Concrete-filled Double-skin Steel Tube Columns Research Articles

Numerical study on dynamic responses and residual axial bearing capacity of square CFDST columns under lateral impact

This paper presented a numerical investigation into the dynamic responses and residual axial bearing capacity of square concrete-filled double-skin steel tube (CFDST) columns under lateral impact using ABAQUS. The finite element analysis (FEA) model for lateral impact and post-impact axial compression of square CFDST members was established, where the material nonlinearity and strain rate of steel tubes and concrete were considered and validated by existing experiments and studies. Based on the verified FEA model, the failure mechanism of the impact response process of the square CFDST member was revealed. To further explore the behavior of CFDST under axial compression after lateral impact, the typical axial load-axial displacement curves, axial strains of the steel tube, failure mode, and stress development of different components of the members were analyzed in depth. Parametric studies were conducted to clarify the influence of each parameter (including hollow ratio, slenderness ratio, concrete strength, and steel strength) on the residual capacity of post-impact axial compression in square CFDST members at different impact energies. Results demonstrated a linear correlation between residual axial bearing capacity coefficient and mid-span residual deflection after impact. Finally, a simplified formula for calculating the residual axial bearing capacity of square CFDST columns after lateral impact was proposed, which provided a reference for practical engineering applications.

Ultimate axial strength prediction of concrete-filled double-skin steel tube columns using soft computing methods

In this study, two hybrid models, grey wolf optimizer (GWO) combined with group method of data handling (GMDH) and random forest (RF) optimized by particle swarm optimization (PSO), are proposed to predict the ultimate axial strength of concrete-filled double-skin steel tube (CFDST) columns. 139 sets of data collected from the literature were used to build the proposed models. The proposed hybrid models were compared with other machine learning models as well as empirical equations and design specifications. The results show that for all datasets, the integral absolute error (IAE), root relative squared error (RRSE), mean absolute percentage error (MAPE) and coefficient of determination (R2) predicted by PSO-RF and GWO-GMDH models are 0.057, 0.092, 0.068 and 0.992; 0.068, 0.094, 0.110 and 0.989, respectively. Compared with the single RF and GDMH models, the IAE, RRSE and MAPE values of the proposed PSO-RF and GWO-GMDH models decreased by 3.91%, 13.66% and 6.34%; 37.81%, 48.78% and 39.24%, respectively, and the R2 value increased by 1.24% and 1.67%, respectively. In addition, the factors affecting the failure of CFDST columns are analyzed through the parameter sensitivity analysis.

Experimental study and numerical analysis on the dynamic response of steel tube confined concrete with a circular hollow section (STCC-CHS) under transverse impact loading

The dynamic response to lateral impact of steel tube confined concrete with a circular hollow section (STCCCHS) was investigated via pendulum impact tests in the laboratory. Nine STCCCHS columns and two concrete-filled double skin steel tubes (CFDST) were tested under lateral impact. The hollow ratio, the thickness of the outer steel tube, the impact height, and axial load ratio were varied. The failure pattern was observed along with the time histories of the impact force and the mid-span deflection. Strain development in the outer steel tube was quantified. The failure mechanism was integral bending and local buckling under lateral impact. Increasing the outer wall thickness and adopting an appropriate hollow ratio can effectively improve the impact resistance of such columns. Changes in impact energy have a significant influence on the plastic deformation of the members. Modest axial loading has little effect on the impact resistance. The test results were used to tune a finite element simulation of the process. A whole dynamic response investigation of STCCCHS columns under impact loads was carried out, and compared with the impact resistance of CFDST columns.

Axial compressive behavior of circular composite columns with external confinement

With the advance of externally confined composite columns, a generic model that can predict the ultimate axial compressive strength of both circular concrete-filled steel tube (CFST) and concrete-filled double skin steel tubes (CFDST) short columns with external confinement is very attractive. For this purpose, a rich experiment database of composite columns is assembled, guides to build up this database are outlined, and its limitations are also summarized. Based on some fundamental considerations, a simple formula for estimating ultimate axial compression of circular composite short columns is proposed first. The hoop and longitudinal stresses of outer and inner (if any) steel tubes are then determined by the stress analysis, from which the assumption frequently used in previous studies that the sandwiched concrete in a CFDST column is under uniform confinement is demonstrated to be unreasonable. Besides, the additional external confining pressure provided by external confinement in the form of steel spirals, rings, jackets, or tie bars is thoroughly investigated. Further, to comprehend the triaxial strength of infilled concrete in both CFST and CFDST columns, a novel concept of equivalent confining stress that can reflect both uniform and non-uniform confinement effects is defined, with which a modified Power-law failure criterion is established. Model validations illustrate that this generic model can reasonably estimate the strength of externally confined CFST and CFDST columns. Although it currently slightly underestimates the strength of ring-enhanced CFDST columns, this limitation can be easily overcome by recalibrating some coefficients without rebuilding the proposed framework.

Experimental study on the residual seismic performance of CFDST columns after contact explosion

Bridge columns are susceptible to various hazards throughout their service life, including accidental explosions, terrorist bombing attacks, and earthquakes. To support engineers and owners in making informed decisions regarding post-blast maintenance strategies for damaged bridges, it is crucial to gain a thorough understanding of the residual performance of affected bridge columns following the aforementioned incidents. This paper aims to investigate the residual seismic performance of concrete-filled double-skin steel tube (CFDST) columns through experimental means after being exposed to blast loading. The study consists of two phases: a field blast test and a quasi-static cyclic test, both conducted to evaluate the residual seismic performance of CFDST columns. Two circular column specimens and two square column specimens, each with similar nominal axial load-carrying capacities, were constructed and tested. One circular and one square column were initially subjected to a contact explosion before undergoing lateral cyclic loading. The remaining two columns were directly tested under cyclic loading to examine their seismic performance in an undamaged condition. By comparing the test results of the blast-damaged columns with those of the intact columns, the residual seismic performance of the blast-damaged columns is evaluated. The results indicate that the blast-damaged columns retain satisfactory lateral strength, stiffness, ductility, and energy dissipation capacity, highlighting the resilient performance of CFDST columns in a post-blast earthquake scenario. Furthermore, it was observed that the residual seismic performance of blast-damaged columns is more affected in the loading direction towards the side of blast damage. A residual index of energy absorption capacity is proposed to quantify the residual seismic capacity of blast-damaged CFDST columns. The results demonstrate that circular column exhibits better blast resistance and residual seismic performance compared to the square column. This study contributes valuable insights for the application of CFDST columns in multi-hazard scenarios involving potential contact explosions in seismic regions.

Residual seismic resistance of CFDST columns after a close-in explosion: Experimental study

This paper examines the residual seismic resistance of a concrete filled double-skin steel tube (CFDST) column after a close-in explosion experimentally. First, six 1/4 scaled CFDST columns were fabricated and tested under a close-in explosion with scaled distances ranging from 0.116 m/kg1/3 to 0.232 m/kg1/3. After that, the failure mode and seismic behaviours of the CFDST columns, including one intact specimen, three postblast specimens, and three postblast specimens with retrofitting, were investigated via the cyclic loading test. Additionally, a modified composite damage index was proposed. The results showed that: (i) The CFDST column exhibited a combined localized crater and overall deformation under a close-in explosion, which became more severe as the scaled distance decreased. (ii) As the initial damage degree of the specimen increased, the likelihood of a plastic hinge developing in the blast-induced crater region to consume the hysteretic energy increased. (iii) A weak hinge formed at the crater caused by slight blast damage can assist the plastic hinge induced by the repeated lateral loading to enhance the energy dissipation capacity of the specimen. (iv) In the cyclic loading test, the lateral loading capacity, initial stiffness and energy dissipation of the specimens decreased with the increase in the initial degree of the blast-induced damage. (v) The deformation of the inner steel tube had an unfavourable influence on the stiffness and energy dissipation capacity of the postblast specimen with and without retrofitting. (vi) Among all the specimens, the retrofitted columns exhibited the worst lateral loading capacity, ductility, and energy dissipation capacity due to weld joint failure at the column foot. (vii) When the damage index is less than 0.5, the postblast CFDST column should be retrofitted for further use. In particular, the inner steel tube should be repaired. When the damage index exceeds 0.5, demolishing the postblast CFDST column is suggested.

Non-linear Analysis of Axially Loaded Octagonal CFDST Short Columns

Concrete Filled Double-skin Steel Tube (CFDST) is a form of composite column that consists of outer steel and an inner steel tube and the space between them is filled with concrete. The structural response of CFDST columns is significantly affected by the thickness of outer and inner steel tubes and the compressive strength of infilled concrete. Although many studies have been in the past to understand the effect of inner and outer tubes thickness on the axial compressive response of circular CFDST columns, no such study appears in the context of octagonal CFDST columns. In this paper, the thickness of both inner and outer tubes is varied. Their effect on axial compressive behavior of CFDST columns containing the concrete of different grades is numerically examined. The non-linear FE analysis of CFDST columns is performed using the ABAQUS. The study shows that axial strength and ductility of octagonal section CFDST columns are significantly influenced by steel tube thickness and the grade of sandwiched concrete. Moreover, the study highlights the importance of the proper selection of inner and outer tube thickness for the better axial response of CFDST columns.

Residual axial capacity of concrete-filled double-skin steel tube columns under close-in blast loading

Concrete-filled double-skin steel tubes (CFDSTs) are comprised of two concentrically placed steel tubes with the annulus between the tubes filled with concrete. A number of researches have established the prospect of CFDST columns as ideal alternatives for conventional reinforced concrete (RC) columns subjected to extreme events such as earthquakes, fires and vehicular collisions. Since structural columns are the most critical axial load-bearing members in structures, after experiencing blast loading the residual axial strength of key columns is crucial in preventing catastrophic damage of structural systems. Current knowledge on the residual axial load-bearing performance of CFDST columns is very limited. This paper studies the post-blast performance of CFDST columns after subjected close-in explosions, where the residual axial load-bearing capacity of blast-damaged columns is quantified using a high-fidelity physics-based (HFPB) numerical modelling tool. CFDST columns are built in the nonlinear dynamic analysis program LS-DYNA and are comprehensively validated against experimental results in the aspect of column damage profiles and residual axial capacity of blast-damage columns. Extensive parametric studies are then carried out to investigate the effect of several key design parameters, i.e., charge weight, standoff distance, column diameter, nominal steel ratio, column height and axial load ratios, on the residual axial capacity of CFDST columns after subjected close-in blast loads. The results demonstrated good residual axial load-bearing performance of CFDST columns after subjected to close-in detonations of man-portable improvised explosive devices (MPIEDs). Based on the numerical analysis results, an empirical formula is derived to predict the residual axial load-bearing capacity of CFDST columns after experiencing close-in explosions. The proposed empirical formula can be utilized to quickly evaluate the vulnerability of close-in blast damaged CFDST columns.

Experimental investigation into mild steel circular concrete-filled double skin steel tube columns

Constructing the concrete-filled double skin steel tube (CFDST) column involves placing concrete in between two steel tubes. Such tubes may have the same cross-section shape on the inside and outside, or they may have different cross-section shapes, as in the case of a square outer tube and a square inner tube, a circular outer tube, and a square inner tube, or a square outer tube and a circular inner tube, etc. In this article, the response of concrete-filled double-skinned steel tube columns (CFDST) to axial load is experimentally investigated. To create the CFDST column, two concentric mild steel circular tubes were filled with concrete. CFDST column behavior was illustrated using typical curves of axial load vs deformation and axial load versus longitudinal strain in the outer steel tube, inner steel tube, and concrete core. To further explore how changing the column's geometry and materials can affect its ultimate strength, parametric research was carried out. In the end, the experimental results were compared to the requirements specified in codes like Eurocode-4, AISC-360, ACI, and the equation given by Han et al. (2009).

Non Linear Analysis on CFDST columns with and without GFRP Wrapping

Abstract: Concrete Filled Double Skin Steel Tube (CFDST) columns are modified form of Concrete Filled Steel Tubular (CFST) columns, developed by several researchers. CFDST column composed of two concentric steel tubes in which concrete is packed in between them. It is available in various geometrical shapes. CFDST have many advantages such as high strength, high bending stiffness, good seismic and fire performance. But it is found that CFDST columns have some disadvantages including ageing of structures, corrosion of steel tubes etc. which reduces the strength of the column. So, to prevent these drawbacks and to strengthen the structure, Glass Fiber Reinforced Polymer (GFRP) can be used as an external cover. The present study aims to do nonlinear static analysis of CFDST columns having different hollow ratio wrapped with and without GFRP under axial compression using ANSYS finite element analysis software.

Axial Compression Behavior of Symmetrical Full-Scale Concrete Filled Double Skin Steel Tube Stub Columns

To investigate the bearing behavior of symmetrical full-scale different strength concrete filled double skin steel tube (CFDST) stub columns, 19 full-scale specimens were designed, considering the slenderness ratio (λ); the compression strength of core concrete and sandwich concrete (fcki, fcko), the thickness of the inner and outer steel tubes (ti, to); the diameter of inner and outer steel tubes (Di, Do); and the tensile strength of the inner and outer steel tubes (fyki, fyko) as the main parameters. Nonlinear constitutive models for concrete considering constraint effect were adopted, and a finite element (FE) model was established using ABAQUS software. By comparing the results between simulations and experiments, the rationality of the modeling method was verified. Based on the FE model, the parameter analysis for CFDST columns were conducted, and the force mechanism, stress distribution, and deformation process were analyzed. The results showed that the axial compression bearing capacity (Nsu) increased significantly with an increase in fcki, to, Do, and fcko, while Nsu decreased gradually with an increase in λ. Finally, according to the calculated results of the specimens, the calculation formula for Nsu of full-scale composite columns was statistically regressed using 1stOpt software and showed a good agreement with the FE.

Seismic performance of prefabricated joint for hollow CFDST column

While several studies have been conducted on the performance of concrete-filled double-skin steel tube (CFDST) members, only a few have evaluated the performance of prefabricated CFDST joints. In this study, we propose a prefabricated endplate bolted joint between the steel beam and CFDST column by performing a series of experiments while considering two effect parameters of the endplate type and thickness to achieve rapid construction and avoid wet work and on-site welding. Typical failure modes include plastic deformation of the endplate, local buckling of the flange end, slipping and fracture of the bolts, and fracture of the weld seam. Brittle fractures of bolts usually occur in the extended endplate joint and not in the flush endplate joint. In addition, the displacement ductility coefficients were all between 4.89 and 8.54, which indicated that the proposed joints possessed supreme ductility. Compared to the flush endplate, the extended endplate is capable of increasing the bearing capacity of the specimens by over 20%. Moreover, when the thickness of the endplate was increased to 8 mm, the maximum increment in the bearing capacity of the extended endplate joint and flush endplate joint increased by 35.91% and 30.88%, respectively. Based on the classification method in EC3, all specimens were the semi-rigid and partial strength joints. Furthermore, we propose a prediction method for the ultimate moment of the joint, which showed good agreement with the experimental results. We believe the results acquired in this study can provide a design basis for prefabricated CFDST structures.

Experimental investigation on the residual axial capacity of close-in blast damaged CFDST columns

Concrete-filled double-skin steel tubes (CFDSTs) are comprised of two concentrically placed steel tubes and concrete core filled in the annulus between the two tubes. A number of studies have demonstrated the excellent structural performance of CFDSTs as load-bearing columns under various loading conditions, which have attracted lots of interests of engineers and researchers. Despite the increasing popularity of using CFDSTs as load-bearing structural columns, there is limited knowledge on blast resistant performance of CFDST columns particular under close-in explosion. In this paper, an experimental study was conducted to investigate the residual axial load-carrying capacity of CFDST columns after being subjected to close-in explosion. Field blast tests were firstly carried out on CFDST columns. Then, the blast-damaged columns were subjected to axial compression till failure under quasi-static condition in the laboratory to examine their residual axial load-carrying capacities. Seven half-scale CFDST column specimens were constructed and tested, of which one column serves as a reference column that was directly tested under axial compression to check the ultimate axial load-carrying capacity of CFDST columns without blast damage. The influences of design factors including charge weight, stand-off distance, charge orientation, and section hollow ratio on blast resistant performance of CFDST columns were quantitatively assessed based on the residual axial capacity index. It was found that under close-in blast loading CFDST columns showed localized cross-section denting with slight global deformation. The damaged columns can maintain up to 60% of its ultimate axial load-carrying capacity and exhibited good strength and ductility.

Blast Analysis on In-Filled Steel Tube Columns

In recent years, a large number of studies have been carried out to investigate the behaviours of concrete filled double skin steel tube (CFDST) members due to its increasing popularity in the construction industry. This project aims to study on ultra-high performance concrete filled double-skin tubes subjected to blast loading with cross section being square for both inner and outer steel tubes using ANSYS software. It is evident that the proposed CFDST column was able to withstand a large blast load without failure so that it has the potential to be used in high-value buildings as well as critical infrastructures. The steel tubes and concrete work together well and integrity of steel concrete interface is maintained. Steel tubes in inner and outer can acts as permanent formwork and primary reinforcement. ANSYS results shows that the CFDA column can withstand applied blast load.

Experimental study on residual axial bearing capacity of UHPFRC-filled steel tubes after lateral impact loading

Concrete-filled steel tube (CFST) has been widely used in civil engineering. Numerous studies have shown that concrete-filled double-skin steel tube (CFDST) columns exhibit satisfactory mechanical properties under various loading conditions. Furthermore, ultra-high performance fiber-reinforced concrete (UHPFRC) has been developed as a prominent representative of new construction materials. This study investigates the residual bearing capacities of 10 concrete-filled steel tube specimens (three CFST columns and seven CFDST columns) after impact loading. The investigated parameters include the steel ratio, the concrete type, the section form, axial load ratio, the impact energy and the presence/absence of impact load. The results demonstrate that, when the inner steel tube thicknesses are identical, increasing the steel ratio from 0.296 to 0.467 (the outer steel tube thickness increases from 5 mm to 8 mm) can effectively improve the residual performance. For the non-impacted specimens, the bearing capacity of the CFDST columns is increased by 47.1% as the concrete type is changed from NSC to UHPFRC. For the impacted specimens, due to the influence of the test setup and other reasons, the concrete type has only a slight influence on the residual performance of CFDST columns; however, it has a substantial influence on the residual performance of the CFST columns. Furthermore, with the concrete type in the CFST columns changing from NSC to UHPFRC, the residual performance of CFST columns can be significantly enhanced.

Post-blast performance and residual capacity of CFDST columns subjected to contact explosions

Contact detonations, even with a small charge weight, could cause severe damage to structural columns and result in devastating consequences, associate with significant casualties, economic losses and social impacts. Therefore, to prevent catastrophic structural collapse, the most critical requirement of a blast-damaged column would be the residual axial load bearing capacity. In this paper, the post-blast performance of concrete-filled double-skin steel tube (CFDST) columns subjected to contact explosions was studied by experimental tests and numerical simulations. The experimental study comprised of field blast tests and residual axial load-carrying capacity tests on four CFDST columns. A high-fidelity physics-based numerical model was then developed using the nonlinear dynamic analysis program LS-DYNA and was validated against the test results. It was then used to model the dynamic response and predict the residual axial load-carrying capacity of CFDST columns subjected to contact explosions. An extensive parametric study was then carried out to investigate the influence of charge weight, boundary conditions, axial loading level and column geometries on the column residual axial load-carrying capacity. An empirical formula was derived through multi-variable regression analysis to predict the residual axial bearing capacity of CFDST columns after contact explosions.

Experimental and Analytical Behavior of Square CFDST Column Blind Bolted to Steel Beam Connections

In recent years, steel or composite beam-column connections adopting blind fasteners have drawn increasing praise due to the rapid development of assembled steel structures. However, limited researches have paid attention to the experimental and analytical behavior of concrete filled double-skin steel tube (CFDST) column blind bolted joints. For investigating the structural performance and seismic behavior of this type of connection, cyclic tests on blind bolted joints to square CFDST columns were carried out to explore the effects of column hollow ratio and end plate type. Finite element analytical modelling of the semi-rigid joint was performed considering complex contact interactions and material models. Good agreement between the test and analytical results was observed in terms of the failure modes and the hysteretic behavior. Substantial parametric analyses were conducted on typical CFDST column connections to observe the influence of parameters including strength of steel tube, column hollow ratio and bolt pretension force. Furthermore, certain constructional measures commonly employed in engineering practice were also discussed. It was concluded that the CFDST column blind bolted to the steel beam joint has favorable seismic behavior and is feasible for application in high-intensity earthquake regions.

Seismic performance of concrete-filled double-skin steel tubes after exposure to fire: Analysis

This paper presents numerical analysis on the seismic performance of concrete-filled double-skin steel tubes (CFDST) after exposure to fire. The finite element model with multiple steps is established with considerations of degradations of material properties due to elevated temperatures and post-fire cyclic loading. The numerical model is verified by test results reported in the companion paper and data available from literature. The numerical model is then used to explore the post-fire seismic performance of CFDST columns, where the temperature field, the internal force and stress distribution are analysed. Parametric studies are also carried out, to discuss the influence of the loading path, the duration of heating, the axial load level, the nominal steel ratio, the hollow ratio of the cross section, etc. It is found that the proposed numerical model is capable to simulate the structural behaviour under difference phases, and the loading path could affect the post-fire seismic performance of CFDST columns.

The influence of the inner steel tube on the compression behaviour of the concrete filled double skin steel tube (CFDST) columns

Concrete Filled Double Skin Steel Tube (CFDST) columns which comprise two concentric steel tubes and shell concrete between them is a favourable member of composite columns family. The CFDST columns have the potential to offer equal or superior characteristics compared to the classical Concrete Filled Steel Tube (CFST) counterparts. Although CFDST columns have been the focus of experimental and numerical studies, there is limited knowledge on the contribution of the inner steel tube to the compression performance. Since the inner steel tube is the key member which motivates the deployment of the CFDST columns in the field, it is imperative to characterize its contribution. This paper conveys the findings of an experimental study which was conducted to explore the effect of the inner steel tube on the mechanical response of CFDST columns under axial compression. Two different D/t ratios and concrete classes (normal strength and high strength) were utilized and in total sixteen specimens were tested to failure. To be able to analysis the impact of the parameters, failure modes, compression loading versus end shortening curves and the concrete-steel contribution ratios were detailed. The results show that the CFDST columns could attain greater performance over CFST columns when a proper configuration of the inner steel tube is used. Furthermore, the inner tube works consistently with the shell concrete and plays an important role in mitigating failure mode of CFDST even with high strength concrete (HSC). Finally, the prediction formulations were employed and examined against the test results to specify the convenient method for design processes.

Mechanical behaviour of concrete-filled double-skin steel tube (CFDST) with stiffeners under axial and eccentric loading

This paper presents an experimental investigation of the mechanical behaviour and working mechanism of the concrete-filled double-skin steel tube (CFDST) with stiffeners. A total of 40 CFDST columns were prepared and subjected to axial and eccentric loading. The failure modes and ultimate load-bearing capacities, together with the deformation and strain responses of the specimens, were analysed. The effects of the stiffener type, slenderness ratio, and eccentricity ratio were researched in this study. The test results indicated that the ductility of the CFDST specimens was improved greatly by the presence of stiffeners, which gave a lower load-bearing capacity owing to the reduction in the effective area of the inner concrete. Moreover, an analytical-numerical model was developed to analyse the mechanical behaviour of the CFDST specimen, and an improving mechanism was considered. In addition, theoretical estimations to predict the ultimate load-bearing capacities of CFDST specimens subjected to axial and eccentric loading were established. These estimations showed good agreement with the theoretical results and the test results.