Hybrid Double-skin Tubular Columns Research Articles

Durability assessment of hybrid double-skin tubular columns (DSTCs) under simulated marine environments

Fiber-reinforced polymer (FRP)-concrete-steel hybrid double-skin tubular columns (hybrid DSTCs), consisting of an outer FRP tube, an inner steel tube, and concrete infill between them, are a relatively new structural system. Despite DSTCs being proven to have enhanced structural capabilities over traditional concrete columns, their durability in aggressive environments remains unclear. This study presents a durability assessment of hybrid DSTCs with a glass fiber-reinforced polymer (GFRP) tube. Hybrid DSTCs were immersed in artificial seawater at different temperatures (i.e., room temperature, 40 °C, and 60 °C) for a duration of up to 540 days. The time-dependent behaviors, including axial load-strain (axial and hoop) curves, compressive strength, and ultimate axial and hoop strain, are systematically investigated at different aging times. Test results reveal that the compressive strength of DSTCs exhibits a slight reduction at room temperature and 40 °C, while an obvious reduction is observed at 60 °C. After 540 days, the compressive strengths for hybrid DSTCs with 3-mm-thick and 6-mm-thick GFRP tubes reduce to 79% and 93%, respectively. A decrease in strength is notable for DSTCs with a 3-mm-thick GFRP tube at 60 ℃. The degradation of GFRP tubes is also observed via scanning electron microscope (SEM) analysis, with greater degradation observed in 3 mm-thick GFRP tubes compared to the 6 mm-thick counterparts.

Cyclic axial compressive performance of hybrid double-skin tubular square columns

This paper presents an experimental study on the cyclic axial compressive behavior of FRP-concrete-steel hybrid double-skin tubular columns. The square column specimens were cast with an external Fiber Reinforced Polymer jackets, inner steel tube and concrete in between. The height of the columns was 500 mm and the side dimension was 150 mm. The effects of loading scheme, void ratio and diameter-thickness ratio on axial compression behavior were investigated. A total of eight columns were tested under monotonic and cyclic axial compression. The experimental results show that the effect of loading scheme on axial stress-strain envelope curve and the peak load were not significant, and the ultimate state of the square columns subjected to cyclic axial compression was very similar to that of specimens subjected to monotonic axial compression. Besides, compared with void ratio, the diameter-thickness ratio of the inner steel tube has significant influence on the peak load of the columns when subjected to cyclic axial compression.

Hybrid FRP-concrete-steel double-skin tubular columns of varying slenderness ratios under eccentric compression

Hybrid fiber reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) are an emerging form of hybrid column with excellent mechanical performance and durability. Such columns consist of an outer FRP tube, an inner steel tube and a concrete infill between the two tubes. While extensive experimental and theoretical studies have been conducted on such columns, these studies have been focused on short specimens under concentric compression. This paper presents the results of a series of tests on hybrid DSTCs of varying slenderness ratios subjected to eccentric compression. The effects of FRP confinement stiffness, load eccentricity, and slenderness ratio on the behavior of hybrid DSTCs were examined. The test results showed that the test hybrid DSTCs exhibited excellent performance with great ductility. A theoretical model with incorporations of existing stress-strain models for FRP-confined concrete was then adapted to simulate the behavior of the test hybrid DSTCs. The comparisons showed that the direct use of a concentric loading stress-strain model led to underestimation of the deformation capacity of the test hybrid DSTCs. Compared with the other two models, the eccentric-loading stress-strain model previously developed by the authors' group provided more accurate predictions for the ultimate deformation of the test hybrid DSTCs.

Behavior of large-scale hybrid FRP-concrete-steel double-skin tubular columns subjected to eccentric compression

Hybrid FRP-concrete-steel double-skin tubular columns (DSTCs), consisting of an outer FRP tube, an inner steel tube and a concrete infill between the two tubes, are a novel form of hybrid columns which possess excellent mechanical performance and durability. Extensive studies have been conducted on such columns under concentric axial compression. However, limited research has been focused on the behavior of hybrid DSTCs subjected to eccentric compression which is a common loading case in practice. Therefore, an experimental program on nine large-scale hybrid DSTCs subjected to concentric or eccentric compression was conducted in the present study. The load eccentricity and the thickness of the filament wound FRP tube were the main test variables. The test results demonstrated that the hybrid DSTCs exhibited a highly ductile behavior under eccentric compression. A theoretical column model was then adapted by incorporating three stress–strain models for FRP-confined concrete for predicting the behavior of test DSTCs under eccentric compression. It was found that all the three stress–strain models provided reasonably accurate predictions for the axial load-carrying capacities of the test columns. However, the concentric loading stress–strain model led to significant underestimation in the deformation capacities of the test columns due to the neglect of the effect of eccentric compression.

Behavior and modelling of FRP-concrete-steel hybrid double-skin tubular columns under repeated unloading/reloading cycles

Fiber-reinforced polymer (FRP)-concrete-steel hybrid double-skin tubular columns (hybrid DSTCs) consist of an outer FRP tube, an inner steel tube and a concrete infill between the two tubes. Extensive research has been conducted on the monotonic behavior of hybrid DSTCs, but only a limited number of studies have examined their behavior under cyclic axial compression. In particular, no systematic experimental study has been conducted on hybrid DSTCs under repeated unloading/reloading cycles. This paper first presents a systematic experimental study on hybrid DSTCs under two types of loading schemes: (a) a single unloading/reloading cycle to evaluate the relationship between the unloading strain and the plastic strain; (b) repeated unloading/reloading cycles to investigate the effect of loading history. In the present study, hybrid DSTCs were prepared using filament-wound FRP tubes of a relatively large scale (up to 300 mm in diameter), and the experimental program covered a wide range of concrete strengths (from 40.9 MPa to 104.4 MPa). The systematic experimental study with extensive instrumentation allowed the cyclic stress-strain behavior of the concrete in hybrid DSTCs to be examined in detail, clarifying the cumulative effect of the loading history on both the stress deterioration and the plastic strain of the concrete. This paper also presents a detailed comparison between the test results and the predictions of an existing cyclic stress-strain model for FRP-confined concrete in solid columns, in terms of the repeated unloading/reloading cycles. The cyclic stress-strain model is shown to provide reasonably accurate predictions, and thus can be used together with a monotonic stress-stress model for the concrete in hybrid DSTCs to predict the complete cyclic stress-strain curve of such concrete.

Circular hybrid double-skin tubular columns with a stiffener-reinforced steel inner tube and a large-rupture-strain FRP outer tube: Compressive behavior

A typical fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular column (DSTC) consists of an FRP outer tube, a hollow steel inner tube and an annular concrete in-fill in between. The existing studies on DSTCs in the past decade have generally confirmed the good structural performance of such column form, while it is worth noting that the possible in-ward buckling of the steel tubes in DSTCs is still a problem to be addressed, especially when DSTCs are subjected to large axial deformation. Against this background, a variation form of DSTCs called R-DSTCs has been recently developed by the authors. An R-DSTC is a DSTC in which the steel inner tube is reinforced by vertical stiffeners on the outer surface and the FRP outer tube can be circular, square or rectangular. The present paper presents the first ever experimental study on the compressive behavior of circular R-DSTCs which is the most common form of DSTCs. For the circular R-DSTC specimens tested in the present study, the outer tubes are made of a type of large-rupture-strain FRP. The vertical stiffeners on the steel inner tube are expected to delay or restrain the inward buckling of the steel tube, and the large-rupture-strain FRP outer tube makes possible a relatively large axial deformation of the specimen. In total, two DSTC specimens, twelve R-DSTC specimens and three bare steel tubes with/without stiffeners were tested, with the studied parameters covering the quantity, the dimensions and the shape of the stiffeners and the thickness of the FRP outer tube. The test results showed that R-DSTC specimens had a much better performance than the corresponding DSTC specimens in terms of both axial loading capacity and ductility, due to the existence of vertical stiffeners on the steel inner tube of R-DSTCs. The effects of the vertical stiffener-related parameters on the compressive behavior of R-DSTC specimens were also carefully examined and discussed in details.

Effect of void Ratio of Inner Steel Tube on Compression Behavior of Double Skin Tubular Column

Hybrid double skin tubular columns (DSTCs) are a composite members that consist of outer FRP tube and inner steel tube and concrete filled in between. These three materials combination in one element have several advantages do not found in common columns. The total 12 circular DSTCs were poured and tested under axial load. Every specimen is average of a pair of samples of the variables, diameter and thickness of steel tube, to investigate effect of inner steel tube void ratio on compression behavior of hybrid DSTC. The void ratio was defined as a ratio between the two inner steel and outer FRP tube diameter. The test results show that the stress-strain behavior of concrete material in the hybrid DSTCs has a better confined by two tubes. While the DSTC with small diameter has similar behavior to concrete filled FRP tube (CFFT). Thickness of inner steel tube is slightly effect on concrete stress inside DSTC.

Axial Compression Behaviour of Hybrid Double-Skin Tubular Columns Filled with Rubcrete

Double-skin tubular columns (DSTCs) have become a competitive candidate for column members due to their important advantages compared with conventional reinforced concrete columns, including their better weight-to-strength ratio and ease of construction. Using Rubcrete in hybrid DSTCs is of great interest due to the potential of this system to overcome the Rubcrete material deficiencies and to add more ductility, toughness, seismic resistance, confinement effectiveness, and environmentally-friendly features to that structural system compared to conventional concrete. In this paper, hybrid DSTCs made out of Rubcrete, sandwiched between a fibre reinforced polymer (FRP) tube and a steel tube, were tested. The examined variables were concrete sand or stone replacement ratio (0% and 20%), FRP wall thickness (1- and 2-layers), steel wall thickness (3.2 mm and 4.5 mm), void ratio (50% and 76%), and void shape (circular or square). The axial and lateral stress–strain responses were monitored, measured, and compared. According to this investigation, using Rubcrete in hybrid DSTCs can enhance the axial and hoop strain capacities, especially with fine rubber particles. It was also observed that the adverse influence of using rubber on column ultimate capacity was much lower in DSTC specimens, compared with that of unconfined Rubcrete columns. Therefore, using Rubcrete with fine rubber particles is recommended in DSTC structural columns.

Compressive behavior of hybrid double-skin tubular columns with ultra-high performance fiber-reinforced concrete (UHPFRC)

This study presents the results of an experimental program on the compressive behavior of hybrid fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) with ultra-high performance fiber-reinforced concrete (UHPFRC). In total 40 specimens, including 32 hybrid DSTCs and eight FRP confined solid concrete (FCSC) specimens, were prepared and tested under axial compression. In addition to hybrid UHPFRC DSTCs, hybrid DSTCs with ultra-high performance concrete without steel fiber addition (UHPC), high-strength concrete (HSC), and normal-strength concrete (NSC) were also tested. The investigated parameters included the FRP tube thickness, steel tube thickness, void ratio, steel fiber addition, concrete type, UHPFRC-filling inside the steel tube, and the column type. The test results indicate that the hybrid UHPFRC DSTCs can exhibit highly ductile behavior when a thick FRP tube is used. However, due to the ultra-high strength and the dense microstructure of UHPFRC, the hybrid UHPFRC DSTCs are likely to exhibit more brittle behavior than the hybrid DSTCs with NSC and HSC. Even though a high confinement level is provided, sudden stress reduction or stress fluctuations can be observed for the UHPFRC in hybrid DSTCs. The influences of FRP tube thickness, void ratio, steel fiber addition, and UHPFRC-filling inside the steel tube on the compressive behavior of the hybrid UHPFRC DSTCs are significant, while the influence of steel tube thickness is insignificant. Moreover, when compared to the FCSC specimens, the presence of an inner void is beneficial for the compressive behavior of UHPFRC in the hybrid DSTCs, especially when a thick FRP tube is used. Furthermore, the performance of existing stress-strain model to predict the compressive behavior of UHPFRC in the hybrid DSTCs is investigated.

Compressive behavior of hybrid double-skin tubular columns with a rib-stiffened steel inner tube

Hybrid fibre-reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) consist of an outer FRP tube and an inner steel tube, with the space inbetween filled with concrete. In hybrid DSTCs, the outward buckling of the steel inner tube is constrained by the surrounding concrete and the outer FRP tube, but its inward buckling is still possible. Existing research has shown that such inward buckling can become significant in non-circular hybrid DSTCs due to the non-uniform confinement, in hybrid DSTCs with a strong FRP tube due to the large axial deformation of the columns, and/or in hybrid DSTCs with a thin steel tube. In these cases, the stiffening of the inner steel tube is necessary to prevent or delay its inward buckling and to minimize its negative consequences to the column behavior. Against this background, a variation of hybrid DSTCs (hybrid R-DSTCs), in which the inner steel tube is stiffened by a number of vertical rib stiffeners, was recently developed at the University of Wollongong, Australia. This paper presents an experimental study on the axial compressive behavior of the new form of columns. The experimental program included a total of 12 R-DSTC specimens and two DSTC specimens for comparison, with the test variables being the number, thickness and width of rib stiffeners as well as the number of plies of fibres in the FRP tube. The test results confirmed that the additional rib stiffeners on the steel tube are effective in delaying the local buckling of the steel tube and in improving the performance of the columns. The test results also clarified the effects of the tested parameters of the rib stiffeners.

Numerical Simulation of Hybrid FRP-Concrete-Steel Double-Skin Tubular Columns under Close-Range Blast Loading

Hybrid fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) are a new form of composite columns that consist of an outer FRP tube and an inner steel tube, with the space between them filled with concrete. Although many studies have been conducted on the hybrid DSTCs, no studies have been conducted on their behavior under blast loading. This study presents the results of a numerical study on the behavior of hybrid DSTCs under close-in blast loading. Numerical models of hybrid DSTCs are developed using finite-element code LS-DYNA, and the reliability of the developed models are validated with available testing results. With the validated models, numerical simulations are carried out to investigate the structural responses of hybrid DSTCs under blast loading. The simulation results indicate that the hybrid DSTCs behave in a ductile manner under blast loading. The outer FRP tube can effectively provide confinement to the infilled concrete, and the inner steel tube plays a key role in resisting the blast loading. Detailed parametric analyses are conducted to investigate the influences of different parameters on the blast behavior of hybrid DSTCs. The blast resistance capacities of the hybrid DSTCs, concrete-filled steel tubes (CFSTs), and concrete-filled double-skin steel tubes (CFDSTs) are compared and discussed based on the simulation results.

Compressive behavior of hybrid double-skin tubular columns with a large rupture strain FRP tube

A hybrid fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular column (DSTC) consists of an outer FRP tube and an inner steel tube, with the space in between filled with concrete. This paper presents results from the first ever experimental study on hybrid DSTCs with a large rupture strain (LRS) FRP tube, namely, polyethylene terephthalate (PET) FRP tube. The experimental program involved the testing of 12 hybrid DSTC specimens with or without additional concrete inside the inner steel tube. The test results confirmed the ample ductility of hybrid DSTCs with a PET-FRP tube despite the severe local buckling of the inner steel tube at large axial deformations. The test results also suggested that the diameter-to-thickness ratio of the inner steel tube is a more critical parameter in such DSTCs than in DSTCs with a glass, carbon or aramid FRP outer tube.

Behavior of square fiber reinforced polymer–high-strength concrete–steel double-skin tubular columns under combined axial compression and reversed-cyclic lateral loading

This paper presents an experimental study on seismic behavior of square fiber reinforced polymer (FRP)–concrete–steel columns. Six hybrid double-skin tubular columns (DSTCs) manufactured using high-strength concrete (HSC) were tested under combined axial compression and reversed-cyclic lateral loading. The main parameters under investigation were the axial load level, size of inner steel tube, provision (or absence) of a concrete-filling inside the steel tube, and the column aspect ratio. The results indicate that, in general, square DSTCs exhibit very ductile behavior under combined axial compression and reversed-cyclic lateral loading. However, the important influence of the axial load level on the column behavior is evident, with an increase in the load level leading to a significant decrease in the lateral deformation capacity of DSTCs. The results also indicate that a DSTC with a larger inner steel tube exhibits lower lateral displacement capacity than that of a companion DSTC with a smaller inner steel tube. It is shown, however, that provision of a concrete-filling inside the inner steel tube leads to a significant increase in the lateral deformation capacity of a DSTC with a larger inner steel tube to a level that is higher than that seen in a companion DSTC with a smaller hollow inner steel tube. Experimental results are presented together with accompanying discussions on the influence of the investigated parameters on the seismic behavior of square DSTCs.

Experimental behavior of hybrid FRP–concrete–steel double-skin tubular columns under combined axial compression and cyclic lateral loading

Hybrid FRP–concrete–steel double-skin tubular columns (DSTCs) are a new form of hybrid columns, consisting of an outer tube made of fiber reinforced polymer (FRP) and an inner tube made of steel, with the space between filled with concrete. In hybrid DSTCs, the three constituent materials are optimally combined to achieve several advantages not available with existing columns, including their excellent corrosion resistance and seismic performance. Although two existing studies have examined the seismic performance of circular hybrid DSTCs, they have been limited to normal strength concrete, small column dimensions, and small void ratios (ratios between the inner diameter and the outer diameter of the annular concrete section). Against this background, this paper presents the first experimental study on hybrid DSTCs filled with HSC subjected to axial compression in combination with cyclic lateral loading. A relatively large column section (with a diameter of 300mm) was chosen to allow reliable experimental modeling of real columns. The test results indicate that hybrid DSTCs possess excellent ductility and seismic resistance even when high strength concrete with a cylinder compressive strength of around 120MPa is used; column damage is concentrated in a small plastic hinge region near the column bottom end; and the performance of hybrid DSTCs can be enhanced by filling the inner void in the plastic hinge region. The test results provide valuable data needed for the formulation and verification of a theoretical model for the hysteric behavior of hybrid DSTCs, particularly when they are filled with HSC.

Behavior of Hybrid FRP-Concrete-Steel Double-Skin Tubular Columns with a Square Outer Tube and a Circular Inner Tube Subjected to Axial Compression

Hybrid fiber-reinforced polymer (FRP) concrete steel, double skin tubular columns (DSTCs) are a new form of hybrid columns. The most common sectional form of hybrid DSTCs consists of a layer of concrete sandwiched between a circular inner steel tube and a circular outer FRP tube whose fiber directions are close to the hoop detection to provide effective confinement to the concrete. Much recent research has been conducted on circular hybrid DSTCs, which has demonstrated that the combination of the three constituent materials leads to several advantages not available with existing forms of columns. In practical applications, for aesthetic and other reasons, square hybrid DSTCs may be needed. This paper thus extends the existing work on circular hybrid DSTCs to square hybrid DSTCs in which the outer FRP tube is square while the inner steel tube is still circular. Results from a series of axial compression tests are presented and interpreted to examine the compressive behavior of square hybrid DSTCs. In these tests, FRP tubes formed through a wet-layup process were used instead of filament-wound FRP tubes because the latter were not readily available to the authors at the time of the study. The test results show that the concrete in these square hybrid DSTCs is effectively confined by the two tubes, and the behavior of the confined concrete is similar to that of concrete in FRP-confined solid columns. A stress-strain model for concrete in square hybrid DSTCs is also proposed and is shown to provide reasonably accurate predictions of the test results.

Compressive Behavior of Large-Scale Hybrid FRP-Concrete-Steel Double-Skin Tubular Columns

Hybrid FRP-concrete-steel double-skin tubular columns (DSTCs) are a new form of hybrid structural members. A hybrid DTSC consists of an inner steel tube, an outer FRP tube and a concrete infill between them. Hybrid DSTCs possess many important advantages over conventional structural members, including their excellent corrosion resistance as well as excellent ductility and seismic resistance. A large amount of research has been conducted on hybrid DSTCs, but the existing experimental studies have been limited to the testing of small-scale columns. This paper presents preliminary results from the first series of large-scale axial compression tests on hybrid DSTCs, which forms part of a larger experimental study currently under way at The Hong Kong Polytechnic University. These test results confirm the excellent axial compressive response of hybrid DSTCs as initially expected.

Behavior of Hybrid FRP-Concrete-Steel Double-Skin Tubular Columns Subjected to Eccentric Compression

Hybrid FRP-concrete-steel double-skin tubular columns (DSTCs) are a new form of hybrid columns recently proposed by the third author. The new column consists of an inner steel tube and an outer FRP tube, with the space between them filled with concrete. The new column possesses many advantages over existing columns including its good ductility, corrosion resistance and ease for construction. This paper presents the results of an experimental study on the behavior of the new hybrid DSTCs subject to eccentric compression as well as results from a corresponding theoretical model using the fiber element approach. A so-called “variable confinement model” was adopted in the theoretical analysis for the concrete in the hybrid DSTCs, to account for the effect of strain gradient on the confinement effectiveness. The theoretical model was shown to provide accurate predictions of the test results, and was used in a subsequent parametric study.

Behavior of FRP-confined concrete in annular section columns

This paper presents and interprets the results of a series of axial compression tests on short FRP-confined circular columns with an inner void to examine the behavior of concrete in such FRP-confined annular sections. This study was motivated by the need to understand and model the behavior of concrete in a new form of double-skin tubular columns (DSTCs) composed of a steel inner tube, an FRP outer tube and a concrete infill between the two tubes. To this end, three types of specimens were tested: FRP-confined solid cylinders (FCSCs), FRP-confined hollow cylinders (FCHCs), and short DSTCs. The main parameters examined include the section configuration, the void ratio, the diameter-to-thickness ratio of the inner steel tube, and the thickness of the FRP tube. The test results show that the presence of an inner void reduces the effect of external FRP confinement, but this loss of confinement effectiveness can almost be completely compensated for through the provision of a suitable steel tube. As a result, the concrete in hybrid DSTCs is very effectively confined by the two tubes and the load–axial shortening behavior of concrete in hybrid DSTCs is very similar to that of FCSCs. For an approximate analysis of DSTCs with a suitable steel tube and a reasonable void ratio, an existing stress–strain model for FCSCs may be used.