ASCE 7 wind pressure on overhangs

[masterdesign]

I understand the C&C wind pressure on roof overhangs is the sum of the pressures on the top and bottom surfaces.

The equation in ASCE 7 is p = qh*(Gcp-GCpi) where Gcp is defined as "the sum of the GCp of the overhang's top and bottom surfaces".

By defining GCp as the sum of the external GCp values for the top and bottom surfaces, it appears the intent is to cancel out GCpi. If GCpi were to be included, it would apply to each surface and should be combined appropriately with the corresponding external GCp. Then the pressures for each surface would be combined after that. Furthermore, in no case would it seem appropriate to sum the external GCp values and then apply the GCpi value singularly.

Also supporting that this may be an oversight is how poorly conceived Figure 30.9-1 (7-16) is. Looks like this figure hasn't been touched or the supporting text cleaned up since someone sketched it initially. This points to a lack of attention to this code section.

This seems very clear to me, but if so, why has this persisted from 7-10 to 7-16 to 7-22 without revision?

This thread asks the same question and did not get any traction. The one response essentially agrees with cancelling GCpi (the carry-on thought of including Gcpi for designing the cladding is analogous to calculating the pressure for a single surface).

 

[DaveAtkins]

I agree that inside an overhang, the internal pressure will push up on the roof and down on the soffit. Thus, the two GCpi values cancel each other out.

DaveAtkins

 

[DanKile]

ASCE 7-22 30.7 clarifies this a bit. Cladding only needs their adjacent GCp value (top surface to roof, bottom surface to wall). Components that support both surfaces would need the combined GCp. I would use a similar approach to the internal pressure as the parapets. The internal pressure cancels out when using the combined GCp. In 7-22 with gables greater than 7 degrees, you end up doing this anyway.

 

[LearningAlways]

I'm going to see if this is code or just my logic.

But the reason that there is no internal pressure on an overhang is because overhangs are not pressurized internally. I am now thinking of an attic and am questioning that logic, but the internal pressure is there because a high-rise building has an internal pressure that matches the external pressure in normal conditions. That is why the internal pressure varies the higher you go in building elevation. In a wind event, the internal pressure stays roughly the same but does increase because of the increased wind pressures.

 

[lexpatrie]

I think wannabe got it right.

I don't follow this question otherwise. Cladding takes an internal pressure coefficient. How are you getting two pressure loads and two internal pressures, that needs two surfaces.

Source: ASCE 7-22
(that's the file I opened. I presume this is 30.9-1 in ASCE 7-16 although that means they deleted two figures. Or maybe renumbered something.)

 

[masterdesign]

Thanks to everyone for the responses.

Dave/Dan - this is what I needed to hear. The simplest explanation is usually correct. I just questioned myself because the equation with the GCpi term has been in the code for 3 cycles.

Learning - to the first part of your comment, some overhangs will see internal pressure and some will not. It depends on the extents of the building envelope. If the overhang does not see internal pressure, then obviously the GCpi term will be zero. But my question pertains to the condition where there is internal pressure.

Lex - when the overhang is taken as a whole, the first surface is the roof and the second surface is the soffit. Each surface would have an external and internal GCpi. Cladding forces are calculated from a single surface standpoint. External and internal GCp would apply to that surface. This is not what I am asking about.

 

[lexpatrie]

I don't see when the "sum" would apply. Component and cladding loads for the roof (with internal pressure) apply to the roof sheathing, the component and cladding loads for the soffit (with internal pressure) apply to the soffit.

What are you designing where there's two cladding loads? If there aren't two cladding loads, how are you getting two "different" GCpi that potentially cancel out?

 

[LearningAlways]

I agree that the internal pressures cancel out. In fact, this happens when analyzing any MWFRS system. The buildings GCpi cancels out, in fact we ignored that value altogether in our MWFRS analysis at my previous place of work.

 

[masterdesign]

Lex -

I don't see when the "sum" would apply.

The sum would apply when you are looking at the attachment of the roof framing to the superstructure, say a roof clip on a wood truss (shown in pink below). The sum I am referring to is the roof pressure and soffit pressure on the overhang portion, which both apply when looking at the demand at the roof clip.

Component and cladding loads for the roof (with internal pressure) apply to the roof sheathing, the component and cladding loads for the soffit (with internal pressure) apply to the soffit.

Right there you've stated there is a GCpi for the roof and a GCpi for the soffit. Those are the two GCpi that I believe cancel out (for the condition of checking the roof clip). They are different in sign only.

 

[lexpatrie]

That's my point, if you have cladding pressures from more than one surface, I don't think that's cladding anymore.

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Switching to MWFRS doesn't really resolve your question, though. That's just semantics as to which load you are calculating.

In the MWFRS case I'd look at if you have a separate space, so a stud wall could hypothetically be depressurized while the attic could be pressurized, though it seems unlikely, but then again, there's roof ventilation and the stud wall typically isn't ventilated. But if both the wind loads are affecting the same interior space (e.g. the attic), and there's no vapor barrier between them, I would think treating the GCpi as effectively zero would be appropriate from a statics standpoint, because they exert pressure in roughly opposite directions. Come to think of it, though, the overhang gets a MWFRS Poh, which I think combines the soffit load, so you're likely including one GCpi (that might not actually happen) to increase the calculated uplift, which is conservative.

There's a lot of commentary in C30 on overhangs, but that's for C&C loads.