A PERFORMANCE-BASED APPROACH FOR THE DESIGN OF CONFINING STEEL IN TIED COLUMNS

A review of the development over the years of the ACI Code provisions for confinement is presented. Based on the available experimental evidence, the current Code requirements for the amount of confinement steel in tied columns are critically evaluated. It was concluded that the behavior of columns designed according to the ACI Code may vary from unacceptably brittle to very ductile. While the amount of Code-required steel can be reduced in many cases, much larger amounts of lateral steel are needed in other cases. A new design procedure is proposed in which the amount of lateral steel required is a function of the column ductility performance. The lateral steel content increases with an increase in the level of axial load, and depends on steel distribution and the extent of lateral restraint provided to the longitudinal bars. For any specific steel configuration, the procedure lends itself to a simple design chart. The proposed method when applied to realistically-sized specimens tested by different investigators yielded excellent agreement with the experimental results.