Stability Capacity And Collapse Resistance Performance Of Steel Pipe Piles After Corrosion Under Marine Environment

With the development of maritime transport infrastructure in China,the construction of cross-sea bridges is carried out in a more severe and complex marine corrosive environment.As a temporary steel structure,the steel trestle is an essential part in the construction of cross-sea bridges.Its service life generally equals to the construction duration of the main bridge structures,and the corrosion protection measures are generally weaker than those of main structures.Meanwhile,steel trestles during service bear the similar external loads to the main structures.Therefore,in the harshly corrosive marine environment,the steel corrosion has extremely unfavorable influence on the structural safety of steel trestles.So far,the morphology and material properties of the corroded steel in the corrosive marine environment are not yet clarified.The influence study of non-uniform corrosion on mechanical properties of steel pipe piles(SPPs)is still in the initial stage.And the failure mechanism and collapse resistance of the steel pipe pile foudation(SPPF)after corrosion are not clear.Therefore,based on the Pingtan Straits Rail-cum-Road Bridge(the first rail-cum-road cross-sea bridge in the East Sea),the mechanical properties of SPPs considering the corrosion effects in the marine environment were investigated in this paper.The main research contents are as follows:(1)Based on the construction project of Pingtan Straits Rail-cum-Road Bridge,onsite investigations of the steel trestle were conducted to obtain the typical macro-scale corrosion modes of SPPs in the strongly corrosive environment.The automatic classification method of steel corrosion zones was established using computer vision technology.The accuracy of different neural network models were analyzed.Based on the adopted classification method and on-site survey corrosion results,the SPP with the most serious corrosion was sampled in different corrosion zones.After the corrosion morphology scanning of the sampled steel surface,the geometric model reconstruction and the extreme value analysis of steel maximum residual thicknesses,it was found that the surface corrosion morphology of the steel samples in different corrosion zones were different,and steel samples in the splash zone corroded the most.The maximum residual thickness of corroded steel samples in different zones was fitted to Weibull distribution.(2)Uni-axial tensile tests on corroded steel specimens in the splash,tidal,immersion and non-corrosion zones were conducted.It was found that the reduction of the nominal yield strength,nominal tensile strength,nominal modulus of elasticity and elongation after fracture differed in different corrosion zones.Based on the morphology scanning results and the random distribution rule of the steel maximum residual thickness,the finite element(FE)models considering the random steel residual thickness were established,and the uncertainties in the elastic modulus,yield strength and tensile strength of corroded steel specimens were analyzed.(3)A simplified mechanical model of corroded SPPs was proposed considering the non-uniformly distributed corrosion along the pile height.The formulas for the effective length coefficient of corroded SPPs with different step orders were derived using the equilibrium equation method.The FE model of corroded SPPs was established to verify the accuracy of proposed formulas,and the empirical values of the effective length coefficient for stepped SPPs under different boundary conditions were concluded.In addition,the comprehensive parametric analysis and sensitivity discussion on the stability bearing capacity of SPPs under combined compression and bending were carried out with the key parameters such as step orders,the control section and sectional area ratios.to identify the essential parameters affecting the stability bearing capacity of SPPs.The calculation method for the stability bearing capacity of SPPs under combined compression and bending was put forward considering the non-uniform corrosion,and the calculation method was validated through comparative analysis with FE results.(4)The FE model of the steel pipe pile foundation(SPPF)was established considering the geometrical and material non-linear characteristics.Taking the influence of wave height distributions on the loading pattern of structural lateral forces into account,the failure mechanism of SPPF in the different wave incidence directions was investigated using the incremental wave analysis method.The most unfavorable incidence direction,the corresponding characteristic wave height and bearing capacity were determined for the typical SPPF.In addition,Considering the influence of vertical loads that SPPs bore,the diameter-thickness ratio and slenderness ratio of SPPs on the lateral deformation of SPPF,the failure simulation analysis of the lateral force action under multi-variable coupling participation was conducted,and the calculation formula for the ultimate lateral deformation of SPPF was proposed.Furthermore,based on two typical damage modes including the corrosion damage and the embedding failure of pile ends,the influence of different damage degree and positions on the collapse resistance and lateral critical deformation of SPPF was analyzed,and the calculation formulas for the collapse resistance and lateral critical deformation of SPPF considering the structural damage were established.