Bond, Development Lengths and Splices

frizzflowerUrban and Civil

Nov 29, 2013 (3 years and 10 months ago)

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Chapter
-
7

Bond Development Length

& Splices

Lecture Goals


Slab design reinforcement


Bar Development


Hook development



Flexural Reinforcement in Slabs

For a 1 ft strip of slab is designed like a beam
A
s(req’d)

is in units of (in
2
/ft)








inches
in

spacing
bar
in

12
ft
/
b
s
A
A
The table is A
-
9 from
MacGregor’s book.

Flexural 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









7.6.5

Sec.

ACI

in.

18
thickness
slab
3t
of
smaller
max
S
Flexural 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 to
span)

Maximum & Minimum reinforcement requirements

Flexural Reinforcement in Slabs

Maximum & Minimum reinforcement requirements

T&S Reinforcement (perpendicular to span)
ACI Sec 7.12











t
f
t
f
f
t
f
t
A
*
"

12
*
0.0014
ksi

60



*
"
12
*
60
*
0018
.
0
ksi

60




*
"
12
*
0018
.
0
ksi

50
or

40



*
"
12
*
0020
.
0
y
y
y
y
min
s















Flexural Reinforcement in Slabs

T&S Reinforcement (perpendicular to span)

ACI Sec 7.12

Flexural Reinforcement (parallel to span)

ACI Sec 10.54

S
max

from reinforced spacing





18"
5t

of
smaller

max
S








bal
s
max
s
&
min
s
min
s
75
.
0
A
A
A
A
S
T


Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements

A. Concept of Bond Stress and Rebar Anchorage

Internal Forces in a beam

Forces in Rebar

Bond stresses provide mechanism
of force transfer between concrete
and reinforcement.

Forces developed in the beam
by loading.

Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements

Equilibrium Condition for Rebar

m

= bond stress

(coefficient of
friction)

Note
: Bond stress is zero at cracks

m
m


4
.
0

4

.
0
Force

Bond
.
0
F
b
y
d
b
b
y
2
b
d
f
l
l
d
f
d
T












bar

c

f
k
f
k


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.

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.

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.

Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements

Typical Splitting Failure
Surfaces.

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 in
figure (b). Such splitting
comes largely from
wedging action when the
ribs of the deformed bar
bear 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.

Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements

ACI Code expression for development length for
bars in tension/in compression.

B.

Development Length, l
d

Shortest length of bar in which the
bar stress can increase from zero to
the yield strength, f
y
.

( l
d

used since bond stresses,
m,

vary along a bar in a tension zone)

Development Length for Bars in Tension

Development length, l
d

12” ACI 12.2.1

f
c

10000 psi for Ch. 12 provisions for development length in ACI Codes.

Development length, l
d

(simplified expression from ACI
12.2.2)

Clear spacing of bars being developed or
spliced not less than d
b
, clear cover not less
than d
b
, and stirrups or ties throughout ld not
less than the code minimum







or




Clear spacing of bars being
developed or spliced not less than 2d
b

and
clear cover not less than d
b
.

Other cases


No. 6 and smaller No. 7 and larger
bars and deformed bars
wires



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


Development Length for Bars in Tension

Development length, l
d

ACI 12.2.3

2.5 limit to safeguard against pullout type failure.

5
.
2
in which


40
3
b
ct
b
ct
c
y
b
d




















d
K
c
d
K
c
f
f
d
l

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 3d
b

or clear spacing less
than 6d
b

All other epoxy
-
coated bars or wires

Uncoated reinforcement

1.3

1.0

1.5

1.2

1.0

where

< 1.7

Factors used in expressions for
Development Length (ACI 12.2.4)

g 
reinforcement size factor
(Reflects more favorable






performance of smaller

bars)

No.6 and smaller bars and deformed wire

No. 7 and larger bars

 
lightweight aggregate concrete factor
(Reflects lower

tensile strength of lightweight concrete, & resulting


reduction in splitting resistance.

When lightweight aggregate concrete is used.

However, when f
ct

is specified, shall be permitted to be taken as
but not less than

When normal weight concrete is used

0.8
1.0

1.3

1.0

1.0

ct
c
7
.
6
f
f
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.

Factors used in expressions for
Development Length (ACI 12.2.4)

K
ct

= 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, in
2
.

Specified yield strength of transverse reinforcement, psi.

maximum center
-
to
-
center spacing of transverse reinforcement within l
d

in.

number of bars or wires being developed along the plane of splitting.

A
tr

=

f
yt

=

s =

n =

Note: It is permitted to use K
ct

=0 as a design simplification

even if transverse reinforcement is present
.

n
s
f
A
K
*
*
1500
yt
tr
tr

Excess Flexural Reinforcement
Reduction (ACI 12.2.5)

Reduction = (A
s

req’d ) / (A
s

provided )

-

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 l
d

12 in.







provided
n
u
provided
n
d
req'
n
Reduction
M
M
M
M




Development Length for Bars in
Compression (ACI 12.3)

Compression development length l
dc

= l
dbc

* applicable
reduction factors 8 in.

Basic Development Length for Compression, l
dbc








y
b
c
y
b
dbc

0003
.
0

0.02

of
larger

f
d
f
f
d
l
Development Length for Bars in
Compression (ACI 12.3)

Reduction Factors (ACI 12.3.3)

-

Excessive Reinforcement Factor = (A
s

req’d)/(A
s

provided)

-

Spiral and Ties








If reinforcement is enclosed with spiral



reinforcement 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

Note

l
dc

< l
d

(typically) because


-

Beneficial of end bearing is considered


-

weakening effect of flexural tension cracks is not



present for bars in compression
.




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 3d
b

d
b

=


of hooked bar


Note:

Multiplier for ties or


stirrups (ACI 12.5.3.3)

is not applicable for


this case
.

Hooked Bar at Discontinuous
Ends (ACI 12.5.4)

Table A
-
11, A
-
12, A
-
13 (Back of textbook)
-

Basic
Development lengths

Others

Mechanical Anchorage


ACI (12.6)

Welded Wire Fabric


ACI (12.7)

Bundled Bars



ACI (12.4)

Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements

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.

Reinforcement Development Lengths, Bar
Cutoffs, and Continuity Requirements

C.
Use of 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.

Design of Standard Hooks for
Tension Anchorage (ACI 12.5)

Development Length for Hooked Bar, l
db
.

.
in

6

and


8

where
s
multiplier
*
db
b
db
hd
dh



l
d
l
l
l
Basic Development Length for Hooked Bar = l
hb

when f
y

= 60,000 psi

c
b
hd
1200
f
d
l

Design of Standard Hooks for
Tension Anchorage (ACI 12.5)

Conditions

Bar Yield Strength




Bars with f
y

other 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

f
y

/60,000


0.7



0.7




Design of Standard Hooks for
Tension Anchorage (ACI 12.5)

Conditions

Excessive Reinforcement





Where anchorage or development for fy is not
specified required.

Lightweight Aggregate Concrete


Ties or Stirrups




For No. 11 bar and smaller.
Hook enclosed vertically or horizontally within ties
or stirrup
-
ties spaced along full l
dh

no farther apart
than 3d
b
, where d
b

is diameter of hooked bar.

Multiplier

A
s
(req’d) /
A
s
(provided)



1.3


0.8

Design of Standard Hooks for
Tension Anchorage (ACI 12.5)

Conditions

Epoxy
-
coated Reinforcement



Hooked bars with epoxy coating

Multiplier



1.2

Example

Example 4

GIVEN: A #5 Grade 40 bar is in tension as shown below. Use LIGHTWEIGHT

concrete with f’c = 4000 PSI.

REQUIRED: Determine the min. required hook dimensions “X”, “Y” and “Z”