This requirement, however, does not apply at supports of simple spans and at the free end of cantilevers.Ĭontinuing reinforcement should extend at least the development length Ld beyond the point where terminated or bent reinforcement is no longer required to resist tension. Reinforcement should extend a distance of d of 12db, whichever is larger, beyond the point where the steel is no longer required to resist tensile stress, where d is the effective depth of the member and db is the nominal diameter of the reinforcement. Development for All Flexural Reinforcement In addition, these requirements help minimize any loss of shear capacity or ductility resulting from flexural cracks that tend to open early whenever reinforcement is terminated in a tension zone. The following requirements of the ACI 318 Building Code for the development of reinforcement were proposed to help provide for shifts in the location of maximum moment and for peak stresses that exist in regions of tension in the remaining bars wherever adjacent bars are cut off or bent. The critical sections for development of reinforcement in flexural members are located at the points of maximum stress and where the reinforcement terminates or is bent. Hooks can be used to assist in the development of tension bars only. The calculated tension or compression force in each reinforcing bar at any section must be developed on each side of that section by a development length Ld, or by end anchorage, or both. The ACI 318 Building Code requirements therefore reflect the significance of average bond resistance over a length of bar or wire sufficient to develop its strength (development length). Despite assumptions made in the past to the contrary, bond stress between concrete and reinforcing bars is not uniform over a given length, not directly related to the perimeter of the bars, not equal in tension and compression, and may be affected by lateral confinement. Steel reinforcement must be bonded to the concrete sufficiently so that the steel will yield before it is freed from the concrete.
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