Comparative Effectiveness of Tightening Techniques for A490 1 1/4- in. Diameter Bolts

Abstract

Since the approval of the first high-strength bolt specifications by the Research Council on Riveted and Bolted Structural Joints (RCRBSJ) in 1949 there have been concerns of ensuring that the bolts are properly tightened. In certain applications, such as slip critical and direct tension, current practice requires that all bolts be tightened to reach or exceed a minimum specified preload. Considerable research has been invested on high-strength bolts and the techniques used to tighten them. Most testing on bolt behavior during tightening has been performed using smaller diameter bolts, such as 7/8-in. For larger diameter bolts, the results of these tests have been extrapolated to develop criteria used in establishing the RCSC Specification for Structural Joints Using ASTM A325 or A490 Bolts,2 hereafter referred to as the RCSC Specification. An extensive study at Delft University in the Netherlands explored many different tightening methods including the turn-of-nut as well as load indicating washers. This study detailed the advantages and disadvantages of the different techniques. The significant influence of lubrication on the bolt threads was revealed by Eaves in 1978. Notch showed that hardened washers were needed for the proper tightening of large diameter bolts, and to attain a uniform "snug"ť condition. Early testing of load indicating washers with bolts in the vertical position was performed in 1973 by Struik, Oyeledun, and Fisher. This testing provided a basis for the validity of the washers as load indicators. In 1987, Kulak, Fisher, and Struik published the second edition of the Guide to Design Criteria for Bolted and Riveted Joints and included much of the previous research performed on bolts and bolt tightening techniques. The information on bolt properties and installation contained in this book was extensively used in the course of this investigation.