Structural Concrete - Hassoun

276 Chapter 7 Development Length of Reinforcing Bars
to calculate the basic l d or you can get it directly from Table 7.2: l d = 48 in. For top bars, ψ e = 1.3
and final l d = 1.3(48) = 63 in.
b. Bars with 90 ∘ hook: For no. 8 bars, d b = 1.0 in. development length for f y = 60 ksi
l dh = 1200d b ∕ √ f ′ c = 1200(1.0)∕ √ 4000 = 19in. Because no other modifications apply,
then l dh = 19 in. > 8 d b = 8 in. or 6 in. Other details are shown in Fig. 7.11. The factor ψ e = 1.3 for
top bars does not apply to hooks.
c. Bars with 180 ∘ hook: l dh = 19 in., as calculated before. No other modifications apply; then
l dh = 19 in. > 8d b = 8 in. Other details are shown in Fig. 7.11.
7.8 SPLICES OF REINFORCEMENT
7.8.1 General
Steel bars that are used as reinforcement in structural members are fabricated in lengths of 20,
40, and 60 ft (6, 12, and 18 m), depending on the bar diameter, transportation facilities, and other
reasons. Bars are usually tailored according to the reinforcement details of the structural members.
When some bars are short, it is necessary to splice them by lapping the bars a sufficient distance to
transfer stress through the bond from one bar to the other.
ACI Code, Section 25.6.1.7, gives the provisions for lap splicing of bars in a bundle (tension or
compression). The lap-splice length required for individual bars within a bundle must be increased
20% for a 3-bar bundle and 33% for a 4-bar bundle. Overlapping of individual bar splices with a
bundle is not permitted. Two bundles must not be lap spliced as individual bars. For noncontact lap
splices in flexural members, bars should not be spaced transversely farther apart than one-fifth the
required length or 6 in. (150 mm).
ACI Code, Section 25.5.7, permits the use of mechanical and welded splices. A full mechanical
splice must develop, in tension or compression, at least 125% of the f y of bar (ACI Code, Section
25.5.7.1).
In a full welded splice, the bars must develop in tension at least 125% of the specified yield
strength of bar (ACI Code, Section 25.5.7.1).
ANSI/AWSD1.4 allows indirect welds where the bars are not butted. Although AWSD1.4
does indicate that, wherever practical, direct butt splices are preferable for no.7 bars and larger.
Use of mechanical and welded splices have less than 125% of the specified yield strength of
bars in the region of low computed stresses.
Splices should not be made at or near sections of maximum moments or stresses. Also, it
is recommended that no bars should be spliced at the same location to avoid a weakness in the
concrete section and to avoid the congestion of bars at the same location, which may cause difficulty
in placing the concrete around the bars.
The stresses developed at the end of a typical lap splice are equal to 0, whereas the lap-splice
length, l d , embedded in concrete is needed to develop the full stress in the bar, f y . Therefore, a
minimum lap splice of l d is needed to develop a continuity in the spliced tension or compression
bars. If adequate splice length is not provided, splitting and spalling occurs in the concrete shell
(Fig. 7.12).
Splices in tension and compression are covered by Sections 25.5 of the ACI Code.
7.8.2 Lap Splices in Tension, l st
Depending upon the percentage of bars spliced on the same location and the level of stress in the
bars or deformed wires, the ACI Code introduces two classes of splices (with a minimum splice
length of 12 in.):