Current design procedures for slip resistant connections according to Eurocodes and American specifications are reviewed. Although failure of a slip resistant connection is defined at different levels of slip, 0.15 mm and 0.5 mm respectively, the calculation of the resistance is similar. Most of the research is performed on bolts in normal clearance holes. A testing program was conceived to evaluate the influence of long open slotted holes on the behavior of slip resistant lap joints with tension control bolts because of possible use of such connections in towers for wind turbines. In comparison to specimens with normal clearance holes it is found that the friction coefficient is about 4% lower. This reduction is lower than suggested by the current correction factors. Since none of the design procedures takes the reduction in bolt forces by time into account, the loss of pretension force is experimentally studied and an approximation is proposed. A second testing program was carried out with friction standard specimens in order to determine the slip factor for different surface treatments and steel grades in range between S275 and S690. The achieved slip factors for different surface preparations are in accordance with the classification of friction surfaces in EN 1090-2:2008 and differences obtained are addressed to variations in exposure to weather conditions. The steel grade does not have a significant influence.