= 25. CHAPTER 25 — REINFORCEMENT DETAILS
= 25.7 — Transverse reinforcement
== 25.7.1 Stirrups
=== 25.7.1.1 Stirrups shall extend as close to the compression
and tension surfaces of the member as cover requirements
and proximity of other reinforcement permits and shall be
anchored at both ends. Where used as shear reinforcement,
stirrups shall extend a distance d from extreme compression
fiber.
= R25.7 — Transverse reinforcement
== R25.7.1 Stirrups
=== R25.7.1.1 Stirrup legs should be extended as close as practicable
to the compression face of the member because, near
ultimate load, the flexural tension cracks penetrate deeply
toward the compression zone.
It is essential that shear and torsional reinforcement be
adequately anchored at both ends to be fully effective on
either side of any potential inclined crack. This generally
requires a hook or bend at the end of the reinforcement as
provided by this section.
American Concrete Institute – Copyrighted © Material – www.concrete.org
494 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
=== 25.7.1.2 Between anchored ends, each bend in the continuous
portion of a single or multiple U-stirrup and each bend
in a closed stirrup shall enclose a longitudinal bar or strand.
=== 25.7.1.3 Anchorage of deformed bar and wire shall be in
accordance with (a), (b), or (c):
(a) For No. 16 bar and MD200 wire, and smaller, and for
No. 19 through No. 25 bars with fyt ≤ 280 MPa, a standard
hook around longitudinal reinforcement
(b) For No. 19 through No. 25 bars with fyt > 280 MPa, a
standard hook around a longitudinal bar plus an embedment
between midheight of the member and the outside end of
the hook equal to or greater than 0.17db fyt/(λ sqrt(fc')), with λ
as given in Table 25.4.3.2
(c) In joist construction, for No. 13 bar and MD130 wire
and smaller, a standard hook
=== R25.7.1.3 Straight deformed bar and wire anchorage is
not permitted because it is difficult to hold such a stirrup in
position during concrete placement. Moreover, the lack of a
standard stirrup hook may make the stirrup ineffective as it
crosses shear cracks near the end of the stirrup.
For a No. 16 or MD200 or smaller stirrup, anchorage is
provided by a standard hook, as defined in 25.3.2, hooked
around a longitudinal bar.
For a No. 19, No. 22, or No. 25 stirrup with fyt of only
280 MPa, a standard stirrup hook around a longitudinal bar
provides sufficient anchorage. For a No. 19, No. 22, or No.
25 stirrup with higher strength, the embedment should be
checked. A 135-degree or 180-degree hook is preferred,
but a 90-degree hook may be used provided the free end
of the 90-degree hook is extended the full 12 bar diameters
as required in 25.3.2. Because it is not possible to bend a
No. 19, No. 22, or No. 25 stirrup tightly around a longitudinal
bar and due to the force in a bar with a design stress
greater than 280 MPa, stirrup anchorage depends on both the
type of hook and whatever development length is provided.
A longitudinal bar within a stirrup hook limits the width of
any flexural cracks, even in a tension zone. Because such a
stirrup hook cannot fail by splitting parallel to the plane of
the hooked bar, the hook strength as used in 25.4.3.1(a) has
been adjusted to reflect cover and confinement around the
stirrup hook.
In joists, a small bar or wire can be anchored by a standard
hook not engaging longitudinal reinforcement, allowing a
continuously bent bar to form a series of single-leg stirrups
along the length of the joist.
=== 25.7.1.4 Anchorage of each leg of welded wire reinforcement
forming a single U-stirrup shall be in accordance with
(a) or (b):
(a) Two longitudinal wires spaced at a 50 mm spacing
along the member at the top of the U
(b) One longitudinal wire located not more than d/4 from
the compression face and a second wire closer to the
compression face and spaced not less than 50 mm from
the first wire. The second wire shall be permitted to be
located on the stirrup leg beyond a bend, or on a bend with
an inside diameter of bend of at least 8db.
=== R25.7.1.4 The requirements for anchorage of welded wire
reinforcement stirrups are illustrated in Fig. R25.7.1.4 .
American Concrete Institute – Copyrighted © Material – www.concrete.org
PART 8: REINFORCEMENT 495
25 Detailing
No further reproduction or distribution is permitted.
Fig. R25.7.1.4—Anchorage in compression zone of welded
wire reinforcement U-stirrups._
=== 25.7.1.5 Anchorage of each end of a single leg stirrup of
welded wire reinforcement shall be with two longitudinal
wires at a minimum spacing of 50 mm in accordance with
(a) and (b):
(a) Inner longitudinal wire at least the greater of d/4 or 50
mm from d/2;
(b) Outer longitudinal wire at tension face shall not be
farther from the face than the portion of primary flexural
reinforcement closest to the face.
=== R25.7.1.5 Welded wire reinforcement for shear reinforcement
is commonly used in the precast, prestressed
concrete industry. The rationale for acceptance of straight
sheets of welded wire reinforcement as shear reinforcement
is presented in a report by the Joint PCI/WRI Ad Hoc
Committee on Welded Wire Fabric for Shear Reinforcement
(1980).
The provisions for anchorage of single-leg welded wire
reinforcement in the tension face emphasize the location
of the longitudinal wire at the same depth as the primary
flexural reinforcement to avoid a splitting problem at the
level of the tension reinforcement. Figure R25.7.1.5 illustrates
the anchorage requirements for single-leg welded
wire reinforcement. For anchorage of single-leg welded
wire reinforcement, the Code permits hooks and embedment
length in the compression and tension faces of members
(refer to 25.7.1.3(a) and 25.7.1.4), and embedment only
in the compression face (refer to 25.7.1.3(b)). This section
provides for anchorage of straight, single-leg, welded wire
reinforcement using longitudinal wire anchorage with
adequate embedment length in compression and tension
faces of members.
American Concrete Institute – Copyrighted © Material – www.concrete.org
496 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
Fig. R25.7.1.5—Anchorage of single-leg welded wire reinforcement
for shear._
=== 25.7.1.6 Stirrups used for torsion or integrity reinforcement
shall be closed stirrups perpendicular to the axis of the
member. Where welded wire reinforcement is used, transverse
wires shall be perpendicular to the axis of the member.
Such stirrups shall be anchored by (a) or (b):
(a) Ends shall terminate with 135-degree standard hooks
around a longitudinal bar;
(b) In accordance with 25.7.1.3 (a) or (b) or 25.7.1.4, where
the concrete surrounding the anchorage is restrained
against spalling by a flange or slab or similar member.
==== 25.7.1.6.1 Stirrups used for torsion or integrity reinforcement
shall be permitted to be made up of two pieces
of reinforcement: a single U-stirrup anchored according to
25.7.1.6 (a) closed by a crosstie where the 90-degree hook of
the crosstie shall be restrained against spalling by a flange or
slab or similar member.
=== R25.7.1.6 Both longitudinal and closed transverse reinforcement
are required to resist the diagonal tension stresses
due to torsion. The stirrups should be closed because inclined
cracking due to torsion may occur on all faces of a member.
In the case of sections subjected primarily to torsion, the
concrete side cover to the stirrups spalls off at high torsional
moments (Mitchell and Collins 1976). This renders lapspliced
stirrups ineffective, leading to a premature torsional
failure (Behera and Rajagopalan 1969). In such cases, closed
stirrups should not be made up of pairs of U-stirrups lapping
one another.
When a rectangular beam fails in torsion, the corners of
the beam tend to spall off due to the inclined compressive
stresses in the concrete diagonals of the space truss changing
direction at the corner as shown in Fig. R25.7.1.6 (a). In tests
(Mitchell and Collins 1976), closed stirrups anchored by
90-degree hooks failed when this occurred. For this reason,
135-degree standard hooks or seismic hooks are preferable
for torsional stirrups in all cases. In regions where
this spalling is prevented by an adjacent slab or flange,
25.7.1.6 (b) relaxes this requirement and allows 90-degree
hooks because of the added confinement from the slab (refer
to Fig. R25.7.1.6 (b)).
American Concrete Institute – Copyrighted © Material – www.concrete.org
PART 8: REINFORCEMENT 497
25 Detailing
No further reproduction or distribution is permitted.
Fig. R25.7.1.6—Spalling of corners of beams subjected to
torsion.
=== 25.7.1.7 Except where used for torsion or integrity reinforcement,
closed stirrups are permitted to be made using
pairs of U-stirrups spliced to form a closed unit where lap
lengths are at least 1.3ℓd. In members with a total depth of at
least 450 mm, such splices with Ab fyt ≤ 40 kN per leg shall
be considered adequate if stirrup legs extend the full available
depth of member.
=== R25.7.1.7 Requirements for lapping of double U-stirrups
to form closed stirrups control over the lap splice provisions
of 25.5.2. Figure R25.7.1.7 illustrates closed stirrup configurations
created with lap splices.
Fig. R25.7.1.7—Closed stirrup configurations.
American Concrete Institute – Copyrighted © Material – www.concrete.org
498 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
== 25.7.2 Ties
=== 25.7.2.1 Ties shall consist of a closed loop of deformed
bar with spacing in accordance with (a) and (b):
(a) Clear spacing of at least (4/3)dagg;
(b) Center-to-center spacing shall not exceed the least
of 16db of longitudinal bar, 48db of tie bar, and smallest
dimension of member.
=== 25.7.2.2 Diameter of tie bar shall be at least (a) or (b):
(a) No. 10 enclosing No. 32 or smaller longitudinal bars;
(b) No. 13 enclosing No. 36 or larger longitudinal bars or
bundled longitudinal bars.
==== 25.7.2.2.1 As an alternative to deformed bars, deformed
wire or welded wire reinforcement of equivalent area to that
required in 25.7.2.1 shall be permitted subject to the requirements
of Table 20.2.2.4(a).
=== 25.7.2.3 Rectilinear ties shall be arranged to satisfy (a)
and (b):
(a) Every corner and alternate longitudinal bar shall have
lateral support provided by the corner of a tie with an
included angle of not more than 135 degrees;
(b) No unsupported bar shall be farther than 150 mm clear
on each side along the tie from a laterally supported bar.
== R25.7.2 Ties
=== R25.7.2.2 These provisions apply to crossties as well as
ties.
=== R25.7.2.3 The maximum permissible included angle of
135 degrees and the exemption of bars located within 150
mm clear on each side along the tie from adequately tied
bars are illustrated in Fig. R25.7.2.3a . Limited tests (Pfister
1964 ) on full-size, axially-loaded, tied columns containing
full-length bars (without splices) showed that ties on alternate
longitudinal bars within 150 mm clear of a laterally
supported longitudinal bar are adequate in columns subjected
to axial force.
Continuously wound bars or wires can be considered as
ties, provided their pitch and area are at least equivalent to
the area and spacing of separate ties. Anchorage at the end
of a continuously wound bar or wire should be by a standard
hook as for separate bars or by one additional turn of the tie
pattern (refer to Fig. R25.7.2.3b ). A circular, continuously
wound bar or wire is considered a spiral if it conforms to
25.7.3; otherwise, it is considered a tie.
American Concrete Institute – Copyrighted © Material – www.concrete.org
PART 8: REINFORCEMENT 499
25 Detailing
No further reproduction or distribution is permitted.
Fig. R25.7.2.3a—Illustrations to clarify measurements
between laterally supported column bars and rectilinear tie
anchorage.
American Concrete Institute – Copyrighted © Material – www.concrete.org
500 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
Fig. R25.7.2.3b—Continuous tie anchorage.
==== 25.7.2.3.1 Anchorage of rectilinear ties shall be provided
by standard hooks that conform to 25.3.2 and engage a
longitudinal bar. A tie shall not be made up of interlocking
headed deformed bars.
=== 25.7.2.4 Circular ties shall be permitted where longitudinal
bars are located around the perimeter of a circle.
==== 25.7.2.4.1 Anchorage of individual circular ties shall be in
accordance with (a) through (c):
(a) Ends shall overlap by at least 150 mm.
(b) Ends shall terminate with standard hooks in accordance
with 25.3.2 that engage a longitudinal bar
(c) Overlaps at ends of adjacent circular ties shall be staggered
around the perimeter enclosing the longitudinal bars
==== R25.7.2.3.1 Standard tie hooks are intended for use with
deformed bars only and should be staggered where possible.
=== R25.7.2.4 While the transverse reinforcement in members
with longitudinal bars located around the periphery of a
circle can be either spirals or circular ties, spirals are usually
more effective.
==== R25.7.2.4.1 Vertical splitting and loss of tie restraint are
possible where the overlapped ends of adjacent circular ties
are anchored at a single longitudinal bar. Adjacent circular
ties should not engage the same longitudinal bar with end
hook anchorages (refer to Fig. R25.7.2.4.1 ).
American Concrete Institute – Copyrighted © Material – www.concrete.org
PART 8: REINFORCEMENT 501
25 Detailing
No further reproduction or distribution is permitted.
Fig. R25.7.2.4.1—Circular tie anchorage.
=== 25.7.2.5 Ties to resist torsion shall be perpendicular to the
axis of the member anchored by either (a) or (b):
(a) Ends shall terminate with 135-degree standard hooks
or seismic hooks around a longitudinal bar;
(b) In accordance with 25.7.1.3(a) or (b) or 25.7.1.4, where
the concrete surrounding the anchorage is restrained
against spalling.
R25.7.2.5 Refer to R25.7.1.6.
== 25.7.3 Spirals
=== 25.7.3.1 Spirals shall consist of evenly spaced continuous
bar or wire with clear spacing conforming to (a) and (b):
(a) At least the greater of 25 mm and (4/3)dagg
(b) Not greater than 75 mm
=== 25.7.3.2 For cast-in-place construction, spiral bar or wire
diameter shall be at least 9.5 mm.
=== 25.7.3.3 Except for transverse reinforcement in deep foundations,
the volumetric spiral reinforcement ratio ρs shall
satisfy Eq. (25.7.3.3).
ρs >= 0.45 (Ag/Ach-1)(fc'/fyt)
(25.7.3.3)
where the value of fyt shall not be taken greater than
690 MPa.
== R25.7.3 Spirals
=== R25.7.3.1 Spirals should be held firmly in place, at proper
pitch and alignment, to prevent displacement during concrete
placement.
=== R25.7.3.2 For practical considerations in cast-in-place
construction, the minimum diameter of spiral reinforcement
is 9.5 mm (No. 10 deformed or plain bar, or MD70 deformed
or MW70 plain wire).
Standard spiral sizes are 9.5, 12.7, and 16 mm diameter
for hot-rolled or cold-drawn material, plain or deformed.
=== R25.7.3.3 The effect of spiral reinforcement in increasing
the strength of the concrete within the core is not fully realized
until the column has been subjected to a load and deformation
sufficient to cause the concrete shell outside the core
to spall off. The amount of spiral reinforcement required by
Eq. (25.7.3.3) is intended to provide additional strength for
concentrically loaded columns equal to or slightly greater
than the strength lost when the shell spalls off. The deriva-
American Concrete Institute – Copyrighted © Material – www.concrete.org
502 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
=== R25.7.3.3 Continuation
tion of Eq. (25.7.3.3) is given by Richart (1933). Tests and
experience show that columns containing the amount of
spiral reinforcement required by this section exhibit considerable
toughness and ductility. Research (Richart et al. 1929;
Richart 1933; Pessiki et al. 2001; Saatcioglu and Razvi
2002) has also indicated that up to 690 MPa yield strength
reinforcement can be effectively used for confinement.
=== 25.7.3.4 Spirals shall be anchored by 1-1/2 extra turns of
spiral bar or wire at each end.
=== R25.7.3.4 Spiral anchorage is illustrated in
Fig. R25.7.3.4 .
Fig. R25.7.3.4—Spiral anchorage.
=== 25.7.3.5 Spirals are permitted to be spliced by (a) or (b):
(a) Mechanical or welded splices in accordance with
25.5.7
(b) Lap splices in accordance with 25.7.3.6 for fyt not
exceeding 420 MPa;
=== 25.7.3.6 Spiral lap splices shall be at least the greater of
300 mm and the lap length in Table 25.7.3.6 .
American Concrete Institute – Copyrighted © Material – www.concrete.org
PART 8: REINFORCEMENT 503
25 Detailing
No further reproduction or distribution is permitted.
Table 25.7.3.6—Lap length for spiral reinforcement
== 25.7.4 Hoops
=== 25.7.4.1 Hoops shall consist of a closed tie or continuously
wound tie, which can consist of several reinforcement
elements each having seismic hooks at both ends.
== R25.7.4 Hoops
R25.7.4.1 Refer to R25.7.2.4.
=== 25.7.4.2 The ends of the reinforcement elements in hoops
shall be anchored using seismic hooks that conform to 25.3.4
and engage a longitudinal bar. A hoop shall not be made up
of interlocking headed deformed bars.
American Concrete Institute – Copyrighted © Material – www.concrete.org
504 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
[ Lanjut Ke 25.8—Post-tensioning anchorages and couplers ... ]

Back To Contents