Liu, Chi, and Garlock 2008
The authors explained post tensioned column base in this paper; in terms of system concept, design considerations for the PT column base connection based on a performance-based design approach, and evaluation of PT column base connections, using validate finite element models. A seismic resistant post tensioned steel frame system was developed as an alternative to a welded steel moment frame system. The PT column base connection allows for self-centering behavior of columns, while welded connections do not.
System Concept
The components of PT column base are PT high strength bars, buckling restrained steel plates, reinforcing plates, keeper plates and shim plates. Buckling restrained plates and keeper plates are welded together. Post-tensioned bars are anchored between half-height of the first story columns to close to the bottoms of the below-grade columns. The initial flexural stiffness before gap opening and post stiffness after gap opening at the PT column bases are associated mainly with the initial level of post-tensioning force and the axial stiffness of the post-tensioned bars.
In the PT frame, the softening behavior of plastic hinges at beam-column connections is replaced by gap opening. So, beam-column connections essentially remain undamaged, provided that beam local buckling is delayed enough to achieve the desired level of rotation. In order to eliminate the damage in columns as well as to enhance the self-centering behavior of the PT frame, a PT column base connection at grade level has been proposed for use in the PT frame.
Design Consideration and Evaluation of PT Column Base Connection
The strength of the PT column base connection is determined by yielding of the post-tensioned bars; and the limit states at the PT column base connections are defined corresponding to the level of ground motions. The decompressions at PT column base connections and yielding of buckling restrained steel plates were allowed under the Design Basis Earthquake. Also, minor shear yielding of column web near contact interface, which is due to the stress concentration after gap opening, was allowed.
In order to evaluate the design of the PT column base connection, finite element analyses of the PT column base subassembly were carried out. Friction and material plasticization, which are key phenomena of surface contact and separation, were incorporated into the model using computer program ABAQUS. The gap opening causes the elongation of PT bars as well as stress concentration near the bottom of column.
Reference
Liu, J., Chi, H., and Garlock, M. (2008). “Design and Analytical Validation of Post-tensioned Column Bases,” Proceedings of the Structures Congress Proceedings 2008, Vancouver, British Columbia, Canada, November 6-8.