ASCE 7-10 Wind Loading with roof overhangs

Question

A textbook is attempting to convince me that the following figure:

Represents the generic zones for a Case B wind loading including roof overhangs based on the ASCE 7-10 Chapter 28, Part 2 (Enclosed Simple Diaphragm Low-Rise Buildings) procedure. This procedure addresses roof overhangs via footnote 8 on Figure 28.6-1:

"Where zone E or G falls on a roof overhang on the windward side of the building, use EOH and GOH f`or the pressure on the horizontal projection of the overhang. Overhangs on the leeward and side edges shall have the basic zone pressure applied."

I absolutely do not agree that the figure is a correct application of the Chapter 28, Part 2 wind loading provisions for Case B wind direction. Please see below for ASCE 7-10 Figure 28.6-1 Case B zoning. You may observe that zones H, E , A and C correspond, while zones G and F have been flipped.

I would like to know if the textbook is not applying the wind provisions properly, or if I am missing something in the code? Thanks.

Assuming ASCE 7-10 Figure 28.6-1 is correct and footnote 8 applies, I believe the following (edited) figure shows the correct distribution.

Answer 1

I believe you are correct, the diagram in your book is incorrect in the way it has labeled Zones F and G. This is a bit of a stretch, but I think this may be related to/as a result of an error in Figure G6-5 of a publication released by ASCE called, Wind Loads, Guide to the Wind Loads Provisions of ASCE 7-10 by Mehta & Coulbourne and published by ASCE Press that I came across last month. This publication is an ASCE officially endorsed "How To" guide for ASCE7-10 wind provisions.

Figure G6-5 in this publication (below) is an attempt to replicate the image shown in Figure 28.6-1 of ASCE7-10 but was pretty horribly botched. You'll notice the same mistake with Zones F and G for Case B (called "Longitudinal in my book's figure) as well as some general drafting errors with the linework and arrow hatches.

There doesn't seem to be a published errata for this publication and when I attempted to contact ASCE about the figure last month I didn't get a response.

In addition to that point, I also think that there are at least three other errors in your figure as well.

1. Note 8 of Figure 28.6-1 of ASCE7-10 states (emphasis mine),

   Where zone E or G falls on a roof overhang on the windward side of the building, use E_OH and G_OH for the pressure on the horizontal projection of the overhang. Overhangs on the leeward and side edges shall have the basic zones pressure applied.

   Therefore, I think that the pressures on the overhangs on the side edges should be E and G instead of E_OH and G_OH as shown in your figure. The application of E_OH and G_OH on the windward side in your figure is correct.

2. There is also an error with the boundary line between E_OH and G_OH on the windward side. The boundary between E_OH and G_OH should project parallel to the axis of the building.

3. There is nothing in Figure 28.6-1 of ASCE7-10 that suggests the length of Zone E on the gable side of a structure is equal to $ 2a$ as implied in your figure. I believe that Zone E (and G) should extend all the way to the ridge as shown in Figure 28.6-1 of ASCE7-10. The width of Zone E is equal to $2a$ only on the eave side of the structure.