Effect of Thread Fabrication Tolerance and Snug-Tight on the Loosening of Anchor Nuts in Highway Ancillary Structures

Loose anchor nuts on ancillary structures in the transportation industry pose a serious safety risk to the traveling public. The exact cause of anchor nut loosening is unknown, but thread tolerances and improper tightening are believed to be some of the potential causes. ASTM F1554 and A563 specify allowable tolerances for galvanization and overtapping on anchor rods and nuts, respectively. Too tight of a tolerance fit would not allow for free turning of the nut onto a rod, whereas a loose tolerance fit could lead to loosening of the nuts upon vibration and reduced pretension. Additionally, the snug-tight condition is vaguely defined in the tightening specifications, with variations leading to under-tightening or over-tightening during the turn-of-the-nut tightening procedure. The thread parameters of anchor rods and nuts procured from three suppliers were investigated using digital image analysis. The thread parameters were found to be within allowable tolerances indicating that nut loosening is unlikely to occur due to thread tolerance related issues. The investigation of snug-tight pretension in a double-nut moment connection revealed that snug-tight stress is highly variable due to the variable wrench length and personnel strength, and can lead to under-tightening or over-tightening of the connection during the turn-of-the nut tightening procedure.