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As a promising strengthening solution, carbon fiber reinforced polymer (CFRP) sheets have been used to improve the structural response of concrete-filled steel tubes (CFSTs) in service loads, owing to the durability and high strength-to-weight ratio provided by CFRP. Despite the wealth of knowledge available in the literature on how CFRP sheets contribute to strengthening CFSTs under static loads, there is a research gap regarding how the strengthened structural components respond to lateral impact loads , due to vehicle and vessel collision, as well as wind and water-borne debris impact. This was the motivation of the current study to establish a computational framework supported by experimental tests to evaluate the performance of CFST beams with and without CFRP under a set of impact scenarios. For this purpose, a range of influential aspects related to CFRP, concrete, and steel, as well as impact energy, are investigated. Such a holistic assessment provides unique information to answer fundamental and practical questions regarding the use of CFRP for strengthening of CFST beams against impact loads. To further assist with the proper configuration of CFRP-strengthened CFST beams, a section analysis method is developed and validated as the outcome of this study.

To investigate the compressive behaviour of carbon fibre-reinforced polymer (CFRP)-confined circular concrete-filled steel tube (CFST) stub columns with inner latticed steel angles, the axial compression test was carried out in this paper. The main parameters investigated included the diameter-to-thickness ratio of the outer steel tube, the cross-sectional area of the inner latticed steel angles and the number of CFRP layers. First, the failure modes, load-displacement curves, ultimate load results and load-strain curves were obtained experimentally. Second, the stress-strain curves of steel tubes, the confining stress on the concrete and the stress-strain curves of the concrete were analysed according to the experimental results. Finally, design equations for the ultimate load of CFRP-confined circular CFST stub columns with inner latticed steel angles were proposed. Research results showed that the latticed steel angles have an excellent effect for improving the ultimate load and ductility of CFSTs, and the CFRP have an obvious confining effect on the local buckling of steel tubes, thus improving the ultimate load of CFSTs with latticed steel angles. It can be concluded that the failure of CFRP-confined specimens were mainly due to the rupture of the CFRP caused by the local buckling of steel tubes, and the latticed steel angles could participate in the overall loading process and provide a certain confining effect on the concrete enclosed by the latticed steel angles. On the basis of experimental results, design equations for predicting the ultimate load of CFRP-confined CFST clumns with latticed steel angles were proposed and compared with experimental results, and conservative prediction results were acquired.

This study examines the use of timber as a filler material for steel and pultruded GFRP tube elements and the performance of these sections is compared to solid timber columns, hollow steel and hollow pultruded GFRP tube element after exposure to elevated temperature. In total, 102 specimens were fabricated and exposure to elevated temperature ranging from 20 to 700 °C was evaluated. It was found out that filled tubes using timber could improve the compressive strength and stiffness of tubular section specimens up to 73 and 212 percent, respectively. Timber filled steel tube (TFST) and hollow steel tube specimens had a better performance at elevated temperature in comparison to other specimens such that they were able to maintain 28 and 44 percent of their compressive strength and axial stiffness at 700 °C. In contrast, timber filled pultruded GFRP tube (TFGT) and hollow pultruded GFRP tube lost more than 25 and 93 percent of their strength and initial stiffness at temperature beyond the glass transition temperature of the resin, 95 °C, and resin decomposition temperature, 288 °C, respectively. Two novel empirical models were developed which accurately predicted the compressive strength and axial stiffness of TFST and TFGT specimens at room and elevated temperature.

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.

This paper for the first time presents a newly developed analysis-oriented theoretical model for steel tube confined ultra-high performance concrete (UHPC). An active-confined UHPC model is firstly developed using experimental data collected from 62 conventional tri-axial compression tests for UHPC. Lateral strain calculation equations for both unconfined and active-confined UHPC are then proposed by using elastoplastic theory. Combined with the constitutive model of steel tube, the analysis-oriented theoretical model for steel tube confined UHPC is established and verified with available experimental results of UHPC filled steel tube (UHPCFST). Furthermore, the model’s performance is evaluated with elaborations on the steel tube confinement effect and the axial stress–strain behavior of the confined UHPC for different steel tube thicknesses, steel yield strengths and UHPC compressive strengths. It is unveiled from the responses of the developed model that the confinement effect of steel tube on the core UHPC tends to be postponed for the case with higher UHPC compressive strength. The impacts on the confining stress follow an order of steel tube thickness > UHPC compressive strength > steel yield strength. Moreover, the pre-peak axial stress–strain behavior of steel tube confined UHPC is primarily determined by the UHPC compressive strength, while its post-peak behavior varies with the change in both UHPC compressive strength and steel tube thickness. The proposed theoretical model can serve as a useful tool for engineering design and nonlinear analysis of UHPCFST.

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.

Fiber-reinforced polymer (FRP)-steel tubed concrete (FSTC) columns have emerged as a new type of compression members. An experimental program was performed to further demonstrate the axial compressive behavior of FSTC stub columns and to clarify the confinement mechanism in such members. The test results showed that the capacities of the axially loaded FSTC specimens were enhanced with the increase of the FRP confinement and the decrease of the steel tube’s diameter-to-thickness (D/t) ratio. After deducting the steel tube’s axial bearing contribution through elastic–plastic analysis, the concrete cores’ axial load–strain responses had a bilinear shape, which was very similar to the axial load–strain responses of the composite columns. Based on the comparison between steel confinement and FRP confinement, a confinement mechanism was proposed to reveal the interaction between the inner concrete core and the outer FRP-steel composite tube. It was divided into three stages: domination of steel confinement, domination of FRP-steel dual confinement, and domination of FRP confinement. The total confining pressure at the third stage could be considered as a translational motion of the FRP confining pressure, which explained why the second stiffness of the confined concrete was more sensitive to the FRP confinement stiffness than the steel confinement.

This study presents an experimental investigation on concrete confined by glass fiber reinforced polymer (GFRP) and perforated-steel tube subjected to uniaxial compression. The compressive behavior of this hybrid structure is compared with GFRP tube encased concrete and steel tube encased concrete. The three key experimental parameters were investigated: the layers of GFRP tube (1, 2, 3 and 4), the perforation diameter of porous steel tube (5 mm and 10 mm), and the grid form of perforated steel tube (axial type and helical type). The effects of these parameters on the failure mode, stress–strain relationship and ductility of the specimens were discussed. A simplified design model was proposed for the specimens and the predictions were in good agreement with the experimental results. In addition, the interactive function between steel and GFRP on the confined concrete was analyzed which considered an influence factor provided by these two confinement materials. This study showed that the hybrid GFRP–perforated steel tube exerts the full potential of these two materials, i.e. GFRP with its high tension strength enhanced the loading carrying capacity and the perforated-steel with its high elastoplasticity enhanced the deformation performance in hardening segment, softening branch and residual strength plateau of the concrete, respectively. This study therefore demonstrated that the combination of GFRP and perforated steel tubes is a new effective method to confine concrete.

Previous studies suggested that the behaviors of confined concrete in concrete-filled steel tube (CFT) columns were dependent on their confining stress paths, which thus should be considered in order to well predict their behaviors. Several confining stress path dependent analytical models had been proposed, which, however, were all developed for fiber polymer (FRP) confined concretes. Although one of the existing models can also be applicable to CFT columns, it mainly focused on the specimens with normal strength concrete. Additionally, existing confining stress path dependent models may result in fluctuation issue of load-deflection curves for CFT specimens. This paper proposed a new confining stress path dependent analytical model that is applicable to normal, high and ultra-high strength concrete-filled steel tube columns. Experimental tests were collected to assess the performance of the proposed analytical model, and it was found that the proposed model performs quite well for normal, high and ultra-high strength concrete-filled steel tube columns and generally can avoid the fluctuation issue of load-deflection curves.

Preload in a cast-in-situ concrete-filled steel tube (CFST) structure is an important parameter that has been studied and considered in design and construction. However, the impact of a preload on fire resistance of CFST has not been taken into account in the existing design method of CFST under fire. In this paper, twelve CFST columns with and without preload in the steel tubes are experimentally studied to investigate their heating process, failure modes, thermal and structural responses. The test results show that preload of the steel tube has significant impact on the fire resistance of some of the columns. Further investigations on this important observation will be done numerically in a companion paper [24] to carry out a more comprehensive parametric study.