Research On Carrying Capacity Of Corroded High-Strength Bolt Shear Connections

In industrial or marine atmospheric environment,the structural performance of steel structures gradually degrades,which will affect the safety of structures.As a result,the safety assessment of existing steel structures has been in a hot topic civil engineering.A joint is the critical and complicated part of a steel frame as its behavior directly affects the overall response of structures.The failure of a joint will cause the collapse of the whole structure.The performances(i.e.,the failure mechanism and design specifications)of high-strength bolt connections,normally utilized in a steel frame,have been investigated a lot.However,these studies mainly focus on the behavior of a new structure,and the performance degradation of in-service high-strength bolts connections is not considered.Corrosion not only leads to the deterioration of mechanical properties of steel,but also causes the degradation of pretension load of high-strength bolts.In addition,anti-slip coefficient on the surface of steel will be affected.All these factors have obvious influences on the bearing capacity of a joint in a steel frame.Therefore,it is necessary and urgent to investigate the degradation performance of high-strength bolt connections,which can provide a reference for the safety assessment of existing steel structures urgently.In this work,the high-strength bolt connections with different corrosion degrees were obtained from Copper-Accelerated Acetic Acid-Salt Spray Testing(CASS).The deterioration laws for mechanical properties of steel,pretension load of high-strength bolts,and anti-slip bearing capacity and ultimate bearing capacity of high-strength bolt connections are investigated.(1)Under the environment of CASS,a power function between the corrosion rate and corrosion time is found for Q235 steel and Q355 steel.Initially,the corrosion speed of Q235 steel is faster than that of Q355 steel.After the corrosion rate is larger than 12%,the corrosion speed of Q355 steel accelerates.Base on the test data in this study,power function models of corrosion rate for Q235 steel and Q355 steel are proposed.(2)The mechanical properties of Q235 steel and Q355 steel are investigated.The results show that the yield strength,the tensile strength,the elongation after fracture,and the elastic modulus of Q235 steel decrease linearly as the corrosion rate increases.Corrosion also has significant influence on the elongation after fracture and the elastic modulus of Q355 steel.However,the effects of corrosion on the yield strength and the tensile strength of Q355 steel are not obvious.The constitutive models of corroded Q235 steel and Q355 steel are suggested based on the Hollomon model.(3)The variation trend of the pretension load of high strength bolts is obtained from the real-time monitoring of high-strength bolt connections.Obviously,the pretension load of high-strength bolts has a power function relationship with the corrosion rate of steel.A model for estimating the pretension load of high-strength bolt is suggested.(4)The experiment of the high-strength bolt connections with different degrees of corrosion under shear is performed.The variation trends of anti-slip coefficient on the surface of corroded steel,anti-slip bearing capacity,ultimate bearing capacity are studied.The results show that the anti-slip coefficient and anti-slip bearing capacity increase as the corrosion rate increases if the value of corrosion rate is less than 5%.After the corrosion rate is larger than 5%,the anti-slip coefficient and the anti-slip bearing capacity decreases as the corrosion rate increases.The effect of corrosion on the ultimate bearing capacity of high-strength bolt connections is not obvious.The effect of friction force on the ultimate bearing capacity of high-strength bolt connections should be considered.Based on the experimental and theoretical analysis,models for the anti-slip bearing capacity and the ultimate bearing capacity of corroded high-strength bolt connections are proposed.