Yield Safety, Cracking Control, and Moment Redistribution

Yield safety is introduced as a variable that controls both the cracking under service conditions and the permissible redistribution of ductile structural concrete frameworks at the ultimate limit state. Limiting values of the yield safety that ensure satisfaction of the code-specified cracking criteria are derived from the results of a comprehensive computer investigation. A parametric study of the yield-safety factor against cracking identifies the reinforcing index, , the allowable crack width, wa (or concrete tensile strength fr), and the degree of prestressing, , as the governing factors in cracking control. These factors, along with the live-to-total service load ratio, WL/W, which affects the overall safety of the structure, determine the permissible moment redistribution of concrete frameworks. Numerical examples of design (with concurrent satisfaction of the ultimate and serviceability limit states) and analysis of cracking serviceability criteria, in agreement with ACI 318-89 code requirements, illustrate the application of the concepts presented.