Thickness from Moment of Inertia

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Discussion Overview

The discussion revolves around calculating the wall thickness of a pipe using the moment of inertia, particularly in the context of predicting the behavior of a sign post under wind load for a university assignment. Participants explore the relationship between wall thickness, diameter, and moment of inertia.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant inquires about the feasibility of calculating wall thickness using the moment of inertia for a pipe in a parametric spreadsheet.
  • Another participant notes that the moment of inertia depends on both wall thickness and diameter, suggesting that a larger diameter requires a smaller thickness.
  • A participant confirms the equation for moment of inertia, PI/64*(Od^4 - Id^4), but points out that there are two unknowns (outside and inside diameters), making it impossible to solve without additional assumptions.
  • There is a suggestion that one must assume one variable or select the lowest weight tube that can support the load to proceed with calculations.

Areas of Agreement / Disagreement

Participants generally agree on the relationship between moment of inertia, wall thickness, and diameter, but there is no consensus on how to resolve the issue of having two unknowns in the equation.

Contextual Notes

The discussion highlights the dependence on assumptions regarding the diameters and the need for additional information to solve the problem effectively.

Mechanical-909
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Hi guys... I am creating a parametric spreadsheet to predict the behaviour of a sign post when subject to a specified wind speed for a uni assignment . Currently half way through creating the spreadsheet and one of the questions requires me to calculate a wall thickness of the pipe using the moment of inertia.. is this possible? if so how? please help guys all solutions are welcome :)!

Thanks
 
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The moment of inertia of the pipe depends upon its wall thickness and diameter. The larger the diameter, the smaller the required thickness. Once you determine I, you may have to choose amongst available sizes and strengths, with economics also in mind, amongst other considerations.
 
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PhanthomJay said:
The moment of inertia of the pipe depends upon its wall thickness and diameter. The larger the diameter, the smaller the required thickness. Once you determine I, you may have to choose amongst available sizes and strengths, with economics also in mind, amongst other considerations.

Okay thank you. Guess my question is that, is possible from the equation: PI/64*(Od^4 - Id^4) Od being outside Diameter and Id being the inside diameter.
 
Mechanical-909 said:
Okay thank you. Guess my question is that, is possible from the equation: PI/64*(Od^4 - Id^4) Od being outside Diameter and Id being the inside diameter.
that equation is correct for the moment of inertia, but you have two unknowns, the outside and inside diameters, so you can't solve for either without assuming one of the variables or choosing the lowest weight available tube that will support the load.
 

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