Repair and reconstruction of engineering structures - Assessment and Refurbishment of Steel Structures - page 217

These measures prolonged the serviceability of the steel structures of the bridges and
prevented them from failing. As the life of the bridges had been exceeded, it was
recommended that a proposal for the gradual replacement of steel structures by new ones
be prepared. Analysis of the technical condition of the steel structures attacked by
corrosion revealed that in the original design of the steel structures the basic
constructional measures for the prevention or restriction of corrosive process had not
been taken. In addition it was shown that traditional types of conveyor bridges that use
lattice structure are not suitable for areas where the environment has a high degree of
corrosive aggressiveness.
The proposal for the new steel structures was based on the actual conditions under
which the bridges would be used in the chemical plant [63]:
• The proposed construction must allow good access for grinding and restoration of the
surface of the steel structure
• The possibility of using steel with an increased resistance to corrosion must be
considered
• The best method of cleaning the metal surfaces before coating must be used, and also
the best method of surface protection for the new structures.
The lattice structures composed of members of closed cross-section meet these
requirements. The three-chord lattice structure (Figure 6.42(a)) can be considered to be a
proper solution for the given conditions. The tubular lattice girder cross-section could
also be a quadripod (Figure 6.42(b)). The covering of transparent glass-fibre laminate that
replaced the windows can also be used to cover the conveyor belts. The sheet side
covering (Figure 6.42(a)) having been corroded was replaced by a new one with an
adequate surface protection. The load-bearing structure of the conveyor bridge was
formed by two web main girders connected by binders, as in Figure 6.42(c).
When examining the possibility of using steel with higher resistance to atmospheric
corrosion such as Atmofix, or Cortne it should be noted that in a very aggressive
corrosive environment the conditions for creating a high-quality protective layer, will not
be met.
The client required that the main girders of all bridges be welded web I girders. The
semi-frames formed from these girders were to be covered with a glass-fibre laminate.
The design solution for the bridges had to respect the existing conditions and due to
connection to technical devices also to allow for the possibility of locating new bridges
on the original structure. The flooring of all bridges was made of 50 mm wooden sheet
piles placed on 140×160 mm cuboids. Wood can be an ideal material in such an
aggressive environment. Given these requirements, the proposal for conveyor bridges
Nos. 1, 2a, 2b, 3, 03, 4a, 4b, 20 and 24 was prepared. By 1990 all the bridges had been
refurbished.
The axial distance between the main girders is 2400 and 4200 mm, the maximum span
of the bridges is 33.16 m. On bridge No. 1, the original two-field bridge with one
conveyor belt was replaced during reconstruction by a three-field web structure with
spans of 38.45+17.45+27.00=82.90 m (Figure 6.43). In the transverse direction there is a
semi-frame made from two main girders 1560 mm high and with I 200 binders. There are
lattice tubular supports. The glass-fibre laminate semicircular shield has diameter 1600
mm. A view of new conveyor bridges is given in Figure 6.44.