Civil Engineering Reference
In-Depth Information
It is evident from Table 3.3 that the flexural capacity is inadequate and additional re-
inforcements are required with regard to supported section of middle span, yet minimal
ratio of tensile reinforcement (
φ
6
200) must be sucient for the negligible insuciency
(
M
z
−
m).
f. Calculation and control of reinforcement stress.
Reinforcement stress
σ
s
1
and
σ
s
2
could be determined in accordance with Eq. (3.7), and
for the middle section of side span, the reinforcement stress is given as
M
u
= 2194
−
1897 = 297 N
·
M
1
0
.
87
A
s
h
0
1284000
0
.
87
σ
s
1
=
=
55
= 127 MPa
×
218
×
β
=0
.
5
1
=0
.
5
1
=0
.
231
h
h
1
70
130
−
×
−
M
2
(1
β
)
0
.
87
h
01
A
s
−
=
158
×
10
4
×
(1
−
0
.
231)
σ
s
2
=
=55
.
7MPa
0
.
87
×
218
×
115
σ
s
=
σ
s
1
+
σ
s
2
= 127 + 55
.
7 = 182
.
7MPa
<
0
.
9
f
y
= 189 MPa
In addition, for supported section, because additional concrete lies on the tensile surface, it
does not have leading stress effect in reinforcements as mentioned above. In contrast, the ad-
ditional reinforcements have the effects of stress lag compared with existing reinforcements.
Therefore, it is generally accepted that stress validation of additional reinforcements at sup-
ported sections of continuous beams is not necessary. The retrofitted slabs are illustrated in
Fig. 3.8.
Added concrete
Added non-structural reinforcement
φ
6@250
30
Existing concrete slab
Fig. 3.8
Construction drawing of retrofit strategy of adding concrete layer.
3.2.3
Retrofitting by Adding Tensile Reinforcement
Retrofitting with tensile reinforcement is to add reinforcements on the tensile side of
the beam to enhance its bearing capacity. The method is applicable where stiffness of the
beam section and the shearing capacity are enough but the tensile strength of the bend
region is insucient and the adding reinforcement is not excessive. In this section, the
procedure, characteristics and construction of retrofitting with adding tensile reinforcement,
as well as the calculation method of the bearing capacity of the retrofitted members, will be
introduced.
1. Introduction of retrofitting with adding tensile reinforcement
Fig. 3.9 shows the retrofitting with adding tensile reinforcement. The connection between
additional reinforcements and existing beams involves three forms: full welding, semi-welding
and bonding connections.
(1) Full welding
Regarding full welding, additional reinforcements are directly welded to original reinforce-
ments, and additional concrete layers are not necessary. The additional reinforcements are
exposed to natural environment and participate in flexural resistance with original reinforce-
ments by the action of weld (see Fig. 3.10).
