# The number of reactions in pinned support

• fonseh
In summary, there are 4 pinned support in the structure and each pinned support can support vertical and horizontal reactions. B is the roller support allowed to slide or roll across the floor. There is something connecting 2 force members together at D, so it must be pinned or roller or other supporting parts.
fonseh

## Homework Statement

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

## 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.

fonseh and CivilSigma
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.

fonseh
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.

fonseh
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 ?

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

PhanthomJay and fonseh
sakonpure6 said:
"cap" on point D
What do you mean ?

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 ?

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.

fonseh

## 1. What is the significance of the number of reactions in pinned support?

The number of reactions in a pinned support is important because it helps determine the stability and structural integrity of a structure. It also affects the amount of load a structure can support and the distribution of forces within the structure.

## 2. How is the number of reactions in pinned support calculated?

The number of reactions in pinned support can be calculated using the equations of static equilibrium, where the sum of all forces and moments acting on the structure must equal zero. It is also important to consider the number of unknown reactions and the number of available equations to solve for them.

## 3. What factors can affect the number of reactions in pinned support?

The number of reactions in pinned support can be influenced by various factors such as the type of support, the type of load applied, and the geometry and material properties of the structure. Any changes in these factors can alter the number of reactions in pinned support.

## 4. How does the number of reactions in pinned support vary for different types of structures?

The number of reactions in pinned support can vary depending on the type of structure. For example, a simple beam supported by two pinned supports will have two reactions, while a three-dimensional truss structure may have multiple reactions depending on the number of joints and supports.

## 5. Can the number of reactions in pinned support be greater than the number of supports?

No, the number of reactions in pinned support cannot be greater than the number of supports. This is because the equations of static equilibrium require that the number of unknown reactions must equal the number of available equations, which is equal to the number of supports for a pinned support structure.

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