This article explains how the capacity is calculated for a concrete ring pole. There are four strength checks that are applied to a concrete pole: normal stress, moment, shear stress, and shear force. The strength of a concrete pole is defined by the maximum tip load, moment capacity diagram, shear capacity diagram as explained below.

For a concrete pole only the tip load and moment capacity diagram can be entered. Hence to check against the normal stress it first needs to be back calculated. This is done using the following steps:

Step 1: The tip load of the concrete pole is converted into a maximum bending moment at the ground line using the formula:

It is important to note here that the default embedment depth in the library is used in this formula as it is assumed that the tip load entered in the library corresponds to the default embedment depth.

Step 2: The calculated maximum bending moment at ground line is used to calculate the normal stress limit (effective modulus of rupture) of the material using the formula:

If there is a bending moment capacity diagram then this will be used to assess the bending moment along the pole. However if there is no bending moment diagram it will be back calculated using the geometry of the ring and the normal stress limit (see Normal Stress section).

To calculate the maximum bending moment at any point along the pole (when there is no moment capacity diagram) the following formula is used:

Shear force is the resultant force acting along a cross section. Consider the following example where there are 2 horizontal point loads on the pole. The top section has no shear force, between the two point loads has a shear force of 10kN and underneath the second point load there is no shear force as the two point loads cancel out.

If the shear force is not horizontal to the pole such as in the following example where a point load of 5kN horizontal, -10kN vertical is applied. In this case below the point load the shear force in the pole will be 5kN.

If no shear force capacity diagram the shear force capacity is back calculated using the following formula:

Otherwise if entered, the a shear force capacity diagram will be used.

Note the shear force is not available in the text Structure Summary FEA (in the FEA panel). However it can be accessed via the inbuilt reports.

Shear stress is not uniform in a non homogenous material such as a steel reinforced concrete pole. However it is still calculated and assessed against the shear stress limit. The shear stress limit is set to the normal stress limit multiplied by 0.25. Note the shear force diagram is not linked to the shear stress calculations.

The shear stress calculation takes into account both forces and torsions (unlike the shear force assessment which does not account for torsion). The following formula is used to calculate shear stress:

Consider the below example where there is a 10kN point load at 2m from the centre of the pole (creating a torque) and another point load half way up the pole (both point loads are horizontal).

The shear stress in the top half of the pole will be calculated as follows (note this is a hand calculation and FEA results will vary from these results based on the amount of deflection in the pole):

The shear stress in the bottom half will be calculated:

Note that in this example the torsion is making up the largest component of shear stress.