Basic straight tension development length is calculated
using equation 12-1. The following factors are used in this equation:
Ψt =
reinforcement location factor
- "concrete
below" is taken as the depth from the rebar center to the bottom of the
concrete section
- 1.3 for concrete below ≥
12 inches
- 1.0 for concrete below
< 12 inches
Ψe =
coating factor
- 1.5 for epoxy coated bars
with cover (to center of bar) less than 3db (spacing is not calculated)
- 1.2 for all other epoxy
coated bars
- 1.0 for uncoated bars
Note: the product
ΨtΨe is never taken as greater than
1.7.
Ψs =
reinforcement size factor
- 0.8 for #6 and smaller
bars
- 1.0 for #7 and larger bars
λ = lightweight aggregate
concrete factor
- 0.75 for lightweight
concrete
- 1.0 for normal-weight
concrete
c = cover dimension, vertical distance from the center of
the bar to the nearest concrete surface (spacing is not considered)
The term (c +
Ktr)/db is never taken greater than 2.5
Ktr is
conservatively assumed to be zero
For development of standard hooks, basic tension
development length is calculated in accordance with the following equation
(12.5.2):
The following factors are used to modify this basic
length:
- lightweight aggregate
concrete factor, λ, as described above
- the coating factor,
Ψe, is taken as 1.2 for epoxy-coated
reinforcement
In accordance with ACI 318 12.10.3, the extension length
used for this Code is the maximum of 12db or the effective depth of the member,
taken as the maximum vertical distance from the center of the reinforcing bar
to the farthest concrete surface.
The minimum development length is 12 in for straight bars
(12.2.1) and 8db or 6 in for hooks
(12.5.1).