"Section, Member and System Level Analyses for Precast Concrete Hybrid Frames"

Abstract

Experimental studies have shown that precast concrete hybrid frames with dry jointed moment connections can provide adequate lateral force resistance for buildings located in high seismic zones. This framing concept, which has been approved for use by code officials, utilizes unbonded post-tensioning and mild steel reinforcement that is debonded over a short distance to establish a moment resisting connection between a precast beam and a precast column. As a result, the hybrid frames have the ability to dissipate energy and sustain minimal residual displacements when subjected to earthquake lateral forces. Due to the use of unbonded steel reinforcement, the hybrid connection concept introduces strain incompatibility between the reinforcement and the surrounding concrete, making the analysis and design of the connection difficult. Consequently, the available analysis and design methods for hybrid frame connections are based on many simplified assumptions. The analytical investigation presented in this report examines the monolithic beam analogy concept and establishes an improved set of expressions for estimating concrete and steel strains at the connection. The accuracy of the improved set of equations is verified using experimental data through section (or connection) and system level analyses. The improved analysis procedure is then demonstrated for a member level analysis and seismic analysis of a five-story precast hybrid building under different earthquake input motions. Using the analytical response of the five-story building, the following are examined: the benefits of using flexible floor links in hybrid frames, the ability of hybrid frames to satisfy limit states when subjected to different earthquake intensities, and suitable response modification (R-) factors for the force based design of hybrid frames.