Seismic behavior of column -base plate connections bending about weak axis.

Abstract

Column-base plate connection is one of the most important structural elements in a steel structure, especially in a Moment Resisting Frame (MRF). The base plate connects the column to the concrete foundation directly so that lateral forces due to wind or seismic effects can be transferred through the base plate and anchor bolts to the foundation. In spite of the importance of the column-base plate connection in steel MRFs, there have been no unified seismic design provisions for this connection in the U.S. In order to understand the complex force flow and stress distribution in the column-base plate connection under large lateral displacements, a few experimental and theoretical studies have been conducted after the Northridge earthquake in the U.S. Recently, Drake and Elkin suggested a design procedure. They adopted an equivalent rectangular bearing stress block, instead of a triangular shape, to apply the AISC LRFD philosophy. However, their design method has not been verified analytically or experimentally for either the strong or the weak axis bending case. For the weak axis bending case, an analytical and experimental study was undertaken by the author to evaluate the Drake and Elkin's design method (D&E method) and to develop a more reliable design method. Two fairly typical types of the column-base plate connection in the U.S. practice (i.e., 6-bolt type and 4-bolt type) were selected. Using these selected two connection types, the D&E method was evaluated through numerical and experimental studies. The results showed several limitations of the D&E method and also pointed to some design considerations needed to develop a more rational and reliable design method. Finally, based on the proposed new design force profile and the concept of relative strength ratios among the connection elements, a new design method was developed. Furthermore, two limit states of the column-base plate connection were defined to prevent undesirable connection failures and to maximize cyclic connection ductility.