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5 — Development of standard hooks in tension

5 — Development of standard hooks in tension

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for 90-degree hook with cover on bar extension

beyond hook not less than 50 mm....................... 0.7

(b) For 90-degree hooks of No. 36 and smaller bars

that are either enclosed within ties or stirrups

perpendicular to the bar being developed, spaced

not greater than 3db along ldh ; or enclosed within

ties or stirrups parallel to the bar being developed,

spaced not greater than 3db along the length of the

tail extension of the hook plus bend .................... 0.8

(c) For 180-degree hooks of No. 36 and smaller bars

that are enclosed within ties or stirrups perpendicular

to the bar being developed, spaced not greater than

3db along ldh ...................................................... 0.8



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(d) Where anchorage or development for fy is not

specifically required, reinforcement in excess of that

required by analysis ....... (As required)/(As provided)



Fig. R12.5—Hooked bar details for development of standard

hooks.



In 12.5.3(b) and 12.5.3(c), db is the diameter of the

hooked bar, and the first tie or stirrup shall enclose the

bent portion of the hook, within 2db of the outside of

the bend.



The development length for standard hooks ldh of 12.5.2 can

be reduced by all applicable modification factors of 12.5.3.

As an example, if the conditions of both 12.5.3(a) and (c)

are met, both factors may be applied.

The effects of bar yield strength, excess reinforcement,

lightweight concrete, and factors to reflect the resistance to

splitting provided from confinement by concrete and transverse ties or stirrups are based on recommendations from

References 12.2 and 12.3.

Tests12.13 indicate that closely spaced ties at or near the bend

portion of a hooked bar are most effective in confining the

hooked bar. For construction purposes, this is not always

practicable. The cases where the modification factor of

12.5.3(b) may be used are illustrated in Fig. R12.5.3(a) and

(b). Figure R12.5.3(a) shows placement of ties or stirrups

perpendicular to the bar being developed, spaced along the

development length, ldh, of the hook. Figure R12.5.3(b)

shows placement of ties or stirrups parallel to the bar being

developed along the length of the tail extension of the hook

plus bend. The latter configuration would be typical in a

beam column joint.

The factor for excess reinforcement in 12.5.3(d) applies

only where anchorage or development for full fy is not

specifically required. The λ factor for lightweight concrete

is a simplification over the procedure in 12.2.3.3 of ACI

318-83 in which the increase varies from 18 to 33 percent,

depending on the amount of lightweight aggregate used.

Unlike straight bar development, no distinction is made

between top bars and other bars; such a distinction is difficult for hooked bars in any case. A minimum value of ldh is

specified to prevent failure by direct pullout in cases where

a hook may be located very near the critical section. Hooks

cannot be considered effective in compression.



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Fig. R12.5.3(a)—Ties or stirrups placed perpendicular to

the bar being developed, spaced along the development

length ldh.



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Fig. R12.5.3(b)—Ties or stirrups placed parallel to the bar

being developed, spaced along the length of the tail extension

of the hook plus bend.



Fig. R12.5.4—Concrete cover per 12.5.4.

Tests12.14 indicate that the development length for hooked

bars should be increased by 20 percent to account for

reduced bond when reinforcement is epoxy coated.

12.5.4 — For bars being developed by a standard

hook at discontinuous ends of members with both side

cover and top (or bottom) cover over hook less than

65 mm, the hooked bar shall be enclosed within ties or

stirrups perpendicular to the bar being developed,

spaced not greater than 3db along ldh . The first tie or

stirrup shall enclose the bent portion of the hook,

within 2db of the outside of the bend, where db is the

diameter of the hooked bar. For this case, the factors

of 12.5.3(b) and (c) shall not apply.



R12.5.4 — Bar hooks are especially susceptible to a

concrete splitting failure if both side cover (normal to plane

of hook) and top or bottom cover (in plane of hook) are

small. See Fig. R12.5.4. With minimum confinement

provided by concrete, additional confinement provided by

ties or stirrups is essential, especially if full bar strength

should be developed by a hooked bar with such small cover.

Cases where hooks may require ties or stirrups for confinement

are at ends of simply supported beams, at free end of

cantilevers, and at ends of members framing into a joint



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where members do not extend beyond the joint. In contrast, if

calculated bar stress is so low that the hook is not needed for

bar anchorage, the ties or stirrups are not necessary. Also,

provisions of 12.5.4 do not apply for hooked bars at discontinuous ends of slabs with confinement provided by the slab

continuous on both sides normal to the plane of the hook.



12.5.5 — Hooks shall not be considered effective in

developing bars in compression.



R12.5.5 — In compression, hooks are ineffective and may

not be used as anchorage.



12.6 — Development of headed and

mechanically anchored deformed

bars in tension



R12.6 — Development of headed and

mechanically anchored deformed

bars in tension



12.6.1 — Development length for headed deformed

bars in tension, ldt , shall be determined from 12.6.2.

Use of heads to develop deformed bars in tension

shall be limited to conditions satisfying (a) through (f):



The development of headed deformed bars and the development and anchorage of reinforcement through the use of

mechanical devices within concrete are addressed in 12.6.

As used in 12.6, development describes cases in which the

force in the bar is transferred to the concrete through a

combination of a bearing force at the head and bond forces

along the bar, such cases are covered in 12.6.1 and 12.6.2. In

contrast, anchorage describes cases in which the force in the

bar is transferred through bearing to the concrete at the head

alone. General provisions for anchorage are given in

Appendix D. The limitation on obstructions and interruptions of the deformations is included in 3.5.9 because there

is a wide variety of methods to attach heads to bars, some of

which involve obstructions or interruptions of the deformations

that extend more than 2db from the bearing face of the head.

These systems were not evaluated in the tests used to formulate

the provisions in 12.6.2, which were limited to systems that

meet the criteria in 3.5.9.



(a) Bar fy shall not exceed 420 MPa;



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(b) Bar size shall not exceed No. 36;

(c) Concrete shall be normalweight;

(d) Net bearing area of head Abrg shall not be less

than 4Ab;

(e) Clear cover for bar shall not be less than 2db ;

and

(f) Clear spacing between bars shall not be less

than 4db .

12.6.2 — For headed deformed bars satisfying 3.5.9,

development length in tension ldt shall be (0.19ψefy /

f c′ )db , where the value of fc′ used to calculate ldt

shall not exceed 40 MPa, and factor ψe shall be taken

as 1.2 for epoxy-coated reinforcement and 1.0 for

other cases. Where reinforcement provided is in

excess of that required by analysis, except where

development of fy is specifically required, a factor of

(As required)/(As provided) may be applied to the

expression for ldt . Length ldt shall not be less than the

larger of 8db and 150 mm.



The provisions for headed deformed bars were written with

due consideration of the provisions for anchorage in

Appendix D and the bearing strength provisions of

10.14.12.15,12.16 Appendix D contains provisions for headed

anchors related to the individual failure modes of concrete

breakout, side-face blowout, and pullout, all of which were

considered in the formulation of 12.6.2. The restrictions on

normalweight concrete, maximum bar size of No. 36, and

upper limit of 420 MPa for fy are based on the available data

from tests.12.15-12.17

The provisions for developing headed deformed bars give

the length of bar ldt measured from the critical section to the

bearing face of the head, as shown in Fig. R12.6(a).

For bars in tension, heads allow the bars to be developed in

a shorter length than required for standard hooks.12.15-12.17

The minimum limits on clear cover, clear spacing, and head

size are based on the lower limits of these parameters used

in the tests to establish the expression for ldt in 12.6.2. The

clear cover and clear spacing requirements in 12.6.1 are

based on dimensions measured to the bar, not to the head.



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Fig. R12.6(a)—Development of headed deformed bars.



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Fig. R12.6(b)—Headed deformed bar extended to far side of

column core with anchorage length that exceeds ldt.

The head is considered to be part of the bar for the purposes

of satisfying the specified cover requirements in 7.7, and

aggregate size requirements of 3.3.2(c). To avoid congestion, it

may be desirable to stagger the heads. Headed bars with

Abrg < 4Ab have been used in practice, but their performance is

not accurately represented by the provisions in 12.6.2, and

they should be used only with designs that are supported by

test results under 12.6.4. These provisions do not address

the design of studs or headed stud assemblies used for shear

reinforcement.

A 1.2 factor is conservatively used for epoxy-coated headed

deformed reinforcing bars, the same value used for epoxycoated standard hooks. The upper limit on the value of fc′ in

12.6.2 for use in calculating ldt is based on the concrete

strengths used in the tests.12.15-12.17 Because transverse

reinforcement has been shown to be largely ineffective in



ACI 318 Building Code and Commentary



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