Circular hybrid double-skin tubular columns with a stiffener-reinforced steel inner tube and a large-rupture-strain FRP outer tube: Compressive behavior
Introduction
Fiber reinforced polymer (FRP) composites have been widely adopted as a type of confining material for concrete in structural engineering in the past two decades because of its structural advantages such as high strength, low density, excellent corrosion resistance and ease in construction [e.g., Ref. [[1], [2], [3], [4], [5], [6], [7], [8], [9]]]. In addition to its most popular application of being used as externally bonded reinforcement in structural retrofitting industry, the use of FRP in new buildings and constructions has become increasingly popular in last decade [e.g., Refs. [6,[10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]]. The FRP-concrete-steel double-skin tubular column (DSTC) proposed by Refs. [10] is one of the most popular applications of FRP in new composite structural members. Typically, a DSTC is comprised of three components: an FRP outer tube, a steel inner tube and a concrete in-fill between the two tubes. The concept and the potential structural advantages of DSTCs have been well demonstrated in Refs. [10], and the extensive studies on DSTCs [e.g., Refs. [6,10,[20], [21], [22], [23], [24], [25], [26]]] in the past decades have generally confirmed the good structural performance of DSTCs and developed a relatively comprehensive and in-depth understanding of the structural behavior of DSTCs. Design methods have also been provided for DSTCs in a Chinese national technical code [27]. Among the existing relevant studies, Ref. [26] tested circular DSTCs under combined axial load and cyclic lateral load, and reported that severe local buckling of the steel inner tubes of DSTC specimens in plastic hinge regions was observed as the concentrated axial deformation happened therein. Ref. [6] tested short DSTCs with a large rupture strain FRP tube under concentric compression and also found that severe local buckling of the steel inner tubes of DSTCs occurred as a result of the large axial deformation of the specimen. These experimental findings indicate that the potential local buckling of the steel inner tubes can be a problem when DSTCs are loaded under large axial deformation, especially when relatively thin steel tubes are used in DSTCs. In addition, when the bending is significant or even becomes the dominant behavior of DSTCs, the superior structural performance of DSTCs could be compromised by the relatively weak bond behavior between the concrete and steel components due to the smooth bi-material interface between them. Finally, when a small void ratio is used for DSTCs, the contribution of the steel inner tube in DSTCs to the second moment area of the cross-section can be limited as its position is close to the bending axes of the cross-section. Local buckling of steel tube is also a common problem for concrete-filled steel tubes (CFSTs) [28]. To tackle this problem, welding vertical stiffeners on the inner surface of the steel tube before pouring concrete has been investigated and proved to be effective in delaying the local buckling of the steel tube and thus improving the axial behaviour of CFSTs [[29], [30], [31]].
Against this background, the compressive behavior of a variation form of DSTCs, namely DSTCs with a stiffener-reinforced steel inner tube (referred to as R-DSTCs hereafter for simplicity), is investigated in the present study. In R-DSTCs, the vertical stiffeners are attached on the outer surface of the steel inner tubes by welding. Due to the similar cross-sectional configurations of R-DSTCs and DSTCs, R-DSTCs obviously have all the structural advantages of DSTCs. In addition, R-DSTCs have the following structural advantages over DSTCs: (1) the inward local buckling of the steel inner tube can be delayed or restrained due to the presence of vertical stiffeners which are encased in the annular confined concrete core; (2) the concrete-steel bond behavior can be improved as the adhesion and interaction between them are enhanced by the embedded stiffeners; (3) the stiffeners in R-DSTCs can make additional contributions to the second moment of area of the cross-section and thus lead to a better seismic performance of R-DSTCs. It should be pointed out that although the addition of stiffeners onto the steel inner tube in R-DSTCs may lead to a lightly complex cross section compared with traditional double-skin tubular columns, the constructional procedure of R-DSTCs will not be much influenced in practice, as the rib-reinforced steel inner tube, to be used as part of the permanent formwork for casting concrete on site, can be prefabricated in factory.
To the best knowledge of the authors, the present study is the first ever experimental investigation into the compressive behavior of circular R-DSTCs, while another work by the authors provides an experimental study on square R-DSTCs [32]. It has been found by Ref. [32] that the vertical stiffeners can largely improve the axial load capacity and axial deformation capacity of square R-DSTCs, and such improvement was found to be influenced by the layout and geometry properties of the stiffeners. It should be noted that although Ref. [32] has shed light on the present study, the non-uniform confinement nature resulted from the use of a square FRP tube in square R-DSTCs implies that main findings from Ref. [32] cannot be directly applicable to the circular R-DSTCs (the most common form of DSTCs), in which the confinement exerted by the circular FRP tube onto concrete is circumferentially uniform. Therefore, the present study, which aims for a better and in-depth understanding of the behavior of and thus a more confident use of circular R-DSTCs, is in necessity.
In the past decade, conventional FRP (e.g., carbon FRP and Glass FRP) are most commonly employed in experimental studies of DSTCs [e.g., Refs. [10,22,26]]. Recently, Ref. [6] conducted an experimental study on DSTCs of which the FRP tubes were made from a type of large rupture strain FRP, namely polyethylene terephthalate (PET) FRP. PET FRP has a rupture strain of over 7%, which is over three times of the rupture strain of conventional FRP. Additionally, PET FRP is a type of environment-friendly material as it can be made from waste PET plastic products (e.g., plastic bottles). Relevant studies [e.g., Refs. [6,15,17,[33], [34], [35], [36], [37]]] have shown that PET FRP can substantially enhance the deformation capacity and ductility of confined concrete owing to its large rupture strain. In this regard, PET FRP tubes were adopted for all the double skin tubular columns tested in the present study to investigate the buckling behavior of the steel inner tube under large axial deformation.
In this paper, short circular R-DSTCs with a PET FRP tube were tested under concentric compression to obtain a better understanding of the compressive behavior of such columns. The studied parameters include the quantity, the dimensions and the shape of the stiffeners, and the thickness of the FRP tubes. Based on the test results, the compression behavior of R-DSTCs are discussed and interpreted in this paper.
Section snippets
Test specimens
A total of 14 specimens were tested in the present study, including one pair of short DSTC specimens and six pairs of short R-DSTC specimens. The two specimens in each pair were nominally identical to each other and thus had the same cross-sectional configuration, which leads to seven different cross-sectional configurations in total in this study. The typical schematic diagrams of DSTC specimens and R-DSTC specimens with four stiffeners and six stiffeners are shown in Fig. 1. All the DSTC and
DSTC and R-DSTC specimens
Fig. 8 shows the typical failure modes of the DSTC and R-DSTC specimens. Most of the specimens failed by rupture of the FRP outer tubes near the mid height, as shown in Fig. 8(a)–(c). Loud and sharp noises were heard at the failure of the specimens. For some specimens, local debonding of the outermost layer of FRP sheet was also observed within the overlapping zone along with the rupture, as shown in Fig. 8(d). Such local debonding of FRP happened at the very late stage of the tests thus is
Conclusions
This paper presents an experimental study on the compressive behaviour FRP-concrete-steel double-skin tubular columns (DSTCs) of which the steel inner tubes are reinforced with longitudinal stiffeners. The studied parameters included the quantity of the stiffeners, the dimensions of the stiffeners (i.e. width and thickness), the shape of the stiffeners (i.e. rectangular and wave-shaped) and the thickness of the FRP jacket. Bare steel tubes (both with and without stiffeners) and normal DSTCs
CRediT authorship contribution statement
L. Huang: Conceptualization, Methodology, Investigation, Data curation, Writing - original draft. S.S. Zhang: Supervision, Methodology, Validation, Writing - review & editing. T. Yu: Conceptualization, Supervision, Writing - review & editing. K.D. Peng: Investigation, Data curation.
Declaration of competing interest
No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication.
Acknowledgement
The authors gratefully acknowledge the financial support provided by the Australian Research Council through its Discovery Projects funding scheme (project ID: DP170102992).
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