# Pin-jointed framework

1. Apr 23, 2013

### MMCS

ƩFx = 0
ƩFy = 0
ƩMoments about a point = 0

Ay + By - 2KN = 0

Momments about A = -(2KN * 2.4) + 1*ReactionC = 0
-4.8KN = reactionC

Sub into first eq

Ay +(-4.8) -2 = 0
Ay = 6.8

To find out the reaction in member AB

I have

6.8 - Tab Cos(tan-10.8/1) = 0
Tab = 6.8/Cos(tan-10.8/1) = 8.738

I have the answer to be 2.56

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2. Apr 23, 2013

### PhanthomJay

What about the horizontal component of the reaction force at C? What can you say about the horiz and vert components of the reaction force at A?

3. Apr 23, 2013

### MMCS

The x and y at A should equal the support AB
The x values for A and C i am unsure how to work out, i have only dealt with problems where one support is a roller support

4. Apr 23, 2013

### PhanthomJay

You mean the vector sum of the x and y components of the reaction force at A should be equal in magnitude to the force in member AB?
Truss members are 2 force members. Since no other member besides AB frame into support A, then the total resultant reaction force at support A must be in the direction of member AB.

I note that you have posted this question in the engineering section also. You should probably respond in that thread instead of this one, and avoid double posting in the future thanks.

5. Apr 23, 2013

### MMCS

If the reaction force at A is equal to the magnitude of vector AB, then i have an answer of 6.8KN, but the answer is 2.56KN can you point out the mistake in my working

6. Apr 23, 2013

### PhanthomJay

The reaction force at A acts in the direction of member AB. Sum moments about C to solve for it or its components.

7. Apr 23, 2013

### MMCS

Ok i realise where my confusion was on this problem thanks for your help

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