Field Study of Pretension in Large-Diameter A490 Bolts

Abstract

A limited field testing program is described in which pretension was measured in 44 large-diameter (25.4 and 28.6 mm diameter) A490 bolts installed by construction crews into slip-critical connections in a multistory steel-framed building. The bolts were installed with direct tension indicators into connections that varied from simple bracing connections to large column splices. The field test results showed that the minimum specified pretension was not achieved in a significant number of bolts with long grip lengths (greater than 178 mm or 7 in.), while bolts with shorter grip lengths were adequately tensioned. A combination of factors, including greater difficulty in snugging the plies in the connections with the longer bolts and the very high pretension forces that the large-diameter, high-strength bolts required to develop specified pretension stresses, are thought to have caused the low pretension stresses measured in the longer bolts.