Civil Engineering Reference
In-Depth Information
In summary, by using surrounded outsourcing method, calculation of the right section
bearing capacity is given by:
f
cm
b
1
x
+
f
y
A
s
−
f
y
A
s
+0
.
9
f
y
1
A
s
1
−
N
0
.
9
f
y
1
A
s
1
(3
.
58)
f
cm
b
1
x
h
01
−
+
f
y
A
s
(
h
01
−
x
2
a
s
)+0
.
9
f
y
1
A
s
1
(
h
01
−
a
s
1
)
Ne
h
1
−
(3
.
59)
When reinforcement of grade I or II is symmetry, Eq. (3.56) can be further simplified as
follows:
N
f
cm
b
1
(
x
−
h
1
)+0
.
9
f
cm
1
b
1
h
1
(3
.
60)
(3) Small eccentric compression columns
In small eccentric compression columns, if the tensile bars away from axial force cannot
reach the yield strength, the bearing capacity of two sides thickening columns could be
computed by:
f
y
A
s
+0
.
9
f
cm
1
b
1
h
1
+
f
y
1
A
s
1
−
f
y
1
A
s
1
h
1
)+
f
y
A
s
−
N
f
cm
b
(
x
−
(3
.
61)
h
1
)
h
01
−
+
f
y
A
s
(
h
01
−
x
−
h
1
a
s
)
Ne
f
cm
b
(
x
−
h
1
−
2
+0
.
9
f
cm
1
b
1
h
1
h
01
−
+
f
y
1
A
s
1
(
h
01
−
a
s
1
)
h
2
(3.62)
where,
σ
s
and
σ
s
1
are respectively the stress in original reinforcement and retrofitting rein-
forcement. They can be calculated approximately equivalent:
ξ
−
0
.
8
σ
s
=
0
.
8
f
y
(3
.
63)
ξ
b
−
x
h
01
where,
ξ
is the coecient of compressive height,
ξ
=
;
ξ
b
is the coecient of compressive
height in critical failure, which equals to 0.61 for grade I steel, 0.55 for grade II steel.
For single-side thickening columns, surrounded outsourcing columns and symmetry rein-
forcement columns, bearing capacity can be computed as for large eccentric columns.
5. Examples
Example 3.6
A nine-story oce building is a two-span reinforced concrete frame-shear
wall structure. Two stories need to be added onto it. Some columns need to be retrofitted
after checking. Calculations are given:
a.
×
500 mm. The concrete strength grade is C20. Longitudinal reinforcement is 8 18. The
story height
H
=6
.
5 m. After retrofitting, this column withstands axial compression, the
magnitude of which is 3600 kN.
b. Retrofitting method. It is better to use surrounded outsourcing to retrofit axial com-
pression columns. First, coarsen concrete on four surfaces of the original column, then
collocate longitudinal reinforcement and
φ
8
Original column conditions.
The section size of this central column is 400 mm
200 stirrup, and spray 50 mm C25 fine con-
crete.
c. Calculation of the new longitudinal reinforcement and length of the column:
l
0
=1
.
0
H
=6
.
5m
l
b
6
.
5
0
.
4+0
.
1
=
=13
