The Strength of Rotary-Straightened Steel Columns

A study was undertaken to determine the strength of rotary-straightened steel columns. Almost all steel sections are now rotary-straightened as standard practice. Current column design formulas are based on tests and analytical studies of unstraightened columns that contain compressive residual stress at the flange tips, typical of the manufacturing processes for most steel column sections rolled during 1950-1970. A rotary-straightened W12x65 grade 50 ksi column-type section was evaluated experimentally to determine residual stresses, out-of-straightness and material properties. The measured out-of-straightness averaged L/ 1400 in the weak direction and L/ 9500 in the strong direction. The residual stresses in the flanges were primarily tensile with the value at the tips exceeding 10 ksi. Analytical column curves were developed using residual stresses and out-of-straightness for measured values and typical values used in previous studies. The result showed that weak axis column strength was improved up to 25%, using the rotarized residual stress pattern, compared to the compressive residual stress pattern typically used. For weak axis stability, the rotarized residual stress pattern gave 6% higher column strength compared to neglecting residual stress. For strong axis behavior, the effect of rotarized residual stress was only 1% compared to neglecting residual stress. Analytical studies on some sample unbraced frames showed that weak axis frame strength improved up to 60% with the rotary-straightened stress pattern compared to frames with 0.3Fy compressive residual stresses. Neglecting residual stresses provided up to 45% increase in strength. This implies that it would be conservative to neglect residual stress when evaluating frame and beam-column behavior for W-shapes and inelasticity has a diminishing effect.

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