Standoff screws functioning as mechanical shear connectors in composite joists

The results of sixty-five double-sided pushout tests utilizing the Elco grade 8, 5/16 in. diameter, standoff screw functioning as a mechanical shear connector are presented. In all tests, the base material, through which the screw was fastened, was fabricated out of back to back angle to simulate the top chord of an open web steel joist. Varied test parameters include: steel deck profile, base angle thickness, screw embedment depth, slab thickness, and slab width. The objectives of the testing were (1) to provide an understanding of the strength of the standoff screw in various test geometries and (2) to provide an understanding of the slip characteristics, or ductility, of the connector.

A review of literature presents several applicable existing welded stud strength models based on post-test observations of failure mechanisms. Modeled failure mechanisms include: concrete splitting, concrete pullout, rib shear, and stud shear. After applying the existing models to the pushout test data, the following conclusions are drawn: (1) the concrete splitting model developed by Oehlers (1989) can be used to predict the strength of the standoff screw in flat slab geometries and (2) no existing model adequately predicts the strength of the standoff screw in geometries utilizing profiled steel deck. An equation, based on a rederivation of a wedged shaped shear-cone pullout model (Lloyd & Wright 1990), is presented which predicts the strength of the standoff screw in geometries with steel deck with acceptable accuracy.