Effect of bolt pre-tension on the bearing behavior of high strength steel connections

Bolt pre-tension is regularly used in the practical case of bearing-type connection. Due to confinement of lap plates, complicate triaxial stress field can be observed near bolt hole, which may affect bearing behavior of the whole connection. Currently, investigations on the related issues are limited. To fill this research gap, this paper presents a combined experimental and numerical study on high strength steel single-bolt connections with bolt pre-tension. Results reveal several meaningful recognitions on the bearing behavior of connection with bolt pre-tension. Firstly, the effect of bolt pre-tension on the final tearout failure mode is negligible. Secondly, distribution of triaxial stress field initiates with a circular pattern and gradually shrinks to a localized pattern between shear fracture lines at ultimate resistance of the connection. Thirdly, out-plane confinement will limit the piling-up of plate material in front of bolt, which will reduce the related bearing resistance. However, increasing bolt grade is still beneficial to the whole connection due to a trade-off between ultimate resistance of the connection and related bearing resistance. Comparison with current code is further conducted. It is found that current Eurocode3 has adequate design safety for connection with bolt pre-tension while extra safety is needed for current AISC 360-16.