# Torsion in a hollow beam cross-section

1. Aug 16, 2013

### beamsandthangs

1. The problem statement, all variables and given/known data

The problem is regarding the question on the attached file called 'QUESTION3.png' This is urgent so please reply if you can.

2. Relevant equations

The attempt is in the file attached. Is my procedure correct? Am I missing something or is something wrong?

Also, how do I find the compressive axial force if N- = 20kN? What equation do I use?

3. The attempt at a solution

Attempt is attached.

Thanks.

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• ###### QUESTION 3.png
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2. Aug 16, 2013

### SteamKing

Staff Emeritus
1. Your section is closed. You are using τ = VQ/It to compute the shear stresses in the walls. This is incorrect. Have you studied how to analyze closed sections in shear?

2. The axial force is applied along the longitudinal axis of the beam. How do you calculate stress in axially loaded members? (Hint: it was probably the first stress formula you learned.)

3. Aug 16, 2013

### beamsandthangs

Edit:

In this example I found through google, the person uses VQ/It to find the shear at N.A :
http://faculty.ksu.edu.sa/modar/CE%20302%20Solved%20Problems/Solved%20Problems%20on%20Ch6.pdf [Broken]

Last edited by a moderator: May 6, 2017
4. Aug 16, 2013

### SteamKing

Staff Emeritus
1. No mention is made of any bending in your problem, so Mx = My = 0.

2. In your example, there is no shear connection between the sides of the wooden box and the top and bottom plates except be the nails which connect them. This is not a good example to use on your problem, since you have a full shear connection at all four corners.

3. The attached example is for a beam which does have a thicker top plate than your problem.
http://www.engr.colostate.edu/~thompson/hPage/CourseMat/Tutorials/Solid_Mechanics/shear.pdf
On reading their description, it appears that if all plates are the same thickness, one can assume equal shear flow in the sides, and thus τ = VQ/It may be valid for your problem after all. I apologize for the earlier confusion. However, the attached method should also work for more general problems, as well as this one.

5. Aug 16, 2013

### PhanthomJay

What happened to the Torsion and Bending part? The loads appear to be eccentrically applied?? Is this correct?