Development of a method for the determination of preload in High Strength Friction Grip Connections of existing structures

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The aim of this thesis is to investigate the feasibility of determining the preload in bolts in High Strength Friction Grip Conections and performing in situ measurements. The magnitude of the preload is important for the behavior of High Strength Friction Grip Connections, as it defines the fatigue life of the connection and limits the displacement of the joint. For this reason, it is important knowing the preload at any point during the service life of the connection. This thesis first analyses the causes that may lead to an insufficient initial preload and the factors which may result in a preload loss over time. Then, the state of the art in the determination of preload in High Strength Friction Grip Connections is reviewed and the strain gauge method is tested to define the potential of this method on performing in situ measurements, for two reasons: 1) so far, the in situ tests with this method were not possible, because of a complicated installation procedure. For the tests presented in this thesis, a new type of strain gauges (BTMC strain gauges) is used, which simplifies the installation procedure and 2) the strain gauge method is less affected by factors which govern the in situ measurements. This is done by imitating the in situ conditions in the lab and determining the uncertainty interval of this method. Then, this method is tested on a connection of an existing structure. The results show that it is possible to do in situ measurements with the strain gauge method. It is also shown that the best way to determine the force with a maximum accuracy is by conducting time consuming calibration tests. Therefore, in the last part of this thesis four approaches are suggested, which aim to reduce the number of calibration tests, without loosing much of the accuracy in the estimation of the force.