ACI 423.10R

Abstract

Scope: ACI 318-11 requires that the design of prestressed concrete members allow for prestress loss; however, the required level of detail for calculating losses is unspecified. The friction loss provisions for post-tensioned construction that first appeared in ACI 318-63 were removed from ACI 318-11. Although ACI 318-11 Commentary indicates that the lump sum method is obsolete, the licensed design professional’s requirement to choose a method to compute losses remains. This guide is intended to aid the designer in this choice by providing an overview of the various methods available. Many participants in the design and construction process need information on prestress losses. The licensed design professional, precasters, and post-tensioners all need an understanding of, and method to estimate, aspects of losses. To which entity is responsible for calculation of each type of loss has to be clearly defined in the contract documents. Total losses, ΔfpT, are losses due to friction and seating ΔfpFS, elastic shortening ΔfpES, creep of concrete ΔfpCR, shrinkage of concrete ΔfpSH, and relaxation of tendons ΔfpRE. This can be expressed as Eq. (1.2) ΔfpT = ΔfpFS + ΔfpES + ΔfpCR + ΔfpSH + ΔfpRE (1.2) This guide presents background information and methods to calculate each type of loss. Following the introduction and a list of notation and definitions, Chapter 3 includes a historical account of the lump sum method, currently recommended values for preliminary design, and a summary of losses that have been measured in field and laboratory studies. Chapter 4 discusses the different types of initial losses and addresses the differences between pretensioned and posttensioned members. Chapter 5 presents a simplified approach to estimate long-term losses due to creep, shrinkage, and relaxation for pretensioned and post-tensioned concrete members. Detailed approaches to estimate long-term losses are presented in Chapter 6, which also addresses changes in prestressing force caused by differential shrinkage and hydration of the concrete deck in composite members. The approaches can be used for pretensioned or post-tensioned members. Chapter 7 discusses the variability of prestress loss calculations caused by concrete material properties, including compressive strength at transfer, modulus of elasticity, and creep and shrinkage. Chapter 8 presents example problems and compares solutions from different methods.