Structural response to axial testing of cold-formed stainless steel angle columns

Recognizing the increasing use of cold-formed stainless steel (CFSS) equal-leg angle columns in construction, several research projects have been initiated over the past few years to generate structural design rules which are able to predict their complex structural behavior. In spite of basic geometrical simplicity and adaptability, the location of the shear centre at the intersection of the angle legs and its non-coincidence with the section’s centroid imply a significantly low torsional stiffness, thus rendering angle columns highly susceptible to buckling phenomena involving torsion effects: buckling in a major-axis flexural–torsional buckling (FTB) mode in the low-to-intermediate slenderness range, and a minor-axis flexural buckling (FB) mode in the high slenderness range. Moreover, short equal-leg angle columns may be susceptible to torsional buckling (TB) associated with a failure mode akin to the local buckling (LB) of a slender equal-leg angle section.

Difficulties in assessing the susceptibility to FTB or FB are particularly noticeable in the case of slender sections. The deformation and local stress redistribution upon the elastic LB of angle legs reduce stiffness and section properties and cause a shifting of the effective centroid along the cross-section major axis, towards the corner. Furthermore, the inevitable presence of initial imperfections and end load eccentricity acting in combination with the effective centroid shift increases the degree of interaction between axial load and additional bending and leads to premature buckling failure. Thus, the ultimate response of cold-formed angle columns basically reflects that of an individual beam-column. In addition, as the distance between the shear centre and the point of application of the axial load increases, the twisting tendency of the angle section also increases, potentially causing FTB failure over the member slenderness range.

As the strength and behavior of CFSS angle columns had not been satisfactorily investigated before, a comprehensive experimental investigation of the structural responses of pin-ended CFSS equal-leg angle columns was conducted at the University of Belgrade, Faculty of Civil Engineering by associated professor Jelena Dobrić, PhD candidate Aljoša Filipović and Professor Zlatko Marković in collaboration with Nancy Baddoo, an associate director of the Steel Construction Institute, in the United Kingdom. The experimental program involved press-braked lean duplex stainless steel angle sections with nominal dimensions of 80 × 80 × 4 mm with three different lengths, short (240 mm), intermediate (1000 mm) and long (2000 mm), which were tested under axial compression load to characterize the failure modes and column strengths. The results, which included failure by LB, FTB and FB, were fully reported and thoroughly discussed in the recently published paper in the journal Thin-Walled Structures.

The authors found that the failure mode types are strongly dependent on the initial imperfections and column slenderness. The failure mode of the short columns in the stub column tests was governed by elastic local buckling, akin to TB. Elastic FTB coupled with LB and minor-axis FB was the critical failure mode in the intermediate slenderness domain. In the high slenderness domain, the failure mode of all specimens was minor-axis FB. However, the test results also suggest that long specimens were not prone to LB. Comparing the experimental results to the current European EN 1993-1-4 and Australian/New Zealand AS/NZS 4673 design specifications indicates that current design procedures offer safe but inaccurate strength predictions of the performance of the tested columns. However, it is also noted that the measured geometric imperfections of the specimens have considerably lower magnitudes and different distribution patterns compared with geometric imperfections based on the maximum allowable fabrication tolerances, usually used to develop design buckling curves.

In summary, the paper presents a detailed experimental investigation of the compressive capacities and structural responses of pin-ended CFSS equal-leg angle columns made from lean duplex stainless steel. The critical failure modes associated with the different slenderness domains were identified. The results will contribute to the development of improved design procedures. In fact, associated professor Jelena Dobrić, in a statement to Advances in Engineering, said that these results are just part of a larger ongoing investigation addressing cold-formed, hot-rolled and laser-welded stainless steel equal-leg angle columns with pin-ended boundary conditions, in which they are studying the influences of initial structural imperfections on the ultimate structural responses caused by different production processes.