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8/10/2019 425-Chp7-Bond Development Length Splices
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Chapter-7
Bond Development Length
& Splices
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Lecture Goals
Slab design reinforcement
Bar Development
Hook development
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F lexural Reinforcement in Slabs
For a 1 ft strip of slab is designed like a beam
As(reqd)is in units of (in2
/ft)
inchesinspacingbar
in12ft/ bs AA
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The table is A-9 from
MacGregors book.
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F lexural Reinforcement in Slabs
The minimum spacing of the bars is given as:
Also, check crack control - important for exterior
exposure (large cover dimensions) - ACI Sec. 10.6.4
7.6.5Sec.ACI
in.18
thicknessslab3tofsmaller
max
S
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F lexural Reinforcement in Slabs
Thin slabs shrink more rapidly than deeper beams.
Temperature & shrinkage (T&S) steel is provided
perpendicular to restrain cracks parallel to span.
(Flexural steel restrains cracks perpendicular tospan)
Maximum & Minimum reinforcement requirements
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F lexural Reinforcement in Slabs
Maximum & Minimum reinforcement requirements
T&S Reinforcement (perpendicular to span) ACI Sec 7.12
t
ftf
ft
ftA
*"12*0.0014
ksi60*"12*60*0018.0
ksi60*"12*0018.0
ksi50or40*"12*0020.0
y
y
y
ymins
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F lexural Reinforcement in Slabs
T&S Reinforcement (perpendicular to span) ACI Sec 7.12
Flexural Reinforcement (parallel to span) ACI Sec 10.54
Smaxfrom reinforced spacing
18"
5t
ofsmallermaxS
balsmaxs
&minsmins
75.0 AA
AA ST
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Reinforcement Development Lengths, Bar
Cutoffs, and Continuity RequirementsA. Concept of Bond Stress and Rebar Anchorage
Internal Forces in a beam
Forces in RebarBond stresses provide mechanism
of force transfer between concrete
and reinforcement.
Forces developed in the beam
by loading.
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Reinforcement Development Lengths, Bar
Cutoffs, and Continuity RequirementsEquilibrium Condition for Rebar
m= bond stress(coefficient of
friction)
Note: Bond stress is zero at cracks
m
m
4.
04
.
0ForceBond.0F
byd
bby
2
b
dfl
ldfd
T
bar
c
fk
fk
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Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements
Sources of Bond Transfer
(1) Adhesion between concrete & reinforcement.
(2) Friction
Note:These properties are quickly lost for tension.
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Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements
Sources of Bond Transfer
(3)Mechanical Interlock.
The edge stress concentration
causes cracking to occur.
Force interaction between the
steel and concrete.
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Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements
Splitting cracks result in loss of bond transfer.
Reinforcement can be used to restrain these cracks.
Splitting Load is Affected by:
Minimum edge distance and spacing of bars
(smaller distance= smaller load)Tensile strength of concrete.
Average bond stress along bar.(Increase in bond
stress larger wedging forces)
1.
2.
3.
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Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements
Typical Splitting Failure
Surfaces.
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Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements
General splitting of
concrete along the
bars,either in vertical
planes as in figure (a) or
in horizontal plane as infigure (b). Such splitting
comes largely from
wedging action when the
ribs of the deformed barbear against the concrete.
The horizontal type of splitting frequently begins at a diagonal crack.
The dowel action increases the tendency toward splitting. This
indicates that shear and bond failure are often intricately interrelated.
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Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements
ACI Code expression for development length for
bars in tension/in compression.
B.
Development Length, ld
Shortest length of bar in which the
bar stress can increase from zero tothe yield strength, fy.
( ldused since bond stresses, m,
vary along a bar in a tension zone)
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Development Length for Bars in Tension
Development length, ld 12 ACI 12.2.1
fc 10000 psi for Ch. 12 provisions for development length in ACI Codes.
Development length, ld(simplified expression from ACI
12.2.2)
Clear spacing of bars being developed or
spliced not less than db, clear cover not less
than db, and stirrups or ties throughout ld not
less than the code minimum
or
Clear spacing of bars being
developed or spliced not less than 2dband
clear cover not less than db.
Other cases
No. 6 and smaller No. 7 and larger
bars and deformed barswires
c
y
b
d
25 f
f
d
l
c
y
b
d
20 f
f
d
l
c
y
b
d
50
3
f
f
d
l
c
y
b
d
40
3
f
f
d
l
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Development Length for Bars in Tension
Development length, ld ACI 12.2.3
2.5 limit to safeguard against pullout type failure.
5.2in which403
b
ct
b
ctc
y
b
d
dKc
d
Kcff
dl
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Factors used in expressions for
Development Length (ACI 12.2.4) reinforcement location factor
Horizontal reinforcement so placed that more than 12 in of fresh concrete
is cast in the member below the development length or splice
Other reinforcement
coating factor (epoxy prevents adhesion &
friction between bar and concrete.)Epoxy-coated bars or wires with cover less than 3dbor clear spacing less
than 6db
All other epoxy-coated bars or wires
Uncoated reinforcement
1.3
1.0
1.5
1.2
1.0
where < 1.7
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Factors used in expressions for
Development Length (ACI 12.2.4)
c = spacing or cover dimension, in.
Use the smaller of either
(a) the distance from the center of the bar or wire to
the nearest concrete surface.
or(b) one-half the center-to-center spacing of the bar or
wires being developed.
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Factors used in expressions for
Development Length (ACI 12.2.4)
Kct= transverse reinforcement index (Represents the contribution
of confining reinforcement across potential splitting planes.)
Total cross-section area of all transverse reinforcement within the spacing s,
which crosses the potential plane of splitting along the reinforcement being
developed with in the development length, in2.
Specified yield strength of transverse reinforcement, psi.
maximum center-to-center spacing of transverse reinforcement within ldin.
number of bars or wires being developed along the plane of splitting.
Atr=
fyt
=
s =
n =
Note: It is permitted to use Kct=0 as a design simplification
even if transverse reinforcement is present.
ns
fAK
**1500
y ttr
tr
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Excess F lexural Reinforcement
Reduction (ACI 12.2.5)
Reduction = (Asreqd ) / (Asprovided )
- Except as required for seismic design (see ACI 21.2.14)
- Good practice to ignore this provision, since use of
structure may change over time.
- final ld 12 in.
providedn
u
providedn
dreq'nReduction
M
M
M
M
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Development Length for Bars in
Compression (ACI 12.3)
Compression development length ldc= ldbc* applicable
reduction factors 8 in.
Basic Development Length for Compression, ldbc
yb
c
yb
dbc
0003.0
0.02oflarger
fd
ffd
l
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Development Length for Bars in
Compression (ACI 12.3)Reduction Factors (ACI 12.3.3)
- Excessive Reinforcement Factor = (Asreqd)/(Asprovided)
- Spiral and Ties
If reinforcement is enclosed with spiralreinforcement 0.25 in. diameter and 4 in. pitch or
within No. 4 ties according to 7.10.5 and spaced 4 in.
on center. Factor = 0.75
Noteldc< 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=of hooked bar
Note: Multiplier for ties or
stirrups (ACI 12.5.3.3)
is not applicable for
this case.
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Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements
C. Use of Standard Hooks for Tension Anchorage
Hooks provide additional anchorage whenthere is insufficient length available to
develop a bar.
Note: Hooks are not allowed to developed
compression reinforcement.
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Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements
C. Use of Standard Hooks for Tension Anchorage
Standard Hooks aredefined in ACI 7.1.
Hooks resists tension bybond 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.
.in6and8 wheresmultiplier* dbbdbhddh ldlll
Basic Development Length for Hooked Bar = lhb
when fy= 60,000 psi
c
bhd
1200
f
dl
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Design of Standard Hooks for
Tension Anchorage (ACI 12.5)
Conditions
Bar Yield StrengthBars with fyother than 60,000 psi
Concrete Cover for 180 Degree Hooks
For No. 11 bars and smaller.
Side cover (normal to plane of hook) 2.5 in.
Concrete Cover for 90 Degree Hooks
For No. 11 bars and smaller.
Side cover (normal to plane of hook) 2.5 in.
Cover on bar extension beyond hook tail 2 in.
Multiplier
fy/60,000
0.7
0.7
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Design of Standard Hooks for
Tension Anchorage (ACI 12.5)
Conditions
Excessive ReinforcementWhere anchorage or development for fy is not
specified required.
Lightweight Aggregate Concrete
Ties or StirrupsFor No. 11 bar and smaller.
Hook enclosed vertically or horizontally within ties
or stirrup-ties spaced along full ldhno farther apart
than 3db, where dbis diameter of hooked bar.
Multiplier
As(reqd) /As(provided)
1.3
0.8
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Design of Standard Hooks for
Tension Anchorage (ACI 12.5)
Conditions
Epoxy-coated ReinforcementHooked bars with epoxy coating
Multiplier
1.2
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ExampleExample 4
GIVEN: A #5 Grade 40 bar is in tension as shown below. Use LIGHTWEIGHT
concrete with fc = 4000 PSI.
REQUIRED: Determine the min. required hook dimensions X, Y and Z
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