Analysis of Structures: Finding Reactions & Forces in Trusses

[drewdiddy]

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]

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! )