Concrete-filled Double Skin Steel Tubular Research Articles

A REVIEW ON CONCRETE FILLED DOUBLE SKIN STEEL TUBULAR COLUMNS

Concrete-filled double-skin steel tubular (CFDST) Columns considered as a new type of Concrete Filled Steel Tubular (CFST) Columns. It consists of an inner and outer steel tube with the annulus between the skins filled with concrete. The concrete is properly compacted and filled in between the steel surfaces. CFDST are high performance composite columns that are increasingly being used in bridges and high-rise buildings as well as to reinforce CFST columns. CFDST has many advantages over CFST such as improved stability, section modulus, lighter weight and better damping characteristics. The inner tube increases their strength and ductility as compared to CFST. The hollow steel tubes can be chosen with any type of cross section, commonly circular and square tubes are preferred.

Analytical behavior and bearing capacity research on out-of-code tapered CFDST members under pure torsion and compression-torsion combination

Facing the apparent enhancement of rated power of offshore wind turbines, the increasing rotor diameter and blade length gradually pose severe challenges to its support structure for resisting extreme loads. Due to the remarkable composite effect, the recently developed tapered concrete-filled double skin steel tubular (CFDST) members can provide a high-performance structural type and competitive application prospect compared to traditional steel tower barrels. Therefore, this paper conducted the numerical and theoretical research on out-of-code tapered CFDST members under pure torsion and compression-torsion combination. The finite element (FE) model was established to examine the full-range mechanism including torque versus deformation response (T-θ curve), interaction behavior and component contribution, indicating that four characteristic points can distinguish the T-θ curve under pure torsion, and slight contact pressure between the inner tube and sandwich concrete exists along the height direction. The parametric study was performed to investigate the influences of geometric-physical parameters, e.g., by increasing tapered angle (ψ) 0.2°, the torsional capacity obtains a reduction of 5%; hollow ratios (χ), axial compression ratios, and diameter-to-thickness (D/t) ratios of double-skin tubes obviously affect the torsion resistance; altering steel yield grade influences torsional capacity than concrete grade. Subsequently, a theoretical multilayered cylinder model as the simplified design tool was proposed to simulate full-range T-θ curves of tapered CFDST members. Moreover, a modified design method for calculating N-T correlation curve was developed and validated by introducing the refined equations of pure torsion resistance (Tu), where the design methods in current codes displayed a quite conservative prediction for tapered CFDST members with out-of-code parameters (e.g., hollow ratios or D/t ratios). The above research on out-of-code tapered CFDST members can offer valuable design guidelines for its engineering applications.

Experimental study on torsional behavior of tapered high strength thin-walled concrete-filled double-skin steel tubular columns

Comparing with traditional straight concrete-filled double-skin steel tubular (CFDST) members, tapered CFDST members have been increasingly used in the domain of power industry due to esthetics and special spatial experience. In addition, the utilization of high strength materials can reduce cross-sectional area and further lower construction cost. In this paper, the torsional behavior of tapered high-strength thin-walled CFDST (THSTW-CFDST) columns with or without longitudinal stiffeners was investigated and a total of twenty tapered THSTW-CFDST specimens were tested. The failure modes, torque-torsional angle curves and torsional capacity of the THSTW-CFDST columns were investigated. The results demonstrate that the installation of stiffeners can significantly improve the local stability of thin-walled steel tubes, but reduce the torsional capacity of the members. Compared with THSTW-CFDST specimens without stiffeners, the torsional capacities of those with stiffeners have been apparently reduced. Based on the experimental results, the finite element (FE) model was developed and verified, and then utilized to ascertain the force-transfer mechanism and the stress development process of each component of THSTW-CFDST columns. In addition, the effects of the various parameters including hollow ratio, diameter-to-thickness ratio, inner steel tube strength to outer steel tube strength ratio, concrete strength the torsional behavior of THSTW-CFDST members were quantitatively assessed. Finally, the accuracy of the existing methods for predicting the torsional capacity of THSTW-CFDST members was evaluated.

Hybrid simulation test and seismic assessment of CFDST moment resisting frame

Concrete filled double skin steel tubular (CFDST) moment resisting frame is an effective structural system owing to the higher strength to weight ratio and better fire performance compared with traditional concrete filled steel tubular (CFST) structures. This paper aims to analyze the seismic response of assembled CFDST moment resisting frame with blind bolted joints by the hybrid simulation test and numerical approaches. A prototype CFDST moment resisting frame was designed and its numerical model was established by Opensees software. After the validation of the numerical model, typical failure modes and dynamic response of the CFDST moment resisting frame observed from the hybrid simulation test were detected and discussed under maximum-considered earthquake (MCE) and 2MCE levels (2 times MCE level), respectively. Detailed behavior of the columns and joints were analyzed for the prototype building. It verified that a large portion of energy was dissipated by the joints under seismic loads. Subsequently, parametric analyses were performed to study the influence of joint stiffness on the global behavior of CFDST moment resisting frames by the endurance time method. The energy dissipated by the columns and joints were investigated in detail, indicating that the energy dissipated by the joints was more than 70% of the total hysteretic energy of the frame under earthquakes. It indicated that decreasing the joint stiffness was beneficial for the joint to dissipate more energy and for protecting the column from being damaged. It also generalized that decreasing the joint stiffness had a positive contribution to the uniform story drift distribution and would not always increase the maximum story drift. The analysis results verified that the CFDST moment resisting frame exhibited excellent seismic behavior and activated the application of CFDST moment resisting in engineering practice.

Experimental and numerical investigation on hysteretic behavior of posttensioned precast segmental Concrete-filled double skin steel tubular piers

The posttensioned precast segmental concrete-filled double skin steel tubular (CFDST) piers proposed in this study combines the advantages of posttensioned precast segmental piers and composite CFDST piers, in such way to reduce on-site construction burdens, increase section modulus, and reduce self-weight. To investigate the hysteretic behavior of this new type of bridge pier, one posttensioned precast segmental CFDST pier was designed and tested under cyclic lateral loading, with one cast-in-place CFDST pier as reference. Test results show that the proposed posttensioned precast segmental pier performs well with good ductility, satisfactory energy dissipation, and small residual displacement. The damage to precast segments is effectively avoided by using CFDST members. Moreover, a numerical study was also conducted to investigate the influences of different design parameters on the hysteretic behavior of posttensioned precast segmental CFDST piers. The design parameters studied in this paper included the pier aspect ratio, amount of energy-dissipating (ED) bars, axial load ratio due to gravity, axial load ratio due to prestressing force, initial stress level in prestressing strands and CFDST cross-sectional parameters. It has been found that the amount of ED bars, axial load ratio due to gravity and axial load ratio due to prestressing force are the more important factors on energy dissipation and self-centering capacities. The pier aspect ratio being greater than 7 has a negative effect on residual drift ratio of posttensioned precast segmental CFDST piers due to P-Δ effect. The initial stress level in prestressing strands has little effect on energy dissipation and self-centering capacity, but influences the failure mode and deformation capacity.

Coupled behaviour and strength prediction of tapered CFDST columns with large hollow ratios for wind turbine towers

Concrete-filled double-skin steel tubular (CFDST) tower has emerged as a promising type of wind turbine support structure with excellent strength and stiffness. As the basic member in CFDST towers, tapered concrete-filled double-skin steel tubular columns with large hollow ratios (TLHR-CFDSTs) are frequently subjected to complex combined compression-bending-torsion loads. This paper presents a comprehensive analytical study on the coupled behaviour of the TLHR-CFDST column under combined loads and the relevant ultimate strength prediction method. A FE model was established to conduct numerical analyses and extensive parameter studies while the data from the previous experimental research was used to provide a verification basis. It is found that the column subjected to combined loads has three typical failure patterns, which are mainly governed by the bending-to-torsion ratio (mt). Columns with moderate mt undergo the bending-torsion failure and show obvious coupled performances. The load-deformation relation, the ultimate characteristics and the effects of important parameters, including axial compressive ratio (n), tapered angle (θ), hollow ratio (χ) and height-to-diameter ratio (λ) were discussed. The ultimate strength correlations of the column were revealed by analysing strength data within the parameter scopes commonly used in wind turbine towers. Lastly, a practical and accurate prediction method for the ultimate strength of the TLHR-CFDST column under combined compression-bending-torsion loads was proposed.

Experimental and analytical research on seismic performance of ECC-filled high-strength double skin steel tubular columns with out-of-code D/t ratios

Against the offshore construction requirement on light weight and high performance, using high-strength (HS) steel in concrete-filled double skin steel tubular (CFDST) members usually results in out-of-code diameter-to-thickness (D/t) ratios and performance uncertainty. A quasi-static cyclic loading test was therefore conducted to investigate the seismic performance of circular-in-circular ECC-filled high-strength double skin steel tubular (EFHDST) columns with out-of-code D/t ratios. Typical failure mode, hysteretic curves, bearing capacity and ductility were analyzed. Subsequently, the finite element (FE) model incorporating material damage effect was established and verified to conduct further parametric study. Influence of geometric parameters (e.g., wall thickness and hollow ratio), material strength and axial compression level was discussed in detail. Finally, a simplified restoring force model was developed to predict the seismic behavior of EFHDST columns. The result indicates that: the EFHDST columns fail by the ECC crushing, steel fracture and local buckling of double-skin tubes; the steel yield strength and thickness of outer tube, axial compression level and hollow ratio obviously affect the hysteretic performance (e.g., the bearing capacity and ductility) through the test and FE analysis, e.g., increasing hollow ratio from 0.54 to 0.82 the bearing capacity and ductility coefficient of test respectively decrease by 11.20% and 26.61%; the dimensionless skeleton curve, unloading stiffness degradation and hysteretic rule are proposed to form the restoring force model, where a strength model considering effect of ECC tension strength is included, and the reasonable agreement is achieved by the comparison results. The displayed research on EFHDST columns can provide significant references to its engineering applications.

Numerical investigation of concrete-filled double skin steel tubular (CFDST) structure subjected to underwater explosion loading

With the increasing applications in the offshore industry such as oil and gas jackets, submarine pipelines and wind turbine foundations, concrete-filled double skin steel tubular (CFDST) structures are encountering the ever-increasing risk of threats to underwater explosions (UNDEX). This study presents a systematical investigation on the structural behaviors and design recommendations of the CFDST structures subjected to UNDEX loadings through finite element analysis (FEA) approaches. Finite element models have been developed, where the non-linear material properties of the constitutive steel and concrete parts and the composite actions in-between have been considered. The FEA models are verified against the experimentally determined shock wave pressure history, the deformation shapes, and the residual strength. The full-range analyses were firstly carried out on the structural responses of CFDST structures, including the typical damage patterns and residual strength of the specimen after UNDEX. Then, the parametric studies show that the cross-section hollow ratio, charge weight, and explosion distance play great roles in determining the residual strengths. Thereafter, damage indexes considering the parameter of the hollow ratio and the scaled explosion distance has been formulated, and design recommendations have been suggested accordingly.

Seismic performance and damage assessment of CFDST frames with SBYC damper

Nowadays, the dual system consisting of moment resisting frame with energy dissipation devices was suitable to be employed for high-rise buildings in the area with high seismic intensities. Current investigations merely concentrated on the story drift as the main index to evaluate the seismic performance of such kind of system, which could not reveal the component damage state in a structure. It is more comprehensive to evaluate the seismic performance through the damage state of components. In this paper, the seismic damage of components in the dual system consisting of concrete filled double skin steel tubular (CFDST) moment resisting frame with shear -bending yielding coupling (SBYC) damper was analyzed. The damage factors of different components including CFDST column, semi-rigid joint and SBYC damper were determined based on existing test results. The nonlinear model of the dual system was established by Opensees and verified by the test results. Afterwards, the 20- and 10-story dual structures were designed with different strength of SBYC damper and moment resistances of joint. Parametric analyses were conducted on these two dual systems through the endurance time method. Further, the uniform damage ratio was defined to represent the uniform level of the damage distribution of the components. It found that increasing the strength of joint was beneficial for achieving a uniform damage distribution of SBYC damper, joint and SBYC damper. The analytical results were beneficial for the application of dual system and provided a reasonable method to evaluate the seismic performance and damage of dual systems.

Flexural behavior of novel CFDST components with external welded corrugated steel tubes

A novel type of concrete filled double-skin steel tubular (CFDST) with external welded corrugated steel tube was proposed, and the flexural behavior of this CFDST type was assessed by testing of 8 specimens. Test parameters include the hollow ratio, the thickness of the inner tube, and the strength of the concrete. Experimental results indicate that the specimens with external corrugated steel tubes failed in a ductile manner, and achieved somewhat higher flexural capacity and significantly improved ductility compared with the members confined by external flat steel tubes. The strength and thickness of the external corrugated steel tubes and the hollow ratios had a considerable effect on the flexural capacity, and the desirable geometrical parameters of the corrugation were further obtained by the finite element methods. Moreover, the performance of four existing design models for the flexural capacity of CFDST was evaluated based on the test results available in the literature and in this study, and a new analytical model for CFDST with external welded corrugated steel tube was proposed. The proposed analytical model provided excellent prediction for the flexural capacity of the tested specimens..

Axial compression behaviour of circular concrete-filled double-skin steel tubular columns with bolted shear studs: Numerical investigation and design

The concrete-filled double-skin steel tubular (CFDST) columns have recently attracted significant attention for practical engineering constructions. The axial resistance behaviour of CFSDTs can be greatly enhanced by installing bolted shear studs as pin supports for the outer. This is a companion paper that expands on the authors' work [1]. The previous paper concentrated on full-scale experimental testing to ascertain the influence of bolted shear studs on the axial resistance of CFSDT columns, while this study focused on finite element (FE) analysis and a simplified design approach for such composite structural members. A three-dimensional FE model for predicting the axial behaviour of circular CFDST columns without and with shear studs was developed and validated using the test results of the specimens' typical failure mechanisms and load-displacement responses. The model was then employed to perform structural analysis by comparing the axial load-displacement curve, internal force distribution, longitudinal stress distribution over the sandwiched concrete, and confinement mechanism of a circular column with bolted shear studs with that of its counterpart without bolted shear studs. Parametric studies were carried out to examine the effects of bolt longitudinal spacing, diameter, length, and yield strength. It was found that the spacing of the bolts-to-width (bs/Do) ratio had significant effects on the axial resistance and ductile performance of columns. Also, a bolt spacing corresponding to bs/Do<0.44 was suggested to attain an acceptable ductile performance in practical application. Verification of the design model revealed that it led to satisfactory predictions of the ultimate strengths of circular CFDST columns without and with shear studs.

Axial compression performance of concrete filled double skin (SHS outer and CHS inner) steel tubular columns strengthened by CFRP

This paper reports the compressive behavior of concrete-filled double skin steel tubular (CFDST) columns strengthened by CFRP under axial loading. The outer skin was made of square hollow sections (SHS), while the inner skin was made of circular hollow sections (CHS). Twelve CFDST columns strengthened by CFRP and three CFDST columns were tested. The failure mode, load-displacement relationship, axial load-carrying capacity and initial stiffness are given and analyzed. The results showed that the CFDST columns strengthened by CFRP have better mechanical properties than the column without CFRP reinforcement (i.e. the CFDST columns). The CFDST columns strengthened by CFRP eventually failed due to CFRP rupture or buckling of the external steel tube near the end and cracked concrete damage from the outer steel tube. The stiffness and axial load bearing capacity of the column can be improved as the number of CFRP layers increases. A simplified design formula of CFDST columns strengthened by CFRP is proposed, which is in good agreement with the experimental results.

Seismic behavior and analytical model for a fully bolted joint between CFDST columns and steel beams

A steel beam to concrete-filled double-skin steel tubular (CFDST) column joint was developed to improve the widespread use of CFDST columns in buildings. The comparative studies of six specimens under pseudo-static cyclic tests were designed and performed to examine the influence of endplate thickness, axial compression ratio, bolt diameter, concrete fill degree, and inner tube shape on the mechanical behavior of the joint. The failure modes were the bending of the endplate, warpage and fracture of bolts, fracture of the weld seam, and buckling of beam flanges. The displacement ductility coefficients exceeded 4.89, indicating that the connection had excellent ductility. Increasing the endplate thickness greatly improved the seismic indices of the joint. Although the higher axial compression ratio increased the bearing capacity and initial stiffness of the joint, the ductility and energy dissipation were unaffected. Improving the bolt diameter did not affect the bearing capacity, ductility, and energy dissipation capacity of the joint; however, it increased the initial stiffness and delayed the strength. The bearing capacity and initial stiffness of the joint were improved by increasing the concrete fill degree and using a square tube instead of the inner circular tube; however, it decreased the ductility and energy dissipation and accelerated the strength degradation of the joint. Finally, a calculation model of the initial rotational stiffness was proposed. The mean and standard deviation of the ratios of the predicted results to the experimental results were 0.95 and 0.14, respectively.

Pushout tests for concrete-filled double skin steel tubes after exposure to fire

The performance of the steel–concrete interaction after fire exposure is essential for the post-fire evaluation of composite action in steel–concrete structures. Pushout tests for the bond–slip behaviour between steel tube and concrete are conducted for concrete-filled double skin steel tubular (CFDST) columns after exposure to fire. The main test parameters include the fire duration, the hollow ratio of the cross section, the diameter to thickness ratio of the steel tube, the surface roughness, and the effective length to diameter ratio. Measured temperature results are compared with the predicted ones from numerical analysis, and the load transfer mechanism before and after fire exposure is also discussed. It is found that the bond between the steel tube and concrete is still available after exposure to fire and, in general, the bond strength after fire exposure can meet the requirement of most current codes of practice.

Numerical analysis on torsional behavior of rectangular and square CFDST members

This paper numerically investigated the behavior of rectangular and square concrete-filled double skin steel tubular (CFDST) members under torsion. The finite element model of rectangular and square CFDST members was established and verified against the test results for the members under cyclic torque. The comparison of the finite element analysis results and experiment results show that the comprehensive finite element model can accurately predict the torsional behavior of rectangular and square concrete-filled double skin steel tubular members. Parametric analyses by using the calibrated finite element model were conducted to investigate the influence of various parameters, such as hollow ratio, concrete strength, yield strength and cross-sectional steel consumption of steel tubes on the torsional behavior of rectangular and square CFDST members. Based on the mechanical analysis and regression analysis methods, the design formula was proposed to predict the torsional capacity of rectangular and square CFDST members. The comparison results show that the proposed formula for predicting the torsional capacity of rectangular and square CFDST members has good accuracy.

Torsional behaviour of tapered CFDST members with large void ratio

This paper investigated the mechanical behaviour of tapered concrete-filled double skin steel tubular (CFDST) members under combined axial compression and torsion. A series of experiments on CFDST specimens were tested including 8 tapered members and 2 normal members with various hollow ratios and axial compression ratios. The torsional performance of tapered members was analyzed on failure modes, torque-rotation angle curves, and torque-shear strain curves. Verified FE models were developed to carry out the full-range analysis, stress and strain distribution, and the contact stresses between the components. The results showed that the failure mode of tapered CFDST columns was the torsional deformation occurred between 0.5H and the top section. The specimens with a high axial compression ratio occurred additionally inclined outward buckling at 0.8H section, giving rise to a decline in torsional capacity. A low level of axial force promotes the torsional bearing capacity. The effects of the taper angle and hollow ratio on the distribution of shear strain and the torsional capacity at different heights are obvious. Furthermore, employing the section with the lowest torsional modulus for tapered CFDST members is conservative as the designed section, because the torsional capacity might be underestimated. Finally, a simplified method defined by a new calculated section for forecasting the torsional performance of tapered CFDST members is proposed.

Numerical analysis of square concrete‐filled double‐skin tubular columns with outer stainless‐steel tube

AbstractStainless steel has increasingly been utilized in concrete‐filled composite columns because of its high corrosion resistance and aesthetic appearance. However, there is little published information on the structural behavior of concrete‐filled double‐skin steel tubular (CFDST) columns made of square outer stainless‐steel skin and circular inner carbon steel tubes. The present study focuses on the nonlinear analysis of axially loaded CFDST short columns with outer square stainless steel tube and inner circular carbon steel tube. The explicit analysis methodology is adopted to develop the finite element model of CFDST columns. The verified model is employed to undertake a parametric study on the geometric and material effects on the performance of CFDST columns. The findings indicate that the outer tube diameter‐to‐thickness should be maintained &lt;40 to ensure the required plastic resistance of CFDST columns with hollow ratio in range from 0.34 to 0.42. The design equation for calculating the ultimate axial strength of CFDST columns with outer stainless steel is proposed.

Axial compressive behaviour of tapered CFDST stub columns with large void ratio

An experimental and numerical investigation on tapered concrete-filled double skin steel tubular (CFDST) stub columns with large void ratio under axial compression is presented in this paper. Twelve tapered CFDST specimens with different parameters, namely the void ratio and the diameter-to-thickness ratio of outer steel tube, are tested. The load versus deformation curves, failure modes, effects of different parameters are studied, respectively. It is found that tapered CFDST specimens with large void ratio experience the strength failure, and severely outward buckling occurs near the column top. The diameter-to-thickness ratio of outer steel tube and the void ratio engender a significant influence on the ultimate capacity and ductility of tapered CFDST columns. A finite element (FE) model is developed to predict test results, and to further analyze the working mechanism of tapered columns. The result shows that changing the tapered angle and void ratio has distinct effects on the position of failure section and distribution of peak load of tapered columns. A method considering the influence of failure section is finally proposed for calculating the ultimate capacity of tapered CFDST columns and a good predicted result is presented.

Experimental investigation and analytical modeling of steel beam-to-CFDST column connection

In this paper, a prefabricated connection between concrete-filled double-skin steel tubular (CFDST) column and steel beam with flush endplates and bidirectional bolts was developed. To investigate the effects of the endplate thickness, axial compression ratio, bolt diameter, concrete fill degree, and shape of the inner tube on the seismic performance of the connection, a series of pseudo-static tests were performed on six specimens. It was observed that the bending deformation of endplates, warpage of bolts, local buckling of beam flanges, and cracking of weld seams were typical failure modes. An increase in the endplate thickness and the replacement of the circular inner tube by the square inner tube significantly improved the ultimate strength of the connection. The ultimate strength of the connection was not sensitive to the axial compression ratio and bolt diameter, and it decreased as the concrete fill degree increased. The displacement ductility coefficients of the specimens were greater than 6.21. Higher ductility and accumulated energy dissipation were obtained by increasing the endplate thickness, axial compression ratio, bolt diameter, and replacing the circular inner tube with the square inner tube. The ductility and accumulated energy dissipation decreased as the concrete fill degree increased. Based on the component method, an analytical model of the initial rotational stiffness of the connection was developed. The mean and standard deviation of the ratios of predicted values to test values were 0.95 and 0.06, respectively.

Lateral response of double skin tubular column to steel beam composite frames

Concrete filled double-skin steel tubular (CFDST) column comprises two inner and outer steel tubes with infill concrete between tubes. CFDST columns are used in many structural systems such as offshore structures and high rise buildings. The aim of this research is to examine the performance of composite frames composed of CFDST columns and steel beam under the influence of lateral loading. The frames were modeled and analyzed utilizing ANSYS finite element (FE) software. The linear and nonlinear behavior of steel and concrete materials and confinement effects of inner and outer steel tubes on the infill concrete were considered in the analysis. Three key parameters were considered in the present study. They are the axial load and slenderness ratios of CFDST column as well as linear stiffness ratio of the beam–column. The effects of these parameters on the behavior of the composite frames were evaluated comparatively. Load-deformation responses were achieved for various cases of the investigation. The verification of the developed FE model was evaluated by considering the analysis results with the experimental data existing in the literature. The findings attained from the FE modeling were in consonance with the experimental results. Besides, it was observed that the above parameters had a substantial influence on the load-displacement relationship and the performance of the studied composite frames.