Title: Reexamination of Service Load Lmit Compressive Stress in Prestressed Concrete Members

Author(s): Xiaming Huo, John M. Savage, and Maher K. Tadros

Publication: Structural Journal

Volume: 92

Issue: 2

Appears on pages(s): 199-210

Keywords: compression; creep properties; deflections; fatigue (materials); prestressed concrete; serviceability; specifications; standards; stresses; structural design; Structural Research

DOI: 10.14359/1144

Date: 3/1/1995

Abstract:
One of the main reasons for setting permissible stresses in prestressed concrete flexural members is to insure that serviceability criteria are met. However, the allowable compressive stress at service seems to be unjustified. The issue of allowable compressive stress has not been emphasized in the past, since it seldom controlled in design. But now the compressive stress limit has become a critical factor and more restrictive in modern designs, such as spliced girder bridges, long-span beams in office buildings, and segmental box girders. This paper discusses the proposed ACI 318-95 Building Code and AASHTO Bridge Specifications allowable compressive stress changes. It is suggested that the ACI allowable compressive stress be 0.60 due to load combinations of transient nature, plus the corresponding prestress forces, and 0.45 due to effective prestress plus permanent (dead) loads. The reasons for the proposed changes and several examples are presented. It is shown that these changes do not adversely affect the acceptable safety margin against compression failure or deflection and fatigue limits.