Solving Strange Structures: Platt Truss Issues

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Discussion Overview

The discussion revolves around the analysis of a Platt truss structure, focusing on the calculation of forces in its members, particularly the diagonal member EK. Participants are examining discrepancies between their calculations and those presented in a textbook, exploring potential errors in their methods or the book's solutions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant calculates the force in a diagonal member of the truss to be approximately 240 kN, while the textbook states it is 96.2 kN, leading to questions about the source of the discrepancy.
  • Another participant suggests verifying the summation of forces in both x and y directions and the calculation of support reactions, implying that an error may exist in the original calculations.
  • A participant outlines their procedure for calculating axial forces, including equations for force balance and moments, but expresses uncertainty about the results.
  • Some participants express skepticism about the textbook's accuracy, with one specifically asking if the book in question is by Hibbeler, suggesting a familiarity with common texts in the field.
  • Another participant confirms that they find the original poster's calculations to be correct, indicating a potential consensus on the method used.

Areas of Agreement / Disagreement

There is no clear consensus on the correctness of the textbook's answer versus the calculations presented by the participants. Some participants believe the calculations are correct, while others suggest the possibility of errors in the calculations or the textbook.

Contextual Notes

Participants have not provided complete details on all assumptions or definitions used in their calculations, and there are unresolved aspects regarding the moment equations and force summations that could affect the results.

Who May Find This Useful

This discussion may be useful for students or professionals engaged in structural engineering or mechanics, particularly those working with truss analysis and seeking to understand common pitfalls in calculations.

physicsCU
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OK, I am having some issues with some trusses.

The first one is a platt truss, h = 8 m, length of horizontal beams = 8 m

F = 340 kN occurs at each of the 5 pts between the supports, one is a pin, the other is a roller.

I get the force in a diagonal to be ~240 kN, but the answer in the book is 96.2 kN, a factor of 2.5.

I did a bunch of searches on google, and I think my procedure is right, even according to the examples. So where does that 2.5 come from? Or is the book wrong? Part of me thinks that the book is wrong, but this isn't the first problem in this chapter that would be true.

Thanks!
 
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If you show me an equation I can spot an error, but I don't feel like doing the problem from scratch right now because its 1:13 am, sorry. Just make sure you sumed the forces in the y and x directions correctly and found the correct support reactions when you sum the moments about one of the supports. The factor of 2.5 comes from you getting the wrong anwser. A picture would help a lot here.
 
Last edited:
OK, we are to find the axial force in member EK.

Point A is a pin support, H is a roller.

F_x = A_x = 0
F_y = H_y + A_y - 5F = 0
M_a = 6LH_y - LF - 2LF - 3LF - 4LF - 5LF = 0

These give A_x = 0, H_y = 850 kN, A_y = 850 kN

Now I cut through DE, EK, and KL.

F_y = H_y - 2F - EKsin(45 degrees) = 0

This is the procedure I did.

I can't get a pic up, but imagine this:
B C D E G
__________
/|\ |\ | /| /|\
/ | \| \|/ |/ | \
A ---------------- H
I J K L M

F acts downward at I,J,K,L,M

Again L = 8 m, F = 340 kN

L is the vertical and horizontal distances, but not diagonals.

Hope this helps.

The bridge won't work out, but you get the idea.
 
your work seems right, book might be wrong. Is the book by hibbeler 9th ed.?
 
No, it is Bedford and Fowler.

I will look at it tomorrow and see what I can do with it.
 
Your work does seem correct, to me at least.
 
Thanks.

I had another issue earlier today that was resolved by an incorrect moment equation. I will work on that tomorrow, and report back here.
 

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