Loss of preload in preloaded bolted connections over the service life

Abstract

AbstractThe design of slip‐resistant connections and fatigue loaded tension connections according to the categories B/C and E of EN 1993‐1‐8 relies on a specified preload level of the bolted connections which has to be ensured over the whole service life of the structure. However, it is well known that preload losses occur after tightening due to several reasons: e. g. setting effects, creep and cyclic loading. In order to ensure not only the operational safety but also the load bearing behaviour of preloaded bolted connections, those influencing factors have to be estimated and considered reasonably.For this reason, systematic investigations were carried out to determine the system reserves due to different tightening methods and preload losses. Herein, preload losses due to setting effects, creep and cyclic loading were experimentally determined, evaluated and extrapolated to the intended service life considering two clamping length ratios Zt/d ≈ 5 and Zt/d ≈ 2 as well as five surface conditions. Furthermore, different types of M20 bolting assemblies and tightening methods were used.The main findings and conclusions are presented and discussed in the paper. On this basis, a critical review of the current regulations and recommendations for improvements can be given. Besides all other interesting results, it could be shown that only the combined method turned out to possess enough system reserves (over 30 % in relation to the required preload level) for covering possible preload losses especially for coated surfaces for the service life of the structure.