HAC_Technical-Guide

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Cast-In Anchor Channel Product Guide, Edition 1 • 02/2019
1. Anchor
Channel Systems
2. HAC
Portfolio
3. HAC
Applications
4. Design
Introduction
5. Base material 6. Loading
7. Anchor Channel
Design Code
8. Reinforcing
Bar Anchorage
9. Special Anchor
Channel Design
10. Design
Software
11. Best
Practices
12. Instructions
for Use
13. Field Fixes
14. Design
Example
8.1 Reinforcing Bar Theory
8.2 Development Length
of Straight Bars
8.3 Pullout Strength of Straight
Reinforcing Barsrs
8.4 Pull Out Strength of
Headed Bars In Tension
8.5 Pull Out Strength of
Standard Hooks
8.6 Rebar Lap Splices 8.7 Concrete Cover
8.4 PULLOUT STRENGTH OF
HEADED BARS IN TENSION
Headed anchored bars in tension
When adequate space is not provided to reach full development
length of a straight reinforcing bar, ACI 318 allows the use of
headed reinforcing bars to reduce the development length. The
development length for a headed reinforcing bar generally will
be shorter than that for a straight or hooked rebar. Headed
reinforcing bars are ideal for applications where there is limited
space available to develop bars in tension.
The transfer of force from the bar to the concrete is assumed
to be achieved by a combination of bond-transfer mechanism
along the straight portion of the bar and bearing forces at the
head.
Figure 8.4.1.1 — Stresses on headed anchored reinforcing bars.
Headed anchored bars in tension, ℓ
dt
per
ACI 318-14, 25.4.4
The provision for headed deformed bars were formulated with
due consideration of the provisions for anchorage in ACI 318-14
Chapter 17 and the bearing strength provisions of ACI 318-14
Chapter 22.
The use of heads to develop deformed bars in tension shall be
permitted if conditions (a) through (g) are satisfied:
(a) Bar shall conform to 20.2.1.3
(b) Bar f
y
shall not exceed 60,000 psi
(c) Bar size shall not exceed No. 11
(d) Net bearing area of head Abrg shall be at least 4A
b
(e) Concrete shall be normal weight
(f) Clear cover for bar shall be at least 2d
b
(g) Clear spacing between bars shall be at least 4d
b
The provisions for developing headed deformed bars give the
length of bar, ℓ
dt
, measured from the critical section to the
bearing face of the head, as shown in Fig.###
Figure 8.4.1.2 — Development length of headed anchored reinforcing bar.
The head is considered to be part of the bar for the purposes
of satisfying the specified cover requirements in 20.6.1.3, and
aggregate size requirements of 26.4.2.1(a)(4).
25.4.4.2 Development length ℓ
dt
for headed deformed bars in
tension shall be the greatest of (a) through (c):
( )
( )
( )
.6
8
,
016.0
'
inc
db
d
f
f
a
b
b
c
ey
d
÷
÷
ø
ö
ç
ç
è
æ
=
l
y
!
With given in ACI 318-14 25.4.4.3 and value
e
y
of shall not exceed 6,000 psi
'
c
f
ℓ
dt
= development length of headed anchored rebar, in.
f
y
= yield strength of bar
ψ
e
= epoxy-coating factor
All other epoxy-coated bars or wires ........................................1.2
Uncoated and galvanized reinforcement .................................. 1.0
f′
c
= concrete compressive strength
f′
c
≤ 10,000 psi
d
b
= nominal diameter of the reinforcing bar
Transverse reinforcement, however, helps limit splitting cracks in
the vicinity of the head and for that reason is recommended.
8.5 PULLOUT STRENGTH OF
STANDARD HOOKS
Behavior of hooked rebar
When adequate space is not provided to reach full development
length of a straight reinforcing bar, the reinforcing bar can be
hooked or bent. The behavior of the hooked reinforcing bar
changes as a result of the bend in the reinforcing bar. Hooked
reinforcing bar resists bond failure by bond strength along the
straight portion, anchorage provided by the hook, and by the
bearing on the concrete inside the hook.
The forces that develop in a 90° hook are shown in Figure 8.5.1.1.
The developed stresses cause the bar to move inwards, leaving
a gap of concrete on the outside of the hook. The bar tends to
straighten out due to the formation of the gap and directional
change in force along the bend, causing compressive stresses
on the outside of the tail. Because the compressive force inside
the bend is not collinear with the applied tensile force, the bar
tends to straighten out, producing compressive stresses on the
outside of the tail. Therefore, the failure of the hook can most
often be attributed to the crushing of concrete inside the hook. If
the hook is close to a side face, the crushing may extend to the
surface of the concrete, removing the side cover. If cracking in
the outside of the tail occurs, the tail may straighten.
Figure 8.5.1.1 — Stresses in standard 90 degree hook. Source: Wight, James
& MacGregor, James. Reinforced Concrete Mechanics & Design, 2012.
The stresses and slip of the reinforcing bar are also dependent
upon the degree of bend of the reinforcing bar, as shown in Figure
8.5.1.2. The slip of the rebar at point A is nearly twice as large in
the 180-degree hook as compared to the 90-degree hook.
The main cause of failure of hooked bars is splitting failure of
the concrete cover in the plane of the hook. Splitting failure
depends on hook cover and anchorage strength can be
improved through confinement provided by stirrups.
Figure 8.5.1.2 — Stresses in 90 and 180 degree hooks. Source: Wight, James
& MacGregor, James. Reinforced Concrete Mechanics & Design, 2012.
Tension development length of hooked
reinforcing bar: ACI 318-14, §25.4.3.1
A minimum value of hook development length (ℓ
dh
) is specified to
prevent failure by direct pullout in cases where a hook may be
located very near the critical section. The development length
for deformed bars in tension terminating in a standard hook
(90°and 180° or between top and bottom bar hooks) shall be the
greater of (a) through (c):
( )
( )
( )
.6
8
,
50
'
inc
db
d
f
f
a
b
b
c
rcey
dh
÷
÷
ø
ö
ç
ç
è
æ
=
l
yyy
!
The modification factors applicable to hooked reinforcing bar
development length are as follows:
ℓ
dh
= development length of a standard hook, in.
measured from the critical section to the outside end (or
edge of the hook)
f
y
= yield strength of bar
ψ
e
= epoxy-coating factor
All other epoxy-coated bars or wires ....................................... 1.2
Uncoated and galvanized reinforcement ................................ 1.0