Research On Shear Design Method Of Steel Shell-Concrete Composite Structure In Immersed Tunnel

The tunnel of the national major infrastructure project "Shenzhen-Zhongshan Link Project" is a two-way eight-lane submarine immersed tube structure.The slab and bottom roofs,side and middle walls of the immersed tunnel are all composed of steel shell-concrete(hereinafter referred to as SSC)composite structures.The SSC composite structure consists of the muti-chamber steel shell and concrete that filled inside each shell,of which the steel shell is formed by top plate,bottom plate,longitudinal diaphragm and transverse web.The immersed tunnel structure is novel and shear force transferring mechanism is complex.Morevoer,the interaction mechanism between infilled concrete,steel transverse webs and steel longitudinal diaphragms,the constrain effects on the infilled concrete from the steel shell,and the composite action width in the structure are crucial to shear mechanism,and has become a key scientific issue in the design.Therefore,by carring out model experiment,numerical simulation and theoretical analysis,the shear failure modes and influencial factors effects on the SSC composite structures are investigated,and the composite action mechanism of steel shell and infilled concrete is revealed.The empirical calculation formulas for the steel webinfilled concrete composite action width,for the influence coefficients of steel web and steel diaphragm on SSC composite structures are suggested,and the calculation formulas of shear bearing capacity are established.The shear design method and design mode guide of SSC composite structures in immersed tunnel are proposed.1)Based on the details of the immersed tunnel slab roof of Shenzhen-Zhongshan Link,model specimens with a reduced-scale of 1:2.5 were tested under shear to study the effects of steel web and its thickness,its spacing,the steel diaphragm spacing and stiffening arrangement on the steel plates.Results showed that the steel web and infilled concrete was mainly under tension and compression diagonally,and the steel diaphragm was tensioned vertically,bearing the shear force together.Comprared to the specimens without steel web,the shear capacity of specimens with steel web and thickened web was significantly improved.Steel diaphragm spacing could affect the angle of diagonal compression,so that the decrease on diaphragm spacing resulted in an increase on the capacity.T-shape ribs in transverse direction could induce the initial crack,but showed little effect on the uitimate shear capacity.2)Based on three-dimensional solid finite element models,which considered the nonlinearity and plastic damage of the materials,and the friction and relative slip at the steel-concrete interface properties,refined simulation analysis on shear experiment are conducted.Results shows a great agreement between numerical simulation and experimental results,indicating that finite element model could reflect the shear behavior of the structure.The steel web is under tension diagually tension gradually as the concrete cracks occurred,and the inclined compression zone is formed in the infilled concrete.Furthermore,the compressive stress in the concrete near steel web within a range of about 0.4～0.5 times the section height is much larger than that in the concrete beyond the range,shows a significant composite action between the steel web and infilled concrete.3)Based on the numerical simulation,parametric analysis on the influencing factors of ultimate shear bearing capacity are further discussed.The results show that the main structural influencing factors are the thickness of the steel web,the length-toheight ratio of the steel shell chamber,etc.The thicker of the steel web is,the greater composite actions could be achieved,while when the thickness exceeds a certain value,the steel web shows a minor effect on the composite actions.As the length-to-height ratio of the steel shell increasing,the ultimate shear capacity decreases gradually.For the SSC structure without the steel web,the failure mode is the diaphragm yielding in tension when the steel diaphragm is relative thinner;while the structural failure is controlled by the infilled concrete when the thicker diaphragm applied.4)Based on the elastic foundation beam method,a shear model considering the constrain effects of the steel shell on the infilled concrete is established,and a simplified calculation formula of the steel web-infilled concrete composite action width is given.The calculation formula for the influencing coefficient of the composite action effect and the steel diaphragm effect is fitted with the parametric analysis results of numerical simulations.A series of formulas for ultimate shear capacity are proposed,considering the composite action width,the effects of composite action and the steel diaphragm.Finally,the design calculation methods and mode guide of the steel shell-concrte composite structure in immersed tunnel is suggested.