Design And Development Of New Bolt Ball Joints And Research On Mechanical Properties

The bolted ball joint space grid structure has been widely used in large roof structures such as industrial plants,stadiums,transportation hubs,and terminals.Among them,the "insufficient tightening" of high-strength bolts is a common safety hazard in the installation process of bolt ball joints.To this end,this thesis proposes a new type of bolt ball joint with octagonal high-strength bolts.This node transmits force through the contact between the high-strength bolt and the octagonal section of the sleeve,and the bolt is screwed into the bolt ball to avoid pin shearing.After the installation is completed,by observing whether the tail of the pin is flush with the outer surface of the sleeve,it can be intuitively judged whether there is "insufficient tightening" of the high-strength bolt.In this thesis,the following research work is done on the type design and mechanical properties of the new bolted ball joint:1.The type design of each component of the new bolt ball jointThe new type of bolt is consistent with GB/T 16939-2016 "High strength bolts for joints of space grid structures",and the bolt polygon is an external octagon with a nominal diameter circle of bolts.The matching relationship between the cone head/sealing plate and the high-strength bolt in the current bolt ball joint accessories library will not be changed.The cone head/sealing plate will only expand the opening to accommodate the polygonal segment of the bolt,and other dimensions will remain unchanged.Make full use of the advantage that the pin will not be broken,and realize the purpose of judging whether the high-strength bolt is "insufficient tightening" through the external observation through the design of the size of the deep groove and the shallow groove of the pin,the sleeve and the high-strength bolt.2.Experimental study on tensile strength of octagonal high-strength boltsFour common types of octagonal high-strength bolts were selected: MY20,MY36,MY45,MY56.A total of 12 samples were prepared for 3 samples of each type.The uniaxial tensile test was carried out using a microcomputer-controlled electro-hydraulic servo horizontal tensile testing machine.The results show: The tensile load of one specimen MY45 exceeded the maximum tensile load specified by the standard by 2.36%.The failure load,failure position,failure mode and elongation of the other octagonal high-strength bolts all meet the requirements of the current standard "High-strength bolts for joints of space grid structures" in Chinese（GB/T 16949-2016）.3.Pipe-cone head/sealing plate and bolt integral specimen tensile test and finite element simulationThe specimens composed of bolts,tapers and corresponding round tubes of common specifications in the project are selected for one-way tensile test.The elastic stiffness,failure mode and failure load of the new bolt-ball joint and the traditional bolt-ball joint were compared by selecting typical bolts from each type of bolt by setting control test.The results show: Three failure modes appeared,including steel pipe failure,bolt failure and weld failure.The shape of the unidirectional tensile load-displacement curve of the integral test piece with steel pipe-cone head/sealing plate and traditional high-strength bolt is the same as that of the integral test piece with steel pipe-cone head/sealing plate and octagonal high-strength bolt,same elastic stiffness,into the plastic load strength and ductility,the ultimate tensile strength of the new joint is slightly higher than that of the traditional joint.Numerical simulation shows that the overall stiffness,ultimate load,plastic deformation,stress distribution and plastic strain zone of the traditional node and the new node are similar.4.Experimental research on compressive bearing performance of new sleeveThe inner section of the new sleeve is octagon,and the pin hole opened by the sleeve is on one side of the octagon.When under compression,the face with the pin hole is the weakened face.Therefore,the experimental research on the compressive bearing performance of the new sleeve is carried out.It is obtained that the failure is the overall buckling,and the ultimate compressive bearing capacity exceeds its compressive design bearing capacity by more than 3 times.5.The new bolt ball joints torsion bearing capacity testTorsion tests were carried out on the M20 and MY20 with matching sleeves and pins in order to compare the torque that the new and traditional joints can undertake during installation.Experiment with a digital torque wrench and the results show: Because the bolts are screwed into the bolt balls by relying on the octagonal contact force transmission between the sleeve and the bolts rather than the shearing of the pins,the torques of the new joints during installation are much greater than the torsional bearing capacity of traditional joints.