# Analysis of structures

I have a question on setting up a problem to find the reactions and forces in members of trusses.

So initially I find the reactions at the pins using a free body diagram, I get that. However, when I need to find the forces of the particular members am I supposed to take an initial guess as to whether or not the forces are compressive or tensile and then based on whether or not I get a negative for my result proves the accuracy of initial assumption?

Or do I analyze the loads being placed on the structure and should it tell me from observation what direction the forces are going?

Any help is greatly appreciated.

tiny-tim
Homework Helper
Hi drewdiddy!
…am I supposed to take an initial guess as to whether or not the forces are compressive or tensile and then based on whether or not I get a negative for my result proves the accuracy of initial assumption?

Or do I analyze the loads being placed on the structure and should it tell me from observation what direction the forces are going?
Presumably you're giving the forces names like F1 F2 F3 etc ? …

in that case, yes you have to make a guess as to which direction the force is (and mark it on your diagram) …

if it turns out to be wrong, that doesn't matter, since, as you say, it'll come out negative.

(incidentally, the same thing happens in electric circuits … you just guess which way the current is flowing, and sometimes it comes out negative! )