Tension Control Bolts: Strength and Installation

Abstract

Tension control bolts are a type of alternative design fastener, permitted by the Research Council on Structural Connections. The bolt has a splined end that extends beyond the threaded portion of the bolt and an annular groove between the threaded portion of the bolt and the splined end. The special wrench required to install these bolts has two coaxial chucks—an inner chuck that engages the splined end and an outer chuck that envelopes the nut. The two chucks turn opposite to one another 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 annual groove. The splined end of the bolt then shears off at the groove. If the system has been properly manufactured and calibrated, bolt pretension is achieved at this point. Factors that control the pretension are material strength, thread conditions, the diameter of the splined end, and the surface conditions at the nut-washer-joint interface. The program reported here investigated the pretension of production tension control bolts as it varies from manufacturer to manufacturer and under different aging, weathering, and thread conditions.