Civil Engineering Reference
In-Depth Information
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32
Fig. 3.48
Roof truss with retrofit of composite style.
(1) Analysis for causes of cracking
After the second retrofitting, the cracks on the original members developed instead of
having been controlled, especially on the lower chords, where those cracks began from the
upper regions. The causes for the irregular phenomenon are listed in the following:
a. The cross-section areas of the lower chords were relatively small (only 200 mm
200
mm), and reinforced bars were arranged in the form of a line, concentrating at the axial
lines. The top and bottom area of cross section was plain concrete, so the tensile force in
that area would be totally carried by concrete only under eccentrically tensile loads, which
would result in great development of cracks once cracking initiated.
b. The cracks after the second retrofitting of sunk style did not decrease but increased
in number. This indicates that the line of tension in prestressing tendons deviates from
the axial line; the prestressing tendons kink at the point of downdip, forming the upward
force, not through the joints of members. The friction between steel channels at the point
of downdip and prestressing tendons restrains the tendons from slipping and causes the
difference in tensile forces which derive from unequal tensile stresses in the tendons at two
sides of the downdip point. In that way the additional tensile difference adds the bending
moment value in the concrete members and consequently both tension in the top region
and compression in the bottom region or relatively greater tensile stress on the top make
cracking begin from the top. And the full-size experimental results have proved it.
The analysis above shows that the members with line-laid reinforcement behave more
sensitively to eccentric force effects and it can be concluded that it is better for the lower
chords with line-laid reinforcement to adopt straight-line style retrofit method when using
prestressing bars; if sunk style is used, the sleek supporting point will permit prestressing
bars slipping without restraint. In addition, it should be noted that the reaction in the
upward direction is to pass the joints of all the members.
(2) Decision-making and full-size test
This building was proposed to use the retrofit method of dual structural system, that is,
a pair of additional steel roof trusses are constructed on both sides of the original structure,
with additional structure supporting on the steel columns for the reason that this building
has been defined as “structure in risk”. Cracking in the lower chords was severe and irregular
and the eastern part was subject to fire accident. Despite reliability, this method is deficient
in economy and construction is dicult. Both theoretical analysis and full-size experiment
should be conducted for the original structure for the sake of safety, economy and rationality.
The result after checks shows that the upper chords roughly satisfy the requirements
except those in both end bays are dramatically insucient in both bearing and anti-cracking
capacity considering all the effects of axial force, bending moment and secondary moment.
The bearing capacity of lower chords is merely 80% of the design value when neglecting the
effects of additional prestressing bars; if the additional prestressing force is considered, both
of the bearing and anti-cracking capacity will meet the criterion. Except for the fact that the
first diagonal one lacks in bearing capacity, most of the original web members measure up. If
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