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rafter support columns, the tanks contain no internal
superstructure, and their external plating provides the sole
structural support. The tank shell is thickest at the bottom and
gradually thins toward the top, which makes the center point of
the tank’s total weight lower than half the tank height.
Due to the amount of steel in the tanks’ composition, the
tanks have considerable weight. For example, a 55,900-barrel
tank has a dry weight of 394,000 pounds, and a 151,000-barrel
tank exceeds 1 million pounds. Consequently, tanks usually are
not tied down to their foundations, and none of CITGO’s tanks
are.
The purpose of tank foundations is to spread the tank’s
weight load to help avoid tank settlement and to keep moisture
and other corrosive elements from deteriorating the tank’s steel
structure. Tank foundations may consist of compacted sand or
soil, concrete ringwalls, crushed stone ringwalls, or concrete
slabs. For sand pad foundations, CITGO replaces the top 3 to 6
inches of the soil with sand or granular backfill. Concrete
ringwalls are circular concrete walls from 12 to 18 inches thick
that line up with the tank’s outer edge. The ringwall is mostly
beneath grade and is filled with sand or other material to permit
sufficient drainage. Similarly, crushed stone ringwalls are
circular gravel rings filled with sand or other drainage
material. Concrete slab foundations are more infrequently used
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Last modified: May 25, 2011