Influence of Edge Bearing on the Front-edge Shear Breakout Capacity
Edge bearing stresses on shear-loaded anchorages can enhance the shear capacity because of the confining stresses provided by the compressive bearing stress. However, too much bearing stress can have a negative influence on the shear breakout strength; the higher bearing stress cracks the concrete in a pattern similar to the failure from a pure bearing load, compromising the shear breakout when the anchorage is near an edge. A pilot test program of 11 tests was conducted to examine this effect; this work was initiated from an investigation of a precast wall panel anchorage failure (gravity and wind connection), where the bearing stress on an embedded anchorage likely reduced the concrete shear breakout capacity when the anchorage was near the free edge of the concrete member.
The ACI 318-14 Building Code, Chapter 17 recognizes the interaction of tension and shear on a connection, yet the interaction between bearing (compression) and shear does not appear to have ever been studied experimentally. This pilot test program examined plain steel plates (plates resting on the top of the concrete with a thin layer of grout beneath the plate) loaded in compression and plates with headed studs embedded in concrete loaded in compression with the compression load near an edge. The test specimens were not limited by any lateral side boundary conditions. This paper examines the second phase of testing where headed-stud connections were loaded toward a free edge with an orthogonally applied edge-bearing load. Bearing stress ranged from 0 to 16.4 MPa (2,375 psi) and the front-edge distance to the studs ranged from 51 to 76 mm (2 to 3 in.). One edge of the anchorage plate in bearing was located flush with the edge of the concrete. Concrete cover requirements were maintained on the embedded anchors.
The test findings showed that the shear capacity of the embedded anchorage increased as the bearing stress increased. However, above a bearing stress of 17.2 MPa (2,500 psi), the influence of bearing stress was adverse and reduced the shear breakout capacity computed by ACI 318 methods. A modification relationship to account for this affect is proposed.
3rd International Symposium on Connections between Steel and Concrete