Inelastic Strength of Laterally Unsupported Top- Loaded Built-Up Slender Beams

Authors: M. Massoud El Sa'adawy, F. F. F. El Dib

Abstract:

Lateral-torsional buckling (LTB) is one of the phenomenae controlling the ultimate bending strength of steel Ibeams carrying distributed loads on top flange. Built-up I-sections are used as main beams and distributors. This study investigates the ultimate bending strength of such beams with sections of different classes including slender elements. The nominal strengths of the selected beams are calculated for different unsupported lengths according to the Provisions of the American Institute of Steel Constructions (AISC-LRFD). These calculations are compared with results of a nonlinear inelastic study using accurate FE model for this type of loading. The goal is to investigate the performance of the provisions for the selected sections. Continuous distributed load at the top flange of the beams was applied at the FE model. Imperfections of different values are implemented to the FE model to examine their effect on the LTB of beams at failure, and hence, their effect on the ultimate strength of beams. The study also introduces a procedure for evaluating the performance of the provisions compared with the accurate FEA results of the selected sections. A simplified design procedure is given and recommendations for future code updates are made.

Keywords: Lateral buckling, Top Loading, Ultimate load, Slender Sections.

Digital Object Identifier (DOI): doi.org/10.5281/zenodo.1335152

References:

[1] T.V. Galambos, A.E. Surovek "Structural Stability of Steel: Concepts and Applications for structural engineers". Published by John Wiley & Sons, Inc., Hoboken, New Jersey,2008.
[2] AISC-LRFD, "Specification for structural steel buildings (AISC-LRFD 360-05)". Chicago (IL): American Institute of Steel Construction, March 9, 2005.
[3] A. Mohebkhah, "Lateral buckling resistance of inelastic I-beams under offshear center loading", Thin-Walled Structures, 2011; 49:431-436.
[4] A. Mohebkhah," The moment-gradient factor in lateral-torsional buckling on inelastic castellated beams". Journal of Constructional Steel Research, 2004;60: 1481-94.
[5] ANSYS, " User-s manual, version 5.4". ANSYS Inc. 201Johnson Road, Houston, PA, 15342-1300. 1998.
[6] C.F. Kollbrunner, M. Meister, "Knicken, Biegedrillknicken, Kippen , Theorie und berechnung von Knickstaeben, Knickvorschriften". Published by Springer Verlag, 1961.
[7] NS. Trahair, " Flexural-torsional buckling of structures". Boca Raton. London: E&FN Spon. 1993.
[8] G Y Grondin, J J R Cheng , "Sidesway Web Buckling of Steel Beams". Engineering Journal, 1999(fourth Quarter): 169-179
[9] Serna et al., "Equivalent uniform moment factors for lateral-torsional buckling of steel members". Journal of Constructional Steel Research 2006;62:566-580.
[10] Choi et al., " Inelastic buckling of torsionally braced I-girders under uniform bending" I. Numerical parametric studies. Journal of constructional Steel Research, 2010;66:304-316.
[11] Choi et al, " Inelastic buckling of torsionally braced I-girders under uniform bending", II:Experimental study. Journal of Constructional Steel Research, 2010;66:1128-1137.
[12] Nguyen et al., " Lateral_torsional buckling of I-girders with discrete torsional bracings". Journal of Constructional Steel Research, 2010;66:170-177.
[13] Nguyen et al. , "Flexural-torsional buckling strength of I-girders with discrete torsional braces under various loading conditions". Engineering Structures , 2012; 36: 337-350.