Abstract
Ocean engineering and construction are promising foundations for the implementation of China's maritime power strategy.Compared to land engineering,ocean engineering requires more complex and demanding construction environments and conditions,poses new challenges to structural engineering.Steelconcrete composite structures have broad application prospects in ocean engineering with significant performance advantages and comprehensive economic benefits as they successfully combine the respective advantages of steel and concrete.
In the field of cross-sea tunnel structural engineering,reinforced concrete structures and steel-shell concrete structures have long been the main structural forms.In the 1980s,with the development of composite structure as well as its application and experience in practice,steel-concrete-steel(SCS)composite structures began to be used in cross-sea tunnels.The steel-concrete-steel composite immersed tube structure is composed of inner and outer steel plates,partition webs,stif feners and internal concrete,as shown in Figure 1.
Figure 1 Steel-concrete-steel composite immersed tunnel structure
The steel-concrete-steel composite immersed tunnel structure makes good use of steel properties during construction and operation stage,and has superior bending,shear-resistance,impact-proof,and waterproof performances,while saving costs to a large extent.Besides,it is constructed via the immersed tube method;the immersed tunnel sections are prefabricated in dry docks or large barges,then floated to the construction sites,buried in the design positions,and fixed by the connection measures to finally complete the submarine tunnel.If the steel structure part of the composite tunnel has sufficient out-of-plane stiffness in design to resist the out-of-plane deformation caused by concrete pouring and transportation during the construction process,and the complex temperature effect in the pouring process is accurately analyzed and well controlled,the concrete pouring can be carried out on the water at the construction site,greatly saving the cost associated with the site and transportation.
This study focuses on the mechanical performance of steel-concrete-steel composite immersed tube structure and the complex temperature effect analysis duringits pouring process.The engineering background of this study is the immersed tunnel of the Shenzhen-Zhongshan crossriver tunnel under construction in China,which has already adopted the type of steel-concrete-steel composite immersed tube.
For the part of mechanical performance,a series of studies are conducted that reveal the mechanisms of bending,shear and the interface connection,and propose the corresponding design method.
Firstly,the four-point bending test is used to study the bending performance of seven specimens of the steel-concrete-steel composite structure with a scale ratio of 1∶2,with the research focused on effects of the factors such as local buckling and concrete casting imperfections[1,2].The typical phenomena are shown in Figure 2.The test results show that the main failure mode of the specimen is the yield failure of the tensile steel plate.
Figure 2 Four-point bending test failure modes
Secondly,the three-point bending test is used to study the shear performance for 16 specimens of compartment steelconcrete-steel composite structure with a scale ratio of 1∶2[3],with the research focused on the ef fect of the factors such as shear span-to-depth ratio,concrete width,and bidirectional web layout.The test results show that the structure has good shear performance,high bearing capacity and ductility.
Finally,for the steel shear connectors,push-out test is completed for a total of 78(three identical pieces per group,26 groups in total)full-scale models.It is recommended to use T-shape connectors due to their improved force bearing performance.
For the part of temperature effect analysis during the pouring process of steel-concrete-steel composite immersed tunnel structure,a temperature-structural coupling analysis method considering temperature field simulation and structural deformation analysis in the whole process of construction is proposed.The heat source and heat transfer modes considered are shown in Figure 3.
Figure 3 Heat source and heat transfer modes
Firstly,the ANSYS finite element is adopted to simulate the construction process through the birth and death element,taking into account the two temperature loads of hydration heat release and solar radiation during concrete pouring,and the boundary conditions of heat transfer between structure,air and seawater.The simulation of temperature field distribution of time-domain and space-domain during the whole process of tunnel construction is successfully achieved.The simulation results show that the maximum temperature of the roof concrete during the pouring process is about 72℃(as presented in Figure 4),which meets the Chinese codes'requirement.Besides,solar radiation has a limited effect on the peak temperature of the concrete of tunnel,but has a significant effect on the roof and outer surface of the tunnel.
Secondly,the parameter analysis shows that the reasonable selection of construction period with lower environment temperature,and the control of the concrete pouring temperature,can effectively reduce the peak temperature of the concrete.If the outside temperature or pouring temperature reduces by 10℃,the peak temperature of concrete can be reduced by 3～4℃.
Figure 4 Temperature distribution of roof concrete
The above research and practice demonstrate that this new type of composite structural system has significant advantages in both mechanical performance and construction performance,thus providing new ideas and choices for ocean engineering construction and effectively promoting the application of steelconcrete composite structures in ocean engineering.Follow-up study will be carried out around the construction monitoring of the full-scale test model of the tunnels.