Anchor Bolt Steel Strength in Annular Stand-Off Base Plate Connections

Abstract

Anchor bolts in annular stand-off base plates connecting cantilever sign and signal structures to concrete foundations may experience high shear from base plate torsion and direct shear forces. A three-phase experimental study evaluated the steel shear strength of anchor bolts in grouted and ungrouted annular stand-off base plate connections to concrete. Phase 1 included flush-mounted and ungrouted stand-off base plates with ⅝- and 1-in.-diameter bolts loaded in direct shear. Phase 2 employed a novel torsion test approach on 10 circular groups of six ⅝-in.-diameter bolts installed in flush-mounted, ungrouted stand-off, and grouted stand-off base plates, along with three ungrouted groups of three 1-in.-diameter bolts. Phase 3 comprised one ungrouted and three grouted full-scale annular base plate tests of six 1.25-in.-diameter bolts under predominantly torsion loading. Results from this study suggest that AASHTO's provisions allowing bolt bending stresses to be ignored for anchor bolts with an exposed length of less than one bolt diameter overestimate ultimate strength. The beam model suggested by AASHTO accounting for anchor bolt bending fits experimental data well and is recommended for all ungrouted annular stand-off base plates. The American Concrete Institute's 0.8 reduction factor for steel shear strength of anchor bolts in grouted connections was found to be conservative and is also recommended.