Moment about a point in trusses

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SUMMARY

The discussion centers on calculating moments in trusses, specifically the moment about point C using the forces and distances provided. The author initially calculated the moment contributions from forces 3.2 kN and 6 kN, noting that the moment 3.2(4) is indeed relevant to point C despite being applied at point A. The key takeaway is that the perpendicular distance from the line of action of a force to the point of interest is crucial in moment calculations. Additionally, the reactions at point A were found to be incorrectly calculated, necessitating a reevaluation by summing moments about point B.

PREREQUISITES
  • Understanding of static equilibrium in trusses
  • Familiarity with free body diagrams
  • Knowledge of moment calculation techniques
  • Basic principles of forces and distances in mechanics
NEXT STEPS
  • Study the method of summing moments about different points in trusses
  • Learn how to correctly draw and interpret free body diagrams
  • Explore the calculation of reactions at supports in truss systems
  • Review the principles of static equilibrium and their application in structural analysis
USEFUL FOR

Engineering students, structural analysts, and professionals involved in mechanics and structural design will benefit from this discussion, particularly those focusing on truss analysis and moment calculations.

chetzread
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Homework Statement



for moment about C, the author gave [-F_(FE)](5) +3.2(4) +6(5) = 0
i don't understand the working...Why there are moment 3.2(4) and also moment 6(5)?

Homework Equations

The Attempt at a Solution


for moment 3.2(4) , it's centered at F, so ,it's not moment about C ,am i right?
for moment 6(5) , it's moment about A, am i right?

[/B]
 

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The author has isolated the bot left section in a free body diagram. In order to determine the force in FE, you need to sum moments about C because summing about A or B wil not allow solution of the FE force because the force action line passes through those joints. The 6(5) moment you talk about is the moment of the horizontal reaction force at A about C.
 
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PhanthomJay said:
The author has isolated the bot left section in a free body diagram. In order to determine the force in FE, you need to sum moments about C because summing about A or B wil not allow solution of the FE force because the force action line passes through those joints. The 6(5) moment you talk about is the moment of the horizontal reaction force at A about C.
how about moment 3.2(4) ? it's moment about F,am i right? why the author included it into calculation?
 
chetzread said:
how about moment 3.2(4) ? it's moment about F,am i right? why the author included it into calculation?
Well it is a moment about F, but it also is a moment about C, which is the point of interest . Remenber that moment is force times perpendicular distance from line of action of that force to the point of interest.
 
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PhanthomJay said:
Well it is a moment about F, but it also is a moment about C, which is the point of interest . Remenber that moment is force times perpendicular distance from line of action of that force to the point of interest.
i still don't understand...
the perpendicular distance is the 4m(see red labelled part), it's span from F to A, so moment is about F, not C, am i right?
 

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The perpendicular distance from the line of action of the 3.2 kN force to C is also 4 m. Just extend the 3.2 kN force arrow straight up and measure the perp distance from that line to C. This is one of several methods to calculate moments.
 
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PhanthomJay said:
The perpendicular distance from the line of action of the 3.2 kN force to C is also 4 m. Just extend the 3.2 kN force arrow straight up and measure the perp distance from that line to C. This is one of several methods to calculate moments.
but,the force 3.2kN is being applied at A, not 4m away from C, how can we dothat?
 
PhanthomJay said:
why not?
The force is not applied at C, but at A
 
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you are not understanding how to calculate moments using perpendicular distances from the force line of action.

Now I just noticed the reactions are incorrectly calculated by the author, based on the dimensions and loads given. . You might want to find the correct vertical reaction at A by looking at the entire truss and summing moments about B and report your result.
 

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