Chou and Chen 2009
As an alternative to steel special moment frames, a PT Self-Centering Moment Frame was developed, which uses PT steel to compress steel beams against column. A challenge discovered by previous research was the issue of slab and column restraints. When more than one column is constrained by outer columns, the beam compression force differs from the applied PT force. This paper provides a new analytical technique by evaluating bending stiffness of column and compression forces in the beams based on a deformed column space which matches the gap-opening. The focus of the research is on the restraining effect of columns in a low-rise PT frame. The analytical formulation is supported by a cyclic analytical analysis as well as cyclic test of a full-scale, two-bay by first-story PT frame.
System Concept
The analytical method consist of three basic procedures: determine column deformation in accordance with specified lateral displacements at all beam-to-column interfaces along the column height, compute the column bending stiffness at each story by the reaction force divided by the specified lateral displacement, and compute beam shortenings and compressive forces.
The analytical model consisted of a 3-story PT prototype frame, which was required to self-center at both seismic hazard levels of the DBE and the maximum considerable earthquake. Three two-bay PT frames provided lateral load resistance in the east-west direction and were examined in this study. The analytical model was verified using full scale cyclic testing. The experimental model represented the substructure of the three-story frame.
Experimental Study, Results, and Discussion
The experimental set up had three RC columns extending from the foundation to mid height of the second story. Each column was PT to the foundation. A total of 12 PT strands were strung along the beam webs through each of the columns and anchored to the outside columns. Two actuators were positions between the reaction wall and the frame and one actuator was positioned along each beam span. A quasi-static cyclic loading with increasing displacement amplitude was conducted during the experiment.
The research showed that earlier analysis methods using a pin-supported boundary condition at upper story columns represented an upper bound and was overly conservative. It was found that obtaining the column bending stiffness by the deformation of the frame provided more accurate beam compression force estimates. The author concluded the study by noting the proposed analysis procedure based on using deformed shape of columns to assess the restraining effect of columns had been validated through the analytical model and the experimental work, but only followed the first mode predominantly.
Reference
Chou, C.C. and Chen, J.H. (2009). “Column Restraining Effects in Post-Tensioned Self-Centering Moment Frames,” Proceedings of the 14th World Conference on Earthquake Engineering, Beijing, China, October 12-17.