How do you identify compression and tension in members in a truss?

Question

You know the direction and value of the loads applied

Answer 1

When you do your calculations ( balance of x, y and moment) your answer will be positive or negative. If at the point your analyzing the force is negative (towards the point) then you are pulling and are in tension if your answer is positive (traveling towards the center of the member and away from the point you are analyzing) then the member is in compression.

Answer 2

It's a just a matter of practice. Try starting at the supports. You know the direction of the reaction there. Now, either draw, or just visualize, the free-body diagram for all the members at that joint. You have to have equilibrium, so you start there. If the reaction is upward, any members with a vertical component to their orientation (any member other than a horizontal) will have a force opposing the reaction (vertically downward)*. If the member has any angle to it, there will be a commensurate horizontal force. Use that to determine the direction of the forces in any horizontal members at that joint. Work your way across from the supports toward midspan.

*Most of the time. There are some truss configurations with zero-force members. To be sure, look at the joint at the other end. There has to be at least one member whose orientation can provide an opposite force to satisfy equilibrium (for example, there can't be any force in the vertical member at a T joint, because there is no other member with any vertical component).

Answer 3

Compression pushes

Tension pulls.

Concrete is excellent in compression and holds up well, but it is horrible in tension.

Steel is excellent in tension, but horrible in compression so the combination of the two in steel reinforced concrete is an excellent solution.

When you calculate the stresses in a beam (like part of a truss) then you balance the forces to see how the beam is stressed and you use that to create a shear and moment diagram which will give you the forces of stress and tension involved.

I am not going to give the mathematical equations here this article will give you more information:  http://en.wikipedia.org/wiki/Strength_of... basically you take the force times the distance to calculate the stress or tension at that point and do that for all loads on the beam and then add them in the shear and moment diagram (http://en.wikipedia.org/wiki/Shear_and_m...  From the shear and moment diagram you can calculate the torsion diagram that tells you torsion stresses on the beam.

Read the articles or take the class for more specific details.