Calibration of the high strenght bolts for measuring of the pretension force

When analysing slip resistance connections, which are commonly used in-situ assemblied joints of structures dominantly loaded by cyclic loading, for example in bridges, telecommunication towers, towers for wind turbines, we may say that preloaded high strength bolts are their most important part. A level of pretension force in a bolt influences the strength of a slip resistance connection, therefore it is very important to attain a design level of a pretension force. This paper tackles most frequently used methods for measuring of a pretension force in high strength bolts both on site and in a laboratory. Primarily usage of ultrasonic methods, washer like load cells (with strain gauges and piezoelectric), strain gauges installed on the bolt shank and strain gauges installed inside of the bolt shank. The emphasis is both on a use of strain gauges which measure pretension force inhigh strength bolts and on a bolt calibration procedure prior to pretensioning. Installation process of strain gauges in the bolt shank is described in detail and all its specificities are bring to the attention based experimental research conducted on 126 bolts.