# Homework Help: Which ones are zero force members?

1. Feb 25, 2010

### Kristofeles

Picture in attachments

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Last edited: Feb 25, 2010
2. Feb 25, 2010

### Brian_C

Are we supposed to do the work for you?

3. Feb 25, 2010

### Kristofeles

Haha. No. I have to find the load in each member. I know some of those members are probably zero force members but which ones?

4. Feb 25, 2010

### Kristofeles

LN is a zero force member?

5. Feb 25, 2010

### Studiot

Look again at joint L

If LN is redundant then so is LC.

What makes you think this framework is redundant?
What do you know about the conditions for static indeterminancy?

6. Feb 25, 2010

### Gannet

The members that are perpendicular to the slope are zero force members, because the pin joints must be in equilibrium and zero moments

7. Feb 25, 2010

### Studiot

Do you mean like LC?

Look again at joint L

8. Feb 25, 2010

### Gannet

No member LC is not a zero force member

The members that are zero force members are: BM, DN, FO, and HJ; because at pin joints B, D, F, & H these members cannot have load because there would be no way of obtaining force equilibrium at these pin joints.

9. Feb 25, 2010

### Kristofeles

Okay. I will double check with my teacher. Thanks

10. Feb 25, 2010

### PhanthomJay

That would be the case only if there were no load applied at B, D, F, & H. Since there is a load applied at these joints, these are not zero force members.

11. Feb 26, 2010

### Gannet

You are correct Jay I didn't notice the external loads at Joints B & D; Then, the only zero force members are FO and HJ. Thanks Jay

12. Feb 26, 2010

### Studiot

I did pass comment on your last effort, which you seem to have ignored. The same comment applies again.

If the force in HJ is zero then, by vertical equilibrium of joint J, GJ is also zero.

I ask again why do you think there are any redundant members.

and again

What do you know about the conditions for static determinancy/ indeterminancy?

13. Feb 26, 2010

### Kristofeles

I did asked my teacher and he said that there is no zero forces on this structure

14. Feb 26, 2010

### Studiot

Kris I was referring to Gannet.

I hope your teacher also told you the formula for a perfect frame (statically determinate, no redundant members) - which this is

m = 2j -3

Where m = number of members
j = number of joints.

You can work out for yourself that this frame conforms.

I leave the task of calcualting the member forces to you - thre are many methods available.

15. Feb 26, 2010

### nvn

It currently appears the zero-force members are EO, FO, GJ, GK, GO, HJ, and KO.

16. Feb 26, 2010

### Studiot

This false statement helps the student in precisely what way?

17. Feb 26, 2010

### Kristofeles

Probably he just don't want us to include the zero force members.

18. Feb 26, 2010

### Gannet

Because of the question, I was assuming that Kris was just starting "Theory of Structures" and was using Method of Joints for analyzing Statically Determinant Truss Structures. If Kris is analyzing Statically Indeterminant Truss Structures then he would be using Deflection Methods.

Kris, I am sorry that I mislead you because I missed the two external loads at Joints B and D, the sketch is really light.

19. Feb 26, 2010

### Studiot

Certainly an idea of what you are studying and at what level would be useful to pitch responses and explanations to.

20. Feb 27, 2010

### PhanthomJay

So light, in fact, that the loadings on the right side of the frame appear to be missing. If the frame is symmetrically loaded, there are no zero force members; if the frame is loaded only on the side left of center, then there are many zero force members (see nvm's response). It appears there is a 24 unit force reaction at the left support, implying that the frame is symmetrically loaded,and that the applied joint loads on the right side of the frame are missing.

21. Feb 27, 2010

### Studiot

1) The truss was symmetric and symmetrically loaded. Because of symmetry there was no need to replicate on the right hand half so it was 'greyed out'. this is common practice.

3) No wind loads were considered.

It should be pointed out that some inaccurate statements have been made about the conditions for recognising redundant or 'zero force' members. In particular the fact that a member joins at right angles is no guarantee of redundancy.
In a plane truss if both diagonals in a quadrilateral are present and crossing one will be redundant.
But the only guaranteed method is to apply the formula or work it out from first principles.

22. Feb 27, 2010

### PhanthomJay

Might be common practice, but that intrinsically is not made clear.
I thought they were tiny birds perched on the joints. Is there a difference in the analysis if the loads come from birds or purlins?
I didn't see any lateral loads either, but the print was so light, maybe we missed 'em...
but a truss can be statically determinate and still have zero force redundant members

23. Feb 28, 2010

### Studiot

Since what is obviously some form of coursework is posted in the general engineering section I assume the originator is a budding engineer or architect.

Having responded in a way intended to help the OP, and others viewing this thread, a better understanding of his subject in general and this problem in particular we resolved the issue at hand between all of us.

There were some incorrect statements made by some and I did not want to let these lie to confuse future viewers.
It is helpful to learners to experience and assimilate real action so I posted my outset assumptions, provided a few pointers that a keen student might follow up, and corrected the incorrect.

It is open for any to ask for more explanation or seek it out themselves.

And yes a manifold redundant truss may be statically determinate or indeterminate. I never said otherwise.

Last edited: Feb 28, 2010
24. Feb 28, 2010

### Redbelly98

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