# Internal forces in members of structure

1. Mar 26, 2013

### endy_kami

what are the internal force in the hinged member 1-4?

#### Attached Files:

• ###### 1d2d3d4.docx
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2. Mar 29, 2013

### afreiden

Interesting problem.

I've attached an image that illustrate my thoughts on how to go about solving it.

I'm assuming the beam is "rigid" and the bars all have the same "stiffness."

Take a look at the image.

Finding the bar forces in the first step is trivial, as the structure is symmetric.

Once you've found the bar forces from step 1 and the bar forces from step 2 (based on my attached image), you can sum them to arrive at your solution.

I drew a red arrow pointing at the second step in the process, which I think needs further explanation. You may find example #2 useful on this website: http://utsv.net/mechanics-of-materials/2-statically-indeterminate-structure-axial

Hope that helps

#### Attached Files:

• ###### PF.png
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3. Mar 29, 2013

### endy_kami

tx alot I'll try it

4. Mar 29, 2013

### endy_kami

dear afreiden,
I've tried the clue that you've given me
I found N1=0,6P N2=0,3P N3=0,03P N4=0,07P
but the answer should be N1=0,4P N2=0,3P N3=0,2P N4=0,1P (according to the writer)
the writer said use the force method or displacement method or
the matrix stiffness method, any idea?

5. Mar 29, 2013

### afreiden

Show us your work and we can help. Your answer immediately looks wrong to me since I'd expect the forces to be larger in the rods that are nearer to the point of load application. In the author's solution this is indeed the case.

6. Mar 31, 2013

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7. Apr 1, 2013

### afreiden

Your equation that you wrote at the top right of your page (I think it might be labeled (1) ) is definitely wrong.

In general, I see several procedural mistakes.

In step 1, all bar forces should be the same.

In step 2:
What is your value of "M" ?
How did you determine that N1=2N2? (I expect to see some axial deformation equations)
Do you understand how I was able to draw that triangle?

Your triangle at the bottom of your page does not make physical sense.

8. Apr 1, 2013

### endy_kami

can you show me how to get to the right answer than?