Civil Engineering Reference
In-Depth Information
point of the roof floor, where the new one is located. Another scheme is binding one bay
of roof truss that is usually made of steel or lightweight steel at each side of the original
structure.
For such reasons like diculty in construction, high cost of steel material and unsatis-
factory compatibility between new and original structures, the proposed method is rarely
used.
3. Measures of improving durability of concrete roof truss systems
Insucient durability often leads to serious erosion in members and spalling of concrete
covering induced by carbonization of concrete in tensile members or over-wide cracks, and
decreased safety of the roof truss structure.
Retrofit for durability should include two types of measures, one to prevent or retard the
steel bars from further erosion; and the other to strengthen seriously eroded tensile members.
The approaches to preventing deterioration are detailed in other related materials. Fur-
thermore, three steps of strengthening are reasonable for a small area of cross section of
members: first, seal the cracks with waterproof airtight material; next, cover the surface
with waterproof paint; and wrap the member using the “one-cloth, two-glue” method, that
is, brushing epoxy resin while binding the gauze, and a second layer of epoxy resin follows.
Such method is suitable when cracks are developed in one side and the reinforced bars are
located near that side.
For eroded lower chords, the method of adding tensile bars is recommended on account
that the high-strength reinforcement which is commonly arranged in the roof truss struc-
ture is prone to fracture after having pitting, and the lower chords become crucial to the
performance of the entire roof. Thus, substitute for original members not only enables the
strengthening bars perform well in carrying tension, but also prevents the roof truss from
collapse even when some of the steel bars in the lower chords rupture. Strengthening is
fulfilled through the technique of prestressing.
3.4.3
Practical Examples of Retrofitting of Concrete Roof Trusses
Example 3.10
A certain steel-casting foundry is 108 m in length and 18 m in span,
which is divided into eastern and western parts by the dilatation joint. The eastern part is
for electric furnace smelting and the western part is the area of molding and casting. The
roof truss system was designed as the
Φ
TM-18 trapeziform reinforced concrete structure by
the former institute of design for iron and steel industry. The construction was finished in
1959 and concrete cracked before use due to poor construction quality. Consequently, the
lower chords were simply strengthened by means of adding
φ
30 reinforced bars at both ends
and fixing the top of truss with nuts. In 1967, it was found that there were a total of 57
cracks on 16 bays of the truss, the widest one measured 0.5 mm and that those cracks mainly
distributed on two sides of the lower chord joints and diagonal web chords. These problems
fostered the second retrofit that included replacing the reinforced bars in lower chords with
2 32 prestressing bars in sunk style and adding 2 22 bars to both sides of diagonal web
chords accompanied by screw-induced tension. See Fig. 3.48.
After 10 years, the examination concerning the 16 retrofitted trusses indicated that a great
number of cracks appeared, especially in the region of lower chords, where a large number
of cracks initiated from the tops of chords. The number of cracks added up to over 300, of
which 56 were at both sides of the lower chord joints, 145 lay along the lower chords, and
105 were in the inclined web chords. Those cracks more than 0.3 mm in width totaled 28,
the widest of which equaled 0.6 mm. This structure was assessed as a “structure at risk” in
accordance with standard. In 1980, it was decided to conduct the third retrofit design and
construction, which had been determined by internal force analysis.
