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! )

 

[charng]

I understand your concern about finding the correct assumptions for the forces in truss members. In order to accurately determine the forces in truss members, it is important to consider both the initial assumptions and the loads being placed on the structure.

Firstly, it is important to have an initial guess for the forces in the members. This can be based on the geometry and symmetry of the truss, as well as any known external forces acting on the structure. However, this initial guess should not be solely relied upon for determining the forces.

Secondly, it is crucial to analyze the loads being placed on the structure. This includes both external forces and internal forces from adjacent members. By considering the direction and magnitude of these loads, you can make more accurate assumptions for the forces in the truss members.

In addition, it is important to keep in mind the material properties of the truss members, as this can also affect the direction of the forces. For example, if a member is made of a material that is more prone to compression, it is likely that the force in that member will be compressive.

In summary, to accurately determine the forces in truss members, it is important to consider both the initial assumptions and the loads being placed on the structure. By carefully analyzing these factors, you can make more informed decisions about the direction and magnitude of the forces in the truss members. I hope this helps with your analysis of truss structures.