| With the wide application of steel truss bridges in highway bridges,especially in urban roads,under the increasingly high-speed and heavy-load vehicle loads,high-strength bolted joints are prone to bolt loosening and other diseases.For steel truss bridges with high-strength bolts,the failure of joints often begins with the loosening of bolts under reciprocating fatigue loads.Therefore,it is very important to study the mechanical properties of high-strength bolted joints under reciprocating loads and to judge the safety of joints after the failure of bolts.Based on the actual problem of high strength bolt loosening of highway steel truss bridge,this paper evaluates the safety performance of the joints in the bridge and puts forward the disposal of bolt loosening by discussing the causes and influencing factors of joint bolt loosening.Through the combination of finite element simulation and experimental research,the research successively carries out the finite element simulation and test of single high-strength bolted connection,the finite element simulation of the whole bridge relying on engineering,and the reciprocating loading test of high-strength bolted joints.The main conclusions are as follows:(1)Under reciprocating loads,the loosening of high-strength bolts is caused by material loosening caused by plastic deformation between contact surfaces and structural loosening caused by fretting slip between threads.With the increase of cycle times,when the torsional displacement of high-strength bolts reaches a certain limit,the bolts will loosen spontaneously.(2)Relative slip between joint plates is a necessary condition for the loosening of high strength bolts.Load amplitude at the end of joint plates is the main factor affecting the loosening degree of high strength bolts.By comparing the maximum displacement at the elastic stage?_a with the displacement at the end of the joint plate?_l,the slip load F_a and the load amplitude?F,the loosening of high strength bolts can be classified as follows:Class?,?_l??_a,no change of preload and no self-loosening of high-strength bolts happens;Class?,?_l>?_a and?F/F_a?2.0,the preload decreases periodically and the high strength bolts loosen partially;Class?,?F/F_a>2.0,the preload continued to decline,high-strength bolts continued to loosen.(3)In the reciprocating loading test of high-strength bolt joints,with the increase of the number of bolts falling off,the load-displacement curve at the loading end gradually presents a bow shape,and the joints gradually change from elastic to elastic-plastic.When some bolts in the joints loosen,stress redistribution occurs near the bolt holes.According to the safety level of joints,the loosening of bolted joints can be divided into three categories:Type A bolted joints,which do not have bolt loosening and are safe;Type B bolted joints,whose loosening coefficient is less than 10%,they are prone to loosening and destruction;Type C bolted joints,whose loosening coefficient is more than 10%,these bolts have been loosened and destroyed.(4)In view of the present situation of the failure of bolts in bridge joints,the following countermeasures are put forward:daily patrol inspection,regular inspection and large-scale maintenance of bridges should be carried out,it is suggested that large-scale maintenance be carried out every 20 years,and the traffic volume of Heavy-haul vehicles should be strictly controlled to be less than 1.5 times of the current traffic volume of Heavy-haul vehicles;the safety performance of the joints is evaluated by the ultrasonic bolt preload test system,and the safety performance of the joints is evaluated according to the test results.According to the measured joint loosening coefficient,the joint bolts should be re-twisted.It is suggested that multi-point dampers should be arranged along the span direction of the bridge to reduce the vibration of the bridge caused by vehicles and the loosening of the joint bolts. |
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