Repair and reconstruction of engineering structures - Assessment and Refurbishment of Steel Structures - page 261

driving a set of rollers. The distance covered by the tester is controlled from the ground
by means of a joystick. The measurement results are shown on a digital display and in a
form of computer printout. Plate thickness and measurement localization can thus be
specified in detail. However, this method has not yet been widely used in Poland due to
its high cost.
6.2.1.3 Determination of permissible settlement of the tank bottom
circumference
The settlement of a tank depends on the type of foundation used. Until quite recently,
tanks were usually set on a foundation of sand, encircled by a reinforced-concrete ring.
The internal diameter of the ring was larger by ten centimetres than the diameter of the
tank. Thus the load of the shell was transferred to a sand foundation of low elasticity. In
the case of large capacity tanks with shells made of thick steel plates, this load is
significant and can affect the settlement of the circumference of the bottom (Figure 6.99).
This type of settlement is also characteristic of underground tanks constructed in a
concrete lining where there is an additional loading from a layer of soil on the roof of the
tank.
Figure 6.99 Typical settlement profile
of the circumference of the bottom of a
tank resting on a sand foundation
Figure 6.100 illustrates some values for the settlement profile of the bottom
circumference of a tank of V =50000 m 3 capacity, which needed repair after 20 years of
operation. Settlement of the bottom circumference was responsible for additional bending
stresses in this area, which in extreme cases can result in emergency cracking. Such an
event was involved in a large leak that occurred in Edmonton in 1985, where a crack
developed in the bottom of a tank of 12650 m 3 capacity [80].