Civil Engineering Reference
In-Depth Information
However, some experiments showed that when such reinforced beam was destroyed, the
steel plate was still below the yield strength. Destruction is due to avulsion of the concrete
and the end steel plate. Stress in rebar is suddenly increasing and entering the strengthening
stage as soon as the end plate is loose, then brittle failure will occur without obvious signs.
(2) Force analysis
a. Avulsion reasons. In the second case mentioned above, why is the plate avulsed before
yielding as the strength of the adhesive is rather high? Upon analysis the main reasons are
the following:
a) Compared with the rebar in concrete, stick plates have more disadvantages. Tensile
stress in the plates is only balanced with the single-sided bond stress.
b) The force couple, formed by the composite force of plates and the bond stress are not
in a line, make steel plates deform in the opposite direction of the beam bending and avulse
the plate.
c) The bonding layer is under shear and tensile combined stress.
d) Lack of anchorage between end plates and concrete.
e) Adhesive quality and construction process affect the bond quality.
b. Stress hysteresis in steel plates. Generally, structures are retrofitted without unloading.
So, certain stress has existed in rebar of the original beam, whereas it may start to be
generated under new adding loads in steel plate. Therefore, before steel plate yields, rebar
has already yielded, and deflection and cracks of the beam will develop fast when the sticking
steel plate yields.
4. Calculation of bearing capacity and specifications
When bond strength of adhesive has met the
Technical Specification for Strengthening
Concrete Structures
through testing, the bond strength can be obtained from tables.
(1) Calculation of retrofitting tensile region of flexural members
a. Calculation of bearing capacity. The following expressions are proposed to compute
the bearing capacity of the retrofitted beam:
f
y
A
s
f
cm
bx
=
f
y
A
s
+
f
ay
A
a
−
(3
.
47a)
M
u
=
f
cm
bx
h
01
−
+
f
y
A
s
(
h
01
−
x
2
a
s
)
(3
.
47b)
where
f
ay
is the factored tensile strength of the stick steel plate,
A
a
is the section area of the
steel plate,
A
s
and
f
y
are respectively for section area and factored tensile strength of lon-
gitudinal tension reinforcement in original beam,
A
s
and
f
y
are respectively for section area
and factored tensile strength of longitudinal compression reinforcement in original beam,
a
s
is the covering layer thickness of compression reinforcement.
b. Calculation of anchorage length. Anchorage length
L
1
refers to the stick steel plate
extension length outside the beam section that has no need to be retrofitted. When stress
distribution coecient equals two, the anchorage length of tensile steel plate is given as
follows:
2
f
ay
t
a
f
cv
L
1
(3
.
48)
where
t
a
is the thickness of tensile sticked steel plate,
f
cv
is the factored shear strength of
concrete, obtained from in Table 3.7.
Table 3.7 Concrete shear strength
Grade
C15
C20
C25
C30
C35
C40
C45
C50
C55
C60
Types
Test Value
f
cv
2.25
2.70
3.15
3.55
3.90
4.30
4.65
5.00
5.30
5.60
Nominal Value
f
cvk
1.70
2.10
2.50
2.85
3.20
3.50
3.80
3.90
4.00
4.10
Factored Value
f
cv
1.25
1.75
1.80
2.10
2.35
2.60
2.80
2.90
2.95
3.10
