ACI 550.7-19 Requirements for Design of a Special Unbonded Post- Tensioned Precast Shear Wall Satisfying ACI 550.6 (ACI 550.7) and Commentary

ACI 550.7-19 Requirements for Design of a Special Unbonded Post- Tensioned Precast Shear Wall Satisfying ACI 550.6 (ACI 550.7) and Commentary. R6.3.1 For the wall to return to the upright position when the seismic motions cease and force any crack that develops at the wall-foundation to close, the compressive force exerted by the effective prestress force and the axial load on the wall should be large enough to overcome the maximum tensile force that can develop in the energy-dissipating reinforcement. The lateral load-lateral displacement response of the wall of Fig. R1.1(a) can be idealized as shown in Fig. R6.1 for walls designed in accordance with this standard. At “a” on that response, the tension side of the wall starts to separate from the foundation. At “b,” the extreme tensile layer of the energy-dissipating reinforcement reaches its yield stress, and at “c,” the extreme tensile layer of the energy-dissi- pating reinforcement effectively reaches its tensile strength when the strain in it reaches the plateau associated with that strength at approximately a strain of 0.7 ε u . From that point on, the stiffness of the system is essentially determined by the stiffness of the prestressing tendons. At “d,” the energy- dissipating reinforcement fractures. Section 6.3.2 requires that the conditions for “b” occur before the stress in the prestressing steel exceeds 0.95f py . The strain requirements of Section 6.6.3.1 are intended to ensure that the conditions for “d” occur before the confined concrete crushes. 6.3.2 The value for f se , in combination with the location for the energy-dissipating reinforcement, shall result in the energy-dissipating reinforcement yielding before the prestressing steel stress reaches 0.95f py . 6.4—Energy-dissipating reinforcement 6.4.1 The energy-dissipating reinforcement provided at the base of the wall shall provide at least 25% of the nominal flexural strength of the wall at the wall-foundation interface. R6.4.1 The 25% requirement is intended to ensure that the energy-dissipating reinforcement provides a relative energy-dissipation ratio of at least 1/8 as required by ACI 550.6. This 25% value is based on the work of Kurama (2002). Alternatively, the required amount of reinforcement could be calculated using the procedures developed by Restrepo (2002). 6.4.2 The energy-dissipating reinforcement shall be anchored by: 1) being cast into the concrete of the foundation or wall panel during the placement of that concrete; 2) being grouted in ducts positioned in the foundation or wall panel prior to the placement...