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?

 

[piercas]

Hello! The method of joints is a common method used to analyze truss structures. It involves breaking down the truss into individual joints and analyzing the forces acting on each joint.

To answer your question about the reaction forces at joint A, there can be both horizontal and vertical components to the reaction force at a pin joint. The vertical component will depend on the load and geometry of the truss, and it can be either tension or compression. In this case, the vertical component at joint A is 5*sqrt(2) kN tension, as indicated by the answer for AC.

The horizontal component of the reaction force will always be zero for a pin joint, as it can only transmit forces in the direction of the member it is attached to. Therefore, the horizontal component of the reaction force at joint A does not need to be included in your analysis.

I hope this helps clarify the method of joints for you. Remember to always draw free body diagrams for each joint and apply the equations of equilibrium to solve for the unknown forces. Good luck!