Out-of-Plane Shear Strength Of Steel-Concrete-Steel Sandwich Composite Members

Steel-concrete-steel(SCS)sandwich structural members consist of a concrete core sandwiched between two steel skins.Composite action between concrete and steel skin is achieved through studs,tie bars and other forms of ribs.The composite structure combines the advantages of reinforced concrete(RC)and steel structures.In recent years,SCS structures have found their application in large sized structures like bridges,tube tunnels,nuclear power plants and core tube of high-rise buildings,which are all strictly required on their crack,impact and seismic performances.In an SCS member,the direction parallel to the steel skin is defined as the in-plane direction,and the direction perpendicular to the steel skin is defined as the out-of-plane direction.The shear forces acting on the member include in-plane and out-of-plane forces.The in-plane shear resistance of SCS members is strong.In contrast,the out-of-plane shear strength is relatively weak,and currently there are fewer studies on the out-of-plane properties.So this paper focuses on the out-of-plane shear behavior of SCS members.SCS members subjected to out-of-plane shear forces include beams and slabs.In SCS beams the shear forces are transferred one dimensionally,and in two-way slabs the shear force is transferred through two perpendicular directions simultaneously.This paper studies the shear performance of SCS beams and two-way slabs respectively.The study starts from experimental investigations,and tries to explain the essential mechanism in SCS general members.The shear resistances are derived through lower-bound limit analysis,considering equilibrium conditions,boundary conditions and failure criteria of the materials.If the described stress distributions are close to the real ultimate state,the lower-bound solution would be sufficiently accurate.The study begins with the one-dimensional beam members.The flexural behavior of SCS beams is similar to that of doubly reinforced concrete beams,provided that composite action is well achieved through shear connectors.The mechanical model is clear.By contrast,the shear transfer mechanism is much more complex.In large-scale thick members such as that used in offshore structures and road decking,the problem of shear becomes rather critical,so this study focuses mainly on the shear performances.Based on different shear transfer mechanisms,beams can be classified into deep beams and slender beams(ordinary beams).A total of 18 deep beam specimens and 9 slender beam specimens were tested under static loads.The experimental parameters include the loading pattern,shear span/depth ratio,the configuration of steel plates,shear connectors and studs.Meanwhile,another 3 supplementary tests of reinforced concrete beams were also conducted,in an attempt to study the similarities and differences between SCS and RC members.Based on the comparisons of the shear behavior between SCS and RC members,this paper analyzes the stress redistribution process during diagonal cracking,and the shear resisting mechanism before and after critical cracking.Theoretical models are developed to predict the shear resistances of deep and slender SCS beams,both including the critical cracking strength V_cr,and the strength after critical cracking V_u.After critical cracking,the theory focuses on the resisting mechanism resulting from the composite action between the concrete strut and steel plate,and the resultant failure modes.It forms the fundamental difference between SCS and RC members.In practical engineering the SCS panels are usually supported on all four sides,and out-of-plane shear forces are transferred in two perpendicular directions simultaneously.On the basis of the research works on SCS beams,the performances of SCS two-way slabs are further studied.Eight SCS and SC slabs with different shear spans,section depths,top steel plate and tie bar configurations were tested under concentrate loads applied at the center.The observed modes of failure include flexural yielding and shear punching.The former was initiated by tensile yielding of the bottom steel plate,while the latter was primarily due to punching shear failure of the concrete core around the concentrate load.The flexural capacity can be calculated with the yield-line method.The punching shear capacity is analyzed with a tangentially symmetric radial sector model,which considers the tangential and radial stress distribution,the equilibrium conditions and the failure criteria of concrete.Finally,the problem of out-of-plane strength of SCS members is raised to a generalized SCS element,subjected to proportional loadings of axial force N_x,out-of-plane flexural moment M_x,and shear force V_xy.Firstly the elastic state under service conditions is analyzed,and on this basis,a total of six kinds of ultimate limit states are discussed respectively.The ultimate capacities are present through a series of interaction curves of axial forces and moments.At last,some practical applications are obtained according to the experimental and theoretical analysis.For example,simplified calculation methods are derived for rapid calculation.Considering the experimental observed deficiencies,some suggestions are proposed to improve the structural behavior.The potential of using the SCS sandwich system in deep coupling beams is discussed,taking advantage of its outstanding shear capacity,deformability and ductile performance.On the whole,the theoretical models are based on the experimentally observed limit states,thus they give reasonable predictions.Accuracy can be ascertained by comparing with the test results.The calculation methods are applicable to the analysis and design of SCS structural members.