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13 — Development of web reinforcement

13 — Development of web reinforcement

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CHAPTER 12



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COMMENTARY



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Fig. R12.12—Development of negative moment reinforcement.

12.13.2.1 — For No. 16 bar and MD200 wire, and

smaller, and for No. 19, No. 22, and No. 25 bars with

fyt of 280 MPa or less, a standard hook around longitudinal reinforcement.



R12.13.2.1 — For a No. 16 bar or smaller, anchorage is

provided by a standard stirrup hook, as defined in 7.1.3,

hooked around a longitudinal bar. The 1989 Code eliminated

the need for a calculated straight embedment length in addition

to the hook for these small bars, but 12.13.1 requires a fulldepth stirrup. Likewise, larger stirrups with fyt equal to or

less than 280 MPa are sufficiently anchored with a standard

stirrup hook around the longitudinal reinforcement.



12.13.2.2 — For No. 19, No. 22, and No. 25 stirrups

with fyt greater than 280 MPa, a standard stirrup 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 0.17dbfyt /(λ f c′ ).



R12.13.2.2 — Since 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 value of the

hook and whatever development length is provided. A



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longitudinal bar within a stirrup hook limits the width of any

flexural cracks, even in a tensile zone. Since such a stirrup hook

cannot fail by splitting parallel to the plane of the hooked

bar, the hook strength as utilized in 12.5.2 has been adjusted

to reflect cover and confinement around the stirrup hook.

For stirrups with fyt of only 280 MPa, a standard stirrup

hook provides sufficient anchorage and these bars are

covered in 12.13.2.1. For bars 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 7.1.3.



12.13.2.3 — For each leg of welded plain wire

reinforcement forming simple U-stirrups, either:



R12.13.2.3 — The requirements for anchorage of

welded plain wire reinforcement stirrups are illustrated

in Fig. R12.13.2.3.



(a) Two longitudinal wires spaced at a 50 mm

spacing along the member at the top of the U; or



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(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

not less than 8db .

12.13.2.4 — For each end of a single leg stirrup of

welded wire reinforcement, two longitudinal wires at a

minimum spacing of 50 mm and with the inner wire at

least the greater of d/4 or 50 mm from d/2. Outer

longitudinal wire at tension face shall not be farther

from the face than the portion of primary flexural reinforcement closest to the face.



R12.13.2.4 — Use of welded wire reinforcement for

shear reinforcement has become commonplace 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 a joint

PCI/WRI Ad Hoc Committee on Welded Wire Fabric for

Shear Reinforcement.12.29



Fig. R12.13.2.3—Anchorage in compression zone of welded

plain wire reinforcement U-stirrups.

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Fig. R12.13.2.4—Anchorage of single leg welded wire

reinforcement shear reinforcement.

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 tension

steel level. Figure R12.13.2.4 illustrates the anchorage

requirements for single leg, welded wire reinforcement. For

anchorage of single leg, welded wire reinforcement, the

Code has permitted hooks and embedment length in the

compression and tension faces of members (see 12.13.2.1

and 12.13.2.3), and embedment only in the compression

face (see 12.13.2.2). Section 12.13.2.4 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.

12.13.2.5 — In joist construction as defined in 8.11,

for No. 13 bar and MD130 wire and smaller, a standard

hook.



R12.13.2.5 — 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 in the joist.



12.13.3 — Between anchored ends, each bend in the

continuous portion of a simple U-stirrup or multiple Ustirrup shall enclose a longitudinal bar.

12.13.4 — Longitudinal bars bent to act as shear

reinforcement, if extended into a region of tension,

shall be continuous with longitudinal reinforcement

and, if extended into a region of compression, shall be

anchored beyond mid-depth d /2 as specified for

development length in 12.2 for that part of fyt required

to satisfy Eq. (11-17).

12.13.5 — Pairs of U-stirrups or ties so placed as to

form a closed unit shall be considered properly spliced

when length of laps are 1.3ld. In members at least

450 mm deep, such splices with Abfyt not more than

40 kN per leg shall be considered adequate if stirrup

legs extend the full available depth of member.



R12.13.5 — These requirements for lapping of double

U-stirrups to form closed stirrups control over the provisions

of 12.15.



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12.14 — Splices of reinforcement — General



R12.14 — Splices of reinforcement — General



12.14.1 — Splices of reinforcement shall be made only

as required or permitted on design drawings, or in

specifications, or as authorized by the licensed design

professional.



Splices should, if possible, be located away from points of

maximum tensile stress. The lap splice requirements of

12.15 encourage this practice.



12.14.2 — Lap splices



R12.14.2 — Lap splices



12.14.2.1 — Lap splices shall not be used for bars

larger than No. 36 except as provided in 12.16.2 and

15.8.2.3.



R12.14.2.1 — Because of lack of adequate experimental

data on lap splices of No. 43 and No. 57 bars in compression

and in tension, lap splicing of these bar sizes is prohibited

except as permitted in 12.16.2 and 15.8.2.3 for compression

lap splices of No. 43 and No. 57 bars with smaller bars.



12.14.2.2 — Lap splices of bars in a bundle shall be

based on the lap splice length required for individual

bars within the bundle, increased in accordance with

12.4. Individual bar splices within a bundle shall not

overlap. Entire bundles shall not be lap spliced.



R12.14.2.2 — The increased length of lap required for

bars in bundles is based on the reduction in the exposed

perimeter of the bars. Only individual bars are lap spliced

along the bundle.



12.14.2.3 — Bars spliced by noncontact lap splices

in flexural members shall not be spaced transversely

farther apart than the smaller of one-fifth the required

lap splice length, and 150 mm.



R12.14.2.3 — If individual bars in noncontact lap splices

are too widely spaced, an unreinforced section is created.

Forcing a potential crack to follow a zigzag line (5-to-1

slope) is considered a minimum precaution. The 150 mm

maximum spacing is added because most research available

on the lap splicing of deformed bars was conducted with

reinforcement within this spacing.



12.14.3 — Mechanical and welded splices



R12.14.3 — Mechanical and welded splices



12.14.3.1 — Mechanical and welded splices shall be

permitted.

12.14.3.2 — A full mechanical splice shall develop in

tension or compression, as required, at least 1.25fy of

the bar.



12.14.3.3 — Except as provided in this Code, all

welding shall conform to “Structural Welding Code—

Reinforcing Steel” (AWS D1.4).

12.14.3.4 — A full welded splice shall develop at

least 1.25fy of the bar.



R12.14.3.2 — The maximum reinforcement stress used in

design under the Code is the specified yield strength. To

ensure sufficient strength in splices so that yielding can be

achieved in a member and thus brittle failure avoided, the

25 percent increase above the specified yield strength was

selected as both an adequate minimum for safety and a

practicable maximum for economy.

R12.14.3.3 — See R3.5.2 for discussion on welding.



R12.14.3.4 — A full welded splice is primarily intended

for large bars (No. 19 and larger) in main members. The

tensile strength requirement of 125 percent of specified

yield strength is intended to provide sound welding that is

also adequate for compression. See the discussion on strength

in R12.14.3.2. The 1995 Code eliminated a requirement that

the bars be butted since indirect butt welds are permitted by

AWS D1.4, although AWS D1.4 does indicate that wherever

practical, direct butt splices are preferable for No. 22 bars

and larger.



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12.14.3.5 — Mechanical or welded splices not

meeting requirements of 12.14.3.2 or 12.14.3.4 shall

be permitted only for No. 16 bars and smaller and in

accordance with 12.15.5.



R12.14.3.5 — The use of mechanical or welded splices of

less strength than 125 percent of specified yield strength is

permitted if the minimum design criteria of 12.15.5 are met.

Therefore, lap welds of reinforcing bars, either with or

without backup material, welds to plate connections, and

end-bearing splices are allowed under certain conditions.

The 1995 Code limited these lower strength welds and

connections to No. 16 bars and smaller due to the potentially brittle nature of failure at these welds.



12.15 — Splices of deformed bars and

deformed wire in tension



R12.15 — Splices of deformed bars and

deformed wire in tension



12.15.1 — Minimum length of lap for tension lap

splices shall be as required for Class A or B splice, but

not less than 300 mm, where:



R12.15.1 — Lap splices in tension are classified as Type A

or B, with length of lap a multiple of the tensile development

length ld calculated in accordance with 12.2.2 or 12.2.3. The

development length ld used to obtain lap length should be

based on fy because the splice classifications already reflect

any excess reinforcement at the splice location; therefore,

the factor from 12.2.5 for excess As should not be used.

When multiple bars located in the same plane are spliced at

the same section, the clear spacing is the minimum clear

distance between the adjacent splices. For splices in

columns with offset bars, Fig. R12.15.1(a) illustrates the

clear spacing to be used. For staggered splices, the clear

spacing is taken as the minimum distance between adjacent

splices [Fig. R12.15.1(b)].



Class A splice................................................... 1.0ld

Class B splice................................................... 1.3ld

where ld is calculated in accordance with 12.2 to

develop fy, but without the 300 mm minimum of 12.2.1

and without the modification factor of 12.2.5.



Fig. R12.15.1—Clear spacing of spliced bars.



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