Seismic behavior of moment -resisting steel column bases.

Steel column bases designed following the US standards were damaged during the recent 1994 Northridge earthquake, as well as in the 1964 Anchorage earthquake. It is hard to evaluate the causes of this damage because the seismic response of US steel column bases is not well understood. Most of the available experimental data pertain to monotonic load tests or small-scale specimens. There are only a few analytical studies. More importantly, the design provisions in the US codes are limited to the axial load case allowing the designers to use any of the several textbook procedures to design steel column bases for moment and for combined loads. The main objective of this dissertation is to investigate the seismic behavior of full-scale steel column base connections designed following US practice. The research goal is to establish the conditions under which column base connections can reliably deliver the required design strength, stiffness, ductility, and plastic rotations. The performance of the connections was examined through parametric studies, both experimental and analytical. These studies focused on typical exposed column base connections. Based on various tests described in the literature and herein, three conceptually different behavior mechanism groups were defined. The first group is distinguished by plastic hinging of the column only. The second group is distinguished by simultaneous yielding of the column and one or more elements of the connection. The third group is distinguished by inelastic deformation of one or more elements: anchor rods, concrete foundation, and/or base plate. The first. second and third group were labeled weak column/strong connection, balanced mechanism, and strong column/weak connection, respectively. The key ingredients in the classification of these mechanisms were the strength, stiffness, deformation capacity, and ductility of the column base. A connection strength ratio Gamma was introduced to help designers choose the desired failure mode of the column base connection. This ratio is defined using plastic analysis of the column base connection. The ability of the connection strength ratio Gamma to predict the column base failure mechanism was verified by comparing the results of 52 tests conducted by various researchers. The experimental results showed that the weak column/strong connection mechanism maximizes the post-yield deformation and strength of the connection and can be considered as desirable. The experiments also demonstrated that the boundary conditions have a marked effect on the stress flow in the column base connection. The differences between the classical design assumptions and the observed stress distributions are attributed to column cross-section warping, Poisson's effect, and the ratio of column, plate and anchor bolt stiffnesses. The weld metal was shown to have a great effect on the connection behavior. Connection in which the notch toughness of the weld metal was not controlled failed prematurely. Finally, the finite element model used in this study was shown to give results, which are in good agreement with the experimental results.PhDApplied SciencesCivil engineeringMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132346/2/9963779.pd