This article explains how the capacity is calculated for a concrete pole. Concrete poles have a single strength check performed which the moment capacity check. Note concrete poles do not return results for normal stress or shear stress since the concrete pole material is not homogeneous (it is a mixture of concrete and steel reinforcement).

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 an equivalent moment capacity diagram will be generated using the Breaking Tip Load, Constant Moment Capacity Length and the actual embedment of the pole in the model (1). The bending moment at the base of the pole is calculated with the formula tip load x length of pole above ground. Using the maximum bending moment at the ground line a straight line is drawn to the tip of the pole so that the tip of the pole has a moment of zero. However it will then use the parameter Constant Moment Capacity Length to keep the moment capacity above this distance from the tip to be constant. To illustrate, consider the following example where the pole has a tip load of 10kN, constant moment capacity length of 1.5m and is 10m out of the ground in the model. The moment capacity at the ground line is 10kN x 10m = 100kNm. Then a straight line is drawn to the tip of the pole so that at the tip it is 0kNm. However at a distance of 1.5m from the top of the pole the moment capacity is calculated as 100kNm * 1.5/10 = 15kNm. This moment capacity of 15kNm then remains constant from 1.5m from the tip to the tip.

Note (1): By using the modelled embedment depth rather than the library default embedment it means that regardless of the embedment depth if a point load is placed at the tip of the pole with magnitude equal to the tip load limit in the library then the pole utilisation will be close to 100%. The reason it will not be exactly 100% is due to taking into account p-delta effects.

Note the above example of 10kN tip load and 1.5m constant moment capacity length and 10m out of the ground is equivalent to having the flowing moment capacity diagram (if both poles are out of the ground 10m then they will have the same moment capacity diagram).

Rectangular concrete poles behave the same way as the ring poles however they have strength defined in two directions and then use the diamond interpolation to calculate the moment utilisation at any point. In essence the moment capacity diagram would looks like a rectangular pyramid (with the top becoming a rectangular prism if Constant Moment Capacity Length is non zero).

The shear force will be assessed if there is a shear force capacity diagram for the concrete pole. 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.

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.

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