Tightening Procedures for Large-Diameter Anchor Bolts (1472-1F)

Recent failures of cantilever overhead sign structures in Michigan and elsewhere, coupled with the absence of standards for tightening methods for the double-nut system used with large-diameter anchor bolts in similar structures in Texas have been the motivation for this study. Practices of various field offices were reviewed, a literature study was performed, and both laboratory studies and field studies were performed. Three questions were raised: How tight do the anchor bolt nuts need to be? What techniques and procedures are best suited for tightening? How can tightness best be inspected? Standards for tightening procedures for large-diameter anchor bolts are proposed.In current TxDOT practice, the contractor is left to his or her own judgment concerning how tight to tighten the nuts on the double-nut anchor bolt system. Recent bolt failures in other states have raised concerns about nut tightening procedures. When the nuts of a double-nut system are not tightened sufficiently, fatigue loading, and even impact loading, can be factors in the performance of the bolts.Three full-size test specimens were built in the laboratory to evaluate tightening methods and the fatigue performance of the double-nut system used for anchorage of cantilever overhead sign structures (COSS) and on high mast illumination poles (HMIP). Analytical and numerical studies were performed to identify and study parameters affecting the stresses in the anchor bolts. Field studies were conducted to measure the wind-load induced anchor bolt reactions and stresses in one specimen of each type. This report details these studies and the findings.