FIELD STUDIES OF BOLT PRETENSION

Pretensioned bolts must be used when it is required that an axially loaded connection does not slip when force is applied or for connections loaded in direct tension. Studies that report on the attainment of bolt pretension are generally available only for laboratory conditions, however. This paper reports on the results of two independent investigations that used an ultrasonic bolt length measurement device to establish the pretension in bolts installed in the field. Six different sites were used by one researcher to obtain the pretensions in 232 bolts of different sizes, grades, and which used different installation methods. A total of 104 bolts at three different sites were monitored by the other team. In all cases, installation was done by the erector of the structure: the research teams were not involved in the installation process in any way.