Solve Shear Flow Problem: Find Correct Area

[Precursor]

Homework Statement

The correct answer is circled.
[PLAIN]http://img80.imageshack.us/img80/976/94926993.jpg


The attempt at a solution

I used the equation $$q = \frac{VQ}{I}$$

But for Q = Ay, what is the area? My area was 80 x 20 = 1600 mm^2. However, this gave me the wrong answer.

 

[pongo38]

Sketch the shear stress distribution and ask yourself whether the area (of the flange) isn't (less than 80)*20. Also, if you write again, give us your values of Q and I, with units.

 

[Precursor]

I tried the question again and got it right with a Q value of 7.2*10^-5 m^3 and an I value of 52.693*10^-6 m^4. However, I am still confused as to why we only take an area of 40x20 than 80x20.

 

[pongo38]

If you draw the shear stress distribution for the flange, you will see two triangles with zero in the middle, and maximums over the webs. The nail at A takes approximate the left triangle, and the nails at B the other half. The formula is based on thin sections, which this is not, and so you shouldn't believe the results too precisely. If you were to work this from the same first principles as were used to derive the formula VQ/I then I think you would have a better understanding.

 

[nvn]

Precursor: Why do you think (ii) is the correct answer? Why do you think Q = 72 000 mm^3? Or why did you think A = 1600 mm^2 in post 1?