Fiber-reinforced Polymer Tube Thickness Research Articles

Seismic performance of external diaphragm joints of steel beam-to-CFST column encasing fiber reinforced polymer confined UHPC core

A novel composite column was introduced by integrating a concrete-filled steel tube column encasing a Fiber Reinforced Polymer (FRP) confined ultra high performance concrete (UHPC) core. In this paper, the seismic performance of four SCF-UHPC composite column-to-steel beam connections using external diaphragm joints was experimentally investigated under the quasi-static test. All specimens failed due to the formation of plastic hinges at the beams and the tearing failure on the beam flanges at the plastic hinge zone. The local buckling at the column end and the panel zone was observed for the specimens receiving the higher axial compression ratio. The increase of FRP tube thickness is more effective in improving the ductility and energy dissipation capacity of the joints than increasing the UHPC core diameter. On the contrary, increasing the UHPC core diameter led to better resistance to the shear deformation of the panel zone. The proportion of shear deformation to the total deformation of the panel zone and the strain distributions were investigated to determine the effect of each component. The composite column in the panel zone mainly carried the compression-bending effort, and the external diaphragm mainly took the shear effort. All joints were eventually classified as semi-rigid type joints, according to the initial rotational stiffness.

Chord axial compressive behavior of hybrid FRP-concrete-steel double-skin tubular member T-joints

Hybrid fiber reinforced polymer (FRP)-concrete-steel double-skin tubular members (DSTMs) are a promising form of structural members with superior mechanical performance and durability, which have great potential for application in ocean structures. Such hybrid DSTMs consist of three components: an inner steel tube, an outer FRP tube, and concrete filled between the two tubes. Despite a significant number of studies demonstrating the excellent performance of hybrid DSTMs, no studies have been concerned with the joints of these members. The lack of a reliable design method for DSTM joints is certainly a huge obstacle to their wide application in practice. Against the above background, a research project has been proposed to understand the static behavior of circular DSTM T-joints through a combined experimental, modeling, and theoretical study. This paper presents the results of an experimental program on the axial compressive behavior of the chord in DSTM T-joints. The effects of various important factors, such as FRP tube thickness, steel tube thickness, brace-to-chord diameter ratio, void ratio, and concrete strength, on the performance of the DSTM T-joints were examined in detail. The test results demonstrated that the DSTM T-joints exhibited a ductile behavior provided that the joint region was appropriately strengthened and the presence of a brace did not significantly affect the compressive behavior of the chord in the joints with a brace-to-chord diameter ratio of 0.5. Finally, a simple method was proposed to predict the axial load-axial strain behavior of the DSTM T-joints.

Compressive behavior of FRP-confined cruciform steel-reinforced normal- and high-strength concrete columns

Fiber-reinforced polymer (FRP)-confined steel-reinforced concrete column (FCSRC), which consists of an external FRP tube and a steel-reinforced concrete (SRC) core, have been proposed to avoid the brittleness of SRCs. Capitalizing on the benefits of FRP-confine concrete, it is expected that full strength capacities of high-strength materials (i.e., high-strength concrete and high-strength steel) in FCSRCs can be exploited. Axial compression tests on FCSRCs with different strengths of concrete and steel were conducted in this study. A cruciform steel was adopted in FCSRCs. The parameters investigated in this study included the concrete strength, the steel strength and the FRP tube thickness. The test results demonstrated that the excellent performance of FCSRCs. Compared with normal-strength cross-shaped steel, the high-strength cruciform steel delays the development of hoop strains in the FRP tube for FCSRCs with normal-strength concrete. Additionally, the ultimate axial load of FCSRCs is generally larger than the direct summation of those of the steel section and FRP-confined concrete section (i.e., CFFT), demonstrating that the interaction between the three components (concrete, steel profile and FRP tube) in FCSRCs are in a benificial manner.

Cyclic Compressive Behavior and Load-Strain Model of FRP-ECC-HSC Composite Columns

An innovative composite column, which consists of a high strength concrete (HSC) core, engineered cementitious composites (ECC) ring, and fiber-reinforced polymer (FRP) tube, has recently been developed and subjected to monotonic axial compression testing by the authors. In this study, the cyclic compressive behavior of this proposed FRP-ECC-HSC composite column was examined. Test parameters, including HSC core strength, FRP tube thickness, and ECC ring thickness, were investigated. Typical failure modes, dilation behavior, and axial load versus axial strain behavior were discussed and analyzed. It was found that the FRP-ECC-HSC composite columns exhibited improved deformability, compared with the counterpart traditional FRP-confined HSC columns, with the ultimate axial compressive strain increased by 0.7%–69.1% for the tested specimens. Meanwhile, the ultimate axial strain for cyclically loaded specimens is larger than that for monotonically loaded specimens in general, indicating a delayed column failure. Cyclic axial load-axial strain models, including the envelope model, unloading and reloading models, plastic strain equation, and stress deterioration equation, were proposed to predict the cyclic compressive behavior of the tested specimens. The proposed model was verified with the test results and exhibited good performance.

Experimental study on concrete-filled steel tube columns with inner distributed seawater and sea sand concrete-filled fiber-reinforced polymer tubes under axial compression

To effectively solve the problem of the corrosion of undisturbed seawater and sea sand concrete (SSC) in practical applications, a new structure of concrete-filled steel tube (CFT) columns with inner distributed SSC-filled fiber-reinforced polymer (FRP) tubes was proposed. A total of 40 specimens were tested to investigate the axial compression behavior according to three parameters. The results indicated that the ultimate strength of CFT columns with inner distributed SSC-filled FRP tubes increased with the increase of FRP tube thickness, FRP tube diameter and steel tube thickness. The arrangement of inner FRP tubes effectively improved the axial compressive behavior and ductility of CFT columns. The ultimate strength of the CFT columns improved by 6.23 %–91.14 % due to the inner FRP tube. The failure of the specimens was always accompanied by fracture of one of the inner FRP tubes, while the remaining inner FRP tubes continued providing confinement. Lastly, two models based on different theories were proposed and evaluated to predict the ultimate load of this structure. The evaluation results indicated that both models provided accurate and reasonable predictions for the design of this structure.

Compressive behaviors of FRP-confined concrete prepared using spontaneous combustion gangue as coarse aggregate

Thirty-six concrete specimens composed of spontaneous combustion gangue (SCG) were tested under monotonic axial loading. To evaluate the mechanical behaviors of fiber reinforced polymer (FRP) confined SCG concrete, different concrete strengths and the glass FRP (GFRP) tube thickness were considered as variables. The compressive strength and ductility were significantly enhanced when the SCG concrete was confined with GFRP tube. In addition, SCG concrete has a specific expansion characteristic, which exhibits apparent volumetric compaction behavior during loading. Based on the existing model, a new design model of FRP-confined SCG concrete that uses piecewise functions was proposed, and its accuracy was verified.

Nonlinear analysis of FRP-concrete-steel solid columns

In order to study the axial compression performance of the FRP (fiber reinforced polymer)-concrete-steel solid columns (FCSSC), the nonlinear analysis program of FCSSC was compiled in this paper. The program was verified by existing tests, and the impacts of FRP tube thickness, steel tube thickness, outer radius of steel tube, steel tube strength and concrete strength were examined. The outcomes showed that the calculated results agreed with the experimental results. It demonstrated the way that the program could precisely reflect the strain of FCSSC under various degrees of load and gauge a ultimate load of FCSSC. The increment of steel tube thickness, outer radius of steel tube and concrete strength on the bearing capacity of FCSSC were chiefly reflected in the increment of initial stiffness. The improvement of FRP tube thickness could provide greater constraining force. The increment of steel tube strength could expand the load at the inflection point on the load-longitudinal strain curve. The research results can give a reference to the use of the new structure.

Seismic behavior of FRP-concrete-steel double skin tubular columns with a rib-stiffened Q690 steel tube and high-strength concrete

Fiber-reinforced polymer (FRP)-concrete-steel double skin tubular columns (DSTCs) are a form of novel hybrid columns with a particular merit of lightweight due to the existence of the void core. Utilization of high-strength materials (e.g., high-strength steel (HSS) and high-strength concrete (HSC)) tubes in DSTCs amplifies the advantage of DSTCs by earning a further decrease in the dead weight. In this study, behavior of DSTCs with HSC and a rib-stiffened HSS tube (referred to as R-DSTCs in this paper) under seismic loadings was experimentally investigated. The influences of the quantity of stiffeners, the FRP tube thickness and the axial load ratio on the seismic behavior of R-DSTCs were investigated. The test results demonstrate that DSTCs with HSC and HSS tubes exhibit a good seismic behavior, and an increase in the number of ribs leads to an increase in the lateral loading capacity, a small increase in the stiffness, a significant increase in the resistance to stiffness degradation and an increase in the energy dissipation of the R-DSTCs. Additionally, the axial load ratio and the GFRP tube thickness has a small effect on seismic performance of DSTCs.

Stress-strain behaviour of spontaneous combustion gangue coarse aggregate concrete under FRP tube confinement

Due to its poor mechanical properties and working performance, spontaneous combustion gangue coarse aggregate concrete (SCGAC) is currently limited to non-load-bearing structures. However, these defects can be effectively improved by fiber reinforced polymer (FRP). To capture the axial compression behaviour of FRP-confined SCGAC, a total of 32 cylinders specimens (150 mm in diameter and 300 mm in height) were tested under monotonic axial compression with replacement ratio and FRP tube thickness as test variables. Test results show that, in comparison to ordinary concrete in similar conditions, the strength of FRP-confined SCGAC decreases by 5.6%–39.8% while the ultimate compressive strain increases by 11.7%–110.8%. When SCGAC confined with FRP tube, its strength increases by 1.87–4.44 times, and the ultimate strain of each specimen increased by 6.12–16.19 times. In addition, the crack forms and dilation properties of SCGAC also show significant differences when compared with ordinary concrete. Finally, a new design-oriented model for FRP-confined SCGAC is generated within the framework of the existing models for FRP-confined ordinary concrete, with necessary modification to compensate for the effect of aggregate characteristics.

Seismic performance of FRP-confined steel-reinforced RAC columns

Effective utilization of construction waste is a topic of concern for the research community and the government and society. Recycled aggregate concrete (RAC) is one of the possible approaches to reuse waste concrete. Among the different approaches aiming at effective use of RAC, the use of RAC in composite structure based on the idea of taking advantages of the composite structure to overcome the shortcomings of RAC, appears as an attractive and cost-effective choice. Fiber-reinforced polymer (FRP) confined steel-reinforced RAC column (FCSRAC) is one typical example of such applications. This paper reports an experimental study on the seismic performance of FCSRACs. Eight FCSRACs were tested under constant axial load and cyclic lateral loading, allowing the effects of axial load ratios, replacement of RCA, FRP tube thickness, and steel ratio on the seismic behavior of this composite structure to be fully investigated. Test results show that although the use of RAC has a certain adverse effect on the load-capacity and stiffness degradation behavior of the FCSRACs, their deformation capacity, energy dissipation capacity, and ductility can be improved with the use of RAC, making FCSRAC outperform its counterpart using normal aggregate concrete (NAC) in terms of seismic performance.

Compound concrete-filled FRP tubular columns under cyclic axial compression

The direct use of large pieces of crushed demolition concrete (referred to as recycled concrete lumps or RCLs) for mixing with fresh concrete to create a new kind of recycled concrete (referred to as compound concrete), has obvious advantages in terms of recycling efficiency, cost-effectiveness and maximum recycling ratio compared with the recycling of concrete as aggregates. Existing research has revealed certain performance concerns with such compound concrete, including reductions in strength and durability, due to the presence of RCLs. The confinement of compound concrete with an external fiber-reinforced polymer (FRP) confining tube has recently been explored as an effective technique to improve its mechanical properties and durability. This paper presents the results of the first ever experimental study on compound concrete filled FRP tubular (CCFFT) columns aimed at the understanding and modelling of the cyclic stress–strain behavior of FRP-confined compound concrete. The effects of RCL mix ratio, FRP tube thickness, and loading scheme are examined. A monotonic stress–strain model and two cyclic stress–strain models previously developed for FRP-confined normal concrete are used to predict the test results. It is shown that the inclusion of RCLs has a marginal effect on the cyclic stress–strain behavior of FRP-confined concrete.

Axial compressive behavior of FRP-concrete-steel double skin tubular columns with a rib-stiffened Q690 steel tube and ultra-high strength concrete

Fiber-reinforced polymer (FRP)-concrete-steel double skin tubular columns (DSTCs), which possess an outer FRP tube, an inner steel tube and sandwiched concrete between both tubes, are a kind of hybrid column with a particular merit of lightweight. It is expected that using high-strength materials (i.e., ultra-high strength concrete (UHSC) and high-strength steel (HSS) tubes) in DSTCs magnifies the merit of DSTCs. This paper presents an experimental study on axial compression behavior of DSTCs with ultra-high strength concrete and a rib-stiffened Q690 steel tube. The rib-stiffened steel tubes, which are less susceptible to local buckling compared with those without stiffeners, were used in the present study. The parameters including the quantity of stiffeners, the length of stiffeners, the FRP tube thickness, the steel tube diameter and the concrete strength were carefully designed and investigated through the experimental study. The test results demonstrate that DSTCs with UHSC and HSS tubes exhibit a ductile behavior and the axial load-axial strain curves exhibit an ascending-descending-ascending-failure behavior. Additionally, the stiffeners are beneficial in enhancing the performance of DSTCs. The comparisons between the test results and theoretical results show that the existing model of Yu et al. is inaccurate in predicting the ultimate axial stress and the ultimate axial strain of HSC and UHSC in DSTCs.

Confinement of concrete in double skin tubular members under axial compression loads

A hybrid double-skin tubular members (DSTM) consist of three composite materials in one section, include fiber-reinforced polymer (FRP) tube outward, steel tube inward and concrete is in between. These three materials give the member unique properties like protect member from harsh environment, reduce the amount of concrete by the presence of an inner steel tube that allowed the passage of services. Therefore, consider sustainable structure elements, as well as the rising strengthening of members and excellent corrosion resistance. This paper presents an experimental and analytical study of twenty-four circular columns with two types; concrete-filled FRP tube (CFFT) and double skin tubular column (DSTC), with a dimension of 100 × 310 mm which constructed and testing under an axial compressive load till failure, with four variables: numbers of layers of outer FRP tube, the compressive strength of concrete, thickness to diameter ratio and void ratio of inner steel tube. The experimental results showed that the concrete was effected confinement in hybrid DSTC led to very ductile behavior of stress–strain relationship and the number of outer FRP layers was the most effective parameters on DSTCs behavior. The nominal confinement ratio was affected by the thickness of outer FRP tube and compressive strength of concrete inside DSTCs.

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.

Effect of concrete strength and tube thickness on the flexural behavior of prestressed rectangular concrete-filled FRP tubes beams

This paper presents the results of an experimental study aimed at evaluating the effect of important design parameters on the flexural behavior of unbonded post-tensioned rectangular concrete-filled fiber-reinforced-polymer (FRP) tubes (CFFTs) beams. Four full-scale beams were constructed and tested to failure in four-point bending over a simply supported span of 3,000 mm. The design parameters are the confinement effect of using FRP tube or steel stirrups, the concrete compressive strength, and the FRP tube thickness. The results showed that reinforced concrete (RC) rectangular CFFT beams prestressed with unbonded steel strands can achieve substantially higher flexural strength, inelastic flexural deformation, ductility, and energy absorption capacity than conventional prestressed RC beam without FRP tube. The ultimate strength, deformation capacity, ductility, and energy absorption were increased by 2.43, 1.89, 1.63 and 3.87 times than the prestressed RC beam. The results also showed that increasing the FRP tube thickness of the prestressed CFFT beams from 6.0 to 7.4 mm has led to the increase of the ultimate capacity, ductility, and energy absorption by 19%, 28% and 18%, respectively. Prestressed CFFT beams with high strength concrete (HSC, 70 MPa) enhanced the initial stiffness before cracking, increased the ultimate flexural moment capacity by 7%, with no significant change in the ultimate deflection, compared to normal strength concrete (NSC, 46 MPa). Based on the results of this study, the effect of increasing the FRP tube thickness is more effective in enhancing the flexural strength and stiffness of the prestressed CFFT beams than increasing the concrete strength. An analytical model based on plane sectional analysis using partially confined and unconfined concrete models as well as an empirical design equation are proposed to predict the flexural moment capacity of the tested beams. The proposed models using partially confined and unconfined concrete models and empirical equation successfully predict the flexural moment capacity with satisfactory accuracy on average of 1.06 ± 0.03, 1.23 ± 0.04 and 1.01 ± 0.08, respectively. It was also found that ignoring concrete confinement would highly underestimate the flexural strength. More investigations, however, are needed to assess the effect of a wide range of key influencing parameters to better model the flexural behavior of prestressed rectangular CFFT beams.

Effect of fiber angles on normal- and high-strength concrete-filled fiber-reinforced polymer tubes under monotonic axial compression

Concrete-filled fiber-reinforced polymer tubes are a novel form of composite columns, which are particularly attractive for structural members in harsh environments and seismic regions due to their corrosion resistance and ductile behavior. Over the past two decades, many studies have been conducted on concrete-filled fiber-reinforced polymer tubes under axial compression, and many stress–strain models have been proposed. However, existing studies mainly focused on concrete-filled fiber-reinforced polymer tubes with only hoop fibers. In order to investigate the effect of fiber angles (i.e. the fiber angle between the fiber orientation and the longitudinal axis of fiber-reinforced polymer tube), this study conducted axial compression tests of 42 concrete-filled fiber-reinforced polymer tubes with ±80°, ±60°, or ±45° fiber angles. These concrete-filled fiber-reinforced polymer tubes were constructed using normal-strength concrete or high-strength concrete. Fiber-reinforced polymer tube thickness was also investigated as an important parameter. In order to clarify the effect of fiber angles on the properties of fiber-reinforced polymer tubes, axial compression tests on 15 short fiber-reinforced polymer tubes and tensile split-disk tests on 75 fiber-reinforced polymer rings were conducted. Experimental results indicate that fiber angles had significant influences on the hoop properties of fiber-reinforced polymer tube; the confinement effect of fiber-reinforced polymer tube and the peak stress of the confined concrete decreased with the decrease of the absolute value of fiber angles, while the ultimate strain of the confined concrete increased with the decrease of the absolute value of fiber angles. Two existing stress–strain models, which were developed mainly on test results of concrete confined by fiber-reinforced polymer tubes with only hoop fibers, are capable of providing reasonably accurate predictions for concrete-filled fiber-reinforced polymer tubes with ±80° and ±60° fiber angles, but it underestimates the ultimate axial strain of concrete-filled fiber-reinforced polymer tubes with ±45° fiber angles.

Behavior of recycled brick block concrete-filled FRP tubes under axial compression

Waste brick accounts for a significant fraction of China’s construction and demolition (C&D) waste. A new reprocessing approach in which the waste brick is crushed into blocks (recycled brick blocks, RBBs) distinctly larger in size than normal-sized aggregate and mixed with fresh concrete to form a new type of compound concrete (recycled brick block concrete, RBBC) is proposed in this paper. To improve the compressive behavior of RBBC, an outer fiber reinforced polymer (FRP) tube is used in this study as both the confining and protective device for RBBC; it simultaneously serves as a stay-in-place formwork (i.e., RBBC-filled FRP tubes, RBBCFFTs). Compression tests were conducted on RBBC and RBBCFFTs with the mix ratio and size ratio of RBBs and thickness of FRP tube as variables. The test results show that RBBC has a compressive strength between those of the RBBs and the fresh concrete, and it can derive significantly enhanced strength and deformability under FRP confinement. However, the tested RBBCFFTs exhibited increased discreteness in compressive behavior owing to the large difference in strength between the RBBs and the fresh concrete, and the inhomogeneous spatial distribution of RBBs. In addition, the first portion of RBBC’s stress–strain curve appears elongated under FRP confinement. This phenomenon is believed to be due to the delay in formation of the major cracks in RBBC when it is confined in an FRP tube, allowing the integrity of RBBC to be sustained until a higher axial stress level than when the FRP tube is absent. Finally, a design-oriented stress–strain model for RBBCFFTs is developed within the framework of Lam and Teng’s design-oriented stress–strain model for FRP-confined normal concrete. The proposed model is shown to have reasonably good accuracy.

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.

Axial Response of Concrete-Filled FRP Tube (CFFT) Columns with Internal Bars

This paper aims at investigating the general axial behavior of long circular concrete-filled, fiber-reinforced polymer (FRP) tube (CFFT) columns internally reinforced with different longitudinal rebars. A total of seven CFFT and three reinforced concrete (RC) columns served as control specimens for comparisons and were constructed and tested under cyclic axial loading until failure. The test parameters were: (1) internal reinforcement type (steel, glass fiber-reinforced polymer (GFRP) or carbon fiber-reinforced polymer (CFRP)) and amount; (2) GFRP tube thicknesses; and (3) nature of loading. All columns had 1900-mm in height and 213-mm in diameter. Examination of the test results has led to a number of significant conclusions in regards to the trend and ultimate condition of the axial stress-strain behavior, mode of failure of tested CFFT columns, and plastic strains. As expected, an increase in the FRP tube thickness (or stiffness) resulted in an increase in the strength and strain enhancement ratios. The validity of the available design provisions for predicting the ultimate load-carrying capacity of tested columns is also highlighted.

Compressive behavior of double-tube concrete columns with an outer square FRP tube and an inner circular high-strength steel tube

Double-tube concrete columns (DTCCs) are a novel form of hybrid columns which consist of an outer fiber-reinforced polymer (FRP) tube, an inner circular high-strength steel (HSS) tube and the concrete filled in the entire section. In a DTCC, the inner HSS tube not only effectively diminishes the non-uniformity of stress distribution of concrete in a concrete-filled square FRP tube but also enhances the confinement to the core concrete, with its buckling being effectively restrained by the surrounding concrete and the outer FRP tube. This paper presents the results of an experimental study on the axial compressive behavior of DTCCs with an outer square FRP tube. Eight square DTCC specimens were tested under axial compression. The examined column parameters included FRP tube thickness, FRP tube width, and FRP type. The experimental results show that, in square DTCCs, the concrete is well confined by both the FRP tube and the HSS tube, and the buckling of HSS tube is effectively restrained so that its high material strength can be effectively utilized, leading to an excellent column performance. The experimental results also demonstrate that square DTCCs possess considerable post peak strength, indicating excellent ductility and promising application potential of this new form of hybrid column members in earthquake prone zones.