Building Design and Construction Handbook - Merritt - Ventech!

9.60 SECTION NINE
root of the compressive strength of the concrete, �ƒ�; c the diameter of the bar, db; depth of concrete below horizontal bars; bar spacings; concrete cover; and lateral
confinement reinforcement such as stirrups or ties. The ACI 318 Building Code
reinforcements also contain provisions to account for epoxy-coated bars and embedment
of bars in lightweight aggregate concrete. Tension development length can
also be reduced when more flexural reinforcement is provided than the amount
required by analysis.
The ACI 318-99 Building Code provides the designer with a choice of methods
for determining tension development length, Ld —a direct short-cut method; or a
more rigorous method which is applicable to all conditions of bar spacing, concrete
cover and transverse reinforcement. A third method is provided by the commentary
to ACI 318-99, which sanctions use of the provisions in the 1989 Code.
Using the direct short-cut method for determining the tension development
length of deformed bars or deformed wire in tension—with a clear spacing not less
than db, concrete cover not less than db, and stirrups and ties throughout Ld not less
than code minimum; or clear spacing not less than 2db and concrete cover not less
than db—the equations for calculating Ld are:
for #6 and smaller bars and wire
Ld � (0.04ƒy ���/�ƒ�)d c b � 12 in. (9.54)
for #7 and larger bars and wire
L � (0.05ƒ ���/�ƒ�)d � 12 in. (9.55)
d y c b
The direct short-cut method’s equations for determining the tension development
deformed bars and deformed wire for all other cases are:
for #6 and smaller bars and wire
Ld � (0.06ƒy ���/�ƒ�)d c b � 12 in. (9.56)
for #7 and larger bars and wire
In Eqs. (9.54) through (9.57):
L � (0.075ƒ ���/�ƒ�)d � 12 in. (9.57)
d y c b
� � 1.3 for top bars and 1.0 for other bars; ‘‘top bars’’ are horizontal bars with
more than 12 in. of concrete cast below them
� � 1.0 for uncoated bars
� � 1.5 for epoxy-coated bars with cover �3d b; or clear spacing �6d b
� � 1.2 for other concrete cover and clear spacing conditions of epoxy-coated bars
The product of �� need not be taken more than 1.7
� � factor for lightweight aggregate concrete � 1.3
� � 6.7 �ƒ�/ƒ c ct � 1.0 when the splitting tensile strength, ƒct, of lightweight aggregate
concrete is specified.
Under the more rigorous method, tension development length is calculated:
0.075ƒy ���� db
L � (9.58)
d �ƒ�[(c � K )/d ]
c tr b