Abstract：Cave—in failure is apt to occur in joints of trusses made of square hollow sections．In order to turn the failure mode into a strength failure mode of joint members，the idea is proposed that the chord of the truss is grouted to increase the cave—in beating capacity of a hollow tube chord．An experiment of eight specimens of N—joints made of grout—filled square steel tubes is performed．Based on the experimental study，the geometrical parameters of specimens are analyzed，and the effects of the confinement index ξ，the spacing between the two web members g and the ratio of side length of the vertical web member to that of the chord β on the behavior of specimens are investigated through simulation analysis by an ANSYS program. Based on the test results and simulation analysis，the mechanical properties and the failure modes of this kind of joints are analyzed and the formulate to predict the ultimate beating capacities corresponding to different failure modes are developed． The ultimate beating capacity of compressive N-joints is calculated in accordance with the cave—in failure mode of a chord member；the ultimate beating capacity of tension N-joints is calculated in accordance with the punching—
shear failure mode；the ultimate beating capacity of a chord member is calculated in accordance with the shear failure mode in normal sections．
Key words：N-joint；truss made of grout—filled square steel tube；ultimate cave in beating capacity；simulation an analysis.
