Concrete-filled Double Skin Tubular Research Articles

Stress Concentration Factors of Concrete-Filled Double-Skin Tubular K-Joints

Tubular joints are important connecting parts of a welded steel tube structure. The S-N curves based on the hot spot stress (HSS) method are often used to evaluate the fatigue life of tubular joints in practical engineering. The stress concentration factor (SCF) is a key parameter to calculate HSS. In this paper, stress concentration tests of hollow-section and concrete-filled double-skin tubular (CFDST) K-joints were carried out, respectively, and then finite element models of K-joints considering the weld were established. The developed models were validated with the experimental results. The influence of key geometrical parameters, such as the diameter ratio of brace to chord β, the diameter to thickness ratio of chord γ, the wall thickness ratio of brace to chord τ, brace angle θ, and hollow section ratio ζ on the distribution and key position of SCFs along the weld toe, was discussed. Parametric studies were conducted to obtain the calculating equations for the SCF values of CFDST K-joints. The results demonstrate that infill concrete can effectively reduce SCFs along the weld on the chord. When the hollow section ratio was reduced to 0.317, the SCF was reduced by 77.2%. Notably, the SCF reduction rate was sensitive to γ and θ, with a decrease observed as γ increased. The hollow section ratio ζ had a less pronounced effect on SCF distribution patterns, but as ζ decreased, the chord’s stiffness improved, suggesting a potential approach to enhance joint performance. The distribution of SCFs is similar for joints of the same type but different geometric configurations. The innovatively integrated hollow section ratio in the CFDST design equation significantly simplifies and enhances the precision of SCF calculations for CFDST K-joints.

Testing, numerical analysis and design of CFDST cross-sections with square stainless steel outer tubes in bending

The structural performance and design of concrete-filled double skin tubular (CFDST) cross-sections with square stainless steel outer tubes are studied herein. A total of 17 four-point bending tests on CFDST cross-sections with varying concrete grades, together with accompanying material tests, were first conducted. The details of the test rig and procedures, as well as the key experimental results are reported. Following the physical testing, a numerical modelling campaign was carried out. A finite element (FE) model was initially validated against the tests, and then adopted to conduct a parametric study to acquire further FE data, covering a broader spectrum of material strengths and cross-section slendernesses. The obtained test and FE results were used to evaluate the applicability of the general design provisions for concrete-filled carbon steel members in the current European and American design codes. Overall, the examined design codes are shown to provide unduly conservative (less so for the higher concrete grades) and rather scattered moment resistance predictions, though some moment resistances predicted using the European code were on the unsafe side. Modifications to the European design treatment in relation to the assumed stress distribution, to take due account of the partial spread of plasticity in the outer tube, and the effective compressive strength of the concrete infill, to reflect the reduced relative effectiveness of using higher concrete grades, are proposed and shown to improve the consistency of the resistance predictions.

Study on axial behaviour of concrete filled steel tubular columns

Concrete-filled steel tube (CFST) columns have gained wide acceptance in the construction of high-rise buildings due to their exceptional ability to withstand axial load. They are commonly employed as columns in large-span high-rise buildings, bridges, and piers. This research aims to investigate the performance of concrete-filled steel tube (CFST) columns when subjected to axial load. One key advantage is that the steel tube serves as a permanent confinement for the concrete. The concrete filling inside the tube prevents local buckling and enhances the lateral stability of the member. Moreover, the external steel tube acts as a formwork. Eight composite columns were constructed, comprising four CFST columns and four CFDST (Concrete-filled Double Skin Tubular) columns, and were subjected to axial loading for evaluation. The experimental study was conducted to determine the relationship between load, axial displacement, and lateral displacement, as well as the ultimate load-carrying capacity. Furthermore, finite element analysis using Abaqus was performed to investigate the composite columns analytically. A parametric study was also carried out, considering the diameter-to-thickness ratio and grade of concrete. Confinement significantly enhances the load-carrying capacity, and the presence of concrete infill reduces the lateral deflection of long columns.

Cross-wind dynamic response of concrete-filled double-skin wind turbine towers: Theoretical modelling and experimental investigation

Wind turbine towers are growing taller to obtain larger thrust load and capture more wind energy at higher elevations. To address the excessive vibration problem of conventional steel tube (ST) towers, a concrete-filled double-skin tubular (CFDST) tower can be a great alternative owing to the advantages of excellent mechanical properties and cost efficiency, especially for tall and slender wind turbines. This paper developed a theoretical model of the CFDST-based wind turbine system by combining the Lagrangian method with the Galerkin method. Subsequently, the analytical solutions of both fundamental frequency and cross-wind displacement response were obtained. The wind tunnel tests of scaled CFDST and ST wind turbine systems, with variations in hollow ratio, tube length, and wind speed, were used to verify the proposed model. The predicted results from the analytical model showed reasonable agreement with the test results. Finally, the validated analytical solutions were employed to further analyze the influence of hollow ratio, length-to-diameter ratio, and tip-mass ratio. This study makes a contribution to the integration analysis of the CFDST-based wind turbine system in theory and experiment, and develops a simplified method to predict its dynamic behavior.

Comparative study on seismic performance of concrete-filled double skin tubular piers and hollow concrete piers: Experimental and analytical

Hollow bridge piers with concrete-filled double skin tubular (CFDST) section could offer more efficient fabrication and better confinement effect, but increase construction cost with the use of steel tubes if compared to regular hollow concrete bridge piers. Although hollow concrete piers and CFDST piers have been the focus of the existing experimental and numerical studies, there is limited information available regarding to the seismic performance comparisons of the two systems while considering the cost perspective. Therefore, a comparative study was carried out in the current study to compare the seismic performance of the two designs quantitatively, while considering the cost effectiveness. The experimental results of the two designs were compared in terms of lateral force bearing capacity, deformation capacity, energy dissipation, stiffness degradation, residual displacement, and performance per cost ratio. It has been found that the lateral force bearing capacity, the displacement ductility, the effective pier flexural stiffness and the performance per cost ratio of CFDST pier increased by 50.2%, 16.7%, 22.9% and 36.5%, respectively, while the residual drift ratio reduced by 33.3% at a large drift ratio of 3.0%. In addition, the seismic performance of another two CFDST piers with different diameter-to-thickness ratio of outer steel tubes and hollow ratio was studied through cyclic lateral loading tests. A verified finite element model was also developed to further conduct parametric study to fully investigate the influence of the diameter-to-thickness ratio of outer steel tubes and hollow ratio on the seismic performance of CFDST piers. Experimental and numerical results have found that the diameter-to-thickness ratio of outer steel tubes is one of the most important factors that influences the seismic performance of CFDST piers. In contrast, the hollow ratio significantly influences the deformation capacity of CFDST piers without significantly affecting the lateral force bearing capacity. The diameter-to-thickness ratio of outer steel tubes is suggested to take between 60 and 150, and the hollow ratio is suggested to take between 0.5 and 0.7 for cost effectiveness and ductile seismic design purposes of CFDST bridge piers.

Design of stainless steel CHS-concrete infill-carbon steel CHS double-skin stub columns

Concrete filled double-skin tubular (CFDST) stub column with outer stainless steel circular hollow section (CHS) and inner carbon steel CHS is presented in this paper. It combines the strength of stainless steel, concrete and carbon steel, and also provides the stainless steel's durability, ductility and corrosion resistance properties along with the lighter weight due to the sectional feature. Finite element analysis (FEA) is done in this paper and the developed FE models are verified with the available test results on the basis of failure modes, full load-deformation curves and ultimate strengths of the specimens. Afterward, a series of parametric analysis are conducted to investigate the structural performance of each component and the overall column as well as the confining stresses between the tubes and concrete. In addition, the effect of stainless-steel grade, concrete strength, carbon-steel strength, diameter-to-thickness ratio of outer tube, diameter-to-thickness ratio of inner tube and hollow ratio on confining stress and mechanical property of sandwiched concrete is studied. The constitutive models of sandwiched concrete and outer stainless steel tube under plane stress are proposed. Based on these proposed models, the fiber element model is used to predict the mechanical properties of this composite column and then checked with the finite element modelling (FEM) outputs. Different design methods such as ACI code, European code, continuous strength method (CSM), Han et al. method and Nie et al. method are assessed to evaluate the applicability and accuracy. Finally, a new design method for calculating the ultimate bearing capacity of the CFDST stub column is proposed, which exhibits good agreement with the results of the FEA.

Influences of the strengthening methods on axial and eccentric compressive behaviors of circular concrete-filled double-skin tubular columns

This study presents a comprehensive investigation on the compressive behaviors of concrete-filled double-skin tubular (CFDST) columns with two different types of strengthening methods (installations of reinforcement rings and steel nails). A total of five specimens were tested under axial and eccentric compression, and their damage patterns, load-displacement curves, and strain developments were studied. Results revealed that under axial compression, the ultimate load bearing capacity could be enhanced by around 15% by installing the reinforcement rings, and the occurrence of peak load could be postponed after the yielding of steel tube. This is because the reinforcement rings restrained the steel tube buckling at the loading ends. The steel nails had more contributions in ductility performance, when the column was under eccentric compression. The steel nails helped maintain the integrity of steel tubes which could provide confinement to the annular concrete to delay or alleviate the concrete deterioration and crushing failure, ultimately reducing the lateral deflection of the column. A finite element model has been developed and verified against the test results with less than 10% error in predicting the peak load capacities. The model may be used in future studies in the behavior of CFDST columns with less investigation costs.

Corrosion effect on the flexural behaviour of concrete filled steel tubulars with single and double skins using engineered cementitious composite

Concrete filled double skin tubular (CFDST) members are considered one of the most advanced construction composite forms, consisting of inner and outer tubes with concrete sandwiched between them. This paper presents an experimental study and theoretical analysis of the flexural performance of corroded concrete filled steel tubular (CFST), compared to CFDST beams with engineered cementitious composite (ECC) and normal concrete (NC). ECC is characterised by excellent tensile and flexural behaviour with crack control capacity, which shows great potential for enhancing the flexural behaviour of CFDST, especially in aggressive marine environments. In this study, a 500 µA/cm2 current density was implemented for 170 and 250 h to produce accelerated corrosion with mass losses of 6 % and 9 %, respectively, to investigate the effect of corrosion on the flexural behaviour of regular CFDST and CFDST with different types of infill material. Moreover, due to the shortcomings of normal concrete in preventing both the outward and inward buckling of steel plates, in this study an ECC was incorporated as the infill material between steel tubulars. The experimental results of all samples, including CFST and CFDST, showed that corrosion affects the flexural stiffness, ductility index, and energy absorption more than the ultimate moment strength. In addition, the highest impact of corrosion was for CFST with NC (CFST-NC), whereas the lowest effect was observed in CFDST with ECC (CFDST-ECC). Moreover, there is a lack of consideration regarding the impact of corrosion on the predicted flexural stiffness provided by existing codes. Hence, in this study, a theoretical flexural stiffness analysis was conducted and compared with various codes such as EC4, AIJ, BS5400, and AISC.

Compressive behaviour of double skin sections with stainless steel outer tubes and recycled aggregate concrete

An experimental and numerical study into the behaviour of concrete-filled double skin tubular (CFDST) stub columns is presented. A total of eight axial compression tests were carried out, four utilising conventional concrete and four with recycled aggregate concrete. The stub columns were circular in cross-section and each comprised an austenitic stainless steel outer tube and a carbon steel inner tube, of varying dimensions. Accordingly, hollow ratios of 0.67 and 0.55 were considered. The recycled coarse aggregate was made by crushing test specimens from a previous research project, and a replacement ratio of 50% was adopted. During the experiments, similar structural behaviour and failure modes were observed between the specimens with conventional and recycled aggregate concrete. To investigate the behaviour further, a finite element model was developed in ABAQUS; validation of the model against the experimental results from the current work as well as data available in the literature is described. The finite element model was employed to conduct a parametric study to examine the load-bearing contributions of the constituent components of CFDST sections and to assess the influence of the hollow ratio on the structural behaviour. The experimental and numerical ultimate loads are compared with the capacity predictions determined using available design procedures. Overall, the results show that CFDST stub columns with recycled aggregate concrete can achieve similar capacities to their conventional concrete counterparts, demonstrating the potential for the wider use of recycled aggregate concrete, towards more sustainable structural solutions.

Compressive Behaviour of Square CFDST Short Columns with Double Inner Steel Tubes

This study focuses on the experimental analysis of concrete-filled double-skin tubular (CFDST) short columns with double circular inner steel tubes under concentric axial loading. This cross-section layout promises to increase the ductile behavior of the compressive element and its energy absorption capacity, not to mention its ultimate axial strength. This research analyses main twofold variables: (i) Hollow ratio and (ii) eccentricity ratio (i.e., distance ratio between the inner tube’s separation and the sandwiched concrete width). As a result, load Vs. Axial deformation, load Vs. Axial strain curves, ductility index, strength index, energy absorption capacity, and ultimate axial capacity formulae for square CFDST columns with double circular inner tubes are reported. Key findings of this study show that (a) ductility is an intrinsic property of these types of sections; (b) the variation effect of hollow ratio influence inversely in the confined concrete strength of the element, (c) Eccentricity ratio has proved to be the least ultimate strength capacity influencer. However, its impact increases when it is jointly analyzed with the hollow ratio values. (d) The formulae proposed for predicting the ultimate capacity of CFDST columns showed good agreement with the experimental results. Thus, these expressions could be extended to the design of composite CFDST elements, provided a resistance factor based on a reliability analysis is incorporated.

Estimating the Axial Compression Capacity of Concrete-Filled Double-Skin Tubular Columns with Metallic and Non-Metallic Composite Materials.

This research focuses on estimating the ACC (axial compression capacity) of concrete-filled double-skin tubular (CFDST) columns. The study utilised algorithms and ‘six’ evaluation methods (XGBoost, AdaBoost, Lasso, Ridge, Random Forest Regressor and artificial neural network (ANN) architecture-based regression) to study the empirical formulae and utilise the parameters as the research’s features, in order to find the best model that has higher and accurate reliability by using the RMSE and R2 scores as performance evaluation metrics. Thus, by identifying the best model in empirical formulae for estimating the ACC of CFDST, the research offers a reliable model for future research. Through findings, it was found that, out of the existing evaluation metrics, the ABR for AFRP, GFRP and Steel; RFR for CFRP; and RR for PETFRP were found to be the best models in the CFDST columns.

Performance of CFDST beams using high-strength steel under bending

The high-strength steel (HSS) has been used in steel–concrete composite constructions as it is beneficial for improving structural performance and reducing structural self-weight. This study presents experimental and numerical studies on the bending behaviour of concrete-filled double-skin tubular (CFDST) beams using the HSS with yield strength of 618 MPa. The results showed that the bending resistance of CFDST member was substantially increased and the high strength material was fully exploited. A parametric study was performed using the verified finite element model. In addition, the compatibility of various concrete classes and steel grades was discussed. The design methods from current code of practice were also evaluated for the prediction of bending resistance of circular CFDST beams using HSS tube.

Axial behavior of novel CFDST columns with outer welded corrugated steel tubes

A novel type of welded corrugated steel tube-concrete-flat steel tube composite structural column was proposed, and the axial compressive behavior of this column type was evaluated by testing of 14 short column specimens. The test parameters included hollow ratios, inner tube diameter-thickness ratios, and concrete strength and reinforcement ratios. The test results indicated that the outer corrugated steel tube could provide good hoop confinement to the core concrete. The welded corrugated steel tube-concrete-flat steel tube composite structural columns failed with serious extrusion of the wave crests and inward buckling of the inner steel tubes. The effects of the test parameters on the failure modes, strength, and ductility and strain responses were also discussed. Moreover, some existing design models for concrete-filled double skin tubular (CFDST) columns were assessed based on the test results available in literature and this study, and new models for CFDST columns and welded corrugated steel tube-concrete-flat steel tube columns were proposed. The proposed analytical models were found to be in good agreement with both the test results of the specimens with outer flat steel tubes and corrugated steel tubes under axial compression.

Experimental Assessment of Stainless and Carbon Steel Double‐Skin Tubular Stub Columns Filled with Recycled Aggregate Concrete

AbstractThis paper presents an experimental study into the behaviour of concrete filled double skin tubular (CFDST) stub columns. A total of eight axial compression tests were carried out, half of which were filled with conventional concrete and the remainder had recycled aggregate concrete. The columns were circular in cross‐section and comprised an austenitic stainless steel outer tube and a carbon steel inner tube, of varying dimensions. Accordingly, hollow ratios of 0.67 and 0.55 were considered. The recycled coarse aggregate was made by crushing test specimens from a previous research project. A replacement ratio of 50% was adopted in these tests. Both the natural coarse aggregates and the recycled aggregates had a maximum diameter of 9.5 mm. During the tests, similar structural behaviour and failure modes were observed between the columns with conventional and recycled aggregate concrete, and the faiure modes were identical. The experimental results are compared with available design procedures and it is ultimately shown that using recycled aggregate concrete rather than regular concrete in CFDST stub columns does not have a significant effect on the performance.

STUDY ON THE FIRE PERFORMANCE OF CONCRETE-FILLED DOUBLE-SKIN TUBULAR COLUMNS WITH EXTERNAL STAINLESS STEEL TUBES

To investigate the fire performance of concrete-filled double-skin tubular (CFDST) columns with external stainless steel tubes, the fire resistance of this kind of composite members was analyzed by ABAQUS. The differences in the fire mechanisms between CFDST columns with external stainless steel tubes and those with external carbon steel tubes were studied. The effects of the cross-sectional dimension, hollow ratio, load ratio and material strength on the fire resistance of components were analyzed. The results show that the CFDST columns with external stainless steel tubes present better fire resistance than those with external carbon steel tubes. The fire resistance of members was greatly affected by the cross-sectional dimension and load ratio. The hollow ratio and material strength had a moderate influence on the fire resistance of the members. Based on a parametric analysis, a design method for the fire safety of CFDST columns with external stainless steel tubes was suggested.

Experimental investigation on blast furnace slag aggregate concrete filled double skin tubular (CFDST) stub columns under sustained loading

As significant growth of iron production in China in recent decades, plenty of blast furnace slag is generated. Meanwhile, the policies on the natural aggregate exploitation has been getting more restricted as it threats the environmental conditions. There is an alternative solution to the above issues that furnace slag aggregate concrete is filled in middle between double-skin tubular (DST) columns with external stainless steel tubes. This study conducted a series of tests on furnace slag aggregate concrete filled double-skin tubular (CFDST) stub columns under sustained loading condition. The experimental investigations were conducted on circular specimens with furnace slag aggregate replacement ratio of 100%. The hollow ratio of the specimens ranged from 0.5 to 0.75. A total of 8 specimens were tested to capture the behaviours of the creep, shrinkage, ultimate strength before and after 6 months sustained loading. The results indicated that the shrinkage strain of the sandwiched furnace slag aggregate concrete tended to stabilise after 30 days and the creep developed constantly throughout the 180 days of the test. The furnace slag aggregate CFDST specimens with stainless steel tubes have demonstrated well load carrying capacity and ductility under sustained loading. The existing design methods were evaluated on the applicability on the stainless steel-furnace slag aggregate specimens under sustained loading and reasonable predictions were obtained.

Compressive behaviour and design of CFDST cross-sections with stainless steel outer tubes

A finite element (FE) investigation into the compressive behaviour of concrete-filled double skin tubular (CFDST) cross-sections with lean duplex and ferritic stainless steel outer tubes is presented. FE models were initially developed and validated against available test results reported in the literature. Upon successful replication of the ultimate capacities, load–deformation histories and failure modes exhibited by the tested CFDST stub columns, a parametric study was undertaken to investigate the influence of key variables, including the local slendernesses of the outer and inner tubes, the concrete strength and the adopted grade of stainless steel, on the ultimate response of the studied CFDST stub columns. Based on the generated FE data pool and the available test results, the applicability of the existing European, Australian and American design provisions for composite carbon steel members to the design of the studied CFDST cross-sections was evaluated. All the examined design rules are shown to yield unduly conservative (less so for the higher concrete grades) and rather scattered capacity predictions. Modifications to the design treatment in relation to the effective area of the outer tubes, to take due account of outward-only local buckling, and the effective compressive strength of the infilled concrete, to reflect the reduced relative effectiveness of higher concrete grades, are considered. The modified design rules are shown to improve the accuracy and consistency of the design capacity predictions. Finally, statistical analyses were carried out to demonstrate the reliability of the modified design approaches.

CFDST sections with square stainless steel outer tubes under axial compression: Experimental investigation, numerical modelling and design

The use of concrete-filled double skin tubular (CFDST) cross-sections for compression members has become increasingly popular in construction. A recently proposed innovative form of CFDST cross-section, ultilising stainless steel for the outer tube, offers the combined advantages of the composite action seen in CFDST member alongside the durability and ductility associated with stainless steel. CFDST sections with stainless steel outer tubes, for which there are currently little experimental data, are the focus of the present study. A comprehensive experimental and numerical investigation into the compressive behaviour of CFDST sections with square stainless steel outer tubes is presented in this paper. A total of 19 specimens was tested under uniform axial compression, and the test observations are fully reported. The ultimate loads, load-displacement curves and failure modes from the tests were used for the validation of finite element (FE) models. Parametric finite element analyses were then performed. The combined set of experimentally and numerically derived data was employed to assess the applicability of the existing European, Australian and American design provisions for composite carbon steel members to the design of the studied CFDST cross-sections. Overall, the existing design rules are shown to provide generally safe-sided (less so for the higher concrete grades) but rather scattered capacity predictions. Modifications to the current design codes are also considered—a higher buckling coefficient k of 10.67 to consider the beneficial restraining effect of the concrete on the local buckling of the stainless steel outer tubes, as well as a reduction factor η to reflect the reduced relative effectiveness of higher concrete grades. Overall, the comparisons demonstrated that improved accuracy and consistency were achieved when the modified design rules were applied.

Analytical Behaviour of Stiffened and Unstiffened CFDST Short Column

Concrete filled steel tubular (CFST) members comprise of a hollow steel section of circular or rectangular shape filled with plain or reinforced concrete. Concrete-filled double-skin tubular (CFDST) columns are formed by sandwiching concrete between two concentric hollow steel tubes. When compared to a traditional concrete-filled steel tube (consisting of a single hollow steel tube instead of two), the CFDST column is found to have greater axial, flexural, and torsional strengths as well as improved strength-to-weight ratios. They are widely used in high-rise and multi-story buildings as columns and beam-columns. This paper presents the analytical results obtained by using ANSYS software and to develop various kinds of CFDST columns by varying cross-sections, by providing stiffeners and by changing the position of the reinforcements. And also to plot the load-deformation curve and to evaluate the load-carrying capacity of the different models. From the analysis, it is concluded that the square–squaren CFDST with plain stiffeners attached to the outer steel tube has the best axial load carrying capacity.

Experimental Study of Square and Rectangular CFDST Sections with Stainless Steel Outer Tubes under Axial Compression

AbstractA comprehensive experimental investigation into the axial compressive response of concrete-filled double skin tubular (CFDST) sections with stainless steel square and rectangular outer tube...