Flexural behaviours of steel-UHPC-steel sandwich beams with J-hook connectors
Introduction
Steel-concrete-steel (SCS) sandwich structures (SCSSSs) typically comprise two external steel faceplates and a sandwiched concrete with inter-bonding methods to make sure them working compositely. SCSSSs have a wide range of structural advantages over conventional RC structures in terms of reduced fabrication details, reduced construction labour force, increased construction efficiency, and improved strength as well as ductility. After the pilot application as the undersea oil containers [1], SCSSSs have been successfully applied as shield tunnel in Japan [2], immersed tunnel in China [3] and UK [4], LNG container [5], bridge deck [6], building shear walls [[7], [8]], marine structures [9], offshore platforms [10], and ice walls [11].
Different bonding measures have been developed and proposed for SCSSSs, e.g., adhesive materials and different mechanical shear connectors. Pilot bonding method using adhesive materials (e.g., epoxy) was developed by Solomon et al. [6] for SCSSS bridge decks as shown in Fig. 1(a). Despite sufficient faceplate-concrete bonding was provided and failed the SCSSS plate in flexure, the adhesive materials could not offer out-of-plane shear resistance to SCSSSs. Similarly, the SCSSSs with shallow angles, as shown in Fig. 1(b), were with the same disadvantage [12]. Mechanical shear connectors through the whole depth of the concrete core in SCSSSs overcame this shortcoming, e.g., overlapped headed studs [13], friction-welded rods [14], laser-welded corrugated strip connectors [15] as shown in Fig. 1(c)–(e),hollow steel tubes [16], tilted angle connectors [17] and pipe shear connectors [18]. However, the bonding provided by headed studs in SCSSSs mainly relies on the anchoring concrete core. The failed or spalled concrete core would make this bonding workless. Though friction- or laser- welded connectors offer higher interfacial separation and shear bonding to SCSSSs, their welding equipment restricts their geometries, e.g., the depth of SCSSSs with friction-welded rods is restricted to 0.2–0.7 m due to friction-welding machine, and the thickness of faceplate for SCSSSs with laser-welded strip are restricted to be less than certain value depending on the welding machine (usually less than 6 mm) [14,15]. In addition, these two types of welding tend to be costing for civil engineering constructions. These previous studies have shown that these shear connectors, which can be used in SCSSSs, possess both advantages and potential disadvantages. It is necessary to develop a new type of connectors for SCSSSs that offers excellent structural performances but low costing. J-hooks, as shown in Fig. 1(f), offer a type of semi direct-link connectors to two faceplates in SCSSSs [19]. Moreover, they significantly reduce the costing of SCSSSs compared with friction- or laser- welded connectors. Previous studies have proved the excellent structural performances of SCSSSs with J-hooks (SCSSSJ) under different kinds of loads and effective bonding at steel-concrete interface [8,10,[20], [21], [22], [23], [24], [25], [26]]. The shear and tensile behaviours of J-hook connectors have been extensively studied by Yan et al. [[20], [21]] through 102 push-out tests and 79 tensile tests, respectively. In addition, these studies also cover the shear and tensile behaviours of J-hooks in SCSSSJs with UHPC. Yan et al. [22] studied the ultimate strength behaviour of SCS sandwich beams adopting J-hooks and ultra-lightweight cocnrete, which confirmed the excellent structural performance of SCSSSJs. This paper continues the study on developing high performance SCSSSs with this novel connector and ultra-high performance concrete (UHPC), i.e., steel-UHPC-steel sandwich structures with J-hooks (SUSSSJ).
Recently, UHPC has been adopted for the high performance composite structures. Liew et al. [27], Xiong et al. [[28], [29]], and Hoang et al. [30] applied the UHPC in composite columns of buildings. The UHPC-steel composite beams or decks have been developed for the bridge deck that was extensively reported by Yin et al. [31], Luo et al. [32], Cao and Shao [33], Wang et al. [34], and Zhu et al. [35]. More recently, Lin et al. [36] developed the high performance SCSSSs using UHPC for nuclear shielding walls. This paper develops the high performance SCSSSs with J-hook connectors and UHPC, i.e., SUSSSJ. Therefore, it is of interest to study the structural behaviours of SUSSSJ with J-hooks and UHPC.
Firstly, this study reported two-point loading tests on nine steel-UHPC-steel sandwich beams with J-hooks (SUSSBJs) for the investigations of their ultimate strength behaviours. Key parameters in this testing program were shear span ratio (λ), thickness of steel faceplate (ts), and spacing of J-hooks (S). Influences of these key parameters on structural behaviour of SUSSBJs were checked and analysed. Followed, this study also proposed theoretical models to estimate the strength and stiffness indexes of SUSSBJs that include initial stiffness (K0), elastic stiffness (K1), yielding resistance (Py), and ultimate resistance (Pu). Validations against the reported test results were performed to check the accuracy of developed theoretical models.
Section snippets
Details of SUSSBJs
Fig. 2 shows the fabrication details of a typical SUSSBJ. Firstly, J-hooks were fabricated and welded to the two steel faceplates, and Fig. 2(a) shows the geometric details of a J-hook. After fabrication and welding J-hooks, the two steel faceplates with the welded J-hooks were assembled to produce a steel skeleton for UHPC casting [see Fig. 2(b)]. The casting of UHPC for SUSSBJs is illustrated in Fig. 2(c). Finally, Fig. 2(d) plots the geometric details of a representative SUSSBJ after
Failure modes
Fig. 5(a)–(h) presents the deformed shapes and cracks in the concrete core of SUSSBJs J1–J8. These figures show that all the SUSSBJs failed in flexure mode that show characteristics of vertical cracks in the UHPC core. These vertical cracks in UHPC core started developing in the mid-span and their quantity gradually increased between two loading points, i.e., pure bending region of SUSSBJs. Moreover, all these vertical cracks initiated from the bottom and continued developing to the top
Analysis on stiffness and strength of SUSSBJs
The developed theoretical models developed herein consider the tensile strength of the UHPC. Fig. 13 shows the idealized constitutive models for UHPC under compression and tension whilst Fig. 14 plots the adopted three-linear constitutive models for steel materials.
Conclusions
This study performed nine tests on ultimate strength behaviours of SUSSBJs. The failure and load-transferring mechanisms of SUSSBJs were detailed reported. Moreover, the influences of key parameters on ultimate strength behaviour of SUSSBJs were also reported. This paper also developed theoretical models to estimate the strength and stiffness indexes of SUSSBJs, and these models were validated by the reported test results. These experimental and analytical studies support following conclusions;
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Author contribution statement
Jia-Bao Yan: Conceptualization, Methodology, Analysis on test data, Writing original draft of the manuscript.
Huitao Hu.: Performing tests, analyzing test data, preparing figures.
Tao Wang: Funding the tests, revising first and second round of the draft.
Declaration of Competing Interest
The authors declare that they have no conflict of interests.
Acknowledgment
The authors would like to acknowledge the research grant 51608358 received from National Natural Science Foundation of China and Peiyang Scholar Foundation (grant no. 2019XRX-0026) under Reserved Academic Program from Tianjin University for the works reported herein. The authors gratefully express their gratitude for the financial supports.
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