Strength of Screw Connections in Cold-Formed Steel Construction

This paper presents a review of the design provisions for screw-fastened cold-formed steel connections in the United States, including criteria for testing and evaluating the strength of screw fasteners. In addition, the paper examines the relationship between the connected elements/components computed connection strength and the strength of the connector (screws). Fastener strength test data from three independent manufacturers is presented and it is shown that as the thickness or yield strength of the connected elements/components is increased, the ratio of the fastener strength to the bearing/tilting strength decreases to values less than one-half in some cases. Analysis of the test data showed that even when the 2001 North American Specification (NASPEC) for the Design of Cold-Formed Steel Structural Members statistical requirements for distribution of the test data are met, the computed safety and resistance factors can exceed the recommended limits in Section E4 of the 2001 NASPEC. On this basis, it is recommended that the current language in the 2001 NASPEC be revised to specify, if necessary, limits on manufacturer computed safety and resistance factors. Finally, it is recommended that fastener tilt during screw shear strength tests be eliminated to facilitate an accurate evaluation of the shear strength of a fastener.