Minor thesis --- Preload loss in pretensioned bolts is an inevitable phenomenon. This thesis focuses on the different causes for bolt relaxation and their relation to joint design. Investigating bolt relaxation is relevant because there is a direct relation between the bolt preload and the resistance of the connection. In order to assess the loss of preload in pretensioned bolts, test results of extended creep tests (cf. EN 1090-2) have been used. It was found that bolt relaxation is mainly a function of coating thickness of the joint members. The larger the coating thickness, the more bolt relaxation occurs. The influence of bolt relaxation due to flattening of surface roughness could not be quantified due to the governing influence of coating thickness, but the surface roughness is of large importance for the slip resistance. External static loading has an influence on short-term bolt relaxation which can be modelled using an equation derived in this thesis. Proper joint design is key to achieving a high strength friction grip connection with a high preload level and thus a high slip resistance over the entire service lifetime. Higher ratios of clamping length over bolt diameter lead to less bolt relaxation. The results of this thesis can be used to gain insight in the behaviour of preloaded connections, as well as to have an indication on how to achieve a preloaded connection with relatively small preload losses.