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Components and Cladding wind pressures on roof

  • 1.  Components and Cladding wind pressures on roof

    Posted 01-09-2023 11:03 AM
    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
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  • 2.  RE: Components and Cladding wind pressures on roof

    Posted 01-10-2023 08:03 AM
    Hi Dwayne,

    My company develops ASCE 7 wind software, and so we go through the standard in quite a bit of detail.  I don't ever recall anything in the standard that indicates that the size of an overhang is a consideration, I have always interpreted it as any overhang uses the Overhang figure.
    Regards,

    Chris Rosencutter
    www.mecaenterprises.com

    ------------------------------
    William Rosencutter P.E., M.ASCE
    Engr
    Meca Enterprises Inc
    Broken Arrow OK
    ------------------------------



  • 3.  RE: Components and Cladding wind pressures on roof

    Posted 01-10-2023 10:53 AM
    William,

    Thank you for the  information.  The lack of a definition is a little frustrating as an 8" overhang would certainly have a smaller overall affect than a 10'-0" overhang.  I just feel we are penalizing ourselves maybe a bit too conservatively by painting with such a broad brush once we have an overhang.  I would assume the testing agencies used a defined overhang when they developed these coefficients.  I wonder what that overhang was?

    Thanks again,

    Dwayne

    ------------------------------
    Dwayne Moench P.E., M.ASCE
    Senior Structural Engineer
    Rice MN
    ------------------------------



  • 4.  RE: Components and Cladding wind pressures on roof

    Posted 01-10-2023 04:29 PM
    I would assess the whole depth-to-length ratio for an overhang made up of an individual component. More than likely, I would do the same for an overhang that is an assemblage of structural elements, except I would probably look at the bottom component - tinkering on a global check for which it is insignificant and the local check where it can be significant. 
    Sort of like elastic beam theory where under a certain ratio, the principles are no longer valid. 
    I wonder if there should also be a ratio check of the entire length to overhang. Interesting.

    ------------------------------
    James Williams P.E., M.ASCE
    Principal/Owner
    POA&M Structural Engineering, PLC
    Yorktown, VA
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  • 5.  RE: Components and Cladding wind pressures on roof

    Posted 01-17-2023 11:46 AM
    Dwayne,

    Per Don Scott, chair of the ASCE 7-16 wind committee, "Any extension of the roof structure beyond the face of the building wall is considered an overhang.  The overhang has higher wind loads because of the combination of suction pressures on top of the roof surface along with positive pressues acting on the bottom surface of the overhang."

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    Ronald Hamburger P.E., F.SEI
    Principal
    Simpson Gumpertz & Heger
    Oakland CA
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  • 6.  RE: Components and Cladding wind pressures on roof

    Posted 01-18-2023 10:15 AM
    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
    ------------------------------



  • 7.  RE: Components and Cladding wind pressures on roof

    Posted 01-31-2023 01:34 PM
    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
    ------------------------------



  • 8.  RE: Components and Cladding wind pressures on roof

    Posted 01-31-2023 03:40 PM
    Christian,

    I would tend to believe what you said about affecting the overhang only, but why the different GCp values for the same area of roof between the two figures?  If you note Area 1 and 2t are definitely not in an overhang area yet they have different GCp values whether you are looking at the Roof only figure or Overhang figure.  This again tells me that the ASCE code officials are saying if you have an overhang your area 1 (looking at Figure 30.3-2D) goes from at GCp of -1.8 with no overhang and Area less than or equal to 10 sq. ft. to -2.6 when you have an overhang.  This is a 44% increase in pressure, sorry, but not buying it but I will apply it.  

    Thanks,

    Dwayne

    ------------------------------
    Dwayne Moench P.E., M.ASCE
    Senior Structural Engineer
    Rice MN
    ------------------------------



  • 9.  RE: Components and Cladding wind pressures on roof

    Posted 02-02-2023 08:02 AM
    Dwayne,

    You are referencing ASCE 7-16 Fig 30.3-2D, and I agree the value of GCp is -1.8 for the roof and -2.6 for the overhang.  If you refer to ASCE 7-22 Fig 30.3-2D, I think it sheds some light on this topic.  In ASCE 7-22 Fig 30.3-2D they removed the overhang chart and instead refer you to Section 30.7 for Overhangs.  


    If you look at Section 30.7, it indicates that GCp for the top side of the overhang is exactly the same as the roof, and then you add it to the pressure on the bottom side of the overhang, which is the same as the GCp for the wall.  

    In your example if the A < 10 sq ft then the GCp for zones 4 and 5 are both 1.0, and so on the overhang you would end up with -1.8-1.0 = -2.8.  If I did this right, the value for overhang is a little higher (-2.8) in ASCE 7-22 than it was in ASCE 7-16 (-2.6); however, I think the point was that theGCp on the top surface of the overhang is identical to the roof, but you have to add to it the GCp from the bottom side of the overhang.

    Regards,

    Best Regards, 

    Chris Rosencutter
    www.meca.biz
    chris@...


    ------------------------------
    William Rosencutter P.E., M.ASCE
    Engr
    Meca Enterprises Inc
    Broken Arrow OK
    ------------------------------



  • 10.  RE: Components and Cladding wind pressures on roof

    Posted 02-03-2023 09:06 AM
    Chris,

    Yes, that is exactly what I needed.  I purchased ASCE7-22 this morning.  We work with no jurisdictions that have referenced it here so I many be a little behind.  Thank you very much,

    Dwayne

    ------------------------------
    Dwayne Moench P.E., M.ASCE
    Senior Structural Engineer
    Rice MN
    ------------------------------



  • 11.  RE: Components and Cladding wind pressures on roof

    Posted 01-31-2023 08:00 PM
    Christian Parker's interpretation is correct.

    ------------------------------
    Ronald Hamburger, SE
    Consulting Principal
    Simpson Gumpertz & Heger
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  • 12.  RE: Components and Cladding wind pressures on roof

    Posted 02-01-2023 10:15 AM
    Mr. Hamburger, 

    I see you are the chair of the ASCE7 Standards Committee.  If Christian's interpretation is indeed correct, would you agree that some clarification notes are necessary on the Figure 30.3-2A to 30.3-2G pages for the Overhang?

    Thanks,

    Dwayne

    ------------------------------
    Dwayne Moench P.E., M.ASCE
    Senior Structural Engineer
    Rice MN
    ------------------------------



  • 13.  RE: Components and Cladding wind pressures on roof

    Posted 02-02-2023 08:03 AM

    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
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  • 14.  RE: Components and Cladding wind pressures on roof

    Posted 02-02-2023 01:53 PM
    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
    ------------------------------



  • 15.  RE: Components and Cladding wind pressures on roof

    Posted 02-02-2023 03:11 PM

    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
    ------------------------------



  • 16.  RE: Components and Cladding wind pressures on roof

    Posted 02-03-2023 09:05 AM
    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
    ------------------------------