The Structural Engineer's Corner

Eng. Onorio Francesco Salvatore

Embedment depth for utility services poles according to Australian Standard 4676

Written By: Lexatus - Oct• 13•13

Embedment depth AS 4676 - ONORIO

When dealing with methods for proportioning footings and poles, an useful reference is the Australian Standard 4676, titled “Structural design requirements for utility services poles”.

For direct-planted poles, hence not on square pads, the Engineer can make some reasonable assumptions on the location of the centre of rotation of the footing and the vertical distribution of bearing stress in the foundation material.

In order to define the embedment depth ‘D’, the following assumptions are considered:

–          The centre of rotation of the footing as located at two thirds of embedment depth below ground surface;

–          The vertical distribution of bearing pressure above the centre of rotation is in the form of symmetrical parabola with its axis of symmetry located at one third of the embedment depth below the ground surface with its maximum value taken as 1.5 fb; and

–          The vertical distribution of the bearing pressure below the centre of rotation is a skewed parabola for which the resultant horizontal reaction force is located at eight ninths of the embedment depth below ground level.

Based on the above assumptions, we can calculate the embedment depth (D) from the following equation:

Embedment depth

C is the resistance of the foundation. Its value is:

– fbu x b for ultimate limit state;

– fb x b for serviceability limit state.

Where:

– fb = the nominal maximum bearing strength of the foundation material (kPa);

– fbu = 1.5 fb (this means that the ultimate value is given by the serviceability one increased by 50%);

– b = the effective width of the footing, projected on a plane perpendicular to the direction of the resultant horizontal force acting on the pole (m);

– M = the overtuning moment acting on the pole at ground level (kNm);

The overtuning moment is given by:

HR x ht, where:

– HR = the resultant of the horizontal forces acting on the pole (kN);

– ht = the height above ground level at which HR acts (m).

For any issues or questions, you can contact the author at:

onorio@strutturista.com

Eng. Onorio Francesco Salvatore

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One Comment


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