Method of Joints Truss Analysis

[member 392791]

Hello,

I was curious, for the method of joints truss analysis, when considering a joint, is it assumed that all forces at that joint are radiating away from the joint (meaning tension), even if it was already found from a previous joint that the member is acting in compression?

I'm not sure if this question is clear or not, I mean say you have a joint A and B..it was found that member AB is in compression. Does that mean when analyzing joint B, you still consider the force of member AB as tension, or do you keep the negative sign when trying to find the member AC?Also, say member AF is in tension, thus has a positive sign for force. Does that mean member FA is tension as well, or is it now compression?

 

[PhanthomJay]

A member can be in tension or compression, but not both at the same time! Tension forces on members pull away from the joint always. Don't assign a positive number to them. Sometimes they act upward and sometimes downward.
So if a tension force in member AB pulls upward on joint A, the tension force in member BA pulls downward on joint B, away from the joint. If you don't use this convention ( actually an application of Newtons laws) correctly, you will be hopelessly devoured by the plus and minus sign.

 

[timthereaper]

When you draw your diagram, keep the arrows in the conventional directions. Don't flip them when you find out the quantity is negative. Build all your equations with the arrows in their conventional directions and then once you've solved everything, then you can see what is tension or compression.