How Do Reaction Forces Work at a Pin Joint in Truss Analysis?

[alex_the_seal]

Hi there!

Can anyone take me through the method of joints procedure for this truss? I am having trouble working out the reaction forces at the pin at joint A. Is there supposed to be a vertical component to the reaction force as well a a horizontal (seeing as it is a pin)? And if so where does it go? is it part of the member AD or outside of it? I am confused.

P.S. The answers are AB = 5 kN T, BC = 5*sqrt(2) kN C, CD = 15 kN C, AC = 5*sqrt(2) kN T, AD = 0. Where T = tension, C = compression.

 

[PhanthomJay]

Hi there!

Can anyone take me through the method of joints procedure for this truss? I am having trouble working out the reaction forces at the pin at joint A. Is there supposed to be a vertical component to the reaction force as well a a horizontal (seeing as it is a pin)?

Yes. A pin can support both a horizontal and vertical component of force. The roller at D, on the other hand, can only support a force normal to its surface; in this case, the roller at D can support a horizontal force only.

And if so where does it go? is it part of the member AD or outside of it? I am confused.

All and any support reactions are external to the system; they are outside of the members. So at A, you have an unknown support reaction of Ax in the horizontal direction, and Ay in the vertical direction. At D, you have an unknown support reaction Dx in the horizontal direction. Use the 3 equilibrium equations of statics to solve for these unknowns. Then proceed with the method of joints to get the member forces. But first, what are the reactions?