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
204 205
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
8.2 DEVELOPMENT LENGTH
Establishing the required reinforcing bar
length
Although bond stress varies along the length of a bar anchored
in a tension zone, ACI uses the concept of development length
rather than bond stress. The development length concept is
based on the attainable average bond stress over the length of
embedment of the reinforcement.
Figure 8.2.1.1 — Stresses in concrete and rebar.
Development length
As stated before, the ACI concept of development length is
based on the attainable average bond stress over the length
of embedment of the reinforcement. Development length (ℓ
d
)
can be defined as the shortest length in which the bar stress
increases from zero to the nominal yield strength (f
y
). Providing
the minimum development length of a bar ensures adequate
load transfer from the reinforcing bar to the concrete.
Development lengths are required to avoid splitting of the
substrate (especially thin substrates) when rebars are highly
stressed. The development length concept requires minimum
lengths beyond all points of peak stress in the reinforcement. In
other words, the reinforcement needs to be anchored properly
beyond the point of peak stress. Figure 8.2.1.2 illustrates rebars
developed beyond all points of peak stress.
Figure 8.2.1.2 — Development length of a reinforcing bar in a cantilever
member. Source: Wight, James & MacGregor, James. Reinforced Concrete
Mechanics & Design, 2012.
The development length concept incorporates two very
important concepts — reinforcing bar stress and nominal yield
strength. Bar stress is the force per unit area of the bar cross-
section. The nominal yield strength is the minimum bar stress at
which permanent (inelastic) deformation occurs.
Structural reinforced concrete design is based on the
assumption that the reinforcing bar will develop its yield strength
before premature failure occurs due to inadequate bond.
Development length is intended to ensure that the nominal yield
strength of the bar can be developed under structure loading.
Orangun, et al. [13] proposed an expression for determining the
development length ℓ
d
of deformed reinforcing bars in tension.
The ACI bond committee simplified the design expression
Development length in accordance with the
provisions of ACI 318-14
ACI 318-14 Chapter 25 contains provisions for reinforcing bar
development lengths. Development lengths are assumed to
preclude concrete splitting and reinforcing bar pullout failure
prior to “development” (attainment) of bar yield stress.
In all cases, the development length of a reinforcing bar in
tension should not be less than 12 in. Moreover, the values of
√f′
c
used to calculate development length shall not exceed 100
psi
Development length for straight deformed
bars in tension given in §25.4.2.3 of
ACI 318-14 as follows:
Deformed bars or deformed wires, development length (ℓ
d
) shall
be calculated as follows:
b
b
trb
set
c
y
d
d
d
kc
f
f
÷
÷
÷
÷
÷
ø
ö
ç
ç
ç
ç
ç
è
æ
÷
÷
ø
ö
ç
ç
è
æ
+
=
yyy
l
'
40
3
!
where:
ℓ
d
= development length, in.
ℓ
d
≥ 12 in.
f
y
= yield strength of bar
ψ
t
= bar-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 ........................................................................................ 1.3
Other reinforcement ................................................................ 1.0
ψ
e
= epoxy-coating factor
Epoxy-coated bars or wires with cover less than 3db or clear
spacing less than 6d
b
............................................................... 1.5
All other epoxy-coated bars or wires ....................................... 1.2
Uncoated and galvanized reinforcement ................................ 1.0
The product ψ
t
ψ
e
need not exceed 1.7
ψ
s
= bar-size factor
No. 6 and smaller bars and deformed wires ........................... 0.8
No. 7 and larger bars ............................................................... 1.0
λ = modification factor for lightweight concrete
When any lightweight-aggregate concrete is used ...............0.75
When the splitting tensile strength f
c,t
is specified, shall be
permitted to be taken as f
c,t
/6.7√f′
c
but not more then ........... 1.0
When normal-weight concrete is used .................................... 1.0
ACI 318-14, §R25.4.2.4 The lightweight factor λ for calculating
development length of deformed bars and deformed wire
in tension is the same for all types of lightweight aggregate
concrete. Research does not support the variations of this factor
in Codes prior to 1989 for all-lightweight and sand-lightweight
concrete. Section 25.4.2.4 allows a higher factor to be used
when the splitting tensile strength of the lightweight concrete is
specified. Refer to 19.2.4.
f′
c
= concrete compressive strength
f′
c
≤ 10,000 psi
c
b
= reinforcing bar cover factor
Reinforcing bar cover factor is:
a) the least of the side cover
b) the concrete cover to the bar or wire
c) One-half the center-to-center spacing of the bars.
In all cases, c
b
is measured from the center of the bar.
Figure 8.2.1.3 — Minimum reinforcing bar cover. Source:Wight, James &
MacGregor, James. Reinforced Concrete Mechanics & Design, 2012.
d
b
= nominal diameter of the reinforcing bar
k
tr
is a confining reinforcement across potential splitting
planes factor
40 A
tr
=
_______
s·n
where
A
tr
= total cross-sectional area of all transverse reinforcement
within the spacing s, which crosses the potential plane of
splitting along the reinforcement being developed within
the development length, (illustrated in Fig. 8-11)
Figure 8.2.1.4 — Transverse reinforcement (A
tr
).
s = maximum center-to-center spacing of transverse
reinforcement within ℓ
d
, in
n = number of bars or wires being developed or spliced
along the plane of splitting.
For simplicity, K
tr
can be taken equal to zero, even if there
is transverse reinforcement. This assumption results in
conservative development lengths.
A limit of 2.5 is placed on the term (c
b
+ K
tr
)/d
b
. When
(c
b
+ K
tr
)/d
b
is less than 2.5, splitting failures are likely to occur.
For values above 2.5, a pullout failure is expected, and an
increase in cover or transverse reinforcement is unlikely to
increase the anchorage capacity.