Investigation of Minimum Longitudinal Reinforcing Requirements for Concrete

Provisions in the ACI 318 Building Code (Ref. 1) for minimum longitudinal reinforcement in columns are based on conclusions from testing programs that were performed several decades ago. The tests employed materials with strengths that are no longer consistent with today’s materials, and so it is uncertain if the provisions are appropriate for use today. The presence of the minimum requirement for longitudinal steel was to ensure that "passive yielding" of the reinforcement would not occur. A limit of 1% limit was first published in a committee document by the American Concrete Institute - American Society of Civil Engineers (ACI-ASCE) Joint Committee 105 in 1933.

This investigation was designed to identify the minimum longitudinal reinforcement ratio for concrete columns that would preclude passive yielding of the reinforcement. It was anticipated that a lower ratio would be suitable in certain applications. This experimental program and related analyses were designed to determine the possibility of such a reduction.

Several conclusions were drawn from the data produced by this investigation. Many factors were found to affect the amount of steel needed to prevent passive yielding. Strain response predictions made using the ACI 209 method agreed reasonably well with measured data but tended to under-predict strains. In general, it appears that it may be acceptable to reduce the minimum reinforcement requirement for certain conditions, but in general, it cannot be reduced.