Seismic behavior of high-performance fiber reinforced cementitious composite coupling beams

Coupling beams may significantly improve the seismic behavior of structural wall systems, depending on their span-to-depth ratio and their ability to transfer shear between coupled walls during earthquake ground motions. Current design requirements for deep reinforced concrete (RC) coupling beams include intricate diagonal reinforcement detailing to ensure a stable behavior during earthquakes, leading to reinforcement congestion and construction difficulties. A research program was conducted at the University of Michigan to develop a simple design for coupling beams through the use of strain-hardening or high-performance fiber reinforced cement composites (HPFRCCs).; Four coupling beam specimens were tested, including an RC control specimen. Specimens were proportioned to approximately 3/4 of full scale and a span-to-depth ratio of l.0 was selected to ensure a shear dominant behavior. The main variables investigated were the type of material used in the coupling beams, fiber type, and reinforcement detailing. The HPFRCC beams were cast separately as prefabricated members to ensure adequate material quality and to ease the construction process at the job site.; Test results indicate that in order to achieve a large displacement capacity, diagonal reinforcement must be used in the proposed HPFRCC coupling beams. However, the use of strain-hardening fiber composites allowed the elimination of transverse reinforcement required for confinement in RC coupling beams, thus significantly simplifying the reinforcement details.; In the analytical phase of the research program, a method to predict the shear force versus shear distortion envelope curve of HPFRCC coupling beams was developed. Key shear distortion levels were predicted based on the tensile strain capacity of the HPFRCC material. The corresponding shear strength was estimated based on the contributions from the HPFRCC material and steel reinforcement. Using the predicted envelope curve, an existing hysteretic model was modified to accurately simulate the behavior of HPFRCC coupling beams under displacement reversals. As a result, good agreement between the predicted and experimental response was obtained. In the final phase of the analytical study, a damage index analysis was performed and a range of values was proposed to predict the performance of HPFRCC coupling beams during earthquakes.