Abstract:
High Strength Friction Grip (HSFG) property class 8.8 bolts are widely used in construction of steel structures. When fully tensioned, they enable rigid steel systems to be built without the need for site welding. In new developments, the design and behaviour of a generation of seismic energy dissipaters such as the Asymmetric Friction Connection (AFC) in the Sliding Hinge Joint (SHJ) are also dependent on the use of tensioned HSFG bolts, albeit to a lower level of tensioning. According to NZS 3404, HSFG property class 8.8 bolts, when installed in the Tension Bearing (TB) and/or Tension Friction (TF) bolt modes, must be tensioned based on the part-turn method of tightening or by using a direct-tension indication device. The part-turn is identified by NZS 3404 with respect to the bolt length-to-diameter ratio and contact angle of the outer faces of the bolted parts. There is also a requirement identified by the most recent (2007) amendment to NZS 3404 regarding the minimum number of clear threads run out beneath the nut after tightening for fully tensioned HSFG bolts. This necessitates the existence of five, seven, or ten free threads at nut loaded face with respect to different bolt length-to-diameter ratios. These aspects have been the subject of experimental and analytical research and this paper presents the results of that research and proposed changes to the existing NZS 3404 recommendations as follows: • Relaxing the minimum number of clear threads run out beneath the nut after tightening, to level that will deliver dependable performance but will make installation easier. • Proposing a more precise definition for the bolt length when determining the required part-turn of the nut. • Proposing a method of pre installation bolt inspection to increase the reliability of the installed bolt tension.
