ASCE7-16 C&C Chapter 30 part 1

[Insp-struc]

I am looking at ASCE7-16 Chapter 30 Part 1 Components and Cladding calculations and I have a question about the external pressure coefficients.

For example purposes - Figure 30.2-2B is for Gable Roofs 7deg to 20deg. There are two tables, one for Roof and the other for Overhang. I was under the impression that you use the "roof" table for the roof span and then use the "Overhang" table for the overhang span. But further review in the commentary Table C30.3-3 says "Negative without overhang" and "Negative with overhang"

So my question is:

1: if the roof has an overhang, no matter the overhang span do you design the whole roof with the "Overhang" external pressure coefficient table? and then if there is no overhang you use the "roof" external pressure coefficient table?

Or

2: Design the roof span with the "roof" table and the overhangs separately with the "overhang" table? If this is the case though why do some of the "overhang" tables in Chapter 30 Part 1 have zones 1, 1', etc in them when they would never see an overhang.

I hope this makes sense and thank you for the help

 

[ProgrammingPE]

Note 6 in Figure 30.3-2A (and other identical notes) says that "Values of (GCp) for roof overhangs include pressure contributions from both upper and lower surfaces".

This means that you can't use the values from the Overhang chart for the rest of the roof, since the values include pressure from below, which cannot occur, so your original method was correct. If a roof has an overhang, you design the roof using the graph labeled "Roof" and design just the overhang portion using the graph labeled "Overhang".

This also makes intuitive sense since adding a tiny overhang shouldn't immediately trigger a change in the pressure for the entire roof. The length of the overhang would also have a big effect on the pressures, and the charts don't consider that at all.

why do some of the "overhang" tables in Chapter 30 Part 1 have zones 1, 1', etc in them when they would never see an overhang.

I'm not sure exactly what you mean here. If the overhang is large enough, all zones would be possible.

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[Insp-struc]

Thank you for clarifying @ProgrammingPE

and your right, if the overhang is large enough it could be in all zones as, a (zone dim), is taken from the edge of the overhang.

Thank you again sir!