Study On Wind-induced Fatigue Of High Strength Bolts Of Lightning Rod Structures

Lightning rods belong to high-rise structure,which adopt circular steel tube structure.The steel tube sections are connected by flanges.Due to the small fundamental frequency of the structure,large displacements and vibrations are generated under the action of wind load,which is a fatigue load for steel structures.High-strength bolts are the main connectors in the lightning rods,and their fatigue strength directly affects the bearing capacity,safety performance and fatigue life of the lightning rods.Large preload is applied on lightning rods in practical work.It is easy to form stress concentration region at the bottom of thread,and due to the dynamic action of the wind load,the bolts are subject to fatigue failure.Therefore,the fatigue analysis of the high-strength bolts of the lightning rod flange is particularly important.In this paper,ANSYS was used to establish the beam element model of three different kinds of lightning rods,and the wind-induced response analysis was carried out.Acquiring the stress time history that beam element model was calculated by the dynamic time.Using rain-flow counting method counted the stress time history,obtaining variable amplitude stress spectrum.The fatigue damage value of the cylinder was analyzed based on the Miner cumulative damage theory and S-N curve.A local refined solid model was established for connection part of lightning rod flange.Solid model was calculated by using the time history of shear force and moment proposed from the beam element model calculated by dynamic time history.The stress time-history of the maximum stress point on the high-strength bolt was obtained.Using rain-flow counting method counted the stress time history,obtaining variable amplitude stress spectrum.Its fatigue damage value is obtained according to the Miner cumulative damage theory,S-N curve and wind speed distribution probability.Finally,the influence of different preload value and flange thickness on the fatigue performance of high-strength bolts was analyzed.The main conclusions of this paper are as follow:(1)The modal analysis of the beam element model of three kinds of lightning rods was performed that the natural vibration frequency of the structure was obtained.The fundamental frequencies of the three kinds of lightning rods finite element models obtained by modal analysis were basically consistent with the field measured data,which verified the correctness of the finite element model.(2)Under design wind load,wind-induced fatigue analysis of single-span and three-span frame lightning rod cylinders and flange high-strength bolts showed that in the single-span frame lightning rods,the bending wind vibration coefficient of two-support lightning rods was greater than that of three-support lightning rods.In the same time,the damage values of cylinder and the flange high strength bolt of the second support lightning rod were larger than the three support lightning rod.In the three-span framework lightning rod,the fatigue damage value of the outer lightning rod cylinder and the flange high-strength bolt was greater than the inner lightning rod in the same time.(3)Under design wind load,as the preload increases,the stress amplitude of the bolt decreases and the average stress increases.The fatigue damage value of the bolt will decreased in the same time,and the higher preload can effectively improve the bolt’s fatigue performance.However,the preload reached 80% of the product of material’s yield strength and the cross-sectional area.Under the wind load,the stress in the local area of the bolt exceeded the yield strength of the material.Therefore,the bolt preload should be less than 80% of the product of the material’s yield strength and cross-sectional area.(4)The increase of flange thickness would reduce the bolt stress and improved the fatigue performance of the bolt.The double row bolt arrangement can reduce the amplitude of the bolt and the average stress that improved the fatigue performance of bolts.When some bolts were damaged,the stress amplitude and mean value of adjacent bolts would increase and the fatigue performance of bolts would decrease.