Repair and reconstruction of engineering structures - Assessment and Refurbishment of Steel Structures - page 280

connected by a coupling ring, and there are five purlins along the ribs, located at 1.45 m
intervals. Every fifth rib in the roof structure has a truss bracing; altogether there are four
truss braces. The roof cover is made of 3 mm thick plate. Due to an error during
operation, negative pressure was generated in the tank. As a result, the shell was
deformed at two places around the tank circum-
Figure 6.119 Zone of deformed shell
requiring replacement, prepared on the
basis of the deformation map
ference. Some more extensive deformations occurred below the service platform
connecting the roof of the tank to another nearby tank. Deformations were found on the
second and third plating strake from the top (both plating strakes were 5 mm thick), and
extended over a length of approximately 12 m (Figure 6.120). The arch rise of the
concavities and convexities amounted to about 150 m, and in the second plating strake
from the top, there was a crack along a sharply bent plate of length approximately 140
mm (Figure 6.121). Deformations in the second strake of the shell circumference were
sig-nificantly smaller and of a less destructive nature. The roof was not deformed owing
to its rigid construction (described below). However, the tank, though was generally in
good technical condition, required repair. The deformed plates were to be cut out and the
new plates fitted. Hydraulic expansion techniques could not be applied because the
deformations had sharp edges and there was a crack in one of the plates. From the repair
assessment it was decided that the plates 12 m long in two strakes of the shell (i.e. those
at 3.0 m height) should be replaced.