Retrofitting Design of RC Structures - Retrofitting Design of Building Structures - page 73

relatively large, and so is the crack width. This shows that the higher the value of prestress
that is applied on retrofitted reinforcement, the more the stress condition of retrofitted beam
can be improved. Consequently, stretching control stress σ con should not be too high, or
the internal force of some steel bars will arrive at or exceed the yield strength and result in
danger during over-stretching period. Stretching control stress is listed in Table 3.4.

Table 3.4 Allowed stretching control stress σ con

Item number Steel category Stretching control stress σ con

1 Carbon wire, indented wire, steel stranding wire 0.70 f ptk

2 Cold-drawn low-carbon wire, heat treated steel bar 0.65 f ptk

3 Cold-drawn hot-rolled steel bar 0.85 f pyk

4 Hot-rolled steel bar 0.90 f pyk

As retrofitted reinforcement produces small prestress to original beam's concrete, the
concrete creep loss is accordingly small, or even does not exist. That is to say, under the
same condition of stretching control stress, the ultimate stress of retrofitted reinforcement
is higher than that of prestressed reinforcement.
(2) Calculation of prestress loss
Constitutions and techniques of external prestressed reinforcement of retrofitted beams
are both different from ordinary prestressed concrete beams, and so is prestress loss. For
convenience, the following text will make the most of signs regulated by Code for Design of
Concrete Structure .
a. Anchoring loss σ l 1 . σ l 1 can be calculated as the following formula:

a
L E s

σ l 1 = (3 . 42)

where L is effective length of prestressed reinforcement when straight retrofitted reinforce-
ment is stretched at the end or in the middle by jack; and is half of effective length of
prestressed reinforcement when curved reinforcement is stretched at the end (often both
two ends are stretched simultaneously). a is anchor device distortion value and steel re-
inforcement retraction value. Values in Code for Design of Concrete Structure are shown
in Table 3.5. In retrofitting, the anchor device is much more complex than that in ordi-
nary prestress concrete beams. When anchor device is not included in Table 3.5, a can be
determined by referencing Table 3.5 and adapting to actual situation.

Table 3.5 Anchor device distortion and steel reinforcement retraction values

Steel reinforcement
retraction value (mm)

Item number Anchor device distortion

Anchor device with screw cap (including cone screw anchor
device, cylinder anchor device, etc.)
Gap between screw caps
Gap between additional pads

1

1
1

2 Heading anchor device of steel tendon 1

Steel cone anchor device of steel tendon
JM-12 anchor device

3

5

When prestressed reinforcement is steel bar
When prestressed reinforcement is steel stranding wire

3
5

4

5 Cone anchor device of single cold-drawn low-carbon wire 5

Distortion loss caused by bar stay or screw can be neglected. These kinds of distortion
exist in stretching, and prestress value is constant before and after anchoring.
b. Prestress loss of friction in turning point σ l 2 . When down-supported prestressed rein-
forcement is applied, friction at lower-supporting point will make internal force of retrofitted