The number of reactions in pinned support

[fonseh]

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...

 

[PhanthomJay]

r is the number of external support reactions. Note that one is a pinned support and the other is a roller support.

 

[fonseh]

Where are the roller ? Can you point out ? ?? I didnt see any roller in the structure

 

[fonseh]

Where are the roller ? Can you point out ? ?? I didnt see any roller in the structure

 

[CivilSigma]

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...

 

[PhanthomJay]

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.

 

[fonseh]

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 ?

 

[CivilSigma]

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.

 

[fonseh]

How

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 ?

 

[CivilSigma]

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

 

[fonseh]

"cap" on point D

What do you mean ?

 

[CivilSigma]

What do you mean ?

Basically a piece of metal is soldered to to help connect members AD and CD.

Edit, or it can be plate connection: https://www.google.ca/search?q=trus...onnecting+truss+members&imgrc=NvDUwadH2ASccM:

 

[fonseh]

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 ?

 

[CivilSigma]

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.