How to derive equation of deflecting curve for a simple beam

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Discussion Overview

The discussion revolves around deriving the equation of the deflecting curve for a simple beam, specifically focusing on the relationship between deflection, load (q), length (L), and flexural rigidity (EI). The context is primarily homework-related, involving the application of differential equations to solve for deflection.

Discussion Character

  • Homework-related, Mathematical reasoning, Technical explanation

Main Points Raised

  • One participant presents a bending moment diagram and attempts to derive the deflection curve using the second order differential equation, indicating specific moment functions for different segments of the beam.
  • Another participant questions the completeness of the first post, noting that the calculations are not provided, which hinders the ability to identify errors.
  • A subsequent reply reiterates the need for the original calculations to assess the correctness of the moment functions presented.
  • One participant expresses uncertainty about the accuracy of their moment functions and seeks confirmation on whether they are correct before proceeding with the integration.
  • A later post provides a link to an image showing the moment calculations, suggesting a visual aid to clarify the participant's approach.
  • Another participant advises that the moment expressions should align with the bending moment diagram, indicating a discrepancy between the equations and the expected curve.

Areas of Agreement / Disagreement

Participants generally agree that the moment functions are crucial for deriving the deflection curve, but there is no consensus on the correctness of the moment functions presented. Multiple competing views remain regarding the accuracy of the calculations and the approach to solving the problem.

Contextual Notes

There are limitations in the discussion due to missing detailed calculations from the initial post, which are necessary to fully evaluate the correctness of the moment functions. The discussion also depends on the definitions and assumptions related to the beam's loading conditions and boundary conditions.

Blugga
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Homework Statement



Obtain deflection curve in terms of q, L, and EI
fu8i06.jpg


Homework Equations



Use the second order differential equation of the deflection curve to solve.
Meaning that the M(x) is the second derivative and you integrate twice to get V1 and V2

The Attempt at a Solution


From the bending moment diagram I've found that:
(0 ≤ X ≤ 36) → M1(x) = 1800*X which is [(L/8)q](x)
(36 ≤ X ≤ 72) → M2(x) = -100(x-72)2 - 5400(x-72) which is
(-q/2)(x-L)2-(3L/8)q(x-L)

From the back of the book, I can see that the answer is going to be the same as the table values (just for the opposite side). I Have tried, but can't get the right answer. Can anyone help me?
Table values:
33m98ae.jpg
 
Last edited:
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You have shown the back of the book answer, not your own calculations. How can we see what is wrong with your calculations if we don't have them?
 
SteamKing said:
You have shown the back of the book answer, not your own calculations. How can we see what is wrong with your calculations if we don't have them?

yes that is what we're supposed to get, but I'm not getting close to it. Pretty much all I have to do is get the moment of the beam as a function of x for the first half of the beam and the second half. From there, we take the integral twice.

Anyway, I think my first question is, are the moment functions right? Then we can go from there. Thanks.
 
If you evaluate your moment expressions, you should get the same values on your bending moment diagram. Your equations don't match the curve.
 

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