Inelastic behavior of laterally supported steel beams subjected to cyclic loading
Whaley, Larry Emerson
Master of Science
This thesis reports an experimental study.of the load-deflection behavior and the failure characteristics of laterally supported wideflange steel beaus which are subjected to alternating inelastic strains. It is essentially an extension of a study conducted by L. D. Ozier on similar beaus which had no lateral support. The primary purpose of the thesis is to investigate the effects of the lateral bracing upon the performance of the beans. The cantilevered test specimens are cycled between equal positive and negative values of end deflection by a concentrated cyclic load applied at the free end. The boundary conditions for lateral deflection are approximately fixed-pinned. Specimens composed of 6B16 and 8B15 sections are tested at two different overall lengths, with the nominal ratios between the length and the lateral radius of gyration being chosen as 30 and 60. Lateral support is provided at the midpoint along the length for each short specimen, and at the one-third points for each long specimen. At each lateral support location, lateral displacement is prevented at a point just below the bottom flange, but rotation about this point is allowed. Ramberg-Osgood parameters are determined for analytically describing the initial hysteresis loops for end load versus end deflection and for end load versus end rotation. Also, theoretical and experimental relationships between ductility factors for deflection, rotation, and strain are examined. After the first two or three cycles of loading it is found that, in general, there is a decline in load capacity, stiffness, and energy dissipated per cycle. Also, the hysteresis loops become asymmetric as the cycling progresses because one flange is supported laterally and the other is not. All of these trends are related to the development of local, lateral, and web buckling. Lateral bracing is always found to improve the performance of a beam to some degree, but the improvement is not great in those cases in which web buckling is more critical than lateral-torsional buckling. When other factors are comparable, both the load-deflection and failure characteristics are generally more favorable for a stockier section (6B16 rather than 8B15), a shorter length, and a smaller amplitude of cyclic end deflection. The failure characteristics are studied in terms of the number of cycles for which the nominal plastic load can be sustained, the number of cycles prior to fracture, and the total energy absorbed prior to fracture.