High Strength Bolts for Bridges

An extensive experimental program was conducted to evaluate the performance of both black and galvanized high strength bolts for bridges. Seventy-two A325 black bolts, 145 hot-dip galvanized A325 bolts, 85 mechanically galvanized A325 bolts, and 83 black A490 bolts from several different suppliers, were tested under direct and torqued tension in order to evaluate the current installation practices and ASTM Standard requirements for high strength bolts and to develop guidelines which will ensure proper installation and satisfactory performance of these bolts. An important parameter in the test program was the thread conditions of the bolts and nuts. Four thread conditions were examined: as-received, cleaned, weathered and lubricated. Several types of lubricants were considered in the course of the experimental program.

The results indicate that a great deal of the problems associated with the performance of high strength bolts lies in the vagueness of the current ASTM Standards and the failure of the bolt suppliers to follow the requirements stated in these standards. A major recommendation in this study is to establish a unified standard which will cover the performance of fastener assemblages (bolt-nut-washer) under the responsibility of one committee. Guidelines for proper installation of high strength bolts are also given.