Experimental and analytical assessment of LTB resistance of built-up steel I-section members

Analytical and experimental research investigations conducted subsequent to the development of the unified AISC 360–05 Section F4 and F5 and AASHTO [3] LRFD provisions for built-up I-section members have identified several areas where improvements are needed. However, the experimental data is relatively scarce within the design space for inelastic lateral-torsional buckling (LTB) of these member types. This study presents the results of eight experimental tests of doubly symmetric built-up I-section members and a large number of full-nonlinear shell finite element analysis (FEA) simulations validated against the experimental results. The study was conducted to supplement existing data and to evaluate three recommended changes to the AISC 360 Specification: (1) reduction of the ordinate at the transition between the elastic and inelastic LTB equations to ML = 0.5RpgMyc for the base uniform bending case, where Rpg is the web bend-buckling strength reduction factor and Myc is the yield moment to the compression flange; (2) use of the more general equation in AISC 360–16 Table B4–1 Case 16 for the compact-web limit, λpw; and (3) use of a variable crw factor from the current AASHTO LRFD Specifications for the calculation of the noncompact-web limit, λrw. Comparisons were also made to the predictions from the first generation of Eurocode 3. Results from the study support the recommended changes to the AISC 360 Specification and confirm the significant impact of flange lateral bending from the amplification of initial compression flange sweep for members with LTB slenderness close to the noncompact bracing limit associated with ML. •The current AISC 360 provisions overestimate the LTB resistance near the unbraced length Lr.•Significant flange lateral bending occurs as the theoretical elastic LTB moment is approached for unbraced lengths near Lr.•Flange lateral bending causes early flange yielding, thereby limiting the moment resistance for unbraced lengths near Lr.•The AISC 360 compact and noncompact web limits need minor enhancements to improve the Specification predictions.•Simple modifications to the LTB strength parameters can improve the AISC 360 predictions.