To calculate U for a wide flange bolted in the web, Segui says that since you haven’t yet discussed a plantic neutral axis, just use Case 7 on page [16.1-28] in the manual. However, since he forgot that there weren’t 4 bolts along the load axis, that case does not apply.
Thus the simplest thing to do is just do what the manual says, and we will explain plastic neutral axis concepts later.
The manual says on page [16.1-28] to use case 2. But that doesn’t help since I don’t know what he means by Xbar on the figure.
That’s what the commentary is for. Since we are having trouble applying Part 16, Chapter D3 in the Specifications [16.1-28] we will go get help from the corresponding section in the Commentary on page [16.1-282].
Note that it is critical that you see what just happened. We went to the same Chapter D3 in the Specifications to D3 in the Commentary.
Here he says exactly what to do for this case on page 16.1-284:
Now do I really understand all that? Not really, at least not yet. Do I clearly understand what the manual says to do? Absolutely. I simply go to page [1-22] where information on a W16x40 is listed, find the Zy = 12.7 in^3 and the cross sectional area A = 11.8 in^2 and calculate Xbar = 1.076″.
I also, as a practical engineer I check to see if I think this is reasonable by noting that the flange width = bf = 7″ wide and thus sticks out about 3.5″ from the neutral axis. From the above calculations the manual is making me say the centroid of the unconnected flanges is pretty close to half of how far they stick out. That’s probably reasonable, since the centroid of the flange alone would be about 3.5/2 = 1.75″, and the web area draws the centroid back in towards the axis of symmetry of the W shape.