Truss Method of Sections (2013 q23 a engineering) (1 Viewer)

[Jig]

That's the sample answer, and it says BC=0. What I am I doing wrong in finding BC? I'm using method of sections where I've cut through BC. And this is my equation (which is wrong):

 

[Jig]

And also, they're analysing joint C?? I though BC goes through C so how can you analyse joint C since BC goes through C?

 

[StudyOnly]

Just cut the beams AB, BC and CD diagonally. Then if you take the moments about A, F(AB) and F(CD) go through A so they don't count. The only force that counts is F(BC), but we know the moment about A must be 0, meaning F(BC) = 0.

OR

You can do method of joints at C, since there are no vertical forces F(BC) = 0, because sum of vertical forces has to equal 0 at every joint.

 

[Jig]

Just cut the beams AB, BC and CD diagonally. Then if you take the moments about A, F(AB) and F(CD) go through A so they don't count. The only force that counts is F(BC), but we know the moment about A must be 0, meaning F(BC) = 0.

What about that 2.5kN force? Why do we disregard that?

 

[StudyOnly]

When you do the method of sections, I believe you're only analysing the forces at where the cuts are made. You're cutting through the members, and hence you're only looking at the axial forces along those members. You're not cutting through point B, so the force at B isn't taken into consideration.

I could be wrong (it's been a decade since I've taken an engineering statics course). I always prefer to use the method of joints since its more intuitive.

This.

Since we are analysing the left hand side of the diagonal cut. I'll attach an image to make it easier.

 

[Jig]

When you do the method of sections, I believe you're only analysing the forces at where the cuts are made. You're cutting through the members, and hence you're only looking at the axial forces along those members. You're not cutting through point B, so the force at B isn't taken into consideration.

I could be wrong (it's been a decade since I've taken an engineering statics course). I always prefer to use the method of joints since its more intuitive.

This.

Since we are analysing the left hand side of the diagonal cut. I'll attach an image to make it easier.

Ok wow that helps a lot thanks!