Cylindrical steel tanks


ABOVEGROUND STEEL TANKS

 

Types of steel tanks

Stiffening of membrane of single deck floating roof

Methods of erection

Typical damages of aboveground steel tanks

Inspection of steel tanks

Realized projects

Typical drawings

Publications

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FOUNDATION


Design and building of steel construction of the tank can not be done without taking into account the relation between upper steel construction and soil basement. Inclusion of coefficient of the bed Kb in solution of contact problem in the joint shell - bottom is not sufficient.The badly designed or badly executed foundations can cause failure condition of the tank and at least it can lead to differential settlement. Welded spatial constructions with thin walls are flexible and they are very sensitive to the settlement.

1. Shape of the bottom

a) cone up bottom
This is more often used bottom shape. It allows water drains from inside of the tank to the situated near to the shell draw-off  sump. The atmospheric waters do not move toward to the center of the tank and are not kept bellow the bottom.
b) cone down bottom.
This type of bottom is applied when the tank often must be emptied / filled. Value of the slope is the same as the slope of the cone up bottom. It assures full drain of the water dropped inside in the tank and minimal contact surface between the water inside and the bottom. One of the weaknesses is the possibility for keeping and storing the atmospheric water felt below the bottom. Outside water can move to the center and increase the speed of bottom corrosion.
c) flat bottom without slope
It can be applied in the small tanks where the removing of the water dropped inside into the tank is not required.
d) flat bottom with single slope.
Can be applied where the diameter D ≤ 30 m. This type of bottom requires sloped foundation and the construction of the shell must assure its vertical position.


2. Type of foundation construction

a) foundation from compacted soil (earth type foundation).

This is most often applied scheme of foundation construction because it is cheapest and easiest for execution. It is made according to the scheme shown in API Std. 650. It is used when the soil can bear the pressure of the upper steel construction and when the anchorage is not necessary. Even when there is small leak moving out
of the soil is possible. It can leads to destruction of the tank. In this reason the diameter of the tank must be bigger than the diameter of the tank with not less then 1,8 m. Earth type foundation does not allow good leveling of the bottom i.e. of the shell of the tank. When they are used it is possible the uneven settlement which cause additional efforts in the tank's elements.

Earth type foundation of steel tanks
Earth type foundation

b) reinforced concrete ring which is not placed under the shell.

The tanks which are subject of this research are the above ground facilities. They are placed on 0,3 1,2 m above the soil. This level difference is remarkable in the fuel oil tanks where pump always must be under the liquids (Δh ≥ 0,7m). If the classical earth type foundation reaches this height the facility must occupy remarkable surface on the site. The reinforcing of such different leveled surfaces bears a risk for landslide (when the earth is covered by grass or asphalt) or it is slow, expensive and work consuming process (when the earth is paved).
In order to avoid this inconvenience appears the idea of small foundation ring between the ground and the bottom level which ring is a combination free sand pillow and reinforced concrete ring in the periphery. The proposed construction is similar to the API Std. 650, but the foundation ring is moved in the outside direction where it can not be influenced of the load of the shell and the tank roof upon it. When there is soil settlement under the tank, the reinforced concrete ring does not allow full drain of the water so that this solution should not be applied to the new build tanks.
Concrete ring out of shell

Reinforced concrete ringwall foundation which is not placed under the shell

c) reinforced concrete ring wall foundation.
The trend in the tanks building shows that the volumes of the facility increase. Spatial steel construction of the tanks stands more flexible. In this reason bigger attention must be paid to the shell settlement and to the prevention measures. The use of the rigid reinforced concrete ring increase around the world. When the tanks are bigger the dimensions of the rings are: largeness not less then 0,6 m and height 1,5 – 2,0 m.This type foundation construction allows very good leveling of the periphery of the bottom and the shell which is positioned on it. The uneven settlement of the tank is limited. It is possible anchors to be put there.
Foundation of steel tank as concrete ring

Reinforced concrete ringwall foundation

d) reinforced concrete slab
They are applied when the tanks are relatively small because this type of foundations is very expensive.
This type of foundation is more favorable for upper steel structure. They do not allow the uneven settlement of the tank. The reinforced concrete slab is very recommendable when the level of the underground water is high.

Foundation of steel tanks as reinforced concrete slab
Reinforced concrete slab

The special foundation constructions as the foundation in the weak soil must be executed according to the concrete projects after the technical and economical analysis made on the ground of results of the geological and engineer research.