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Trouble understating Trusses

by wahaj
Tags: trouble, trusses, understating
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wahaj
#1
Nov21-12, 10:00 PM
P: 142
I missed a class and now I am having some trouble understanding whats going on. I am trying to solve for the components of forces in the truss and whether they exert a compression force or a tensile force. I have attached an image taken from my book and the free body diagrams provided by the book for the truss in question. The question is long and painful but I will really appreciate it if you help me out

*In the diagram forces F and angles θ are the same. There are pin joints at all the points labeled*
The FBD drawn at A looks alright. Fairly easy to to find the direction of the forces in those trusses just be inspecting it. The FBD drawn at C is where things start to get confusing. First of why did the vector CD switch directions? Since CD switched directions as a result so did BC which means that there is now a compression force acting on BD. If CD had remained the same direction as it was in the FBD drawn at A then there would have been a tensile force along BD. Another thing that bothers me is that the effects of the horizontal component of ED was cancelled out by CD in the first diagram so why does CD show up again? Pretty much all of the same questions arise again in the last FBD. I forgot to label some parts in the last diagram so lets just leave it be as if I understand the second diagram I am confident I will understand the third one as well.
Attached Thumbnails
20121121_213907.jpg  
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Studiot
#2
Nov22-12, 02:38 AM
P: 5,462
Hello wahaj, I am sorry to hear that you missed a class and I hopw you are now past whatever the reason was.

I am also sorry to tell you that you need to go back a stage and review some fundamentals or you will continue to struggle with this stuff.

To help with this look at the bar CD in my attachment.

It is any old bar in any old framework, not necessarily yours.

Can you draw the arrows to show the forces in the bar when it is subject to (a) tension (b) compression?

Understanding this is vital to understanding your frame.

Are the free body diagrams you have drawn really copies from your book?
I would be suprised, what is that book?
Attached Thumbnails
bar1.jpg  
wahaj
#3
Nov22-12, 09:17 PM
P: 142
in your diagram there would be tension on the bar if the forces applied at the end of the bar points away from the center. If they point towards the center then there is a compression force acting on the bar. The textbook that I use the the Hibbeler engineering mechanics, statics. The diagram I showed you is from the exercise that I was trying to do to catch. The FBDs are from the solution manual. Tension and compression are intuitive so I don't have any trouble with those. The big thing I don't understand is why CD changed direction. And the other stuff I said but the big thing is why CD changed directions, twice


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