# Internal Forces of a Truss

1. Jul 12, 2015

### smrgn92

1. The problem statement, all variables and given/known data

Hi there, I've been having problems with this kind of truss in the attached file. I'm used to resolving forces where the supports are located along the same x-axis, but this time I have the supports on the y-axis and a point load in the same direction. I haven't shown it, but there is a pin joint located at joint A and C. I would like to just simply know if my method is the correct solution.

2. Relevant equations

See attached file.

3. The attempt at a solution

Firstly I resolved the forces for the reactions at the support in the x direction using the moment of equilibrium equation around joint A. However, I cant seem to find a logical solution for resolving the reactions in the y direction, but upon further analysis I don't think this is entirely necessary, since I'm able to resolve the internal forces without these reaction forces. Have a made a mistake in my analysis?

Thank you forum :)

#### Attached Files:

• ###### Internal Forces.pdf
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2. Jul 12, 2015

### PhanthomJay

Your results look good, however, please indicate whether the members are in compression or tension, because it is not clear from your calculations. Note also that since members of a pure truss are 2-force members, there can be no vertical support reaction at the pin at C.

3. Jul 12, 2015

### smrgn92

Thank you, didn't realise there would be no vertical reaction at the pin joint at C, but makes perfect sense. I also indicated tension or compression in the image with arrows on the internal members. I guess another question I had that you answered by confirming my results was that not all reaction forces are necessary to calculate internal forces. :)

4. Jul 12, 2015

### PhanthomJay

Hmm your arrows look good I guess as long as they are joint forces and not member forces so please indicate (C) for compression in member or (T) for tension in member, to remove all doubt.
Yes you can solve for member forces as you did without first getting that 0 vertical reaction force at C, but generally trusses member forces are more easily solved when you find support reaction forces first, so it's always a good idea to solve for them as a first step.