Single Member Axial Tests
An experimental testing program has been carried out on axially loaded cold-formed lipped cee-studs to determine the required flexural and torsional bracing strength and stiffness demand of the stud. Conventional bridging or nodal bracing has been simulated in the experiments using steel wires attached to the stud flanges at mid-height. A range of brace stiffness from less than 30 lbs/in. to greater than 4000 lbs/in. was simulated in the testing frame by using various diameters and lengths of wire. Brace strength was determined from the cross-sectional area of the steel wire and it’s experimentally determined yield strength. The axial load, individual brace forces, axial shortening, and in-plane (weak-axis) and out-of-plane (strong-axis) lateral displacements were measured in each test. The required bracing stiffness was experimentally determined by varying the brace stiffness for a given stud size and was based on the ability of the stud to develop it’s nominal axial compressive capacity as predicted by the 1996 AISI Cold-Formed Steel Specification including Supplement No. 1 (AISI 1996, 1999). The experimental results were compared to existing nodal bracing models, analytical prediction models, and the current column bracing provisions that are part of the 1999 AISC-LRFD Specification for Structural Steel Buildings (AISC 1999).
Thirty-four cold-formed steel cee stud sections were loaded in axial compression to determine their ultimate load carrying capacity, examine their deformation characteristics throughout the entire axial load-axial shortening history, report on the affects of varying the bracing stiffness on the axial strength of lipped cee studs, and determine the required strength of the bracing to achieve the measured results.
The following general observations may be drawn from the testing:
The field installation end conditions have significant impact on the long-wave buckling capacity of the stud. The stud mounted in a standard track assembly provides some degree of weak axis flexural restraint, and a significant degree of strong axis and torsional restraint. The strong axis and torsional restraint provided by the track connection approaches that of a fixed end condition. Ultimate capacities far in excess of those predicted by the usual stud design assumptions of pinned ends were obtained.
Torsional and/or flexural bracing stiffness has very little influence on the load carrying capacity of the compression member if distortional buckling of the cross section is the controlling strength limit state. The bracing system only needs to be stiff and strong enough to develop the distortional buckling strength limit state.
Global geometric imperfections have an observable influence on the behavior of the stud. Larger imperfections result in greater lateral deflections and higher brace forces, but have little influence on the ultimate load carrying capacity. This observation verifies the analytical predictions of Winter (1960). |
