Civil Engineering Reference
In-Depth Information
in Fig. 3.18) should be set between supporting steel pad and prestressed reinforcement.
Sometimes steel cylinder, of which length equal to the width of beam, is enveloped out of
steel bar to reduce friction prestress loss (shown in Fig. 3.24(b)).
retrofitted beam
steel cylinder
retrofitted beam
Steel pad
tension rod
Steel pad
Steel bar
Steel bar
tension rod
(b)
(a)
Fig. 3.24
Construction of turning point of prestressed reinforcement.
When a continuous slab is retrofitted by prestress, the turning point of prestressed re-
inforcement is better to be positioned around inflection point, which can produce obvious
upward force and reduce the span of slab (Fig. 3.25).
slits chiseled on slab pivot of steel stranding wire (concentrated force of girder)
stretch
upward force
steel stranding wire without bond
Fig. 3.25
Retrofitting continuous slab by prestress method.
h. Oblique hole through which reinforcement goes at turning point of prestressed rein-
forcement of continuous slab can be at 45
◦
, and its position should avoid reinforcements in
slab. From oblique hole, slits should be chiseled on slab face and bottom, respectively, along
direction of prestressed reinforcement. The depth of slits depends on the requirements of
upward force at turning point: the shallower the slits, the larger the upward force, and more
prestress loss at turning point.
i. Two-end stretching is better to stretch prestressed reinforcement of continuous slab, in
order to reduce friction prestress loss.
7. Retrofitting design examples
(1) Calculation steps
For the retrofitted beams with prestressing reinforcements in external, calculation steps
can be generalized as follows:
a. Draw internal force map under residual load and whole load.
b. First check the value of compression region height,
x
, considering the member as
a flexural member with bending and then, moment M, obtain the moment Δ
M
resulting
from retrofitting reinforcement in the mid-span section of the beam, finally estimate the
cross-sectional area of retrofitting reinforcement
Ap
, according to Δ
M
.
c. Determine tension control stress, and calculate the loss of prestress.
d. Based on the tension control stress, calculate prestress internal forces.
e. Act prestress as external force on the original beam, check the normal section bearing
capacity of the original beam considering it as an eccentric compressive members. If the re-
sults cannot meet the demands, increase the area of prestressing reinforcements and recheck
it.
f. Check the oblique-sectional bearing capacity of the beam.
g. Proceed calculation of the prestressing effect and extension value.
