Comparison of Hybrid and Emulative Precast Concrete Shear Walls for Seismic Regions

This paper compares the measured lateral load behavior of a 0.4-scale “hybrid” precast concrete wall test specimen with the behavior of a second precast specimen designed to emulate monolithic cast-in-place reinforced concrete shear walls. Both walls have the same overall geometry and are constructed by placing rectangular wall panels across horizontal joints. The lateral resistance at the joints of the “emulative” system is provided by mild [i.e., Grade 400 (U.S. Grade 60)] steel bars, whereas the hybrid wall features a combination of mild steel bars and highstrength unbonded post-tensioning (PT) strands. The mild steel reinforcement at the base of both systems is designed to yield and provide energy dissipation, while the unbonded PT steel in the hybrid system also provides self-centering capability to reduce the residual lateral displacements of the structure after a large earthquake. The comparisons between the two walls focus on the applied lateral load versus displacement behavior, energy dissipation, behavior of the steel reinforcement, and behavior along the critical horizontal base-panel-to-foundation joint. Due to the lack of PT steel, the emulative specimen developed a residual uplift along the entire basepanel-to-foundation joint, resulting in excessive horizontal slip, large strength and stiffness degradation, and significant residual lateral displacements. In contrast, the hybrid specimen showed significantly smaller residual uplift, horizontal slip, and strength and stiffness degradation, resulting in excellent re-centering and energy dissipation with ductile behavior over much larger lateral deformations. INTRODUCTION AND BACKGROUND As described in detail in Smith et al. (2011), the hybrid precast wall system investigated in this research utilizes a combination of mild [i.e., Grade 400 (U.S. Grade 60)] steel bars and high-strength unbonded post-tensioning (PT) strands for lateral resistance across horizontal joints. The general test configuration for the hybrid specimen is shown in Figure 1a. Under the application of lateral loads into the nonlinear range, the primary mode of displacement in these walls occurs through gap opening at the base joint. The PT steel is provided by multi-strand tendons placed inside un-grouted ducts to prevent bond between the steel and concrete. Thus, the tendons are connected to the structure only at end anchorages. The emulative wall configuration (Figure 1b) uses only mild steel reinforcement with no PT steel. In both systems, the mild steel bars crossing the horizontal joint at the base are designed to yield and provide energy dissipation through the nonlinear lateral displacements of the structure. A pre-determined length of these bars is unbonded at the bottom of the base panel (by wrapping the bars with plastic sleeves) to prevent lowcycle fatigue fracture. In the case of the hybrid wall, both the PT steel and mild steel contribute to the lateral strength of the structure. Upon unloading, the PT steel provides a restoring force to close the gaps and reduce the residual (i.e., permanent) lateral displacements of the wall after a large earthquake. The use of unbonded PT tendons (as compared to bonded tendons) delays the yielding of the strands and reduces the tensile stresses transferred to the concrete (i.e., reduced cracking) as the tendons elongate under lateral loading. According to Chapter 21 of ACI 318 (2008), “a reinforced concrete structural system not satisfying the requirements of this chapter shall be permitted if it is demonstrated by experimental evidence and analysis that the proposed system will have strength and toughness equal to or exceeding those provided by a comparable monolithic reinforced concrete structure satisfying this chapter.” The hybrid precast wall system investigated in this research falls within this category of structures that require experimental validation and analysis prior to their use in practice as a primary lateral load resisting system. The roadmap for the seismic performance assessment of hybrid walls is provided by ACI ITG-5.1 (2007), which lays out the minimum experimental evidence needed for the classification of these walls as special reinforced concrete shear walls based on ACI 318. The emulative system in Figure 1b is also not directly permitted by ACI 318, since unlike the requirements for a monolithic cast-inplace reinforced concrete structure, the longitudinal mild steel reinforcement is lumped near the ends of the wall (i.e., there is no distributed steel crossing the horizontal (Front Face of Wall) NORTH