Civil Engineering Reference
In-Depth Information
other one is removal of load, which fits in with guaranteeing bearing capacity of the global
structure. If the roof structure is damaged beyond the capabilities of retrofitting, it should
be removed and reconstructed. For further purpose, retrofit can be categorized as follows:
prestressing
changing force path
sticking steel or
enlarging section area
reducing roof load
dual structural system
strengthening
Retrofit method for RC
roof trusses
removal of load
The damage level of the structure and availability of operation should be considered to
determine which scheme to adopt from those above. Features and availability for each of
these schemes are to be described below.
(1) Prestressing
a. Technique: The method of prestressing is common for its convenience in construc-
tion, low cost in material and remarkable effectiveness. More of the lower chords require
retrofitting as tensile bar members are more likely to fail. Prestressing reduces the internal
force in the tensile chords and raises the bearing capacity, makes crack width narrower, and
even closed. In addition, this method can decrease the deflection of roof truss and alleviate
the stress lag, thus improving the effectiveness of service.
A variety of arrangements for prestressing tendons are adopted, such as straight-line, sunk,
butterfly and composite style.
a) Straight-line style: Fig. 3.44(a) shows how the trapeziform roof trusses in a certain
factory of Nanjing has been retrofitted. The problems of this structure are insucient
strength, cracking of concrete lower chords and erosion in steel bars. Though several pre-
stressing methods of retrofitting are discussed, the method of anchoring prestressing tendon
at the plate with lobes is adopted. The distance between tendon and lower chord is 250
mm. Prestressing is exerted through tightening reinforced bars with U-shaped bolts at the
point 3 m from the anchorage.
b) Sunk style: Fig. 3.44(b) shows the 15 m-long span composite roof truss in a factory.
The deficiency in the structure is due to low carrying capacity should be retrofitted in the
sunk style. In doing so, the load on the upper chords will be reduced. This not only
upgrades the strength of tensile lower chords, but provides reinforced bars with prestressing
force, imposing upward force on the truss as well. Prestressing works through electrical
heating, in which two ends of the steel bars are welded to the truss after heating when the
current is switched on, based on the principle of
thermal expansion and contraction
.
c) Butterfly style: Fig. 3.44(c) shows the trapeziform roof truss in a certain factory of
Shanxi, which uses butterfly type retrofit for inadequate strength. Upward force resulting
from this type of method is much greater than that from the sunk style, so load reduction
is more effective. What is negative is that it might partly transform the mechanical char-
acteristics of some bar members, and even bring about adverse effects. That's why internal
force in the truss should be checked to ensure safety of the members. The procedures for
this example will be further discussed in the following section.
d) Composite style: Fig. 3.44(d) shows how the roof truss in a steel foundry was retrofitted,
where both butterfly type and straight-line prestressing method were adopted. This method
employing two different styles simultaneously was called composite style, in which not only
lower chords but also tensile web chords can be strengthened.
