School of Civil and Environmental Engineering

Ph.D. Thesis Defense Announcement

Advancements in the Characterization of Stability Limit States for Steel I-Section Members Subjected to Flexure

By Ryan Slein

Advisors: 

Dr. Don White (CEE), Dr. Ryan Sherman (CEE)

Committee Members:  Dr. David Scott (CE, Georgia Southern University) , Dr. Arash Yavari (CEE), Dr. Richard Neu (ME)

Date and Time: August 19, 2020,  2:00 pm

Location: Mason 3133

This research addresses various fundamental advancements in the calculation of the strength of steel I-section members subjected to flexure. I-section members classify as thin-walled open-sections having a relatively low lateral flexural rigidity EIy, Saint Venant torsional rigidity GJ, and warping rigidity ECw, relative to the stiffness in major-axis bending. These characteristics make I section members susceptible to lateral-torsional buckling (LTB).

In the United States, design specifications classify the strength of steel I-section members through a LTB curve that is subdivided into three regions. The first region is referred to as the “plateau” where out-of-plane bracing is sufficient such that the member can fully develop its cross-section strength. The third region corresponds to theoretical elastic LTB. The second (middle) region corresponds to an out-of-plane failure but where significant yielding of the cross section occurs prior to the strength limit. This region is represented by a linear interpolation between anchor points corresponding to the elastic buckling and the plateau strengths. Proper determination of the anchor points at the transition between the plateau and the inelastic buckling regions, (Lp, Mmax), and between the inelastic and elastic buckling regions, (Lr, ML), requires consideration of results from beam theory, numerical modeling, and experimental testing.

This research develops the fundamental basis for, and creates various improvements to, the design of general built-up I-section members within the primary US (AISC and AASHTO) standards. These improvements provide: (1) calculated capacities that better fit new and existing data via changes to the anchor points (Lp, Mmax) and (Lr, ML); (2) increases in calculated capacities recognizing inelastic reserve strength in members experiencing early yielding in flexural tension; and (3) substantial shortening and streamlining of Specification provisions for design.

Improvements in the characterization of the LTB strength curves are carried forward to calculation of the resistance of general nonprismatic I-section members subjected to flexure. This research presents a two part methodology for calculating the strength of any general nonprismatic member through (1) the calculation of the elastic buckling resistance, via manual or numerical procedures; and (2) mapping the elastic buckling resistance to a nominal member capacity.