Effect of web distortion on the buckling strength of noncomposite discretely-braced steel I-section members

The influence of the web distortional flexibility typically is not addressed explicitly in standards for flexural design of steel I-section members. This is due in part to the fact that there are no simple closed-form solutions that account for these effects, whereas closed-form solutions are well established for elastic lateral-torsional buckling using thin-walled open-section beam theory. Thin-walled open-section beam theory is of course based on the assumption that the cross-section profile does not distort. Furthermore, accurate strength predictions are obtained for a wide range of experimental tests without explicit accounting for web distortion in the lateral-torsional buckling calculations. Nevertheless, it is clear from prior research studies that the web distortional flexibility can lead to a substantial reduction relative to the beam theory lateral-torsional buckling resistance for I-sections with torsionally-stiff flanges and relatively thin webs. [AASHTO. AASHTO LRFD bridge design specifications. 3rd ed. Washington (DC): American Association of State and Highway Transportation Officials; 2004; AISC. Specification for structural steel buildings. Chicago (IL): American Institute of Steel Construction; 2005 [in press]] give specific limits on cross-section geometry and yield strengths aimed at controlling the unconservative errors due to the neglect of web distortion effects. This paper evaluates the effectiveness of these limits. Potential future directions for improved calculation of I-section member flexural resistances are suggested for cases where the influence of web distortion is significant.