Ultra-long-term slip-resistance and comments to the test procedure determining the slip factor

If a shear loaded joint is subjected to impact, vibration and/or load reversal, slip-resistant connections (Category B or C) with preloaded bolts are recommended to prevent effectively movement of the connected parts. The slip-resistance is mainly influenced by the characteristic of the faying surfaces and the preload of the bolts. For structural stability and durability of the steelwork, the level of preload has to be guaranteed over the whole service life of the structure and the loss of preload has to be estimated in the design of the joints otherwise maintenance-work is required. As a participant of the RFCS-Project SIROCO, Fraunhofer IGP investigates the slip factor under ultra-long-term sustained loading and the related loss of preload in the bolts. For the determination of the slip factor a test procedure is given in EN 1090-2. This paper gives comments on the test procedure, especially on the initial load level and the testing time of the extended creep tests (ECT). Furthermore the relation between the load-displacement of different surface treatments during slip is discussed. Based on the investigations a method called “step test” is developed, which allows the definition of the initial load level for the ECT for the first time. In addition, results are presented with a testing time up to 1009 days. Finally, remaining preloads in bolts are shown for the ultra-long-term test duration. This paper deals with the displacement-time and preload-time behavior of slip-resistant connections over an ultra-long-term period of time