The maximum bending moment (which is used to calculate the maximum tip load) is calculated using the formula:
The section modulus in this formula is dependant on the diameter. Hence there is a relationship between the maximum bending moment, the diameter of the pole and the modulus of rupture (also known as the material strength).
If the box is checked for Derive Modulus of Rupture From Tipload then the software will use the rated tip load (field Rated Tip Load X/L) and ground line diameter (field Nom. Ground Diameter X/L) to automatically calculate the modulus of rupture (field Modulus of Rupture) for the specified. When the box is checked any changes to either the tip load or ground line diameter will result in the Modulus of Rupture being automatically recalculated. If the box is unchecked it will use the modulus rupture and the ground line diameter to automatically calculate the tip load. When the box is unchecked any changes to the modulus of rupture or the ground line diameter will result in the tip load being automatically recalculated.
Note if the box is unchecked and you change the tip load it will automatically check the box and then recalculate the Modulus of Rupture given the new tip load and the ground line diameter.
Example
Consider the following wood pole, 12m out of the ground (15m - 3m), with a modulus of rupture of 85MPa and ground line diameter of 43cm.
Since the box is for Derive Modulus of Rupture From Tipload is unchecked the tip load will be calculated from the modulus of rupture and the ground line diameter.
The max bending moment is calculated as:
The section modulus of a round pole is (this can be found online or in reference text books):
Hence the maximum bending moment is calculated as:
The maximum tip load for a pole is calculated as the maximum bending moment divided by the height above ground:
This value of 55.29kN matches the value in the screenshot above.
If now the ground line diameter is changed to 45cm then the tip load is automatically recalculated to 63.37kN which can be calculated using the formulas above.
