The number of reactions in pinned support

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SUMMARY

The discussion centers on the identification of support reactions in a structural analysis problem involving pinned and roller supports. Specifically, it clarifies that there are four pinned supports, with only three reactions accounted for due to the absence of external support at point D. The roller support is identified as point B, while point A is confirmed as a pinned support. The conversation emphasizes the importance of understanding support types and their implications on reaction forces in structural engineering.

PREREQUISITES
  • Understanding of structural analysis concepts, specifically support types.
  • Familiarity with free body diagrams and their application in engineering.
  • Knowledge of truss member connections and their significance in structural integrity.
  • Basic principles of mechanics related to forces and reactions in structures.
NEXT STEPS
  • Study the differences between pinned and roller supports in structural mechanics.
  • Learn how to create and interpret free body diagrams for various structures.
  • Research the role of connection types, such as soldered and plate connections, in truss design.
  • Explore common practices in structural analysis for identifying support reactions.
USEFUL FOR

Structural engineers, civil engineering students, and anyone involved in analyzing support reactions in truss structures will benefit from this discussion.

fonseh
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Homework Statement


why the number of reaction is 3 ? How to know that ? shouldn't the total number of reaction = 8 ?

Homework Equations

The Attempt at a Solution


There are 4 pinned support in the structure . Each pinned support can support vertical and horizontal reactions...
 

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r is the number of external support reactions. Note that one is a pinned support and the other is a roller support.
 
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Where are the roller ? Can you point out ? ?? I didnt see any roller in the structure
 
Where are the roller ? Can you point out ? ?? I didnt see any roller in the structure
 
fonseh said:
Where are the roller ? Can you point out ? ?? I didnt see any roller in the structure
The roller is labelled 'B' , although I do agree that the difference is not that obvious...
 
It's sort of a different way of showing it, but a is a pinned support fixed to the floor, and B is the roller support allowed to slide or roll across the floor. Look carefully at each and compare the difference.
 
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PhanthomJay said:
It's sort of a different way of showing it, but a is a pinned support fixed to the floor, and B is the roller support allowed to slide or roll across the floor. Look carefully at each and compare the difference.
ok , i accept that the B is roller , then how about D ? Is it a roller or pinned support ? Why we don't have to consider the reactions at part D ?
 
fonseh said:
ok , i accept that the B is roller , then how about D ? Is it a roller or pinned support ? Why we don't have to consider the reactions at part D ?

A is a pin support, at D there is nothing supporting it (restricting it from moving) so there are no reaction forces.
 
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How
sakonpure6 said:
D there is nothing supporting it
How do we know that ? There's something connecting 2 force members together at D , right ? So , it must be pinned or roller or other supporting parts , right ? Why you said there's no support there ?
 
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fonseh said:
How

How do we know that ? There's something connecting 2 force members together at D , right ? So , it must be pinned or roller or other supporting parts , right ? Why you said there's no support there ?

By support, I mean fixed to a medium like Earth. Here, the "cap" on point D is just to connect the truss members together. On a free body diagram, there won't be any external support reactions
 
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  • #11
sakonpure6 said:
"cap" on point D
What do you mean ?
 
  • #13
sakonpure6 said:
Basically a piece of metal is soldered to to help connect members AD and CD
ok , understand . But , how do you know that ? It's not stated in the question . And the author also doesn't stated that the B is a roller , how do we know that ?
 
  • #14
fonseh said:
ok , understand . But , how do you know that ? It's not stated in the question . And the author also doesn't stated that the B is a roller , how do we know that ?

Again , there is nothing externally connected at point D , so no external reaction forces

Usually, the author should state what is a pin/roller support...but it comes with practice.
 
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