Experimental Investigation on Tensile Properties of the Bolt in Sphere Joints Under Fire

As a common type of joint used in spatial grid structures, the tensile performance of bolt-sphere joints directly affects the safety of the spatial grid structure under fire. To study the tensile properties of the bolts in sphere joints under fire, the four sets of 36 bolt-sphere joints consist of M20, M24, M30, and M36 high-strength bolts were conducted in elevated temperature by steady-state test. Therefore, the tensile bearing capacity of the bolt-sphere joint was obtained under different elevated temperatures. Experimental results show the destroyed section of the bolt-sphere joint that occurs in the thread of the high-strength bolt at the intersection of the sphere because of the maximum concentration of stresses that occurred in areas of destruction. The brittle fracture of bolt-sphere joints happened when the temperature is below 400 ℃. As the temperature increased, the neck shrinkage of the damaged section of the bolt becomes more apparent, and the ductile failure of the bolt-sphere joint occurred. The stiffness and the bearing capacity of bolt-sphere joints decreased gradually as the temperature rose. Conversely, the ductility was gradually increasing. When the temperature was greater than 300 ℃, the stiffness and the bearing capacity of the bolts in sphere joint decreased rapidly. The bearing capacity reduction coefficient of the high-strength bolts with different diameters were next to the same at elevated temperatures. According to the experimental data, the formulas of the tensile bearing capacity reduction coefficient of bolt-sphere joints were obtained.