Resultant vectors in cable and beams

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Discussion Overview

The discussion revolves around the differences and similarities between resultant vectors in cables and beams, particularly in static structures. Participants explore the implications of these differences on force analysis and the assumptions that can be made regarding the direction of forces in each case.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asserts that the resultant vector of a cable points in the direction of the angle it is connected, while questioning if the same assumption holds for beams.
  • Another participant suggests that if Tbc is not pulling at its angle, a free body diagram (FBD) should be set up for section BC.
  • There is a debate about whether the analysis should focus on cables or beams, with some arguing that the external forces acting on section BC should be considered regardless of the type of structure.
  • One participant claims that if Tbc is not aligned with the cable or beam, it would lead to movement or rotation, implying that Tbc must be along the cable or beam for static conditions.
  • Another participant posits that the values of the forces remain the same whether dealing with cables or beams, suggesting no significant difference in this context.
  • One participant concludes that in this specific case, cables and beams behave similarly, emphasizing that all forces act at the pins and are directed downwards due to gravity.
  • Another participant clarifies that the joints in beams can transmit moments, which differentiates them from cables that do not transmit moments.
  • A participant provides terminology for structural members, distinguishing between those that carry moments and those that do not, further elaborating on the characteristics of cables, chains, and beams.

Areas of Agreement / Disagreement

Participants express differing views on the assumptions that can be made regarding the direction of forces in cables versus beams. While some agree on the similarities in static conditions, others highlight the differences in how forces and moments are transmitted through these structures. The discussion remains unresolved regarding the implications of these differences.

Contextual Notes

Participants acknowledge the importance of drawing free body diagrams to aid in understanding the forces at play, but there is no consensus on the necessity of calculations versus conceptual reasoning.

Femme_physics
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I know that when I have cable connected, the resultant vector of each cable will point at the direction of the angle it's connected in.

http://img26.imageshack.us/img26/8454/wiresn.jpg

BUT, suppose we switch all these cables to rigid body structures like beams. I cannot make the same assumption (to my understanding), is that correct?

HERE, for instance, I make the assumption that Tbc is pulling the direction of its angle. If these were beams, and not cables, I cannot make such an assumption can I?

http://img36.imageshack.us/img36/4784/wires2.jpg
 
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Suppose Tbc is not pulling at its angle.

Can you set up an FBD for section BC?
 
Wait, in which case? Beams or cables? Because in cable cases it must!
 
Femme_physics said:
Wait, in which case? Beams or cables? Because in cable cases it must!

Does not matter.
Pick one.
Just look at the external forces that act on section BC, whether it be beam or cable.

What is the sum of the forces?
What is the sum of the moments?
They should both be zero.
What does that tell you?
 
That it doesn't matter whether it's cable or beam?
 
Huh... :confused:

EDIT: Uh no, if Tbc is not along the cable or beam, it will always start moving or rotating.
Since the construction is supposedly static, Tbc has to be along the cable or beam.
 
Last edited:
Yes, of course, but the point it the value of the forces stays the same regardless of the fact that in case it's cables and in the other it's beams?
 
Short answer: in this case there is no difference between cables and beams.

But there's really no need to make an assumption about what the direction is.
Draw an FBD of just one section and you can deduce what the direction is.
 
I agree, that's the best way... :)

HOWEVER, I do think I am getting to conceptual understand WITHOUT calculations and I'd like to demonstrate it by words.

The beams will act the same as ropes because

1) All forces are acting in the pins
2) All forces are pointing downwards with gravity in the y direction (no X component)
When you have beams, with forces pointing down from

So there is nothing to differentiate beams from ropes in this case because all the beams are IN TENSION, in fact, like the ropes.

Aha, I got it!

Tell me I'm not brilliant huh? :D
 
  • #10
Tell me I'm not (huh?)brilliant huh?

OK

You are not brilliant.

Actually it is the fact that the joints (rings) act as hinges that make the beam solution the same as the cable one.

Joints at the end of beams can be (partly) rigid and transmit moments, in which case the beam solution would not be the same.

Cable joints cannot be rigid.
 
  • #11
OK

You are not brilliant.

:wink:

Actually it is the fact that the joints (rings) act as hinges that make the beam solution the same as the cable one.

I figured that's "one of the reasons".

Joints at the end of beams can be (partly) rigid and transmit moments, in which case the beam solution would not be the same.

You're right. Cables don't transmit moments, that makes the big difference here. :)

I guess if I'd have calculated I might have seen it for myself, but I'd like to have reached there by words for a change,...if you'd excuse me for that. I appreciate your help :)
 
  • #12
Just a bit of terminology for you.

Structural members that do not carry moments or transverse loads. ie are only subject to axial forces and are only loaded at their ends are called

Compression - struts

Tensions - ties

Structural members that may carry transverse loads and may be loaded at any point are

Cables and chains tension only. No compression or moment.

Beams can carry tension, compression and moment.

:approve:
 
  • #13
Femme_physics said:
I guess if I'd have calculated I might have seen it for myself, but I'd like to have reached there by words for a change,...if you'd excuse me for that.

No need to calculate, but it does help to draw a picture (an FBD of a section). ;)
 
  • #14
Thanks, you two :)
 

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