Detailing for Reinforcement Stability in RC Members

Instability of reinforcement in adequately detailed reinforced concrete (RC) structural elements is a symptom of excessive deformation in compression, and it typically marks the limit of usable ductility of the member. Because the supply of ductility (deformability) is a measure of the effectiveness of the confining reinforcement, detailing necessary for reinforcement stability is intimately related to that required for confinement. Existing design models consider the two problems independently, and associate the initiation of bar buckling with the occurrence of a critical milestone of material behavior under compression. The resulting requirements for lateral support of the reinforcement do not correlate well with those obtained from studies for confinement and rarely control in cases of earthquake design. The objective of this paper is to develop alternative requirements for reinforcement stability that recognize the interaction between displacement ductility demand in the critical section, tie effectiveness, limiting concrete strain, bar size, and tie spacing. To quantify this relationship an extensive database consisting of over 300 column tests was compiled from international literature. Tests considered in the database were conducted under repeated axial and flexural load reversals, simulating earthquake effects on specimens with a variety of confining and longitudinal steel arrangements. The database was systematically analyzed to identify from experimental evidence the critical condition for reinforcement stability and its relationship to the limiting average compressive strain and/or ductility demand in the plastic hinge zone, and to the confinement geometry and effectiveness. Recognizing the interdependence between critical buckling conditions and confinement presents an opportunity to relate detailing requirements for the stability of reinforcement to detailing for ductility, and to distinguish between designs intended to satisfy different levels of deformation demand required for construction in various zones of seismic risk.