Improved guidelines for the drill stop-hole retrofit method of steel structures

Fatigue cracking in steel bridge connections is a prevalent issue faced by our nation's infrastructure. Many techniques have been developed to repair or retrofit these structures in an effort to mitigate damages associated with fatigue cracking. A widely used and effective technique is to drill a hole at the tip of a fatigue crack, removing the sharp notch, which drastically reduces the stress field in the surrounding material. While this method has experienced much success, there is limited guidance on the size of the hole required by the retrofit. Drilling a hole too large removes more material than needed, unnecessarily weakening the section strength and stiffness. On the other hand, drilling a hole too small may insufficiently reduce the stress field, potentially resulting in reinitiation of fatigue cracking. In this thesis, thirty-five finite element models were created to develop a relationship between crack length, hole diameter, and local stress. Using traditional fracture mechanics, a design table is proposed by comparing the local stress intensity to the threshold stress for crack nucleation in structural steel. The table is a function of elastic stress quantities from mechanics formulae and provides guidance toward the selection of an efficiently sized hole diameter to fully arrest crack propagation.