As the weak part of the force and deformation of the whole ring arch structure,the joint type is the key to the structural design,and its mechanical characteristics are very important for accurate evaluation of the overall bearing performance.In order to evaluate the force performance of the bolted endplate joint and the combined sleeve joint of the high-strength steel pipe grid,the bending test of the local full-length joint was carried out,the changeable law of the failure form,vertical deflection,bearing capacity and stress strain of the two joint components was discussed,and the bending stiffness and bearing capacity of the joints were quantitatively evaluated.The results showed that during the whole loading process,the two kinds of joints showed typical elastic growth stage,plastic development stage,stable bearing stage or residual strength stage.The bolted endplate joint showed a smooth curved failure mode,and the combined sleeve joint showed a broken line failure mode.Compared with the combined sleeve joint,and the ultimate bending moment of the bolted endplate joint was 46.4 kN·m higher than that of the combined sleeve joint,the deflection was reduced by 18.5 mm,and the bending stiffness at initial stage and service stage of the bolted endplate joint were also increased by 1 011 kN·m2 and 877 kN·m2.The strain response law of the steel tube of the two kinds of joints was basically the same,showing the distribution characteristics of the outer ring being compressed and the inner ring being stretched,and will enter the yield stage first due to the outer compression.The stress of bolted endplate joint was stable during the loading process,the opening deformation of the endplate was small,and the safety factor of bolts was high.The bearing capacity of the combined sleeve joint was weak,the deformation range was large.The research results could provide a certain reference for the joint design and engineering application of steel pipe grid support structure in tunnel engineering.
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