Steel Shear Strength of Anchors with Stand-off Base Plates

Sign and signal structures are often connected to concrete foundations through a stand-off annular base plate with a double-nut anchor bolt connection, which leaves exposed anchor bolt lengths below leveling nuts used in these connections. Cantilever sign and signal structures may experience high shear forces in anchor bolts due to torsion at the base connection resulting from wind loads. Grout pads, which may or may not be present in existing structures, offer additional restraint against these forces. Motivated by gaps in information and lack of uniformity in addressing anchor bolt steel shear strength in stand-off base plates, this research study was initiated to quantify the reduction in steel shear strength for anchors installed with stand-off base plates and develop draft design and maintenance guidelines for these systems. To satisfy these objectives, a three-phase experimental study was undertaken. Phase 1 utilized direct shear methodology to establish relationships between stand-off distance and ultimate steel shear strength. Phase 2 contained torsion tests of ungrouted and grouted circular groups of 5/8 in. and 1 in. diameter anchor bolts. Phase 3 comprised four full-scale tests containing circular groups of six 1.25 in. diameter bolts. Decreases in anchor steel shear strength were observed for all levels of stand-off distance, including those within the current permissible range for ignoring strength reductions caused by bolt bending. Grouted tests contained higher shear strengths commensurate with flush-mounted strength at high levels of connection deformation, while a fiber-reinforced polymer (FRP) retrofit around the perimeter of the grout pad resulted in higher strength at lower connection deformation. Design and maintenance recommendations include consideration for strength reduction of anchor bolts in all stand-off base plates, allowance for grout pad contributions to ultimate strength in double-nut connections, and use of grout pads as a viable retrofit for increasing the strength of existing ungrouted stand-off base plates.