Understanding Axial Members in Structural Statics - Learn the Basics

[TSN79]

Hi! Just discovered this forum, and it's like the best ever! Anyway, I do have a question. In statics I hear about an "axial member" of a structure. This means that the force in this member works along the rod or wire or whatever, so that the direction of the force need not to be determined. What I'm now wondering about, is when this is the case and when it is not. How can I tell? Is it usually obvious just by looking at the structure?

 

[quantumdude]

Hi! Just discovered this forum, and it's like the best ever!

Damn straight!

Anyway, I do have a question. In statics I hear about an "axial member" of a structure. This means that the force in this member works along the rod or wire or whatever, so that the direction of the force need not to be determined.

Yes, a member is axial if it is a 2-force member. That means that the system of forces acting on the member can be reduced to 2 forces acting at distinct points. To see if a system of forces is equivalent to the 2 forces, just sum the forces and the moments about any point and see if they are the same.

What I'm now wondering about, is when this is the case and when it is not. How can I tell? Is it usually obvious just by looking at the structure?

In general, you would have to try to reduce the system of forces on each member to 2 forces. But if you are looking at a structure composed entirely of beams, joints, and supports then you should be able to determine by inspection whether the members are axial. For instance, if member A is connected to member B in the middle of member B (as opposed to one of the ends), then member A cannot be a 2-force member (and hence it cannot be an axial member). In that case, the two components of the reaction force must be treated as independent variables.

You can find detailed discussions of this very issue in the chapter(s) on structural analysis. Structures whose members are all axial are called trusses and structures that have at least one non-axial member are called frames or machines. (Frame if the structure is intended to be static, machine if intended to move).

 

[TSN79]

Thanks! So would it be correct to say that a member is azial if forces only act on the members ends? Where is this chapter on structural analysis you mentioned?

 

[quantumdude]

So would it be correct to say that a member is azial if forces only act on the members ends?

Yes.

Where is this chapter on structural analysis you mentioned?

It varies from book to book, of course. Look in the index for "truss", "frame", "machine", and you'll be taken to the appropriate chapter(s).

What book are you using, by the way? I might have it, then I can tell you for certain which chapters it is.

 

[TSN79]

It's a Norwegian book so I kinda doubt you've ever seen it, but thanks for your help anyway Tom!