Consider the following wood pole for this example
The maximum bending moment (which is used to calculate the Breaking Tip Load) is calculated using the formula:
The section modulus in this formula is dependant on the pole 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 Derive MoR from Tip Load is checked then the software will use the Breaking Tip Load and Diameter Ground to automatically calculate the Modulus of Rupture. When Derive MoR from Tip Load is checked any changes to either the tip load or ground line diameter will result in the Modulus of Rupture being automatically recalculated. If Derive MoR from Tip Load is unchecked then it will use the Modulus Rupture and the Diameter Ground to automatically calculate the Breaking Tip Load. When Derive MoR from Tip Load is unchecked any changes to the Modulus of Rupture or the Diameter Ground will result in the tip load being automatically recalculated.
Worked Example
Consider the pole in the above screenshots. Since Derive MoR from Tip Load is unchecked the Breaking Tip Load will be calculated from the Modulus of Rupture and the Diameter Ground.
The max bending moment is calculated as:
The section modulus of a round pole is as follows (this formula 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 20.5kN matches the value in the screenshot above.