Mathematical Model for Relaxation in High-Strength Bolted Connections

High-strength bolted connections with protective coatings are used extensively in bridges. The relaxation in high-strength bolts due to creep in the coating can reduce the shear capacity of the connection. In a recent bridge application using high-strength bolted connections with hot-dip galvanized coating, a loss in the bolt clamping force was observed. Tests have been conducted to evaluate the effect of coating thickness on the bolt clamping force. The reduction in the bolt force for coatings up to 0.508 mm (20 mils) has been shown to be as large as four times that for bare steel. A mathematical model is developed to evaluate the relaxation effect in the high-strength bolted connections. The analytical predictions are compared with experimental data, which shows a very good agreement. An approach is presented to predict the bolt-load retention based on the relevant creep data. The model developed in this paper can also be used for other coating systems to predict the loss of the bolt force due to the creep occurring in the bolted connections.