Statics struss tension/compression

[xzibition8612]

Homework Statement

see attachment

Homework Equations

The Attempt at a Solution

Why is DG, DE in tension while HG is in compression according to the FBD? How did the book determine this? Do you just randomly guess tension/compression and then do the calculations and see if you get a positive/negative number? Is there a set procedure? Thanks.

 

[PhanthomJay]

It's ok to make assumptions, consistent with Newtons 3rd law, but limit the number of assumptions you make or else the plus and minus sign will devour you. I notice that the solution shows Ax pointing right, when obviously it must point left, so don't assume when you don't have to. Always calculate support reactions first, numerically, that's a big help. When you look at the right side cut section, since R points up, producing ccw moment about G, then Fde must point inward toward the joint, to produce a cw moment about G . Fde is compression by the way, since it points toward joint . Forces pulling away from joint are tensile.