Chapter-7 Bond Development Length & Splices. Lecture Goals Slab design reinforcement Bar Development Hook development.

Chapter-7Bond Development Length

& Splices

Lecture GoalsLecture Goals

• Slab design reinforcement

• Bar Development

• Hook development

Flexural Reinforcement in SlabsFlexural Reinforcement in Slabs

For a 1 ft strip of slab is designed like a beam As(req’d) is in units of (in2/ft)

inchesin spacingbar

in 12ft / bs AA

The table is A-9 from MacGregor’s book.


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

thicknessslab3t ofsmaller maxS


• 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 SlabsFlexural Reinforcement in SlabsMaximum & Minimum reinforcement requirements

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

t

ftf

ft

ftA

*" 12*0.0014

ksi 60 *"12*60

*0018.0

ksi 60 *"12*0018.0

ksi 50or 40 *"12*0020.0

y y

y

y mins

Flexural Reinforcement in SlabsFlexural Reinforcement in SlabsT&S Reinforcement (perpendicular to span) ACI Sec 7.12

Flexural Reinforcement (parallel to span) ACI Sec 10.54

Smax from reinforced spacing

18"

5t ofsmaller maxS

balsmaxs

&minsmins

75.0 AA

AA ST

Reinforcement Development Lengths, Bar Reinforcement Development Lengths, Bar Cutoffs, and Continuity RequirementsCutoffs, 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.


Equilibrium Condition for Rebar

= bond stress

(coefficient of friction)

Note: Bond stress is zero at cracks

4.

0 4

.

0Force Bond.0F

byd

bby

2b

dfl

ldfd

T

bar c

fk

fk


Sources of Bond Transfer

(1) Adhesion between concrete & reinforcement.

(2) Friction

Note: These properties are quickly lost for tension.


Sources of Bond Transfer

(3)Mechanical Interlock.

The edge stress concentration causes cracking to occur.

Force interaction between the steel and concrete.


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.


Typical Splitting Failure Surfaces.


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.


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 to the yield strength, fy.

( ld used since bond stresses, vary along a bar in a tension zone)

Development Length for Bars in TensionDevelopment 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 2db and clear cover not less than db.

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 TensionDevelopment Length for Bars in Tension

Development length, ld ACI 12.2.3

2.5 limit to safeguard against pullout type failure.

5.2in which

40

3

b

ct

b

ctc

y

b

d

d

Kc

dKcf

f

d

l

Factors used in expressions for Factors used in expressions for Development Length (ACI 12.2.4)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 3db or 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


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 fct 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

ctc7.6 ff


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.


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 ld in.

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

**1500yttr

tr

Excess Flexural Reinforcement Excess Flexural Reinforcement Reduction (ACI 12.2.5)Reduction (ACI 12.2.5)

Reduction = (As req’d ) / (As 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 ld 12 in.

providedn

u

providedn

dreq'nReduction M

M

M

M

Development Length for Bars in Development Length for Bars in Compression (ACI 12.3)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

0.02

oflarger fd

f

fd

l

Development Length for Bars in Development Length for Bars in Compression (ACI 12.3)Compression (ACI 12.3)

Reduction Factors (ACI 12.3.3)- Excessive Reinforcement Factor = (As req’d)/(As 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 ldc < ld (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 Hooked Bar at Discontinuous Ends (ACI 12.5.4)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.

Hooked Bar at Discontinuous Hooked Bar at Discontinuous Ends (ACI 12.5.4)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)


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.


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 Design of Standard Hooks for Tension Anchorage (ACI 12.5)Tension Anchorage (ACI 12.5)

Development Length for Hooked Bar, ldb.

.in 6 and 8 wheresmultiplier* dbbdbhddh ldlll

Basic Development Length for Hooked Bar = lhb when fy = 60,000 psi

c

bhd

1200

f

dl


Conditions

Bar Yield Strength Bars with fy 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

fy /60,000

0.7

0.7


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 ldh no farther apart than 3db, where db is diameter of hooked bar.

Multiplier

As(req’d) / As(provided)

1.3

0.8


Conditions

Epoxy-coated Reinforcement Hooked bars with epoxy coating

Multiplier

1.2

ExampleExampleExample 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”

Chapter-7 Bond Development Length & Splices. Lecture Goals Slab design reinforcement Bar Development Hook development.

Documents

concrete reinforcement

slabs flexural reinforcement

cracks slide

bar cutoffs

ft slide

reinforced spacing slide

bond failure

continuity requirements