Requirements for Design of a Special Unbonded Post-Tensioned Precast Shear Wall Satisfying ACI ITG-5.1 (ACI ITG-5.2-09) and Commentary
|Publication Date:||1 August 2009|
This standard defines design requirements for a certain class of unbonded post-tensioned precast concrete shear walls that can be used as special reinforced concrete shear walls for Bearing Wall and Building Frame Special Reinforced Concrete Shear Wall Systems, as defined in ASCE/SEI 7.
The requirements described in this standard are for special
unbonded post-tensioned precast shear walls with: 1. Essentially
planar proportions in the vertical direction, no significant
discontinuities in plan, in vertical configuration, or in their
Post-tensioning tendons unbonded from anchor to anchor and located in a single duct at the centroid of the transverse cross section of the wall, or in an additional two or more ducts positioned symmetrically on either side of that centroid and within 10% of the shear wall length from that centroid; and
Energy dissipation provided: a) for uncoupled walls by energy-dissipating reinforcement that crosses the interface between the base of the wall and the foundation; and b) for coupled walls by coupling devices that connect adjacent vertical boundaries of shear walls, number at least two for each connected vertical boundary of each precast panel, and are distributed uniformly over the height of the panel.
While the shear walls described in this standard do not fully satisfy the prescriptive requirements of Chapter 21 of ACI 318, analyses and tests, and reporting consistent with the intent of the requirements of ACI ITG-5.1, have established dependable and predictable strength, energy dissipation, stiffness, and drift capacities for the characteristic wall configurations described in this standard. The main features of the wall systems covered by this standard are illustrated in Fig. R1.1(a) and (b). Details for a typical uncoupled cantilever shear wall are shown in Fig. R1.1(a), and details for typical coupled shear wall systems in Fig. R1.1(b). For walls designed in accordance with this standard, the number of upper panels can be greater or less than the two panels shown in Fig. R1.1(a) and each panel can be one or more stories in height.
The uncoupled cantilever shear wall in Fig. R1.1(a) is a planar wall composed of three precast concrete panels and intended for use in a building that is three stories high. Except for two regions where large compressive stresses develop in the concrete and the associated reinforcement near the toes of the wall, the only reinforcement yielding in tension under lateral loading is the energy-dissipating reinforcement that crosses the wall-foundation interface. The unbonded length and prestress level for the central tendon is deliberately selected so that the prestressing steel does not yield as a result of the opening of the gap at the wall base at the design displacement. The deformed bar reinforcement for energy dissipation is anchored by grouting in ducts preformed in the lower-most panel and the foundation. The length over which the energy-dissipating reinforcement is debonded in the panel adjacent to the wall-foundation interface and the position of the two sets of bars either side of the center of the wall are selected deliberately to provide the desired design level of overall performance. The axial load stress level in the wall due to combined gravity load and prestressing is relatively low and, therefore, the neutral axis depth at the wall-foundation interface that gives rise to the region of high compressive strains in the toe of the wall is relatively small.
For the coupled planar two- and three-wall systems shown in Fig. R1.1(b), each wall is composed of three precast panels and each system is for a building five stories in height. The upper panels in each wall are two stories in height while the lower-most panel, where large compressive stresses develop in the toe of the wall under lateral loading, is detailed differently than the upper panels. Energy dissipation is provided by connection devices that can yield and are located, with at least two per panel, along the vertical edges that are coupled. The shearing displacements imposed on the devices are a direct function of the gap opening allowed at the wall-foundation interface under the design displacement. The prestress level for the central tendon in each wall is selected so that the prestressing steel does not yield under the gap opening imposed by the design displacement. The axial load stress level imposed on each wall at the design displacement is a function of the gravity load, the prestress level, and the effective yield strength of the connection devices. Because of the greater weight of the precast panels and the greater length of the tendons for the coupled walls of Fig. R1.1(b) (as compared with the uncoupled wall of Fig. R1.1(a)), blockouts may need to be provided on the line of the tendons and at the bottom of each panel for the location of access to couplers for the prestressing tendons.
All precast and reinforced concrete components and systems for the shear wall, energy dissipation and coupling devices, and associated gravity load systems, shall be designed to satisfy the requirements of ACI 318 except as modified by this standard.
All precast, prestressed, and reinforced concrete components and systems for the shear wall system, including energy dissipation and coupling devices and associated gravity load systems, shall meet the quality assurance requirements of Appendix 11A of ASCE/SEI 7. Requirements of Sections 11A.1.3.2, 11A.1.3.3, and 11A.1.3.4 shall apply for any shear wall panel element constructed on or off site, for assembly and post-tensioning operations on site, and for construction and installation of coupling devices.
For the shear wall systems described in this standard to be accepted as special shear wall systems defined by ASCE/SEI 7, the periodic special and continuous special inspection requirements during steel placement, welding operations, grouting, and concrete placement need to be properly executed by personnel who are qualified to perform the work and such inspection should be specified in the contract documents.