Lecture 18 Bar Development

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Lecture 18- Bar Development

• February 26, 2003
• CVEN 444

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Lecture Goals

• Bar Development
• Hook development

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Example - Development
For the cross section of a simply supported beam
reinforced with 4 8 bars that are confined with
3 stirrup spaced at 6 in. Determine the
development length of the bars if the beam is
made of normal weight concrete fc 3 ksi and fy
60 ksi
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Example - Development
Check if conditions for spacing and concrete
cover are met
For 8 bars, db 1.0 in. Clear cover 2.5 in -
0.5 in. 2.0 in. gt db
Clear spacing between bars
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Example - Development
Bars are confined with 3 stirrups. The
conditions are met. Determine the factorsa
1.0 (bottom bars), b 1.0 (no coating) and l
1.0 (normal weight concrete) and 54.8
psi lt 100 psi
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Example - Development
So ld 54.8(1.0 in.) 54.8 in. 55 in.
Using the more general formula Ktr 0.0
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Example - Development
c 1.17 in. controls
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Example - Development
So ld 55 in.
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Example - Development
If the same beam is made of light weight
aggregate concrete and the bars are epoxy coated
and As required for analysis is 2.79 in2
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Example - Development
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Example - Development
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Critical Sections in Flexural Members
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Critical Sections in Negative Moment Reinforcement
Three sections are critical for the negative
moment reinforcement Section 1 is at the face
of the support, where the negative moment as well
as stress are at maximum value. Two development
lengths, x1 and x2 must be checked.
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Critical Sections in Negative Moment Reinforcement
Section 2 is the section where part of the
negative reinforcement bar can be terminated. To
develop full tensile force, the bars should
extend a distance x2 before they can be
terminated Once part of the bars are terminated
the remaining bars develop maximum stress.
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Critical Sections in Negative Moment Reinforcement
Section 3 is a point of inflection The bars shall
extend a distance x3 beyond section 3 x3 must
be equal to or greater than the effective depth
d, 12db or 1/16 the span, which ever is greater.
At least 1/3 of the total reinforcement provided
for negative moment at support shall extend a
distance x3 beyond the point of inflection.
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Critical Sections in Positive Moment Reinforcement
Section 4 is that of maximum positive moment and
maximum stresses. Two development lengths x1 and
x2 have to be checked. The length x1 is the
development length ld specified by the ACI Code
Section 12.11. The length x2 is equal to or
greater than the effective depth d, 12db .
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Critical Sections in Positive Moment Reinforcement
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Critical Sections in Positive Moment Reinforcement
Section 6 is at the points of inflection limits
are according to section 12.11.3 of the ACI Code.
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Example Problem Bar Details
A continuous beam has the bar details shown. The
bending moments for maximum positive and negative
moments are given. We must check the development
lengths at all critical
sections. fc 3 ksi fy 60 ksi , b 12
in. d 18 in. and the span L 24 ft.
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Example Problem Bar Details
The critical sections are at the face of the
support for tension and compression
reinforcement, at points where tension bars are
terminated within the span and at point of
inflection and at midspan.
Development length for negative moment x1 4.5
ft from face of support where as 3 bars extend to
a distance of 6 ft.
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Example Problem Bar Details
The development length is (d 1.128 in.)
(if
spacing and cover
conditions are met)
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Example Problem Bar Details
The bars are at the top so a 1.3 and the
development length is
So ld 54 in. 4.5 ft gt 12 in. (minimum)
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Example Problem Bar Details
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Example Problem Bar Details
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Example Problem Bar Details
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Example Problem Bar Details
It can not be less than 8 in. The length 15 in.
controls. For 8 bars db 1 in. ld provided is
15 in. which is greater than that required.
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Example Problem Bar Details
The development length for positive moment
reinforcement 3 8 bars extend 6 ft beyond the
centerline, and the other bars extend to the
support. The development length x6 from center
line is
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Example Problem Bar Details
The ld is 37 in. But it can not be less than 12
in.. So x6 provided is 6 ft 72 in. gt 37 in. The
length x7 is equal to d or 12 db, and 18 in. is
provided.
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Development Length for Bars in Compression (ACI
12.3)
Reduction Factors (ACI 12.3.3)
Note ldc lt ld (typically) because - Beneficial
of end bearing is considered - weakening effect
of flexural tension cracks is not present for
bars in compression.
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Hooked Bar at Discontinuous Ends (ACI 12.5.4)
If side cover and top (or bottom cover) 2.5
in. Enclose hooked bar w/ ties or stirrup-ties
Spacing 3db db f of hooked bar
Note Multiplier for ties or stirrups (ACI
12.5.3) is not applicable for this case.
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Hooked Bar at Discontinuous Ends (ACI 12.5.4)
- Basic Development lengths
Others
Mechanical Anchorage ACI (12.6) Welded Wire
Fabric ACI (12.7) Bundled Bars ACI (12.4)
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Standard Hooks for Tension Anchorage
C. Use of Standard Hooks for Tension Anchorage
Hooks provide additional anchorage when there is
insufficient length available to develop a bar.
Note Hooks are not allowed to developed
compression reinforcement.
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Standard Hooks for Tension Anchorage
Standard Hooks are defined in ACI 7.1.
Hooks resists tension by bond stresses on bar
surface and bearing on on concrete inside the
hook.
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Design of Standard Hooks for Tension Anchorage
(ACI 12.5)
Development Length for Hooked Bar, ldb.
Basic Development Length for Hooked Bar lhd
when fy 60,000 psi
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Design of Standard Hooks for Tension Anchorage
(ACI 12.5)
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Design of Standard Hooks for Tension Anchorage
(ACI 12.5)
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Design of Standard Hooks for Tension Anchorage
(ACI 12.5)
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Design of Standard Hooks for Tension Anchorage
(ACI 12.5)
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Design of Standard Hooks for Tension Anchorage
(ACI 12.5)
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Standard Hooks
A hook is used at the end of a bar when its
straight embedment length is less than the
necessary length, ld. The minimum diameter of
bend, measured on the inside of the main bar of a
standard hook D is
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Standard Hooks
The stress distribution for a 90o hook under a
force P is shown.
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Standard Hooks
The basic development length lhb must be
multiplied one of the following factors.
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Standard Hooks
The basic development length lhb must be
multiplied one of the following factors.
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Standard Hooks
The basic development length lhb must be
multiplied one of the following factors.
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Standard Hooks
The basic development length lhb must be
multiplied one of the following factors.
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Example- Hook
Compute the development length required for the
top 8 bar of the cantilever beam that extend
into the column support if the bars are

• Straight.
• Have a 90o hook
• Have a 180o hook

The bars are confined by 3 stirrups spaced at 6
in., and clear cover 1.5 in., and clear
spacing 2.0 in. and fc 4 ksi and fy 60 ksi
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Example- Hook straight bar
Straight bars For 8 bar db 1.0 in., because
the clear spacing 2db and clear cover is greater
than db with the bars confined condition (a) and
(b) are met. For top bars a 1.3
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Example- 90 oHook
Bar with 90o hook For 8 bar db 1.0 in., the
basic equation is No modification apply, than
ldh 19 in. gt 8db 8 in. or 6 in. The factor a
1.3 does not apply to hooks.
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Example- 180 o Hook
Bar with 180o hook For 8 bar db 1.0 in., the
basic equation is No modification apply, than
ldh 19 in. gt 8db 8 in. or 6 in. The factor a
1.3 does not apply to hooks.
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Example- Hook summary
The basic summary of the results for the two
hooks
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Homework
Problem 10.4 10.5