Selection of concrete footings for SAL and MAL aluminium columns

The lighting column with assembled extension arm, luminaire or other equipment during exploitation is loaded by its own weight and from wind 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.

Selection of concrete footings for SAL and MAL aluminium columns

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

   

GROUND CATEGORIES AND CONDITION SELECTED ORIENTATIVE GEOTECHNICAL PARAMETERS OF THE GROUND
φ [°] c
[kNm2]
γ
[kNm3]
C
[kNm3]
f qg
[MPa]
CATEGORY I
Stiff ground
Heaps, rubble, gravels,  gravel shores, coarse and medium-concentrated sands, and medium-compacted, fine sands 35 0 18,5 15000 0,55 0,30
Dusts, compacted clays, clays, loam gravels,  gravel shores, semi-solid sands and hard plastic sands 20 25 20 20000 0,25

CATEGORY II
Medium stiff ground

Heaps, rubble, gravels,  gravel shores,  coarse loose sands, fine and silty sands, medium concentrated 32 0 17,5 12000 0,45 0,25
Dusts, clays, compacted clays, clays, loam gravels,  gravel shores and plastic clay sands 15 15 19 10000 0,30

CATEGORY III
Soft ground

Fine and dusty sands, loose, humus sands moderately concentrated 25 0 15 10000 0,35 0,20
Dusts,  clays, compacted clays, loam gravels,  gravel shores,  soft plastic clay sands 10 5 18 5000 0,10

·

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. Types of concrete footings and their moments of stability for III category of grounds (qg = 0.2 MPa)

Table 2 provides stability moments for individual types of prefabricated concrete footings for III category of ground with typical standard column load.

Concrete footing type Concrete footing side [mm] Height H
[mm]
Spacing and type of bolts Weight
[kg]
The stability moment of concrete footing with a typical load
Ms [Nm]
B-50 240 900 180 - M14 95 8000
B-51 260 1000 200 - M18 122 10000
B-51A 260 1200 200 - M18 148 14000
B-60 320 1000 250 - M18 168 12000
B-71 400 1000 300 - M24 250 14000
B-70 400 1200 300 - M24 296 20000
B-80 400 1500 300 - M24 380 32000

Table 3. General classification of concrete footings for the types of columns assuming a minimum of III category of grounds as in Table 2.

For the assumptions made above, the concrete footing type is assigned to the type of column as follows: 

Concrete footing type Height H
[mm]
Column types - base-plate diameter x base-plate side x bolt spacing Maximum possible heights of the columns used
B-50 900 114 120 with a 224x180 base-plate including SAL SYG 6
B-51 1000 114 D60 120E 146G with a 260x200 base-plate 7
B-51A 1200 114 D60 120E 146G with a 260x200 base-plate  * 7
B-60 1000 146 with a 320x250 base-plate 8
B-71 1000 146H  176 178K 180M with a 400xz300 base-plate with single and double extension arms up to 1.5m outreach length 12
B-70 1200 176 180M with a 400xz300 base-plate including versions wzm 12
B-80 1500 Columns type MAL with a 400xz300 base-plate 16

.

* 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.