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Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

On the preload-time behavior of slip resistant preloaded joints in steel construction under cyclic loads
Maik Dörre a, Ralf Glienke b, Knuth-Michael Henkel c, Fritz Wegener aa

Mechanical Joining Technology, Fraunhofer IGP, Rostock, Germanyb Department of Engineering, Mechanical Engineering, University of Applied Science Technology, Business and Design Wismar, Germanyc Department of Mechanical and Ship Engineering, Joining Technology, University of Rostock, Germany

problems, e. g. due to poor accessibility or friction deviations caused byenvironmental influences. To overcome these problems, mechanicallymaintenance-free concepts are targeted, which do not require checkingof the preload. However, the preload losses over the entire service life ofa structure must be determined and included in the design when
following a mechanically maintenance-free approach. An analytical
determination is already state of the art for uncoated parts (VDI 2230 Part 1:2015-11). Since parts in steel construction must often be coated toprovide sufficient corrosion protection, this analytical approach is notapplicable. In addition, the influence of cyclic operational loads on
preload loss has not yet been fully investigated.
Therefore, the preload losses due to embedding and cyclic loads for slip
resistant connections in a segmented tower concept of a wind turbine
were determined experimentally following (IEC 61400-6, 2020), inlaboratory tests as well as during an online monitoring of a segmented
wind turbine tower. The application chosen is representative for a generaltrend in the onshore wind energy industry (Hau, 2016; Wallasch, Lüers,& Rehfeldt, 2017). To ensure the stability of ever large towers, thediameters at the base of the tower must increase. Segmented tower
concepts are increasingly used for this purpose, which are easy totransport and are assembled on site via slip resistant connections.
The results of laboratory and field measurements were compared, andthe test procedures were optimized. The aim was to provide a practicallyapplicable method for the experimental determination of preload losses
that delivers conservative results with justifiable experimental effort.

ABSTRACT
For the design of mechanically maintenance-free connections, thepreload loss of the bolts must be determined and considered in the design.
Since parts in steel construction must often be coated to provide
sufficient corrosion protection, an analytical calculation is not possible.
For the experimentally determination of the preload losses, (IEC 614006, 2020) suggests two promising concepts - laboratory and fieldmeasurements. Investigations based on both methods were performed.
The results were compared, and the test procedures were optimized. The
aim was to provide a practically applicable method for the experimental
determination of preload losses that delivers conservative results withjustifiable experimental effort.

KEY WORDS: loss of preload; bolted joint; embedding; slip resistantconnection; fatigue; maintenance; monitoring; wind turbine

INTRODUCTION
For bolted connections in constructions under fatigue loads, slip resistantconnections are used since the load transmission by friction reduces
stress peaks at the hole edges and therefore increases the fatigue
resistance (Petersen, 2013). Additionally, they allow for the connectionof final coated parts. The load transfer behavior is largely determined by
the normal force acting in the frictional contact and the static friction
coefficient. Both parameters affect the service load (slip load) of theconnection, which is the most important characteristic of the loaddisplacement behavior. The bolts in slip resistant connections are
therefore preloaded in a defined manner. Unfortunately, despite proper
tightening of the bolts, the operational safety of bolted joints can bejeopardized by a loss of preload. Usually, embedding causes the majoramount of preload losses on slip resistant steel connections. In addition,
preload losses due to cyclic operational loads and transversal contraction
must be considered for this kind of connection. Depending on theapplication, other causes like residual stresses may also have to be taken
into account.
If the preload losses are not known, the connections must be retightened
after a defined maintenance interval according to corresponding
standards like (IEC 61400-6, 2020). This procedure is often time
consuming and expensive. Moreover, the retightening process can cause

DETERMINATIONOFSTANDARDIZATIONPRELOADLOSSESIN

According to the current state of standardization, the consideration ofknown preload losses is permissible in (VDI 2230 - Part 1:2015-11,2015), (DIN 25201-1, 2015), (DIN 18088-3:2019-01, 2019), (DNVGLST-0126:2018-07, 2018), (EN 1090-2:2018, 2018) and (IEC 61400-6,2020). However, methods for actual experimental determination of
preload losses are given only in the latter two standards.
The informative Annex I of DIN EN 1090-2 provides exemplary relative
preload losses for dry film thicknesses greater than 100 µm. The
applicability of these guide values is limited to the joint configuration of
the standard test specimen. For deviating configurations, it is suggested
to perform experimental investigations using the standard test specimen.

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