Concrete-filled Double-skin Steel Tube Research Articles

Manuscript title: Experimental investigations of concrete-filled double skin steel tube column-column grouted connection under axial compression

This paper presents a novel grouted connection joint for prefabricated CFDST columns. In order to ascertain the reliability of this grouted connection, axial compression tests were conducted on 24 CFDST columns with grouted connections and 3 CFDST columns without grouted connections. The test parameters were grout strength, casing tube length, and length-to-diameter ratio. The results showed that the stub columns failed due to local buckling and the slender columns failed due to global buckling with significant lateral flexural deformation. CFDST columns with grouted connections exhibited a similar or even higher ultimate bearing capacity and much higher later bearing capacity than conventional CFDST columns. The ultimate bearing capacity of CFDST columns with grouted connections is influenced by the casing tube length and the length-to-diameter ratio. The increase in casing tube length leads to an increase in bearing capacity, while an increase in length-to-diameter ratio results in a decrease. Enhancing the grout strength does not significantly improve the ultimate bearing capacity. Furthermore, the effects of each test parameter on the stiffness, strength and ductility of the CFDST column-column grouted connections were evaluated. Finally, by considering the interaction between components and the impact of casing tube length on bearing capacity, a prediction equation for the ultimate bearing capacity of CFDST column-column grouting connections was developed. This equation can offer accurate predictions with an average error of less than 1% to provide a reference for the joint design.

Post fire behavior of square and rectangular concrete filled steel tubes with granulated blast furnace slag fine aggregate concrete

AbstractThe present study investigated the behavior of concrete‐filled steel tubes (CFST) and concrete‐filled double‐skin steel tubes (CFDST) of square and rectangular shapes after exposure to elevated temperatures. The infill concrete was prepared with granulated blast furnace slag (GBS) as fine aggregate, and no natural fine aggregate was used. A total of 20 CFST and CFDST specimens of both square and rectangular shapes were prepared. The specimens were heated at 200, 400, 600, and 800°C temperatures for 2 h and allowed to cool inside the furnace after heating. Axial load was applied to the specimens to check their behavior after exposure to fire. The reduction in the load‐carrying capacity was insignificant for the specimens heated at 200, 400, and 600°C for both square and rectangular CFST and CFDST conditions. The decrease in load‐carrying capacity was slightly higher in the case of CFDST specimens compared to CFST specimens. A three‐dimensional finite element model using ABAQUS software was proposed for predicting the behavior of the tested specimens. The proposed model could predict the behavior of both unheated and heated specimens with acceptable accuracy.

Mechanical properties of geopolymer recycled concrete infilled steel tubes with three sectional types

The steel tubes with geopolymer recycled concrete (GRC) infill are actively seeking environmentally friendly and low-carbon building materials as alternatives in the process of promoting sustainable development, maximizing the use of waste materials and resources, and reducing consumption of natural resources and environmental pollution. In this paper, three types of steel tube specimens with GRC infill were proposed, i.e., the geopolymer recycled concrete-filled steel tube (GRCFST), geopolymer recycled concrete-filled double skin steel tube (GRCFDST), and geopolymer recycled concrete-filled double section steel tubes (GRCFDSST). These GRC-steel tube structures were tested under axial compressive loading, and the influence of some crucial factors on bearing capacity, ductility and strain distribution of specimens was considered, such as the slag content, and the types of steel tube section and the replacement rate of recycled aggregate. The experimental results show that the steel tube can provide effective confinement for geopolymer recycled concrete and thus compensate for the adverse effect induced by recycled aggregate. The slag content has less effect on the bearing capacities of the GRCFDSST specimen compared with the GRCFST and GRCFDST specimens due to the stronger confinement effect of the former. The ductility of GRCFDSST specimens is better than the other steel tube specimens under the same parameter conditions. This study, building on existing literature, provides new insights and design recommendations for the mechanical performance of concrete-filled steel tube structures by using geopolymer recycled concrete as a filling material.

Behaviour of high-performance alkali-activated slag concrete-filled double-skin steel tubes under compression loading

Behaviour of high-performance alkali-activated slag concrete-filled double-skin steel tubes under compression loading

Confinement-based direct design of axially-compressed octagonal concrete-filled double skin steel tubes

Prior research has highlighted distinctions in the confinement mechanism for concrete-filled double-skin steel tube (CFDSST) columns with octagonal sections compared to circular and square sections. Current design codes primarily address the models for the design of square and circular sections in columns made of concrete-filled steel tube (CFST). This study addresses this gap by presenting a novel confinement-based direct design model specifically developed for octagonal CFDSST columns, encompassing both unstiffened and stiffened outer steel tubes. Initial data collection involved experimental and numerical results of axially-compressed octagonal CFST and CFDSST columns sourced from literature. Subsequently, ultimate compressive resistance, as determined by international design codes (EC4, ACI-318, AISC-360, GB-50936, T/CCES-7, AS/NZS-2327, CSA-S16, AIJ) and existing analytical models, underwent comparison with test results. The results reveal that existing codes and models generally offered accurate predictions, but their consistency across the entire side width-to-thickness (w/to) ratio range varied. The study proposes a new confinement design model for predicting the compressive resistance of octagonal CFDSST columns. Although the new model does not outperform existing ones, it demonstrates satisfactory prediction scatter throughout the w/to ratio and compressive strength (fc′) range, particularly excelling in high-strength concrete (HSC) and ultra-high-strength concrete (UHSC) scenarios. Notably, the proposed formula incorporates the influence of stiffening ribs on the confinement of the octagonal steel tube on concrete, making it applicable for practical design considerations for both unstiffened and stiffened octagonal CFDSST columns.

Flexural behavior of circular rubberized concrete-filled double-skin steel tubular beams: Experiments

The study aims to assess the impact of RuC on twelve concrete-filled double-skin steel tube (CFDST) specimens. The research explores key variables, including outer tube slenderness ratio, hollow ratio, and rubber replacement ratio. Rubber particles replace natural aggregates at replacement ratios (ρνr) of 0%, 15%, and 30% by mass. The experimental findings are comprehensively discussed, encompassing member flexural capacity, confinement effects, ductility, energy dissipation, and failure mechanisms. The findings reveal that replacing normal concrete (NC) with RuC leads to a reduction in beam flexural strength ranging from 5.0% to 18.1%, depending on ρνr and outer tube diameter. In contrast, RuC members exhibit an increase of up to 70% in ductility and 27.7% in energy dissipation compared to members with NC, depending on rubber content. Increasing the outer tube thickness proves to be an effective method for mitigating strength reduction due to rubber replacement. Specifically, elevating the outer tube thickness from 3.6 mm to 4.5 mm results in strength enhancements of 17.0%, 19.0%, and 23.7% for beams with NC, 15%RuC, and 30%RuC, respectively. Furthermore, the results are applied to assess existing design models and to suggest modifications that offer improved strength estimations for a broad spectrum of rubber replacement ratios.

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

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

Residual bearing capacity of concrete-filled double-skin steel tube K-joints after lateral impact

With the application of jacket offshore wind turbines (OWT), the jackets are exposed to the risk of ship collision. However, concrete-filled double-skin steel tube K-joints (CFDST-K joints) in the jacket can still operate normally after slight impacts. To study the residual axial properties of the joints, eight CFDST-K joints and one hollow K-joint were tested by a long column testing machine, focusing on the failure mode, residual bearing capacity, displacement, and ductility after impact. Results indicated that the typical failure mode of the joints was characterized by overall bending resulting from excessive midspan deflection. The residual properties were negatively correlated with the initial damage. Specifically, the stiffness degradation was worsened as the impact energy and axial compression ratio increased, causing an evident decrease in residual bearing capacity and ductility. Moreover, increasing the outer tube thickness improved the bearing capacity by enhancing the confinement effect of the steel tube on the concrete. Furthermore, the hollow ratio exerted a great effect on ductility. A calculation method for the original loading capacity of CFDST-K joints was also proposed, and the damage index was introduced to assess the damage degree. The results verified that the joints still possessed excellent bearing capacity after impact.

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.

Analytical investigation of multi-joint rotation behavior of PTPS-CFDST column under lateral loading

The post-tensioned precast segmental concrete-filled double-skin steel tube (PTPS-CFDST) column is an innovative composite column that has recently been proposed and verified to exhibit excellent seismic resilience performance, being a promising solution for low-damage seismic design in accelerated bridge construction. Under lateral loading, this type of new column behaves with multiple joint rotations with laterally unconstrained post-tensioned (PT) tendons, which cannot be accurately predicted by the existing analytical models. This study firstly presents a new multi-joint rotation analytical model for the PTPS-CFDST column under lateral push-over loading, with the positional influence of PT tendons considered. The proposed model is then validated against experimental and numerical results, demonstrating its accurate prediction of lateral force capacity, global column deformation, and local joint openings. In addition, a comparison between the one-, two- and three-joint analytical models is conducted, and the results reveal that the two- and three-joint analytical models achieve accurate results. Finally, parametric studies considering the initial prestressing force, thickness of steel tube and hollow ratio are performed, which further validates the accuracy, applicability, and versatility of the proposed model. This research enhances the analytical understanding of the multi-joint rotation behavior of the PTPS-CFDST column, which may provide references for the seismic design of this new type of column.

Compression behavior of the concrete-filled double skin steel tube columns under hydrostatic pressure: Experimental and modeling study

This paper conducted an experimental study focusing on the axial compressive behavior of circular concrete-encased concrete-filled steel tubular (CFDST) columns composed of a circular inner steel tube under active confinement. The investigation aimed to explore the impact of concrete compressive strength grades and diameter-to-thickness ratio of steel tubes in the presence of hydrostatic pressure. The effects of varying outer diameter-to-thickness ratios (44, 45.6, 85.84, and 86) and concrete compressive strength grades (10 and 20 MPa) on the performance of CFDST columns were investigated in this paper. Additionally, the study considered the impact of two inner steel tube diameter-to-thickness ratios (20.5 and 39). The experimental results demonstrated that external pressure can significantly enhance both the load-carrying capacity and ductility of the specimens. The ultimate strength of the columns was improved by a minimum of 34% and a maximum of 92%. Finally, empirical formulas for predicting the axial compression capacity of CFDST under active confinement using gene expression programming were presented. The accuracy of the proposed model was then assessed by the artificial neural network, and the results indicate that both models perform well when compared to the empirical equations that take into account the hydrostatic pressure of wet concrete.

A Seismic Risk Assessment of Concrete-Filled Double-Skin Steel Tube (CFDST) Frames with a Beam-Only-Connection for Reinforced Concrete Shear Walls (BRWs)

The beam-only connected reinforced concrete shear wall (BRW) is used as a reinforcing component to enhance the seismic performance of concrete-filled, double-skin steel tube (CFDST) frames. The effects of the BRW on seismic risks of CFDST frames are investigated. Three performance levels of limit states are defined and described according to the typical failure of test specimens and the existing definition of current guidance. A simplified numerical model is calibrated for CFDST frame-BRW structures, and test results validate it. Nonlinear dynamic analyses on a nine-story CFDST-BRW building are performed to investigate the effects of BRW on reducing the seismic risk of CFDST buildings. The results show that the BRW reduces the probability of collapse of the CFDST frame to 2.76% in 50 years, which can effectively reduce the probability of different degrees of damage in the service cycle of the structure. The results provide information for risk-informed decision-making on the design of CFDST frame-BRW structures.

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.

Analytical model for axially compressed circular concrete-filled double skin steel tubes (CFDSTs): Insights from concrete non-uniformly confined states

Concrete-filled double skin steel tube (CFDST) is becoming a competitive structural concept in deep-sea and deep-ground engineering structures. However, the passive non-uniform confinement of the sandwiched concrete needs to be further clarified and quantitatively described to achieve better design efficiency of CFDSTs in sophisticated application scenarios. This investigation suggests a development approach for the analytical model of the axially compressed circular CFDSTs considering the non-uniform confinement effect. The strength and deformation characteristics of the concrete subjected to constant lateral stresses have been first systematically analyzed through the extensively collected experimental database, and the corresponding constitutive models have been formulated accordingly. The stress distributions of the sandwiched concrete and the influence of the cross-sectional geometric properties have been discussed, and then the calculation method for the non-uniform confining factor has been developed. An efficient analytical model has been established on the basis of an incremental-iterative procedure, with the steel tube-concrete interaction and bonding behavior appropriately considered. The proposed analytical model has been verified through the widely collected experimental data of axially compressed CFDSTs, and the results show that it could effectively simulate the full-range axial load–strain curves and improve the prediction accuracy of ultimate loads by about 5% to 16% compared with the existing design formulas.

Experimental and numerical study on impact resistance of offshore wind turbine CFDST jacket

With the development of large-scale offshore wind turbines (OWT), the jacket foundation widely used to offshore wind turbines are at significant risk of collision damage from vessels and sea ice. In order to improve the impact resistance of hollow steel tube jacket foundation, a new type of concrete-filled double-skin steel tube (CFDST) jacket foundation is proposed. From the analysis of impact force, displacement, acceleration and energy consumption, the peak strain and acceleration of the jacket leg at the impact position are reduced by 57.1 % and 50.4 %, respectively. The deformation mechanism and failure mode of the two foundations under multiple impact loads were obtained by combining LS-DYNA software and test. After two impacts the hollow steel tube jacket was seriously dented and damaged in bending. However, after multiple pendulum impacts on a concrete-filled double-skin steel tube, only the supporting struts were bent, the indentation depth and displacement peak of the jacket leg are reduced by 76.9 % and 72.2 %, respectively, and the entire structure showed excellent impact resistance, overall bending stiffness and energy dissipation capacity. Finally, the experimental basis and theoretical analysis are provided for the application of concrete-filled double-skin steel tube jacket in large offshore wind turbines and the resistance to impact loads.

Numerical study of seismic performance of steel-concrete composite rigid-frame bridge with precast segmental CFDST piers crossing fault-rupture zones

Although seismic performance of bridges crossing fault-rupture zones has been studied in recent two decades, there is still no effective solution to mitigate the hazard of fault-crossing effect. Recently, a new type of post-tensioned precast segmental (PTPS) concrete-filled double skin steel tube (CFDST), known as PTPS-CFDST column, has been proposed and verified to exhibit excellent performance with respect to seismic resilience. Moreover, PTPS-CFDST columns can accommodate torsional deformation through joint slip and are thus considered as a promising structural solution for bridges crossing fault-rupture zones. This study aims to investigate the performance of a steel–concrete composite rigid-frame bridge (SCCRFB) with PTPS-CFDST piers subjected to across-fault ground motions through numerical simulations. Two detailed three-dimensional (3D) finite element (FE) bridge models are developed using the explicit FE code LS-DYNA. In particular, Bridge 1 uses the new PTPS-CFDST pier, while Bridge 2 serves as a reference model with monolithic CFDST piers. Two types of across-fault ground motions with strike- and dip-slip mechanisms are used as seismic inputs, and the influence of fling-step is parametrically investigated. Structural responses, including the global seismic responses, seismic damages, energy absorption, and local joint slip and opening, are systematically discussed. The numerical results indicate that, compared to the monolithic CFDST piers, PTPS-CFDST piers exhibit advantages in terms of seismic resilience, with lower damage and outstanding self-centering performance, which facilitates post-earthquake recovery after experiencing across-fault ground motions. This study provides valuable references for the seismic design of bridges with PTPS-CFDST piers crossing fault-rupture zones.

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.

Analytical and experimental investigation on axial compression capacity of carbon fiber-reinforced plastic-confined concrete-filled double-skin steel tube stub columns

Concrete-filled double-skin steel tubes (CFDSTs) are widely used in civil engineering. During the service life of CFDSTs, their bearing capacity can be easily weakened by the corrosion of outer steel tubes. This study investigated the interaction mechanism and reinforcement effects of carbon fiber-reinforced plastic (CFRP)-confined CFDSTs under axial compression. Typical failure modes, axial load-shortening curves, and axial load-strain behaviors of CFDST and CFRP-confined CFDST (CFRP-CFDST) specimens were obtained. The test results showed that the ultimate bearing capacity of stub columns with concrete strength of C40 and C60 increased by 19.78% and 15.12%, respectively, when wrapped with two layers of CFRP sheets, and by 41.60% and 37.99%, respectively, when wrapped with four layers of CFRP sheets. In addition, the FE modeling of CFRP-CFDSTs was established and validated by experimental results. On this basis, the parametric analysis was conducted. To avoid local buckling and yield failure of the inner steel tubes before the outer steel tubes, a calculation formula was proposed for predicting the minimum wall thickness of the inner tube. Furthermore, the prediction formulas for the bearing capacity of CFDSTs and CFRP-CFDSTs were proposed.

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.