Experimental Research And Finite Element Analysis Of Reinforced Tubular N-joints

Experiments on static behavior of a tubular N-joint(JD-A),a plate reinforced tubular N-joint(JD-B), a concrete filled tubular N-joint(JD-C) and a N-joint reinforced both by concrete and plate(JD-D) under the axial compressive stress on brace and chord were carried out. The design of test specimens,the loading scheme and the testing steps are introduced, methods to estimate the ultimate bearing capacity of the four joints are proposed, the phenomena and the main results of tests are obtained.Experimental results showed that the failure mode of specimen JD-A and JD-B were plastic fracture at the connecting face of chord;the failure mode of specimen JD-C was local buckling failure of the brace,which bear the weight of compressive stress;and the failure mode of the specimen JD-D was rupture of the reinforce plate. The calculation indicates that the ratio of the test value of ultimate capacity of specimen JD-A to the result obtained from code design formula is 0.97,and to the formula suggested by professor Packer is 1.05,it illuminate that for tubular N-joints,even if whose material yield-point preponderate over 355 Mpa,credible results of the ultimate capacity of the joints can still be obtained from the code design formula. The calculation also indicates that the ultimate capacity of joints can be remarkably enhanced by filling concrete in chord, and can be only enhanced to a certain extent by welding a reinforce plate on tubular wall of chord,for filling concrete can considerably augment the ratio of radial stiffness of chord to axial stiffness of braces,whereas the reinforce plate can't. While welding a reinforce plate on tubular wall of chord may weaken the ultimate capacity of the joint when the radial stiffness of chord is sufficient. For reinforce tubular joints,considering the operational difficulties in machining and welding reinforce plate may aggrandize practical cost , filling concrete be given preferential consideration to reinforce tubular joints when within the confines of deadweight of the structure. Therefore the key point in designing of tubular joints and reinforced tubular joints is control the ratio of radial stiffness of chord to axial stiffness of braces to make best use of chord and braces.Making use of finite element package program ANSYS,in which four nodes shell element was employed,elastic-plastic large deflection finite element analysis(FEA) of the experimental joints was conducted. Compared the curves of load in compressive brace with deformation in tubular wall of chord obtained from experiments and finite element analysis,there is a good agreement between the experimental and analytical results,it proves experimental results are reliable,and the method employed in FEA can be used in practical project. The ratio of FEA value of ultimate capacity of specimen JD-A,JD-B,JD-C and JD-D to the results obtained from code design formula is 1.05,1.04,0.97 and 1.08. The main reason for the relative discrepancy of specimen JD-D is that the failure mode of FEA result is local buckling failure of the brace, which is different from test result. Compared the stress distribution and propagation of plasticity of the four joints find that the reinforce plate can diffuse load from braces to lager face of joint's chord and enhance ultimate capacity of the joints, filling concrete can bear most load from braces after connecting zone of chord wall yielded, thus remarkably enhance ultimate capacity of the joint.