Bending Moment of Beam Equation in Terms of d: Tips and Advice

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Discussion Overview

The discussion revolves around deriving the bending moment equation for a beam in terms of the depth variable \(d\). Participants are exploring algebraic manipulations and substitutions related to the bending moment, force, and moment of inertia in the context of beam mechanics.

Discussion Character

  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents the equation \(f/y = M/I\) and seeks assistance in expressing it in terms of \(d\).
  • Another participant arrives at the equation \(2f/12M = d/bd^3\) and asks how to isolate \(d\).
  • A subsequent post questions the correctness of the derived expression \(d = \text{cube.root}(12M/2fb)\).
  • One participant points out a potential typo in the moment of inertia, suggesting it should be \(I = bd^3/12\) instead of \(bd^3/13\), and implies that the issue may lie in algebraic substitution.
  • Another participant confirms using \(bd^3/12\) and expresses confidence in their solution, seeking validation.
  • A participant provides specific values for \(M\), \(f\), and \(b\) and calculates \(d\) using their formula, asking for confirmation of their result.
  • Another participant critiques the calculation, noting the presence of \(d\) in the numerator and \(d^3\) in the denominator, suggesting a misunderstanding in the algebraic manipulation.

Areas of Agreement / Disagreement

Participants express differing views on the correctness of the derived equations and the algebraic steps involved. There is no consensus on the final expression for \(d\), and the discussion remains unresolved regarding the accuracy of the calculations and the algebraic manipulations.

Contextual Notes

Participants have noted potential typos in the moment of inertia formula and have expressed uncertainty about the algebraic steps required to isolate \(d\). The discussion reflects various interpretations of the equations and their manipulations.

jon1984
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Hi,

I have the equation:

f/y=M/I

And y = d/2; I = bd^3/13

However i have been trying for ages to try and get this equation in terms of d. Can anyone help me/ give advice as to how?

Thanks
 
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i've got to:

2f/12M = d/bd^3

How do i collect the d's now?
 
is this correct?

d = cube.root(12M/2fb)
 
The I of a rectangle is bd^3/12, you may have a typo. The rest is algebraic substitution. Is that where your problem lies...in the algebra?
 
yeah, i did use bd^3/12 when working out to find d.

I think my above solution is correct, just need someone to check
 
M = 45Nm
f = 240x10^6
b = 16mm

Im trying to find d.

I used the formula:
f/y = M/I

re-arranged to find d:
d = cube.root(12M/2fb)

Put my values in and got an answer of:
d = 0.041m

Is this correct?
 
No. You have a 'd' in the numerator and a 'd^3' in the denominator. 2f/12M = d/bd^3. That's the same as 2f/12M = 1/bd^2
 

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