Statics - Compression and tension forces in a truss

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SUMMARY

The discussion centers on the forces in members KJ, CJ, and CD of a truss structure. The participant correctly calculated the force in KJ to be 26.8 kN, identifying it as a compressive force, contrary to the textbook which lists all three members as tension. The participant argues that for all members to be in tension, an external horizontal force would be necessary, which is absent, indicating a potential error in the textbook. The conclusion is that at least one of the members must be under compression.

PREREQUISITES
  • Understanding of static equilibrium principles, specifically ΣFx=0
  • Knowledge of truss analysis and member force determination
  • Familiarity with tension and compression forces in structural engineering
  • Basic proficiency in interpreting engineering textbooks and problem statements
NEXT STEPS
  • Study the method of joints in truss analysis
  • Learn about the role of external forces in maintaining equilibrium in structures
  • Explore common errors in engineering textbooks regarding force analysis
  • Investigate the calculation of forces in truss members using software tools like SAP2000
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Engineering students, structural analysts, and professionals involved in truss design and analysis will benefit from this discussion.

jacobthejones
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Homework Statement


I am trying to find the forces in KJ, CJ, and CD.
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Homework Equations


ΣFx=0

The Attempt at a Solution


I found the magnitude of KJ to be 26.8kN, which is correct (according to answers in the back of the book). I believe it is a compressive force. The book lists all 3 forces as tension.

If all 3 members were under tension, there would have to be a reactionary horizontal force external to these three members acting on the right side of the truss to maintain equilibrium. Because there is no reactionary force with a horizontal component on the right side, the three members cannot all be experiencing tension. Therefore, the book has a typo and either one or two of the members are experiencing a compressive force.

Is this logic correct?
 

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I would agree.
If all three of those are tensions, then there would be positive tension across the length of the truss and the anchor points (A and G) would not be simple vertical supports.
 
I confirm. I get 26.8 N compression in KJ and 10.8 N tension in CJ.
 

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