Ultimate strength behavior of curved steel–concrete–steel sandwich composite beams

https://doi.org/10.1016/j.jcsr.2015.08.043Get rights and content

Highlights

  • Investigate the ultimate strength behaviour of curved steel–concrete–steel sandwich beam under concentrated loading

  • Investigate influences of different parameters on shear resistance of curved steel–concrete–steel sandwich beam

  • Develop formulae to predict the shear resistance of curved steel–concrete–steel sandwich beam

Abstract

A concept of using curved steel–concrete–steel (SCS) sandwich structure as the ice-resistant wall has been proposed for Arctic oil and gas drilling platform. In the developed curved SCS sandwich structure, ultra-lightweight cement composite (ULCC) and overlapped headed studs were used as the core material and bonding measures at the steel-concrete interface, respectively. In this paper, quasi-static tests on ten curved SCS sandwich beams have been carried out to investigate their ultimate strength behaviors under patch loading that considers the critical local ice-contact pressure. The test results reported the failure mode and shear resistances of structures, studied the influences of thickness of the steel skin shell, curvature, spacing of the connectors, depth of the cross section, strength of core materials, and boundary conditions on the ultimate strength behavior of the curved SCS sandwich beam. Extensive discussions and analysis were also carried out to provide information for the development of the analytical models. Analytical models were developed through modifying design code provisions. These innovative modifications in the analytical models included redefining the inclination angle of shear failure surface, redefining the effective depth of the section, considering the influence of the thickness of steel skin, and developing analytical models on tensile resistance of the overlapped headed studs. The accuracy of the predictions by the analytical models was checked by the test data. All these efforts were made to provide better predictions on the shear resistance of the curved SCS sandwich beam.

Introduction

The steel–concrete–steel (SCS) sandwich structure, comprising two external steel skin plates and a sandwiched concrete core, is a relatively new type of structure and becomes popular in recent three decades [1], [2], [3]. Compared with the normal reinforced concrete structure widely used in civil constructions, the SCS sandwich structure exhibits distinctive advantages, such as saving formworks of concrete casting, shortening site construction time, relative high resistances to static, impact, and fatigue loadings [4], [5], [6]. This type of structure exhibits superiorities in applications which require high strength, high ductility, high blast and impact resistances [5], [6], [7], [8]. The potential applications of the SCS sandwich structure include submerged tunnels, ship hulls, bridge and offshore decks, shear walls in the high-rise building, walls in the nuclear structures, and protective structures [5], [6], [7], [8], [9].

In steel–concrete–steel (SCS) sandwich structures, mechanical shear connectors or cohesive materials are common measures to enhance the composite action at the steel-concrete interface. Cohesive materials, e.g., epoxy, offer continuous bond at the steel-concrete interface. However, imperfections in the epoxy workmanship compromise the structural performance of SCS sandwich composite structure [10]. Mechanical shear connectors offer discrete point-bonding that depends on their spacing in the structure. The main advantage of the mechanical shear connectors over cohesive materials is bridging the shear cracks in the concrete core to provide transverse shear resistance. Different types of mechanical shear connectors have been developed for the SCS sandwich structure, e.g., headed studs in ‘Double skin’ structure [2], friction welded connectors in ‘Bi-steel’ structure [11], laser-welded corrugated connectors [12], J-hook connectors [3], [4], [5], [6], [7], [8], and angle connectors [13]. Including the innovations of the connectors in the SCS sandwich structure, different types of concrete materials have emerged and evolved in the development of SCS sandwich structures [1], [2], [3], [4], [5], [6]. In the early studies, normal weight concrete (NWC) was used as the core materials [1], [2]. In order to reduce the self-weight and make the SCS sandwich structure as a competitive option for the marine and offshore constructions, lightweight concrete (LWC) made of expanded clay type of coarse aggregate was developed for the SCS sandwich structure [5], [6], [14]. The ultra-lightweight cement composite (ULCC) further increased the strength to 60 MPa but retained the density at 1400 kg/m3 [15], [16]. All these developments in the lightweight concrete materials provide more choices to develop the marine and offshore SCS sandwich structures.

Curved steel–concrete–steel sandwich structure has been proposed as the ice resistant wall in the Arctic offshore structures for the oil drilling and productions as shown in Fig. 1 [3], [17], [18]. This structure consists of two external rolling-formed steel shells and a sandwiched lightweight core. This study used the ULCC as the core material and the headed studs with overlap to bond different components working as integrity. Previous studies focused either on the curved reinforced concrete plates [19], [20], [21] or curved SCS sandwich structures without bond enhancement [22]. There is still limited information on the curved SCS sandwich structure especially for this type of structure with lightweight concrete core and shear connectors. Further investigations are thus necessary to advance the understanding on the strength development and failure mechanism of the curved SCS structures to support design protocols in engineering guidelines, which does not currently include provisions for such composite structures [23], [24].

Localised high-pressure zones (HPZ) [25], [26], [27] have become a critical design consideration for Arctic offshore structures, which experience non-uniform pressure caused by a moving ice floe or ice sheet. In this study, the curved SCS sandwich beams were tested under patch loading that considered this critical scenario under the high-pressure-concentrated ice loading.

This paper reported a test program comprising ten curved SCS sandwich beams to examine the failure modes and ultimate load carrying capacities. The experimental results demonstrate that design equations in Eurocode 4 [23] should include essential modifications on the inclination angle of the shear failure surface, the equivalent depth of the curved SCS sandwich beam, and the consideration of the influence of the steel face skin. These modifications lead to a set of new design equations capable of predicting the shear resistance of the curved SCS sandwich beam under patch loading.

Section snippets

Experimental program

The experimental program comprises ten curved SCS sandwich beams. These specimens cover the variation of the critical parameters that influence the ultimate strength of the structure, including thickness of the steel face plate, curvature of the beam, spacing of the connectors, strength of the concrete core, depth of the cross section, and the horizontal restraint stiffness of the support.

General load-deflection behaviors

Fig. 7 plots the load-central deflection curves of the beams. Overall, the load-central deflection curves exhibit brittle behavior and the load carrying capacity cannot sustain at a level after it achieves the ultimate value, i.e., there is a sharp drop in the load-deflection curves.

Fig. 8 depicts the generalized load-deflection curves of the curved SCS sandwich beams failed in shear mode that exhibits four distinct working stages. At stage OA, the curved beam behaved linearly and no cracks

Analysis on shear resistance of the curved SCS sandwich beam

Current design guideline on SCS sandwich structures has several limitations when they are used to predict the shear resistance of the SCS sandwich structures [24] as the following

  • 1)

    Neglecting the influence of the curvature

  • 2)

    Ignoring the influence of the steel face plate

  • 3)

    Overestimating the tensile resistance of the connectors used in the structure (ultimate tensile fracture strength of the connector is used instead)

The design equation proposed by Narayanan et al. [24] is used to predict the shear

Conclusions

This paper studies the ultimate strength behavior of the curved SCS sandwich beams through ten quasi-static tests. The test program investigates the influences of different parameters on the resistances of the curved SCS sandwich beams, and extensive discussions are made based on these experimental observations. Analytical models have been developed to predict the shear resistance of the curved SCS sandwich beam. Based on the experimental and analytical studies, the following conclusions are

Acknowledgements

The research described herein was funded by the Maritime and Port Authority of Singapore, and supported by the American Bureau of Shipping (ABS) and National University of Singapore under research project titled “Curved steel–concrete–steel sandwich composite for Arctic region” (Project No. R-302-501-002-490).

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