| The Research Council on Structural Connections of the Engineering Foundation provides a specification for the installation of high-strength bolts (1). This Specification is directed primarily at users of ASTM A325 or A490 bolts. The use of so-called alternative design fasteners is also permitted, however.;Tension-control bolts are a relatively new type of alternative design fastener. The bolt itself has a splined end that extends beyond the threaded length of the bolt. It also has an annular groove between the threaded portion of the bolt and the splined end. In order to install these bolts, a special electrically-powered wrench is required. This wrench has two coaxial chucks--an inner chuck that slips over the splined end of the bolt and an outer chuck that envelopes the nut. The two chucks turn relative and opposite to each other to tighten the bolt. At some point, the torque developed by the friction between the nut and bolt threads and at the nut-washer interface equals the shear resistance of the bolt material at the annular groove and the splined end of the bolt shears off at the groove. If the system has been properly manufactured and calibrated, pretension is achieved at this point.;Factors that control the pretension are material strength, thread conditions (such as lubrication, dirt, and thread damage), the diameter of the splined end, and the surface conditions at the nut-washer-joint interface.;The program herein investigated the pretension of production tension-control bolts as it varied from manufacturer to manufacturer as well as under different conditions of aging weathering, and thread conditions. Further objectives were to examine the behavior of these bolts in direct and torqued tension and to establish whether or not these bolts meet the material properties requirements of the governing standard (4). |
