Election of cut placement for Shear flow -- Hibbeler's example

AI Thread Summary
The discussion focuses on the election of cut placement for calculating shear flow in Hibbeler's Mechanics of Materials. It highlights that while a specific cut is chosen for symmetry in Example 7.5, alternative cuts can also be valid if they maintain symmetry, allowing for adjustments in shear flow calculations. However, choosing a cut that results in a centroid location of zero can lead to incorrect calculations, as it nullifies the first moment of area (Q). The conversation emphasizes the importance of the second moment of area, which significantly influences stress flow due to its dependence on the distance from the neutral axis. Ultimately, the placement of cuts affects the shear flow results, and understanding these limitations is crucial for accurate analysis.
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Hibbeler's exercise 7.5 uses a cut for computing shear flow q at a junction. However, if I choose another cut, the shear flow turns out zero. Is there a rule for choosing a good cut? Other than not getting zero.
Hello. I am reading Hibbeler's Mechanics of Materials (ninth edition). Example 7.5 computes shear flow at a segment where there are nails attached to different boards.
He chooses a cut like the one shown here:
book cuts like this.jpg


And gets (by symmetry between C and C') the shear flow q computing the first moment of area Q and the location of the cut area y.

However, I believe on can choose any cut and it will be useful as long as there is symmetry (so as to argue that the shear flow is twice the required one and dividing by two if there are two places where the cut passes through). If I choose this cut:
shear flow cut.jpg

I get y (location of the centroid of the cut area) equal to zero, rendering my Q null. This isn't right.
I am looking to understand what limitations I have, or if there's any rule as to what cuts should be chosen. Or maybe if I'm doing something straight right wrong.

Thanks for any insights.
 
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The shear is caused by the vertical bending of the beam, which cross section is represented.
Each of those cuts dramatically modifies the first and the second moments of area about the neutral axis N-A of the cross-section.

As the second moment of area depends on the square of distance y measured between axis N-A and the centroid of each cut area, its influence on the stress flow is greater than the influence of the first moment of area.
Cuts made farther from the neutral axis should induce a smaller value of shear flow.
 
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