Civil Engineering Reference
In-Depth Information
because
M
2
<M
cr
=0
.
235
bh
2
f
tk
=0
.
235
600
2
×
×
250
×
1
.
5=31
.
7kN
·
m
Thus adopt Δ
S
2
=0.
The close shortened deformation Δ
S
is given by Eq. (3.31)
Δ
S
=Δ
S
1
−
Δ
S
2
=2
.
36 mm
Because of adopting welding to anchor, the deformation of anchorage devices is 0, and
then the value of Δ
S
L
is obtained
Δ
S
L
=Δ
S
+
σ
con
E
s
2
L
2
+2
a
268
2
.
0
=2
.
36 +
10
5
×
2
×
600 = 3
.
97 mm
×
At last, transverse frapping value, Δ
H
, is obtained as
Δ
H
=
L
1
(Δ
L
0
+Δ
S
L
)=
900
×
(6
.
43 + 3
.
97) = 96
.
7mm
From the calculation above, it is noted that the transverse frapping value considerably
influences the stretching effect of prestressed reinforcements. If the influence is ignored, the
tension stress will decrease brought down by 50%, which results in greatly diminishing the
effect of retrofitting. This type of influence increases along with the increase of sectional
area ratio of prestressed reinforcements to original steel bars, and decreases along with the
decrease of loads acted on the original beam when constructing. Hence, it is important
to consider the effect of closing shortened deformation in retrofitting calculation of some
members.
In practical work, prestressing retrofitting is flexible and diverse, and sometimes incorpo-
rating other retrofitting methods is necessary. Hereinafter an example is given to illustrate.
Shanghai Arts and Crafts Service was built in 1915. It was two stories when it was built;
later it was increased to four stories by use of coating portal frame construction with high
columns (three stories high). The crossbeam of the frame adopted thin webbed girder,
spanning 23 m, of which sectional height is 5 m, and thickness of web was 250 mm. At two
ends of the girder, small door opening was established as the access to the fourth floor, and
the third floor was a large-space market without columns. Thereafter, two more stories were
added by adding columns on the beam, and part of the structure became six stories high.
Because of adding stories again and again, the depth of roof was deepened whenever water
leaked; as a result of this, it was found that the depth of roof concrete reached 380 mm by
coring method. Because all the load increase are delivered by the crossbeam, three beams
in four cracked severely, the length of oblique cracks reached 4/5 depth of the beam, and
the widest cracks reached 3 mm.
Owner requests: a) retrofit of crossbeam; b) establish a door opening of 2 m
2.4 m at
the middle of the beam; c) integrally increase to six stories; d) set up a water tank of 60 kN
and a pumping room on the roof of the sixth floor.
After repeated discussion, the comprehensive retrofitting proposal containing retrofitting
the crossbeam and releasing the load on beam was adopted.
The method of retrofitting the crossbeam was to cast a W-shaped truss closely combined
with the original thin webbed beam at each side of the thin webbed beam. 8 30 curved
prestressed reinforcements were established in the longitudinal direction, and the prestress
was transferred to the crossbeam by W-shaped truss. Mechanical stretching was adopted.
At two ends of the beam 8
×
16 vertical prestressed stirrups were established. The ends of the
