scope:

Scope and general requirements

While only ACI Committee 318 can specify the requirements necessary for unbonded post-tensioned precast walls to meet the provisions of 18.2.1.7 of ACI 318, 1.10 of ACI 318 permits the building official to accept precast wall systems, other than those explicitly covered by Chapter 18 of ACI 318, provided specific tests, load factors, deflection limits, construction procedures, and other pertinent requirements have been established for acceptance of such systems consistent with the intent of the Code. This document provides a framework that establishes the specific tests, load factors, deflection limits, and other pertinent requirements appropriate for acceptance for regions of high seismic risk or for structures assigned to high seismic performance or design categories of unbonded post-tensioned precast wall systems, including unbonded post-tensioned precast coupled wall systems, not satisfying all the prescriptive requirements of Chapter 18 of ACI 318.

This document assumes that the unbonded post-tensioned precast wall system to be tested has details that differ from those prescribed by 18.10 of ACI 318 for conventional monolithic reinforced concrete construction. Such walls may, for example, involve the use of precast elements, precast prestressed elements, post-tensioned reinforcement, or combinations of those elements and reinforcement. Life safety and toughness are theoretically enhanced by mild steel reinforcement grouted across the wall to foundation interface. The presence of mild steel reinforcement, however, makes erection more difficult, and may inhibit the self-centering action provided by unbonded post-tensioning crossing the same interface. For an uncoupled wall, mild steel reinforcement, or some other form of energy-dissipating base connection, is necessary to meet the relative energy dissipation requirements of this document. For coupled walls, however, energy-dissipating coupling elements can be used along the vertical boundaries between walls so that only unbonded post-tensioning tendons need to cross the wall to foundation interface. Life safety for coupled walls is then more critically dependent on the unbonded post-tensioning not fracturing under the seismic event. In that case, careful attention should be paid to corrosion protection of the tendon and to stress increases in the tendon during the seismic event.

For monolithic reinforced concrete walls, a fundamental design concept underlying the Chapter 18 provisions of ACI 318 is that walls with h_w/l_w exceeding 1.0 should be proportioned so that their inelastic response is dominated by flexural action on a critical section located near the base of the wall. That same basic fundamental concept is retained in this document. The limiting h_w/l_w value, however, is reduced to 0.5. The basis for that lower limit is discussed in R1.2.2.

Tests on modules, as envisioned in this document, cannot be extrapolated with confidence to the performance of panelized walls of proportions differing from those tested for the development of Chapter 18 of ACI 318 if the shear-slip displacement pattern or excessive joint opening pattern of Fig. R2.3 is significant in the response developed in the test on the module.

Two other fundamental requirements of Chapter 18 of ACI 318 are for closely spaced ties around heavily strained boundary element reinforcement and the provision of minimum amounts of uniformly distributed horizontal and vertical reinforcement in the web of the wall. Ties around boundary element reinforcement to inhibit its buckling in compression are required where the strain in the extreme compression fiber exceeds 0.003 and spalling of the cover concrete may occur. Those ties then provide confinement that maintains the integrity of the boundary element and permits the confined concrete to develop increasing compressive forces with increasing lateral displacements in spite of the loss of the concrete cover. Minimum amounts of uniformly distributed horizontal and vertical reinforcement over the height and length of the wall are required to restrain the opening of inclined cracks and allow the development of the minimum acceptable drift angle capacities specified in Section 5.4. Deviations from those tie and distributed reinforcement requirements are possible only if a theory is developed that can substantiate reasons for such deviations and that theory is tested as part of the validation testing.

This document defines minimum acceptance criteria for unbonded post-tensioned precast structural walls, including coupled unbonded post-tensioned precast structural walls, designed for regions of high seismic risk or for structures assigned to high seismic performance or design categories, where acceptance is based on experimental evidence and analysis.