Civil Engineering Reference
In-Depth Information
reinforcement at the other point smaller than that at stretching end. That is to say, pre-
stress will be reduced because of friction at turning point. Prestress loss of friction
σ
l
2
can
be calculated by formula in
Code for Design of Concrete Structure
.
e
−μθ
)
σ
l
2
=
σ
con
(1
−
(3
.
43)
where
θ
is the included angle from oblique retrofitted reinforcement to longitudinal axis;
μ
is friction coecient between supporting pad and gliding block. When they are both steel
plate,
μ
=0
.
25; when retrofitted reinforcement contacts steel gliding plate directly,
μ
=0
.
4;
and when jacket is placed in supporting point,
μ
=0
.
1.
c. Prestress loss of steel relaxation loss
σ
l
4
. Prestress loss caused by relaxation of
retrofitted reinforcement can be adopted as value that is regulated in
Code for Design of
Concrete Structure
. Cold-drawn hot-rolled steel bar and heat treated steel bar could be
calculated as the following formula:
One-time stretching,
σ
l
4
=0
.
05
σ
con
(3
.
44)
(3
.
45)
Prestress loss of relaxation loss of steel wire and steel stranding wire can be calculated as
the following formula:
Over-stretching,
σ
l
4
=0
.
035
σ
con
σ
l
4
=
ψ
0
.
36
σ
con
0
.
18
σ
con
f
pyk
−
(3
.
46)
where
ψ
is empirical coecient related to stretching technique: One-time stretching,
ψ
=1
.
1;
over-stretching,
ψ
=0
.
9;
f
pyk
is standard strength of prestressed reinforcement.
d. Prestress loss of concrete creep loss
σ
l
5
. Because prestressed reinforcement in retrofitted
member is only one part of tensile steels (or even small part), and it produces very small
compressive pre-stress in concrete, which is not large enough to counteract the tensile stress
caused by external load which is acted at the same time with prestress, concrete creep loss
σ
l
5
can be neglected.
6. Constructional detail requirements
Concrete slab and beam structure retrofitted by prestress should follow constructional
details as below.
a. It is better to adopt steel reinforcement bar or steel stranding tendon with diameter
φ
12
φ
8.
b. When slab is retrofitted by prestress, flexile steel wire, not thick steel bar, is preferred.
c. Clear distance from horizontal segment of straight prestressed reinforcement or lower-
supported prestressed reinforcement to bottom of retrofitted beam is better to be:
<
100
mm, and 30
∼
φ
30; when choosing prestressed reinforcement, diameter is better as
φ
4
∼
80 mm is more suitable.
d. External retrofitted reinforcement needs anti-corrosive treatment after stretching,
which includes gunning cement mortar method and brushing anticorrosion paint method.
e. When stretching transversely method is used, diameter of frapping screw is better to
be:
∼
φ
16, and nut height should be no less than 1.5 times screw diameter.
f. Anchoring of prestressed reinforcement should be firm and reliable, and without dis-
placement.
g. Supporting steel pad should be set on the bottom of original beam, at the turning point
of lower-supported prestressed reinforcement. The height of steel pad should be
10 mm,
the width should be no less than 4 times its height, and the length should be as long as the
widthofretrofittedbeam.
Steel bar (shown in Fig.
20 mm, length not less than
b
+2
d
+40(
b
is beam width,
d
is diameter of prestressed reinforcement) or steel plate (shown
3.24(a)), of which diameter
