10 — Transverse reinforcement for compression members

STRUCTURAL CONCRETE BUILDING CODE (ACI 318M-11) AND COMMENTARY



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(3) epoxy-coated deformed bar or wire,

or zinc and epoxy dual-coated

deformed bar .......................................... 72db

(4) plain uncoated bar or wire, or plain

zinc-coated (galvanized) bar, which have

a standard stirrup or tie hook in

accordance with 7.1.3 at ends of

lapped spiral reinforcement.

The hooks shall be embedded

within the core confined by the spiral

reinforcement.......................................... 48db

(5) epoxy-coated deformed bar or wire,

or zinc and epoxy dual-coated deformed

bar, which have a standard stirrup

or tie hook in accordance with 7.1.3

at ends of lapped spiral reinforcement.

The hooks shall be embedded within

the core confined by the spiral

reinforcement.......................................... 48db

(b) Full mechanical or welded splices in accordance

with 12.14.3.



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or wire smaller than 16 mm diameter, a minimum of two

spacers should be used for spirals less than 500 mm in

diameter, three spacers for spirals 500 to 750 mm in diameter,

and four spacers for spirals greater than 750 mm in diameter. For spiral bar or wire 16 mm diameter or larger, a

minimum of three spacers should be used for spirals 600 mm

or less in diameter, and four spacers for spirals greater than

600 mm in diameter. The project specifications or subcontract

agreements should be clearly written to cover the supply of

spacers or field tying of the spiral reinforcement. In the

1999 Code, splice requirements were modified for epoxycoated and plain spirals and to allow mechanical splices.



7.10.4.6 — Spirals shall extend from top of footing or

slab in any story to level of lowest horizontal reinforcement in members supported above.

7.10.4.7 — Where beams or brackets do not frame into

all sides of a column, ties shall extend above termination

of spiral to bottom of slab, drop panel, or shear cap.

7.10.4.8 — In columns with capitals, spirals shall

extend to a level at which the diameter or width of

capital is two times that of the column.

7.10.4.9 — Spirals shall be held firmly in place and

true to line.

7.10.5 — Ties



R7.10.5 — Ties



Tie reinforcement for compression members shall

conform to the following:



All longitudinal bars in compression should be enclosed

within transverse ties. Where longitudinal bars are arranged

in a circular pattern, only one circular tie per specified

spacing is required. This requirement can be satisfied by a

continuous circular tie (helix) at larger pitch than required

for spirals under 10.9.3, the maximum pitch being equal to

the required tie spacing (see also 7.10.4.3).



7.10.5.1 — All nonprestressed bars shall be

enclosed by transverse ties, at least No. 10 in size for

longitudinal bars No. 32 or smaller, and at least No. 13

in size for No. 36, No. 43, No. 57, and bundled longitudinal

bars. Deformed wire or welded wire reinforcement of

equivalent area shall be permitted.

7.10.5.2 — Vertical spacing of ties shall not exceed 16

longitudinal bar diameters, 48 tie bar or wire diameters,

or least dimension of the compression member.

7.10.5.3 — Rectilinear ties shall be arranged such

that every corner and alternate longitudinal bar shall

have lateral support provided by the corner of a tie



The 1956 Code required “lateral support equivalent to that

provided by a 90-degree corner of a tie,” for every vertical

bar. Tie requirements were liberalized in 1963 by increasing

the permissible included angle from 90 to 135 degrees and

exempting bars that are located within 150 mm clear on each

side along the tie from adequately tied bars (see Fig. R7.10.5).

Limited tests7.15 on full-size, axially-loaded, tied columns

containing full-length bars (without splices) showed no

appreciable difference between ultimate strengths of columns

with full tie requirements and no ties at all.



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with an included angle of not more than 135 degrees

and no bar shall be farther than 150 mm clear on each

side along the tie from such a laterally supported bar.



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Fig. R7.10.5—Sketch to clarify measurements between

laterally supported column bars.

Since spliced bars and bundled bars were not included in the

tests of Reference 7.15, it is prudent to provide a set of ties

at each end of lap spliced bars, above and below endbearing splices, and at minimum spacings immediately

below sloping regions of offset bent bars.

Standard tie hooks are intended for use with deformed bars

only, and should be staggered where possible. See also 7.9.

Continuously wound bars or wires can be used 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. A

circular continuously wound bar or wire is considered a spiral

if it conforms to 7.10.4, otherwise it is considered a tie.

7.10.5.4 — Where longitudinal bars are located

around the perimeter of a circle, a complete circular tie

shall be permitted. The ends of the circular tie shall

overlap by not less than 150 mm and terminate with

standard hooks that engage a longitudinal column bar.

Overlaps at ends of adjacent circular ties shall be

staggered around the perimeter enclosing the longitudinal bars.



R7.10.5.4 — 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. 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.



7.10.5.5 — Ties shall be located vertically not more

than one-half a tie spacing above the top of footing or

slab in any story, and shall be spaced as provided

herein to not more than one-half a tie spacing below

the lowest horizontal reinforcement in slab, drop panel,

or shear cap above.

7.10.5.6 — Where beams or brackets frame from

four directions into a column, termination of ties not more

than 75 mm below lowest reinforcement in shallowest of

such beams or brackets shall be permitted.



R7.10.5.6 — With the 1983 Code, the wording of this

section was modified to clarify that ties may be terminated

only when elements frame into all four sides of square and

rectangular columns; for round or polygonal columns, such

elements frame into the column from four directions.



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7.10.5.7 — Where anchor bolts are placed in the top

of columns or pedestals, the bolts shall be enclosed by

transverse reinforcement that also surrounds at least

four vertical bars of the column or pedestal. The

transverse reinforcement shall be distributed within

125 mm of the top of the column or pedestal, and shall

consist of at least two No. 13 or three No. 10 bars.



R7.10.5.7 — Provisions for confinement of anchor bolts

that are placed in the top of columns or pedestals were

added in the 2002 Code. Confinement improves load

transfer from the anchor bolts to the column or pier for

situations where the concrete cracks in the vicinity of the bolts.

Such cracking can occur due to unanticipated forces caused by

temperature, restrained shrinkage, and similar effects.



7.11 — Transverse reinforcement for

flexural members



R7.11 — Transverse reinforcement for flexural

members



7.11.1 — Compression reinforcement in beams shall

be enclosed by ties or stirrups satisfying the size and

spacing limitations in 7.10.5 or by welded wire reinforcement of equivalent area. Such ties or stirrups shall be

provided throughout the distance where compression

reinforcement is required.



R7.11.1 — Compression reinforcement in beams and

girders should be enclosed to prevent buckling; similar

requirements for such enclosure have remained essentially

unchanged through several editions of the Code, except for

minor clarification.



7.11.2 — Transverse reinforcement for flexural framing

members subject to stress reversals or to torsion at

supports shall consist of closed ties, closed stirrups, or

spirals extending around the flexural reinforcement.

7.11.3 — Closed ties or stirrups shall be formed in one

piece by overlapping standard stirrup or tie end hooks

around a longitudinal bar, or formed in one or two

pieces lap spliced with a Class B splice (lap of 1.3ld)

or anchored in accordance with 12.13.



7.12 — Shrinkage and temperature

reinforcement



R7.12 — Shrinkage and temperature

reinforcement



7.12.1 — Reinforcement for shrinkage and temperature

stresses normal to flexural reinforcement shall be

provided in structural slabs where the flexural reinforcement extends in one direction only.



R7.12.1 — Shrinkage and temperature reinforcement is

required at right angles to the principal reinforcement to

minimize cracking and to tie the structure together to ensure

it is acting as assumed in the design. The provisions of this

section are intended for structural slabs only; they are not

intended for slabs-on-ground.



7.12.1.1 — Shrinkage and temperature reinforcement

shall be provided in accordance with either 7.12.2 or

7.12.3.

7.12.1.2 — Where shrinkage and temperature

movements are significantly restrained, the requirements of 8.2.4 and 9.2.3 shall be considered.



R7.12.1.2 — The area of shrinkage and temperature

reinforcement required by 7.12.2.1 has been satisfactory where

shrinkage and temperature movements are permitted to

occur. Where structural walls or columns provide significant

restraint to shrinkage and temperature movements, the

restrain of volume changes causes tension in slabs, as well as

displacements, shear forces, and flexural moments in columns

or walls. In these cases, it may be necessary to increase the

amount of slab reinforcement required by 7.12.2.1 due to the

shrinkage and thermal effects in both principal directions (see

References 7.7 and 7.16). Topping slabs also experience

tension due to restraint of differential shrinkage between the

topping and the precast elements or metal deck (which has

zero shrinkage) that should be considered in reinforcing the



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slab. Consideration should be given to strain demands on

reinforcement crossing joints of precast elements where most

of the restraint is likely to be relieved. Top and bottom reinforcement are both effective in controlling cracks. Control

strips during the construction period, which permit initial

shrinkage to occur without causing an increase in stresses, are

also effective in reducing cracks caused by restraint.



7



7.12.2 — Deformed reinforcement conforming to 3.5.3

used for shrinkage and temperature reinforcement

shall be provided in accordance with the following:

7.12.2.1 — Area of shrinkage and temperature

reinforcement shall provide at least the following ratios

of reinforcement area to gross concrete area, but not

less than 0.0014:



R7.12.2 — The amounts specified for deformed bars and

welded wire reinforcement are empirical but have been used

satisfactorily for many years. The area of reinforcement

given by 7.12.2.1 may be distributed near the top or bottom

of the slab, or may be allocated between the two faces of the

slab as deemed appropriate for specific conditions. Splices

and end anchorages of shrinkage and temperature reinforcement are to be designed for the full specified yield strength

in accordance with 12.1, 12.15, 12.18, and 12.19.



(a) Slabs where Grade 280 or 350

deformed bars are used .................................0.0020

(b) Slabs where Grade 420

deformed bars or welded wire

reinforcement are used...................................0.0018

(c) Slabs where reinforcement

with yield stress exceeding 420 MPa

measured at a yield strain of

0.0018 × 420

0.35 percent is used ........................... ---------------------------------fy

7.12.2.2 — Shrinkage and temperature reinforcement

shall be spaced not farther apart than five times the

slab thickness, nor farther apart than 450 mm.

7.12.2.3 — At all sections where required, reinforcement to resist shrinkage and temperature stresses shall

develop fy in tension in accordance with Chapter 12.

7.12.3 — Prestressing steel conforming to 3.5.6 used

for shrinkage and temperature reinforcement shall be

provided in accordance with the following:

7.12.3.1 — Tendons shall be proportioned to provide

a minimum average compressive stress of 0.7 MPa on

gross concrete area using effective prestress, after

losses, in accordance with 18.6.

7.12.3.2 — For monolithic cast-in-place posttensioned beam-and-slab construction, gross concrete

area of a beam and tributary slab shall consist of the

total beam area including the slab thickness and the

slab within half the clear distance to adjacent beam

webs. It shall be permitted to include the effective force

in beam tendons in the calculation of total prestress

force acting on gross concrete area.



R7.12.3 — Prestressed reinforcement requirements have

been selected to provide an effective force on the slab

approximately equal to the yield strength force for

nonprestressed shrinkage and temperature reinforcement. This

amount of prestressing, 0.7 MPa on the gross concrete area,

has been successfully used on a large number of projects. In

monolithic beam-and-slab construction, a minimum of one

shrinkage and temperature tendon is required between

beams, even if the beam tendons alone provide at least 0.7 MPa

average compression stress on the gross concrete area as

defined in 7.12.3.2. Any size tendon is permissible as long as

all other requirements of 7.12.3 are satisfied. Application of the

provisions of 7.12.3.2 to monolithic cast-in-place posttensioned beam-and-slab construction is illustrated in

Fig. R7.12.3(a).

Where the spacing of slab tendons used for shrinkage and

temperature reinforcement exceeds 1.4 m, additional nonprestressed reinforcement is required to extend from the slab



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