Building Design and Construction Handbook - Merritt - Ventech!

9.58 SECTION NINE
9.49 DEVELOPMENT, ANCHORAGE, AND
SPLICES OF REINFORCEMENT
Steel reinforcement must be bonded to the concrete sufficiently so that the steel
will yield before it is freed from the concrete. 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. 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).
The calculated tension or compression force in each reinforcing bar at any section
[Eqs. (9.53) to (9.61) and (9.64)] must be developed on each side of that section
by a development length L d, or by end anchorage, or both. Hooks can be used to
assist in the development of tension bars only.
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.
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. 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.
9.49.1 Development for All Flexural Reinforcement
Reinforcement should extend a distance of d of 12d b, 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 d b is the nominal diameter of the reinforcement.
This requirement, however, does not apply at supports of simple spans and
at the free end of cantilevers.
Continuing reinforcement should extend at least the development length L d beyond
the point where terminated or bent reinforcement is no longer required to
resist tension.
Reinforcement should not be terminated in a tension zone unless one of the
following conditions is satisfied:
1. Shear at the cutoff point does not exceed two-thirds of the design shear
strength, �V n.
2. Stirrup area A v not less than 60b ws/ƒ y and exceeding that required for shear
and torsion is provided along each terminated bar over a distance from the termination
point equal to 0.75d. (A v � cross-sectional area of stirrup leg, b w � width
of member, and ƒ y � yield strength of stirrup steel, psi.) The spacing should not
exceed d/8� b, where � b is the ratio of the area of the bars cut off to the total area
of bars at the cutoff section.
3. For No. 11 bars and smaller, continuing bars provide double the area required
for flexure at the cutoff point, and the factored shear does not exceed three-fourths
of the design shear strength, �V n.