Research On Mechanical Behaviors And Design Methods Of Steel-Concrete-Steel Immersed Tunnel Composite Structures

The novel steel-concrete-steel immersed tunnel composite structures(SCSITs)are used in large-scale submarine tunnel projects,exhibiting superiorities in capacity,ductility,impact resistance,blast resistance,and construction efficiency.Studies on the shear,bending,and local buckling mechanisms of SCSITs are carried out,in which experimental programs,numerical models,and theoretical analyses are conducted and developed.The main research works and results are as follows:(1)Shear experimental programs on 16 SCSIT specimens are conducted,in which three-point loading method is applied.The influences of shear span-depth ratio,steel ratio,arrangement of webs,interfacial connection,and dowel action on the shear capacities are investigated.Cracking development and stress state are analyzed.Main failure modes and relative mechanical characteristics are clarified.(2)Bending experimental programs on 7 SCSIT specimens are conducted,in which four-point loading method is applied.The influences of casting imperfection,interfacial property,and local buckling on the bending capacities are investigated.Stain-stress development and buckling evolution process are analyzed.Different failure modes are classified and suggestions are proposed on structural design restrictions.(3)Numerical model to simulate the bending/shear behaviors of SCSITs is developed,based on the general finite element software package MSC.MARC,which will function as an effective tool in the structural analysis of SCSITs.The interfacial setting,material constitutive model,and cracking simulation method are analyzed and discussed.Further,parametric analyses are conducted to reveal the intrinsic structural mechanisms.(4)Theoretical shear model considering different anti-shear mechanisms of composite truss,web pure shear,and dowel action is proposed.Stress decomposition method to distinguish the attendance and contribution of the axial web in different anti-shear mechanisms is developed.Influences of shear span-depth ratio etc.are investigated and a theoretical method to derive the shear capacities considering multiple mechanisms is proposed.The proposed method is verified through the comparisons with the test and numerical model.(5)Theoretical bending model is developed and analyses on the influences of local buckling,shear-bending coupling,partial interfacial connection are carried out.Theoretical solutions on the stress state of the flanges in multiaxial actions are derived both in elastic and plastic ranges.The effect of bidirectional strengthening is revealed and the relative strengthening factor is obtained.The theoretical method to derive the bending capacities considering multiaxial effect is then developed and verified.Further,design suggestions to consider the local bucking,casting imperfection,and material hardening are proposed.(6)Theoretical model to analyze to the buckling process of the composite stiffened plates is developed.A selected series method is proposed to describe the buckling shapes of stiffened plates.Based on the energy theory,the high order explicit solutions on the elastic buckling of stiffened plates are derived.Numerical model is then developed and massive parametric analyses are conducted,based on which a reduction factor is proposed to consider the mixed middle state buckling.The theoretical solutions are verified through the numerical model and a design procedure is suggested.