Transcript No Slide Title

Bond, Anchorage, and Development Length
- Bond Stresses in Beam
- Development Length
- Standard Bar Hooks
- Bar Cutoff of Bend Points in Beams
Bond Stresses in Beam
Concrete
Reinforcing bar
End slip
P
Greased or lubricated
Free slip es น ec
Bond_02
Bond forces acting on concrete
Bond forces acting on steel
Bond_03
Tied-arch action in a beam with little or no bond
(Round bar reinforcing)
Mmax
jd
Little or no bond
Bond_04
Bond Stress Based on Simple Cracked Analysis
C + dC
C
[ SMO = 0 ]
j d (dT ) = V (dx )
jd
V
V
u So dx = dT
O
dx
u
1
T + dT
1
[ SFx = 0 ]
T + dT
T
T
dT V

dx jd
2
2
V
u
So j d
Elastic Cracked
Section Equation
Bond_05
Nominal Bond Strength
เหล็กเสริมข้ ออ้ อยรับแรงดึง:
เหล็กบน
4.51 f c
un 
 39.4 kg/cm2
db
เหล็กอื่น
6.39 f c
un 
 56.2 kg/cm2
db
เหล็กเสริมข้ ออ้ อยรับแรงอัด:
3.44 f c
un 
 56.2 kg/cm2
db
* สำหรับเหล็กข้อกลมผิวเรี ยบให้ใช้กำลังครึ่ งหนึ่งแต่ไม่เกิน 17.6 กก./ซม.2
Bond_06
Steel Force and Bond Stress
M
Cracked concrete segment
T
M
T
u stresses on bar
steel tension T
T
M
jd
bond stress u
1 dT
u
S o dx
Bond_07
Actual Distribution of Flexural Bond Stress
CL
T
M
jd
Actual T
u
V
So jd
Actual u
Bond_08
Source of Bond Strength
R
u
DT
Friction & chemical adhesion
between concrete and steel
R
R
DT
Reaction of concrete on ribs
R
R
Forces exerted by ribs on concrete
Bond_09
Failure modes
R
R
Side-split failure
V-notch failure
R
Vertical cracking of bottom cover
Bond_10
Ultimate Bond Strength
Cylindrical zones of
circumferential tension
Reinforcement
Radial component
of bearing pressure
Circumferential
tensile stresses
Bond_11
Minimum Bar Covering and Spacing
Minimum bar covering ( cb )
cb
2cs
cs
Minimum bar spacing ( 2cs )
Bond_12
Splitting of concrete along reinforcement
Bond_13
Development Length
Embedment length to develop full tensile strength of bar
T=0
ld
a
T = Ab fs
Requirement: l ณ ld : development length
Factors influencing ld
- Tensile strength of concrete fct
- Steel covering
- Bar spacing
- Stirrup
Bond_14
Development of Tension Reinforcement
Bonding Sources:
- Embedment length
- Bar Hook
- Mechanical anchorage
- Chemical bonding
Bond_15
ระยะฝังของเหล็กเสริมรับแรงดึง
ld
a
fs = 0
fs = fy
SFv x  0
un
Ab fy
ld
Ab f y  un S o ld

4
db2 f y  un  db ld
ld 
db f y
4 un
Bond_16
ACI Code for Development Length
ld 0.28 f y 

db
f c  c  Ktr 


 db 
c  Ktr
 2.5
db
Ktr 
Atr f yt
105 s n
= Transverse reinforcement index
s = Max. spacing of stirrup
n = number of bar
c = smaller of edge and one-half c-c distance
Bond_17
Modifiers for ld /db
 Bar location:
 Coating factor:
(a) Top reinforcement
 = 1.3
(b) Bottom bars
 = 1.0
(a) Epoxy-coated cover < 3db or
clear spacing < 6db
 = 1.5
(b) All other epoxy-coated
 = 1.2
(c) Uncoated reinforcement
 = 1.0
Note:  ด  need not be taken > 1.7
Bond_18
 Bar-size factor:
(a) DB20 and smaller
 = 0.8
(b) DB25 and larger
 = 1.0
 Lightweight
(a) Lightweight aggregate
aggregate factor:
concrete
(b) Normal weight concrete
Reduction for excess reinforcement:
 = 0.3
 = 1.0
As,req/ As,sup
Reduction for large cover and wide bar spacing:
0.8
Heavily confined reinforcement:
0.75
Bond_19
Simplified Equation for Development Length
c  Ktr
set
 1.5
db
Case A:
(1) covering = db
clear c-c = db
min. stirrup
DB20 and smaller
( = 0.8)
DB25 and larger
( = 1.0)
ld 0.15 f y


db
f c
ld 0.19 f y


db
f c
ld 0.23 f y


db
f c
ld 0.28 f y


db
f c
(2) covering = db
clear c-c = 2db
Case B: others
Bond_20