Thanks Keith, my ears are burning.
I intended to bring in the MWFRS provisions for comparison purposes, although in hindsight I did not state this clearly. ASCE's testing would have either demonstrated that overhangs influence pressures on non-overhang portions of the roof, or that they do not, so it would be strange for the C&C provisions to penalize the main roof for the overhang when the MWFRS provisions do not.
I think part of the confusion is that when we look at the zone plans in Figs 30.3-2X, we imagine a short overhang along the perimeter that would almost always fall in the outermost zone (Zone 2/3 in 30.3-2A). However, for a low-slung building with a long overhang, there's a chance an overhang could extend into the inner roof zones. For example, a 1-story building with eave height 10' and a 13' overhang would have a 6' zone width per Figure 30.3-2A and would technically include zones 1, 1', 2, and 3 in the overhang alone. I'm sure it's rare to have the interior zones in an overhang, but I see why they would include it. A quick google image search of "long overhang building" found this example:
------------------------------
Christian Parker P.E., M.ASCE
Structural Project Engineer
Washington DC
------------------------------
Original Message:
Sent: 02-02-2023 02:28 PM
From: Keith MacBain
Subject: Components and Cladding wind pressures on roof
Dwayne,
Yes, I think we've all wrestled with aspects that could be clearer but I'd also concede that we're a tough and varied audience. That's why this forum is so useful! I'd suggest carefully going over Chris' reply, which I interpret as it applies to your question may be taken as "only use the chart labeled 'overhang' where there actually is an overhang". In other words, yes the area 1 value is considerably larger in the overhang graph but not applicable in your case because your overhang is only 18" and your area 1 is actually 'roof', not 'overhang'. Many thanks to Chris for the excellent input and helping me migrate to 22 as well!
Hope this helps,
Keith
------------------------------
Keith MacBain Ph.D., P.E., M.ASCE
Associate & Chief Analyst
Geiger Engineers, Campus 2
Suffern NY
Original Message:
Sent: 02-02-2023 08:55 AM
From: Dwayne Moench
Subject: Components and Cladding wind pressures on roof
Keith,
You are exactly right. I was never concerned about the canopy itself, just what appeared to be the results on the rest of the roof because the roof had a canopy. And yes, I'm referring to C&C pressures. These graphs need a better explanation that is for sure.
------------------------------
Dwayne Moench P.E., M.ASCE
Senior Structural Engineer
Rice MN
Original Message:
Sent: 02-01-2023 01:02 PM
From: Keith MacBain
Subject: Components and Cladding wind pressures on roof
Christian,
I agree with Ron that you have the correct interpretation but I'm not clear that you're fully addressing Dwayne's question. However you definitely get two gold stars for the research! I do enjoy and appreciate your positive and sometimes provoking contributions to the discussions!
My impression is that the question is not about MWFRS but instead C&C (because he's in Chapter 30). Further, as I understand it the question is not about the pressure on the overhang itself but instead the C&C pressure on various zones of the roof – i.e., zones over the entire roof not just the overhang (note his mention of area 1 and example values)
As I understand it, the question might also be cast as something like e.g., "is it possible to interpolate b/w graphs in the same Figure based on overhang dimension, particularly if the overhang is small?" Although this is a sensible question, unfortunately there is no allowance or guidance I'm aware of in ASCE for this. Perhaps an ASCE-person can add to this?
Hope this helps,
Keith
------------------------------
Keith MacBain Ph.D., P.E., M.ASCE
Associate & Chief Analyst
Geiger Engineers, Campus 2
Suffern NY
Original Message:
Sent: 01-31-2023 01:07 PM
From: Christian Parker
Subject: Components and Cladding wind pressures on roof
Dwayne,
My interpretation is that when an overhang is present, the overhang pressure need only be applied to the overhang itself; not the entire roof. In MWFRS roof loads, there is no penalty to the roof for having an overhang; only the additional pressure acting on the overhang itself per 27.3.3 (ASCE 7-16). Mr. Scott's response by way of Mr. Hamburger seems to support this, if I've understood correctly.
My impression is that these wind studies are pretty far-reaching; they may have data on sensitivity to overhang size that has not been incorported into the provisions. However, the abstract of the ASCE paper linked below actually suggests that shorter overhangs have higher edge pressures than long overhangs. Just speculating, but this could be due to the greater stiffness of shorter cantilever overhangs compared to more flexible long overhangs. I was too cheap to actually buy the article and only read the abstract, but it may be worth your hard-earned cash if you're curious.
https://ascelibrary.org/doi/abs/10.1061/%28ASCE%29ST.1943-541X.0003477
------------------------------
Christian Parker P.E., M.ASCE
Structural Project Engineer
Washington DC
Original Message:
Sent: 01-18-2023 08:54 AM
From: Dwayne Moench
Subject: Components and Cladding wind pressures on roof
Ronald and all,
I appreciate your responses. Though I am highly skeptical that such small overhangs will produce the forces as instructed by Figure 30.3 I will continue to use this 'with overhang' figure. As instructed by James below I believe a ratio of sorts should be proposed for intermediate cases. You don't have to be a wind tunnel engineer to know that a 8" overhang will have a much smaller overall affect than a 10'-0" overhang, especially when buildings get over 100'+ in width.
Thanks again,
Dwayne
------------------------------
Dwayne Moench P.E., M.ASCE
Senior Structural Engineer
Rice MN
Original Message:
Sent: 01-09-2023 10:53 AM
From: Dwayne Moench
Subject: Components and Cladding wind pressures on roof
Hello,
I work for a building manufacturer and have been puzzled by some of the Figures in chapter 30 of ASCE7-16. Specifically the External roof pressure coefficients found in Figures 30.3-2A thru 2D. There is a distinction between 'roofs' and those 'with overhang'. I haven't found a definition of how large the overhang needs to be to drive these higher wind pressures. We have buildings sometimes over 250+ wide with 8" to 18" overhangs. It seems incredible to believe the wind pressures vary that much with such a small overhang. Is there a good definition of the size overhang that will produce these larger pressures on the roof?
Thanks,
Dwayne Moench
#ASCE7-16
------------------------------
Dwayne Moench P.E., M.ASCE
Senior Structural Engineer
Rice MN
------------------------------