Selection of concrete footings for SAL and MAL aluminium columns

The stability of foundation of the concrete footing in the ground is a complex issue, because its stability is influenced by many factors such as: type, granulometric composition and degree of ground compaction, location of the concrete footing in relation to slopes and ground faults, depth of freezing and others.

The lighting column with assembled extension arm, luminaire or other equipment during exploitation is loaded by its own weight as well as from wind and snow pressure. As a result of these loads, the column is subjected to the following:

- resultant force from wind pressure, which is hooked at a certain height, closer to the top of the column (F),

- bending moment, which is the largest in the plane of the column base-plate (BM),

- the torque that occurs for a column equipped with an asymmetrical extension arm (T)

These loads must be taken over by the concrete footing embedded in the ground.

The stability of foundation of the concrete footing in the ground is a complex issue, because its stability is influenced by many factors such as: type, granulometric composition and degree of ground compaction, location of the concrete footing in relation to slopes and ground faults, depth of freezing and others.

Table 1 gives a simplified division of ground into categories in terms of their characteristics.

Table 1.
Ground category and condition as well as selected geotechnical parameters

Symbols:

φ - angle of internal friction,

c – consistency,

γ – unit weight,

C – ground vulnerability module,

f – coefficient of friction of the ground against the concrete footing,

qg – ground boundary resistance.

 

Due to ensuring adequate stability of the column, a suitable typical prefabricated concrete footing must be used, or cast-in-place concrete made at the place of foundation of the column using a typical reinforcement basket..

The selection of prefabricated concrete footings and making of cast-in-place concrete with the use of baskets can be carried out on the basis of the d'Andree and Norsa formula:

where:

Ms - moment of concrete footing stability,

Nc - total load on the ground (sum of weight: column, equipment and concrete footing),

A - concrete footing side width,

H - concrete footing height,

qg – ground boundary resistance.

 

The stability of the structure will be maintained if the following condition is met:

Ms > Mg + (T*H)

where:

Mg - moment on the column at the base-plate,

T - shear force on the column at the base-plate,

H – concrete footing height.

Table 2 provides stability moments for individual types of prefabricated concrete footings for III category of ground with typical standard column load.
Table 2.
Types of concrete footings and their moments of stability for III category of grounds (qg = 0.2 MPa).

 

For the assumptions made above, the concrete footing type is assigned to the type of column as follows:
Table 3.
General classification of concrete footings for the types of columns assuming a minimum of III category of grounds as in Table 2.

* Higher concrete footings with a higher stability moment should be used in the following cases:

-when assembled on columns equipped with extensions with three or more arms

- when installed on slopes or near the slopes at their top

- in the case of embedding in grounds of very low strength and unpaved with characteristics not sufficiently recognized.

In any other case, when the prefabricated concrete footing is insufficient, a cast-in-place concrete should be made at the place of installation of the column with appropriate dimensions using a Z-type reinforcement basket.

The above selection of concrete footing type for column type is only approximate. The proper use of a suitable concrete footing requires a detailed analysis of ground conditions. The foundation of the concrete footing for the lighting column in each case takes place in a specific, often heterogeneous ground, which can only be recognized in detail by authorized persons. Therefore, the selection of concrete footings contained in this description should be treated only as a guide.