Ultimate Bearing Capacity of Unconfined Bolted Connections in Cold-Formed Steel Members

Abstract

The commentary to the North American cold-formed steel specification recommends physical testing to determine the performance of a bolted connection where the plies are not in contact with each other. This paper presents the laboratory test results of 120 such specimens with sheet thicknesses ranging from 0.55 to 3.0 mm, bolted with 10-, 12-, or 16-mm bolts. Finite-element analyses support the experimental finding that the bearing failure of an unconfined bolted connection is not associated with shear fracture but with crimpling of the critical ply. The bearing capacity varies with the square root of the bolt diameter and nonlinearly with the sheet thickness. Dimensionally consistent and continuous design equations are derived and verified against the results of the laboratory tests and finite-element analyses, the latter supplementing the former with configurations involving 8-mm bolts. The derived relationships between the bearing capacity and the sheet thickness and bolt diameter are also verified against 18 independent test results involving bolted connections through square hollow sections. The presented equation is significantly simpler but more accurate than those proposed in the literature for such connections. A resistance factor of 0.70 is recommended for use with the proposed design equations for determining the bearing capacities of unconfined bolted connections.