEA model is validated against the experimental results from long-term loading tests. By using the FEA model, the behaviour of concrete-encased CFST columns subjected to long-term loading is thoroughly analysed, such as the full-range strain development, the internal force distribution among different components, the confinement effects between the steel tube and surrounding concrete, and the comparisons of three types of columns, i.e. concrete-encased CFST column, RC column and traditional CFST column. The creep deformation of the concrete-encased CFST column is also evaluated by the FEA model and existing design standards. The parametric analysis is conducted to investigate the influence of various parameters on the strength degradation of concrete-encased CFST columns, including the material strength, the steel ratio of CFST component, the sectional configuration, longitudinal rebar ratio, stirrup characteristic value, and the long-term axial load level. Finally, the ultimate strength of the concrete-encased CFST columns after long-term loading is evaluated, with calculation tables provided for the referencing of practical design.

One type of submarine composite pipeline structure, with carbon steel-concrete-stainless steel (CCS) double-skin tube (DST), was introduced in this paper. This composite pipeline was expected to make optimal use of the three types of the materials, and provide significant structural and internal corrosion resistance. During installation and service stage, submarine pipelines may experience significant torsion effects. Global response of the system depends on both the behavior of each constituent part and interactions between them. In this paper, an interaction model considering the friction and the cohesive force between the steel tube and the concrete is introduced, and a finite element model of the submarine pipeline under torsion is established by using this interaction model. The developed finite element model was verified through the comparisons between the numerical and experimental determined results, in terms of torque rotation angle histories, stiffness and ultimate torque. The results show that the ultimate strength and stiffness of the model considering cohesive force are increased by 4.6% and 11.9% respectively compared with the model only considering friction force.

The post-fire performance of concrete-filled stainless steel tubular (CFSST) columns subjected to an entire loading–fire history, including four characteristic phases: (i) ambient temperature loading, (ii) heating, (iii) cooling with constant external loads, and (iv) post-fire loading, is investigated in this paper. Sequentially coupled thermal-stress analyses are performed using ABAQUS to establish the temperature field and structural response of CFSST columns. To improve the precision of the finite element analysis (FEA) models, the influence of moisture on the thermal conductivity and specific heat of the concrete in the heating and cooling phases is considered by using subroutines. Existing fire and post-fire test data on CFSST columns are used to validate the FEA modelling. Comparisons between FEA and test results indicate that the accuracy of the model is acceptable; the FEA model is then extended to simulate CFSST columns subjected to the four characteristic phases. The behaviour of the CFSST columns during the four characteristic phases is explained by analysis of the temperature distribution, load versus axial deformation relations, failure modes and internal force redistribution. The excellent post-fire performance of CFSST columns is examined in comparison with traditional concrete-filled carbon steel tubular (CFST) columns with the same total cross-sectional area. The residual strength index is studied with respect to a series of parametric analyses. It is found that the residual strength of CFSST columns is higher than that of CFST columns after the same fire exposure, and that the diameter of the stainless steel tube, slenderness, heating time ratio and load ratio have a significant influence on the residual strength index.

The life-cycle performance of concrete-filled steel tubular (CFST) columns is investigated through numerical modelling. A finite element analysis (FEA) model is established to simulate the performance of deteriorated CFST structures subject to lateral impact. The imposed effects cover a wide range of factors that may deteriorate the mechanical performance of CFST during its life-cycle, which include: construction issues (preload, residual stress, and imperfection), sustained load (axial load from upper structure), environmental aggressiveness (chloride corrosion) and extreme hazard (impact lateral load). The numerical model is stage-by-stage verified against experimental results and shows good accuracy. It is then employed to analyze the performance of CFST column under the coupled effects, including the failure modes, the full-range load-displacement relationship, and the residual compressive strength. Particularly, the effects of the coupled hazards on the loading capacity of the column are compared with the results when the hazards are separately and independently imposed, which highlights the importance of the life-cycle design for the composite structure. Finally, a parametric study is carried out to analyze the influence of salient parameters on the residual compressive strength of CFST subject to the coupled effects.

For structural components with specific requirement of fire resistance rating for safety reasons, additional fire protection is usually necessary. Intumescent fire coating (IFC) is one of the well-used materials to protect steel structures, and it has advantages such as light weight, simple construction, aesthetic appearance, easy maintenance, etc. In this study, a group of eccentrically loaded concrete-filled steel tubular (CFST) columns with and without IFC protection was tested in standard fire circumstance up to 180 min. Three types of intumescent materials were adopted and different dry film thicknesses of the fire coating were designed. Temperature field of each CFST column and the load bearing behaviour were measured and analysed. Finite element modelling of the columns was also established for heat transfer analysis and fire resisting capacity analysis. The modelling was validated by the test data. This study expands the related experimental database with large scale column specimens. The investigation results provide further evidence that CFST column protected by intumescent fire coating could achieve excellent fire resistance. A technical process to determine intumescent fire coating for CFST columns coupling with experimental verification and FE modelling analysis is presented.

Concrete-filled steel tubular (CFST) column-wall has a promising prospect in the construction field, especially for multi-story residential buildings. It is very important to have a thorough understanding of its mechanical behaviour and fire resistance. This is a companion paper that carries forward the work by the authors [1]. The previous paper focused on full-scale testing to determine the fire resistance of CFST column-wall, whereas this paper deals with finite element analysis and simplified design method for such composite structural elements. Heat transfer mechanism was analysed by comparing the temperature distributions of CFST column-wall sections with different fire exposures. Structural analysis, such as internal force distribution, strain and stress of different components, interfacial behaviour between steel and concrete was given to describe the deformation behaviour and failure mechanism of CFST column-wall. Based on the FEA model, parametric analysis was carried out to investigate the influence of geometric, load and physical parameters on the fire resistance of CFST column-wall. Since no specific design rules can be found for CFST column-walls, a simplified fire design method was proposed in this paper to calculate the fire resistance of CFST column-wall structure.

Special-shaped CFST columns are becoming increasingly attractive as alternative solutions to engineering design. Three-dimensional FE models are developed and verified against experimental results in terms of failure modes, load-deformation curves and ultimate loads, where circular, triangular, Fan-shaped, D-shaped, 1/4 circular and semi-circular sections are considered. In light of the FE simulations, the composite actions between the special-shaped steel tubes and concrete cores have been investigated through load-deformation and interaction stress-deformation histories. Possible parameters affecting specimens loading behaviors have been studied. The studies generally show that the failure modes, composite behaviors and load-deformation histories of the axially loaded special-shaped CFST stub columns are similar to those of SHS/RHS specimens.

This paper reports an investigation into the impact resistance of concrete-encased concrete-filled steel tube (CFST) members with circular sections. A finite element analysis (FEA) model was established to simulate the impact behaviour of concrete-encased CFST under laterally low velocity impact, in which the strain rate effects of steel and concrete, the element erosion criteria of concrete, the interactions between concrete and steel, as well as the combined effects of axial load and lateral impact, were considered. Experimental data on reinforced concrete (RC), CFST and concrete-encased CFST members under drop hammer impact were used to verify the accuracy of the FEA model and a generally reasonable agreement was achieved for all three types of structures. A full-range analysis of the behaviour of concrete-encased CFST members with circular sections was then carried out with the FEA model to investigate the impact behaviour and impact resistance of the composite structure. The failure modes, sectional moment development, stress and strain development, as well as the contact behaviour between different parts were analyzed to highlight the reasons behind the good impact resistance of the composite structure. A parametric study was finally conducted with the FEA model to investigate the major parameters that may influence the impact resistance of the concrete-encased CFST members.

This paper is an attempt to study the mechanical behavior of square concrete-filled bimetallic tubular (CFBT) stub columns subjected to axial compression. The bimetallic tubes in the CFBT columns in this research comprised an outer layer made of stainless steel and an inner layer made of carbon steel. 200-mm square carbon steel tubes (wall thickness tsc = 3.30 mm) were manufactured first, then the bimetallic tubes were fabricated by cladding the carbon steel tubes with stainless steel sheets. In the experimental program, fourteen CFBT columns and two conventional concrete-filled steel tubular (CFST) counterparts were tested to failure under axial compressive loading. The test parameters included the stainless steel grade (Grade 316, 304, and 202), wall thickness of the stainless steel tube layer (tss = 0.84, 1.32, and 1.88 mm), and cube compressive strength of concrete (fcu = 54.5, 68.4, and 80.5 MPa). A finite element analysis (FEA) model was established and validated against the experimental measurements. The failure mode and mechanical behavior of the square CFBT stub columns were then investigated and compared with those of the conventional CFST columns. Finally, the ultimate loads obtained from the experiments were compared to those predicted by the available design codes.

Concrete-encased CFST (concrete-filled steel tube) columns are gaining popularity in constructions especially in high-rise buildings and large-span structures. This type of composite column features an inner CFST component and an outer reinforced concrete component. Extensive literature is available on the structural performance of concrete-encased CFST columns at ambient temperature, however, the fire performance of this type of composite columns has seldom been addressed. To fill in this research gap, this paper thus experimentally studies the performance of concrete-encased CFST columns subjected to full-range fire including heating and cooling. A set of tests, including six fire resistance tests and four postfire tests, respectively, were conducted under the combined effect of load and fire. Test observations and results are presented, including fire resistance, postfire residual strength, failure modes, temperature versus time relationships and deformation versus time relationships. The fire performance of concrete-encased CFST columns is analysed and discussed based on these observations. It is found that due to the insulating effect of the outer reinforced concrete, concrete-encased CFST columns exhibits higher fire resistance in fire and higher postfire residual strength as compared with conventional CFST columns without concrete encasement. The simple calculation model of analysing the fire resistance of composite column in Eurocode 4 was extended and then employed to calculate the fire resistance as well as the postfire residual strength of concrete-encased CFST columns. The predictions are found to be conservative as compared with test data, and this simplified method are applicable to assess the fire resistance and postfire residual strength of concrete-encased CFST columns.

Concrete-filled double skin steel tubular (CFDST) stub columns with external stainless steel and internal carbon steel tubes can be considered as new types of composite members and expected to combine the advantages of all three kinds of materials. This paper presents non-linear finite element (FE) analysis and design of circular and square CFDST stub columns with external stainless steel under axial compression. FE models are developed, where non-linear material property of stainless steel is considered, and verified through comparisons with experiments in terms of failure modes, load-deformation histories and ultimate strength. Behaviors of stainless steel composite columns are compared with that of columns with both carbon steel tubes. Parametric studies are conducted to investigate the influence of the outer stainless steel tube strength, concrete strength, inner carbon steel tube strength and hollow ratio on structural behavior of axially loaded columns in terms of loading and interaction performance.

Ductile fracture of steel components cannot be avoided particularly due to large inelastic strain in extreme loading scenarios. Without considering damage model, conventional elasto-plastic constitutive material model cannot lead to accurate finite element (FE) modelling results when damage due to fracture exists. Metal fracture is a complex damage process in which damage onset, material deterioration and final fracture are related to development of material plasticity. Uniaxial tensile fracture and shear fracture are two typical failure modes of structural members. In this study, experimental investigations of monotonic tensile fracture and in-plane shear fracture under different strain rates in quasi-static loading range are presented for low-alloy structural carbon steel. The characteristic steel material properties are obtained insensitive to the chosen strain rate. A semi-empirical process is used to calibrate the damage properties (damage onset and softening criteria) through FE modelling of the coupon samples based on a continuum damage mechanics (CDM) model. Deterioration of the carbon steel properties was found related to the evolution of equivalent plastic strain, and particularly there is a linear relationship for the in-plane shear scenario. The verified damage onset and shear softening are then used for modelling of a welded steel tubular connection experiencing punching shear fracture phenomenon. It is shown that modelling result of the steel connection is significantly improved when the proposed fracture modelling is adopted.

The hexagonal concrete-encased CFST column consists of a CFST (concrete-filled steel tube) core and a hexagonal-shaped reinforced concrete (RC) encasement. This paper presents the finite element (FE) analysis of hexagonal concrete-encased CFST columns subjected to axial compressive forces and cyclic bending moments. High-fidelity finite element analysis (FEA) model is established and validated by comparison with the test data in terms of failure mode and hysteretic curves. From the FEA model, the hysteretic response of the composite columns, the contact stress between the steel tube and concrete, and the strength contribution of different components during the full range of loading are illustrated. Parametric analysis is conducted to investigate the influences of various parameters on force-displacement envelope curves of the hexagonal concrete-encased CFST columns. The parameters include the material strength, confinement factor of CFST section, stirrup characteristic value, area ratio of CFST core to RC encasement, and axial force ratio. Finally, simplified methods are proposed to predict the flexural strength of hexagonal concrete-encased CFST columns. The predictions from simplified methods showed good agreement with the experimental and analytical results.

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.

The behaviour of dune sand concrete-filled steel tubular (CFST) stub columns under axial compression was experimentally investigated in this paper, where the nominal cube strength of concrete was 60 MPa. A total of 24 specimens including 20 dune sand CFST stub columns and 4 reference hollow steel tubular stub columns were tested. The effects of tube thickness, cross-sectional dimension and concrete infill on the behaviour of the composite stub columns were investigated. The results showed that, similar to conventional CFST stub columns, the tested dune sand CFST stub columns behaved in a ductile manner and displayed an outward buckling failure model with high compressive strength. It indicates that it is feasible to use dune sand to replace normal sand to fabricate CFST columns in regions close to deserts. Comparisons were made between the test results and the predicted ultimate strength using the existing codes AIJ (2008), AISC360-10 (2010), BS5400 (2005), DBJ/T13-51-2010 (2010) and EC4 (2004). Generally, these codes are conservative but acceptable for predicting the ultimate strength of dune sand CFST stub columns.

This paper conducts a numerical investigation on the behaviour of tapered concrete-filled double skin steel tubular (CFDST) stub columns subjected to axially partial compression. A finite element analysis (FEA) model is developed to investigate the partial compressive behaviour of the CFDST column. The full-range load versus deformation relations of tapered CFDST columns under partial and overall compression are both analyzed using the verified FEA model. A parametric study is then conducted to investigate the ultimate strength of the column with various geometric and material parameters. Finally, a simplified model is proposed based on the parametric analysis, which could predict the ultimate strength under partial compression with generally good accuracy.

This paper introduces a series of axial compressive tests on 32 recycled aggregate concrete-filled steel tubular (RAC-FST) stub columns. Apart from the cross-section type, the confinement factor and the replacement ratio of recycled coarse aggregate (RCA) are chosen as the main variables to investigate the passive confinement effect of RAC-FST. Analysis of the typical failure modes, the full-range response, the passive confinement effect, the ultimate strength and the effect of RCA replacement ratio are successively carried out. It is suggested that the confinement factor determines the development pattern of the load–strain curves, while the use of RCA has influence on the ultimate strength, the elastic modulus, the critical strain and the transverse deformation coefficient of specimens. The critical confinement factor for circular specimens is determined. A stress–strain model of RAC in RAC-FST is proposed and verified through finite element analysis based on a database of test results presented in this paper and previous literatures. Parametric analysis is carried out and simplified calculation model is proposed. Based on reliability analysis, partial factors for the axial compressive strength of RAC-FST stub columns are calibrated.

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.

This paper reports a full-scale fire testing of concrete filled thin-walled steel tubular (CFST) column-wall structure. All together eight specimens were designed and tested under axial compression load and subjected to ISO-834 standard fire. Load ratio, fire-exposure time and insulation scenarios were varied in the testing program. Cement mortar, aerated concrete block, and mineral wool were used as the insulation layer materials. Test results including temperature to time relations, axial deformation, and fire resistance as well as failure modes were described. A finite element analysis (FEA) model was established in order to simulate the thermal and structural behavior of CFST column-wall under fire. The FEA model was verified against test data and turned out to be reasonably accurate in predicting the temperature distribution, axial displacement, fire resistance and failure modes of CFST column-walls. Sensitivity studies were carried out to determine some major parameters in the FEA model. Discussion was made on the influence of thermal resistance, spalling of the insulation layer and confinement of inner concrete on the modelling of CFST column-wall structures.

Concrete-filled bimetallic tubes (CFBT) could be recognized as a type of steel-concrete composite members, and have the potential to be applied in engineering structures. A three-dimensional finite element analysis (FEA) model is established to further study the axial compressive behaviour of circular CFBT stub columns in this paper. The preciseness of the FEA predictions is evaluated by comparison with experimental results collected from ten CFBT column specimens tested in a companion paper (Ye et al., 2016) [15]. Good agreement is obtained between the FEA predictions and the test results in terms of failure pattern, load versus deformation response, ultimate strength, and strain development of the two steel tube layers. In light of the FEA results, the stresses of concrete and steel, as well as the interactions and load distribution between different components of the composite section are investigated. Finally,parametric study is performed and a simplified model is proposed to be used to calculate the ultimate strength of circular CFBT stub columns.

Concrete filled double-skin steel tubular (CFDST) members have a large potential to be used in trusses, bridges and lattice structures. Previous research work has focused on the structural performance of individual CFDST components whilst there is a lack of understanding on the behaviour of typical connections formed by such composite members. This paper intends to fill the knowledge gap in this area. A total of 20 K-joints with circular CFDST chords and circular hollow section (CHS) braces were tested under two typical sets of boundary conditions. Effects of important parameters were investigated based on the test results, including brace to chord diameter ratio ( β ), chord hollow ratio ( χ ) and preloads on the chord. Typical failure modes and load-deformation relations were investigated, in-depth study on the interactions between materials and the load-transfer mechanism were conducted, simplified strength-prediction method for this new type of composite joints was also proposed.

This research aims to investigate the cyclic behaviour of composite joints consisting of concrete-filled steel tubular (CFST) columns, steel beams, and through-bolt connections. A total of 10 specimens, including 5 specimens with square CFST columns and 5 specimens with circular CFST columns, were tested under lateral cyclic loading with horizontal displacements imposed at the top of the column. The main experimental parameters were the cross-sectional type of the column, the axial load level in the column and the cross-sectional configuration of the beam. The experimental results are analysed in this paper to evaluate the performance of the through-bolt connections. Based on numerical analysis, the performance of the bolted joint is further compared with that of the counterpart with external diaphragm. In general, the initial stiffness of the bolted end-plate CFST connections investigated is greater than 8 EI b / L b , so a fixed connection can be assumed in modelling non-sway frames. However, the connection rotation in unbraced frames may need to be considered to minimise the modelling error since the initial connection stiffness is usually smaller than 25 EI b / L b .

Natural extreme events such as earthquake and fire are destructive for building structures. Earthquake is not only a destructive extreme event but also can trigger other extreme events such as fire. The possible post-earthquake fire (PEF) can be a more destructive scenario for building structures. For safety reasons, it is necessary to capture the behaviour of structural components under PEF scenario. This study aims to investigate the behaviour of flange-welded/web-bolted steel I-beam to box column connections subjected to PEF scenario. Experimental tests were carried out on two groups of steel connections under ISO fire. In each group, four connections were fabricated with the same specifications in which one connection was tested under fire only while the other three connections were tested under cyclic loading and subsequent fire. During the fire stage, the connections were subjected to constant static load equivalent to 30% of their ultimate monotonic loading strength. The furnace temperature, temperature distribution of connection and the beam deflection of connection were measured during the fire tests. It was found that the load-carrying capacity of the flange-welded/web-bolted connections decreases significantly with the increase of pre-damage level induced by cyclic loading. Additionally, in order to study the temperature distribution of the connections under fire, preliminary finite element modelling was carried out for a heat transfer analysis on the basis of uniform fire exposure.