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.

Analysis on the Tensile Strength of Post-Installed Large-Diameter Anchors in Concrete

“…The failure of the anchoring system under different mechanical states can be divided into failure under axial loads, shearing state failure, and edge failure. In their experimental studies conducted with different anchoring parameters, Cook et al [3][4][5] categorized the failure modes under tensile loads into four types: concrete cone failure: the anchoring depth ( ) ranges between 3d and 5d, and the failure strength is dominated by the tensile performance of concrete; bond failure: the inadequate bonding strength is caused by the poor adhesive or unreasonable anchoring; steel failure: the anchoring depth exceeds the tensile strength of the steel materials of the anchor bolt; and compound failure: the compound failure mode of shallow concrete cone and bonding failure under an anchoring depth of 6d to20d. Among these failure modes, compound failure can be further subdivided into glue-concrete interface failure, glue-bolt interface failure, and glue-concrete-bolt interface failure ( Fig.1), concrete cone failure and bond failure should be avoided in engineering projects, and steel failure is often restricted by the size of the anchoring base.…”