Double Skin Steel Tubular Columns Research Articles

Behavior of large-scale hybrid FRP–concrete–steel double-skin tubular columns under concentric compression

Hybrid fiber reinforced polymer (FRP)–concrete–steel double-skin tubular columns (hybrid DSTCs) are a novel form of columns with superior load-carrying capacity, ductility, and corrosion resistance. Such columns consist of an outer FRP tube, an inner steel tube, and the concrete filled between the two tubes. While they have attracted extensive research attention worldwide, most of the existing studies have been focused on small-scale specimens. In particular, the behavior of large-scale hybrid DSTCs with a filament-wound FRP tube and self-compacting concrete (SCC) has not been systematically investigated. Against this background, this paper presents a systematic experimental study on large-scale hybrid DSTCs under concentric compression with the following important issues being particularly investigated: the use of large-scale specimens, the use of prefabricated filament-wound FRP tubes compared with wet-layup FRP tubes, and the effect of shrinkage of SCC. The test results confirmed the excellent ductility of hybrid DSTCs and revealed that the types of concrete (SCC versus normal concrete) and FRP tube (wet-layup versus filament-wound tubes) significantly affected the compressive behavior of hybrid DSTCs. Finally, the predictions of an existing design-oriented stress–strain model for confined concrete in hybrid DSTCs, which was developed based on test results of small-scale hybrid DSTCs made with normal concrete and confined with a wet-layup FRP tube, are compared with the test results. It is shown that the model provides reasonably accurate predictions for confined concrete in the large-scale hybrid DSTCs with a typically bilinear shape, but it fails to predict the obvious axial stress drop after the first peak load caused by the large shrinkage of SCC.

Experimental and numerical investigations into the compressive behaviour of circular concrete-filled double-skin steel tubular columns with bolted shear studs

Eight medium-length circular concrete-filled double-skin steel tubular (CFDST) columns were examined under axial compression to evaluate the influence of bolted shear studs on the axial performance of CFDST columns. The specimens comprised six columns with bolted shear studs with longitudinal spacings of 100 mm, 75 mm, and 50 mm, and two control columns without bolted shear studs for comparison. The investigated parameters were the failure mechanisms, load-displacement curves, initial axial stiffness, compressive strength, toughness, ductility, residual strength, as well as load-strain responses. Besides, a finite element (FE) model was developed and validated against experimental results to further investigate the performance of columns. The test results demonstrated that the bolted shear studs avoided shear failure and significantly increased the ductility of the specimens. Reducing the bolted shear stud spacing reduced the intensity of local buckling in the outer tube and improved the confinement effect. Additionally, the confinement effect provided by the outer tube was remarkably improved by introducing the bolted shear studs.

Axial compression behaviour of hybrid composite FRP–concrete–steel double-skin tubular columns with various fibre orientations

The hybrid double-skin tubular column (DSTC) is a unique composite column composed of an outer tube made of a fibre-reinforced polymer (FRP) and an inner tube made of steel, with concrete filling the space between the outer and inner tubes. The external and internal tubes provide excellent confinement to the concrete. The column is characterised by the outer FRP tube’s excellent corrosion resistance, improved ductility due to the composite effect, and low weight due to its hollow cross-section. Previous studies were limited to the wet-layup of glass fibre reinforced polymer oriented in the circumferential direction of a double-skin tubular column, both experimentally and analytically. A total of 36 specimens were tested for axial compression loading. The behaviour of the column was inferred from the following parameters: orientation of fibre angle (0°,0°/90°and ± 45°) with respect to the circumferential direction, thickness of the FRP tube (2.1 mm and 4.2 mm), concrete strength (NSC and HSC), and void ratio (0.5, 0.59 and 0.67) of the steel tube. The experimental results showed that the fibres oriented in the hoop direction provided higher ultimate load-carrying capacity in comparison with fibres oriented at 0°/90°and ± 45°. The fibres oriented at ± 45° showed an increase in the ultimate strain than the other orientations. It was also found that the normal-strength concrete (NSC) specimens display better strength and strain enhancement ratio than the high-strength concrete (HSC) specimens. An improvement in resisting compressive stress and strain was observed when an increase was made in the FRP tube thickness and steel tube diameter. The results obtained from the experiments were compared to the theoretical prediction model results. The predicted model was found to be accurate in estimating the ultimate stress and strain of concrete in hollow DSTCs.

Behaviour of tapered CFDST columns with large hollow ratio under combined loads

Tapered concrete-filled double-skin steel tubular columns with large hollow ratio (LHR-CFDST) have a good application potential in wind turbine towers, where they are commonly subjected to the combined action of compression, bending, shearing and torsion. Up to now, research work on the behaviour of tapered LHR-CFDST columns under combined loads is seldom reported and relevant experimental data are limited. This paper presents an experimental and numerical study on the tapered LHR-CFDST column under combined compression-bending-shear-torsion loads. Seven specimens with different parameters including the tapered angle, the hollow ratio, the torsion-to-bending ratio, the axial compression ratio and the height-to-diameter ratio were tested. The experimental results in terms of failure modes, load versus deformation relations and strain responses were discussed. Reliable finite element models were established to conduct numerical analysis in an extended parameter range. The mechanism of columns and the effects of torsion-to-bending ratios, axial compression ratios, height-to-diameter ratios, tapered angles, hollow ratios, diameter-to-thickness ratios of steel tubes and the material strength were revealed. The suitability of the existing ultimate capacity prediction method was evaluated for tapered LHR-CFDST columns. It is found that the failure mode of columns under compression-bending-shear-torsion loads is related to the torsion-to-bending ratio. The columns behave in a ductile manner under moderate axial compression. Tapered angle has a slight effect on the bending capacity of the column, whereas the capacity declines with the increase of hollow ratio and torsion-to-bending ratio. The existing method gives a conservative prediction for the ultimate capacity of tapered LHR-CFDST columns.

Behaviour of tapered concrete-filled double-skin steel tubular columns with large hollow ratio under combined compression-bending-torsion loads: Experiments

Recently, there has been an increasing interest in utilizing circular Concrete-Filled Double-Skin Steel Tubular (CFDST) columns in modern buildings. As a typical member in large-span building structures, the Concrete-Filled Double-Skin Steel Tubular column with Large Hollow Ratio (LHR-CFDST) is characterized by a tapered shape and a hollow ratio larger than 0.75. In engineering practice, this type of column is commonly subjected to a combination of compression, bending and torsion. This paper presents an experimental study on the tapered LHR-CFDST column under combined compression-bending-torsion loads. Specimens with different tapered angles, hollow ratios, bending-to-torsion ratios and axial compression ratios were tested. The failure mode, load versus displacement relation, strain response and effects of important parameters were discussed. The ultimate strength and mechanism of tapered LHR-CFDST columns under combined loads were analysed. The feasibility of predicting the ultimate strength of tapered LHR-CFDST columns using the existing methods was evaluated based on the experimental data.

Ultimate compressive capacity of tapered concrete-filled double skin steel tubular stub columns with large hollow ratio

Recent developments in towers of wind turbine have heightened the need for tapered Concrete-Filled Double Skin Steel Tubular columns with Large Hollow Ratio (LHR-CFDST). It is necessary to further investigate the mechanical behaviour, key parameters' effects and design methods of tapered LHR-CFDST columns. This paper thus presents an analytical study on the tapered LHR-CFDST stub column under axial compression. A comprehensive finite element (FE) modelling is developed and then verified with the experimental data. A parametric study is conducted to reveal the effects of tapered angles, hollow ratios and diameter to thickness ratios of steel tubes. Further analysis is carried out to understand the compressive behaviour in terms of the full-range load-deformation relation, the axial compressive force distribution and the limitation of steel tube's diameter to thickness ratio. A simplified method for predicting the ultimate compressive capacity of tapered LHR-CFDST stub columns is proposed and the prediction agrees well with the test data.

Experimental study on Tapered Concrete-Filled Double Skin Steel Tubular columns under torsion

Recent developments in the high-output wind turbine have heightened the need for CFDST support towers. Tapered Concrete-Filled Double Skin Steel Tubular columns with Large Hollow Ratios (TLHR-CFDSTs) are the typical members in the wind turbine towers, which are usually subjected to torsion during the life cycle. It is necessary to pay more attention to these members under torsion due to their tapered shapes and thin-walled cross sections. This paper thus presents an experimental study on the torsional behaviour of the TLHR-CFDST column. Specimens with different tapered angles, hollow ratios, tubes’ thicknesses and axial compression ratios were tested under pure torsion and compression–torsion. The typical failure modes, torque versus deformation relations, strain distributions and the influence of important parameters are revealed. Based on the experimental results, the mechanism of TLHR-CFDST columns under torsion is discussed. The applicability of existing torsional capacity calculation methods is evaluated and finally a simplified method for predicting the torsional capacity of TLHR-CFDST columns is proposed.

Experimental investigation on behavior of splicing glass fiber–reinforced polymer-concrete–steel double-skin tubular columns under axial compression

Splicing glass fiber–reinforced polymer (GFRP)-concrete–steel double-skin tubular column (DSTC) is to set connection component at the joint of two or more separated GFRP tubes, and then pour concrete in the double-tube interlayer to form a continuous composite member. In this paper, the splicing DSTC composite members based on steel bar connection were designed and tested under axial compression to determine its mechanical performance. The main parameters include the connection steel ratio, the hollow ratio, and the thickness of GFRP tube. The results show that the GFRP tube presents apparent constraint effect on the concrete at about 60% of the ultimate load. The failure of splicing specimen occurred in the non-splicing section at a certain distance from the splice joint, and the stirrups at the splice joint provide effective constraint effect on the internal concrete. The proposed DSTC splicing method based on steel cage connection can satisfy the strength requirements of splice joint. Nevertheless, the increase of axial steel bar ratio cannot improve the bearing capacity of the splicing column, and the steel ratio of 2.44% is suggested for the splice joint of DSTCs under axial compression. The axial bearing capacity of splicing DSTCs significantly increases with the increase of GFRP tube thickness, but the amount of stirrups should be increased properly when a larger tube thickness is used. Two models were selected to calculate the bearing capacity of splicing members and it is found that Yu’s model is more accurate in predicting splicing DSTCs.

Optimization and modeling of axial strength of concrete-filled double skin steel tubular columns using response surface and neural-network methods

The main objective of this study is to optimize and model the ultimate strength of axially loaded concrete-filled double skin steel tubular (CFDST) composite columns having a circular hollow section . By performing the multi-objective optimization analysis through the response surface methodology (RSM), an ultimate strength-based optimal design solution for CFDST composite columns was proposed. Besides, artificial neural networks (ANN) together with RSM techniques were used to develop design formulations that can be involved in the prediction of the ultimate axial strength of such types of composite columns. An experiment data repository compiled from the studies available in the literature was used in both optimization and modeling analysis. The aim of the optimization was to achieve the optimum values for the design parameters based on the maximum capacity. However, in the modeling, attaining a design formula having an accurate and reliable prediction performance was the purpose. Furthermore, a verification study in which the developed models were compared with the modified design formulas of the AISC and Eurocode 4 and some existing models proposed by the researchers was carried out. It was observed that the generated models gave a good estimation capability with a considerably high R-squared value and fewer errors. After statistically analyzing the outputs of the models, it was proved that the models proposed in the current study yielded more reliable, accurate, and consistent prediction performance than the existing formulas. Besides, it was indicated the optimization analysis has a desirability value of 0.942, meaning an almost perfect optimization. • The CFDST composite columns were optimized based on ultimate strength. • The ultimate strength-based modeling of the CFDST columns was carried out. • The axial loading condition was considered in the modeling. • Reliable and accurate numerical models were developed. • Optimum properties for the CFDST column with the CHS were achieved.

Numerical analysis of square concrete-filled double skin steel tubular columns with rubberized concrete

Structural engineers have assessed the possibility of replacing part of natural aggregates with rubber particles in concrete in recent years to obtain sustainable structural members. It has been confirmed that the use of rubber particles can enhance ductility and energy absorption capacity of concrete. By contrast, the compressive capacity of rubberized concrete (RuC) is usually lower than the normal concrete (NC). Confining RuC with steel tubes can be considered as an appropriate solution for reaching to a ductile member with adequate strength. This paper, therefore, investigates the behavior of rubberized concrete-filled double skin steel tubular (RuCFDST) short columns by performing a series of nonlinear finite element (FE) analyses. The developed FE models were verified against the experimental test results by comparing the axial load–displacement curves, ultimate axial strength, and failure mechanism of specimens. The validated FE model was used to investigate the effects of geometric and material properties on the behavior of RuCFDST columns with 5%, 15%, and 30% rubber contents, and to compare the performance of RuCFDST and concrete-filled double skin steel tubular (CFDST) columns. The results showed that using RuC for filling square CFDST columns could significantly enhance the ductility. By contrast, CFDSTs presented greater axial strengths than RuCFDSTs. Reduced confining stresses between the outer tube and the concrete core were displayed when RuC was applied compared to CFDST.

Component based moment-rotation model of composite beam blind bolted to CFDST column joint

This paper aims to explore the mechanical behavior and moment-rotation model of blind bolted joints between concrete-filled double skin steel tubular columns and steel-concrete composite beams. For this type of joint, the inner tube and sandwiched concrete were additionally identified as basic components compared with CFST blind bolted joint. A modified moment-rotation model for this type of connection was developed, of which the compatibility condition and mechanical equilibrium were employed to determine the internal forces of basic components and neutral axis. Following this, load transfer mechanism among the inner tube, sandwiched concrete and outer tube was discussed to assert the action area of the components. Subsequently, assembly processes of basic coefficients in terms of their stiffness and resistances based on the component method by simplifying them as assemblages of springs in series or in parallel. Finally, an experimental investigation on four substructure joints with CFDST columns for validation purposes was carried out to capture the connection details. The predicted results derived from the mechanical models coincided well with the experimental results. It is demonstrated that the proposed mechanical model is capable of evaluating the complete moment-rotation relationships of blind bolted CFDST column composite connections.

Numerical modeling of rectangular concrete-filled double-skin steel tubular columns with outer stainless-steel skin

Concrete-filled double-skin steel tubular (CFDST) columns composed of outer stainless-steel skin have grown increasingly popular in structural applications on account of their attractive appearance, ductility, resistance to corrosion, fire resistance and easy maintenance. However, only one numerical study of the behavior of rectangular CFDST short columns with outer stainless steel tube has been reported. This paper presents the numerical analysis and design of axially loaded rectangular CFDST short columns where the outer tube is made of stainless-steel. A finite element model is created for analyzing the responses of rectangular CFDST short columns. The numerical analysis accounts for the confinement mechanism of concrete, strain hardening of stainless steel and imperfection of steel tubes. A comparative study of numerically predicted and experimentally observed behavior is presented to gain an appreciation for the accuracy of the developed numerical model. A parametric study is undertaken, investigating the performance of a series of rectangular CFDST columns by varying the concrete strength, load distribution, cross-section shape and tube thickness. The ultimate axial strengths predicted using Australia/New Zealand Standard for composite structures are compared against experimental results. It is demonstrated that the numerical model predicts well the responses of CFDST columns where the outer skin is made of stainless steel. Australia/New Zealand Standard yields the accurate predictions of the ultimate axial strengths of rectangular CFDST columns with outer stainless-steel tube.

Concentric compressive behavior of hybrid concrete–Stainless Steel Double-Skin Tubular Columns incorporating High Performance Concretes

This paper investigates experimentally and numerically concentric compressive behavior of hybrid concrete–Stainless Steel (SS) Double-Skin Tubular Columns (DSTCs) employing High Performance Concretes (HPCs). DSTCs can be prefabricated using outer-and-inner double tubes positioned concentrically sandwiching the concrete core in-between. However, the ordinary steel tubes beside the poor mechanical properties of Normal Concrete (NC) may not be appropriate for the environmental attack. This research proposes employing two durable SS tubes sandwiching HPCs in-between to gain satisfied performance. Beside NC, three types of HPCs were suggested: Engineered-Cementitious Composites (ECC), High-Strength Fiber-Reinforced Concrete (HSFRC) and High-Strength Concrete (HSC). Forty specimens were tested experimentally under axial monotonic compressive loading up to failure. The main variables were: void ratio created by inner tube, concrete core type and vertical stiffeners embedded in the concrete core. Parallel to those implemented experimentally, nonlinear three-dimensional Finite Element Models (FEMs) were set-up, executed and validated against experimental outcomes generating an accepted model with about 6% error. Results revealed that the studied parameters could significantly enhance the behavior in terms of load–displacement response, elastic/plastic appearance, ultimate capacity, and capturing ductile failure mode. Finally, both experimental and numerical results were extended deriving new formula could estimate the ultimate capacity with difference about 12%.

Experimental Investigation of the Hybrid FRP-UHPC-Steel Double-Skin Tubular Columns under Lateral Impact Loading

Abstract The lateral impact behavior of hybrid fiber-reinforced polymer (FRP)-ultrahigh-performance concrete (UHPC)-steel double-skin tubular columns (DSTCs) was experimentally investigated in this...

An Experimental Study on Cyclic Performance of the Geometrically Prismatic Concrete-Filled Double Skin Steel Tubular (CFDST) Columns

This paper aims to investigate seismic performance of the concrete-filled double skin steel tubular columns whose geometry is prismatic. To this end, a number of column specimens with inner section of square, diamond and circular shapes as well as outer section of a prismatic geometry (i.e., square shaped), whose inclination grows from top to bottom with slope of 2.1, were built and then, subjected to the loading protocol designated by ATC-24. Based on the results, failure mode of the columns with inner section of square or diamond is similar, but, to the contrary, the specimens with circular section incurred more intense damages. As an important observation, it was found that base of the columns ruptures diagonally with slope of 45° after 20 loading cycles. Moreover, the initial stiffness and ductility ratio of the specimens, whose inner section is diamond-shaped, were found to be greater than those of the other specimens. Another important point achieved herein is that the column with circular inner section exhibits a more stable behavior, and it does not undergo an abrupt reduction in energy absorption capacity compared to the other columns.

Effect of fiber angles on hybrid fiber-reinforced polymer–concrete–steel double-skin tubular columns under monotonic axial compression

Hybrid fiber-reinforced polymer–concrete–steel double-skin tubular columns are a novel form of hollow columns that combine two traditional construction materials (i.e. concrete and steel) with fiber-reinforced polymer composites. Hybrid fiber-reinforced polymer–concrete–steel double-skin tubular columns consist of an inner tube made of steel, an outer tube made of fiber-reinforced polymer, and a concrete layer between the two tubes. Existing studies, however, are focused on hybrid fiber-reinforced polymer–concrete–steel double-skin tubular columns with fibers of the fiber-reinforced polymer tube oriented in the hoop direction or close to the hoop direction. In order to investigate the effect of fiber angles (i.e. the fiber angle between the fiber orientation and the longitudinal axis of the fiber-reinforced polymer tube), monotonic axial compression tests were conducted on hybrid fiber-reinforced polymer–concrete–steel double-skin tubular columns with an fiber-reinforced polymer tube of ±45°, ±60°, or ±80° fiber angles. There were two types of steel tubes adopted for these hybrid fiber-reinforced polymer–concrete–steel double-skin tubular columns. The fiber-reinforced polymer tube thickness was also investigated as an important parameter. Experimental results showed that the confinement effect of the fiber-reinforced polymer tube increased with the increase of the absolute value of fiber angles, whereas the ultimate axial strain of hybrid fiber-reinforced polymer–concrete–steel double-skin tubular columns decreased with the increase of the absolute value of fiber angles. An existing stress–strain model, which was developed on the basis of hybrid fiber-reinforced polymer–concrete–steel double-skin tubular columns with an fiber-reinforced polymer tube of ±90° fiber angles, is verified using the test results of this study. For the compressive strength of the confined concrete in hybrid fiber-reinforced polymer–concrete–steel double-skin tubular columns, the existing model provides conservative predictions for specimens with a ±80° fiber-reinforced polymer tube, overestimated predictions for specimens with a ±60° fiber-reinforced polymer tube, and close predictions for specimens with a ±45° fiber-reinforced polymer tube.

Study of compressive behaviour of FRP-recycled aggregate concrete–steel stub columns

Fibre-reinforced polymer (FRP) recycled aggregate concrete (RAC)–steel columns are a proposed novel form of composite based on hybrid FRP-concrete–steel double-skin tubular columns and, combined with RAC, greatly benefit our sustainable resources and fully exploit the advanced mechanical properties of every component of these composites. To gain an in-depth understanding of the compressive behaviour of FRP–RAC–steel columns (FRSCs) and to examine whether the existing bearing capacity models for other confined concrete columns are applicable to FRSCs, 15 basalt FRSCs were tested by subjecting them to monotonic axial compression. The main parameters investigated were the diameter-to-thickness ratio of the steel tube and the recycled coarse aggregate (RCA) replacement ratio. The test results showed that the compressive behaviour of FRSCs generally enhanced with a decrease in the diameter-to-thickness ratio of the steel tube, and that the FRSCs with RCA replacement ratios of 30% and 70% tended to have better ultimate bearing capacity and axial shortening. Three typical stress–strain models were checked to predict the ultimate bearing capacity of FRSCs, and values calculated from Yu's model coincided well with the test results.

Concrete filled double skin steel tubular columns subjected to non-uniform heating

The concrete filled double skin steel tubular (CFDST) columns have been widely used due to their high strength and good deformability. In the past decade, research was performed on the reliability of CFDST columns subjected to standard heating condition, however, only limited works were conducted on the fire resistance of CFDST columns under non-uniform heating. In the current study, a sequentially coupled thermal-stress finite element analysis was performed and the fire resistance of CFDST columns subjected to non-uniform heating was analyzed. Parametric study was performed on the effects of key engineering parameters affecting the column reliability. An extended Rankine approach was developed to predict the ultimate bearing capacity of CFDST columns under non-uniform heating. The theoretical predictions were compared with the numerical analysis, which shows the proposed interaction approach can predict the fire resistance of CFDST columns subjected to non-uniform heating with reasonable accuracy.

Ultimate Axial Strength of Concrete‐Filled Double Skin Steel Tubular Column Sections

This study aims at proposing a new model for evaluating the ultimate axial strength of concrete‐filled double skin steel tubular (CFDST) composite columns. For this, a total of 103 experimental data regarding the ultimate strength of CFDST columns under axial loading were collected from the previous studies in the literature. All CFDST columns consist of two steel tubes being outer and inner circular hollow section. The model presented herein was developed by using gene expression programming. For this, the yield strength, diameter, and thickness of both outer and inner steel tubes, the compressive strength of annulus concrete, the length of the specimen, and the ultimate axial strength of the columns were utilized as the parameters. Assessment of the obtained results indicated that the generated model had a good performance compared to the existing models by the previous researchers and the equations specified in the design codes. The high value of R2 and narrow ranged fluctuation of the estimation error for the ultimate axial strength of the CFDST columns were also achieved through the proposed model.

Experimental investigation of rubberised concrete-filled double skin square tubular columns under axial compression

Waste tyres are among the largest and most problematic sources of waste in modern society due to their durability and high rate of dumping in landfills. One possible recycling alternative is to incorporate waste tyre rubber as an aggregate replacement in concrete to promote sustainability and utilise the elastic properties of rubber. Rubberised concrete has not reached its full potential because of the decrease in compressive strength and a lack of research to solve such challenge. Recent research suggests that combining rubberised concrete with confinement increases ductility and energy absorption. Specifically, confined rubberised concrete using single skin or double skin square hollow section tubular columns present higher ductility than those made of normal concrete. This study explored experimentally the use of rubberised concrete filled single skin and double skin steel tubes under concentric axial compression. The experimental investigation included changing the confinement of the outer and inner square hollow sections and explored how confinement affected normal concrete compared to rubberised concrete. Four variations of double skin steel tubes with a total of twelve 300 mm long columns of 0%, 15%, and 30% rubber replacement were created and tested concentrically. Three single skin short columns with 0%, 15%, and 30% rubber content were also tested and compared. The compressive strengths were determined theoretically and compared against those measured experimentally. An interesting spring back phenomenon occurred where the infill rubberised concrete moved upwards after testing due to the large confinement of the core and elasticity of the rubber. This study examined the use of rubberised concrete filled double skin steel tubular columns as a promising construction technique for applications such as columns in buildings located in seismic active zones, security bollards and flexible road side barriers.