LOSS OF BOLT TENSION IN STEEL BRIDGES

Tension in each of sixty 7/8" diameter A-325 structural steel bolts was monitored for a period of 45 days. Primary variables were paint treatments on the clamped plates before insertion of the bolts and tightening method. Two types of red lead and an inorganic zinc paint were applied in varying thicknesses. Both the calibrated-impact-wrench and turn-of-the-nut methods of tightening were used. In bolts tightened by the turn-of-the-nut method, initial tensions were higher and more uniform than in those tightened by the calibrated-impact-wrench method, and could therefore tolerate greater losses without falling below the minimum specified tension. /FHWA/