Building Design and Construction Handbook - Merritt - Ventech!

where � � 0.8 for bar sizes #3–#6
� � 1.0 for bar sizes #7–#18
The term (c � K tr)/d b is limited to a value of 2.5
CONCRETE CONSTRUCTION 9.61
c � the smaller of: (1) one-half of the center-to-center spacing of the bars; or (2)
the concrete cover to the center of the bar, in
K tr � A trƒ yt/(1500 sn)
A tr � total area of all transverse reinforcement within the spacing s, which crosses
the potential plane of splitting through the bars being developed in 2
ƒ yt � specified yield strength of transverse reinforcement, psi
s � maximum center-to-center spacing of transverse reinforcement within L d,in
n � number of bars being developed along the plane of splitting.
Increased L d is required for bundled bars: in 3-bar bundles, 20%; in 4-bar bundles,
33%. For determining the appropriate modifying factors for use with bundled
bars, a unit of bundled bars should be treated as a single bar with a diameter derived
from the equivalent total area.
Application of all the various interdependent tension development length requirements
to each structural element in design would be extremely difficult and a
waste of design time. The authors recommend that the designer check the actual
dimensions available for tension development in the connection (or from a cutoff
point established as a fraction of the span on typical design drawing details), compare
to a table of development lengths required for each bar size, and select the
bar size allowable. Table 9.8, which is based on the direct short-cut method, presents
values of tension L d for each size bar for normal-weight concrete with compressive
strengths of 3000, 4000 and 5000 psi. Note that separate values are tabulated
for ‘‘top bars’’ and ‘‘other bars.’’
9.49.5 Anchorage with Hooks
For rebars in tension, standard 90� and 180� end hooks can be used as part of the
length required for development or anchorage of the bars. Table 9.9 gives the minimum
tension embedment length L dh required with standard end hooks (Fig. 9.17
and Table 9.9) and Grade 60 bars to develop the specified yield strength of the
bars.
9.49.6 Development for Welded-Wire Fabric in Tension
For deformed welded-wire fabric (WWF) with at least one cross wire within the
development length not less than 2 in. from the point of critical section (Fig. 9.18),
the tension development length is the length calculated from Eqs. (9.54) and (9.56)
using the direct short-cut method or from Eq. (9.58) using the more rigorous
method and then multiplied by a wire fabric factor. The wire fabric factor is the
larger of
or
(ƒ � 35,000)/ƒ � 1.0 (9.59)
y y
5 d /s � 1.0 (9.60)
b w