With the aging of civil infrastructures, it is urgent to propose more efficient and simple maintenance and management methods. Higher strength bolts have been known to lose axial force due to relaxation, so that a bolt pre-tension of +10% of the designed axial force was introduced at the time of bolt tightening to compensate for the axial force decrease. Therefore, in this research, focus is placed on high strength bolts under long-term service condition. The residual bolt axial force survey of high strength bolts is carried out on steel bridges after 30 to 50 years of construction in order to know whether the bolt’s axial force remains in design level as required in the design specification.
