REEXAMINATION OF SERVICE LOAD LIMIT COMPRESSIVE STRESS IN PRESTRESSEDCONCRETE MEMBERS

One of the main reasons for setting permissible stresses in prestresse d 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 bee n emphasized in the past, since it seldom controlled in design. But no w the compressive stress limit has become a critical factor and more r estrictive in modern designs, such as spliced girder bridges, long-spa n beams in office buildings, and segmental box girders. This paper dis cusses the proposed ACE 318-95 Building Code and AASHTO Bridge Specifi cations allowable compressive stress changes. It is suggested that the ACI allowable compressive stress to be 0.60f(c)' due to load combinat ions of transient nature, plus the corresponding prestress forces, and 0.45f(c)' 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 li mits.