Peer-to-Peer Standards Exchange

 wind C&C Chapter 30, Part 4

  • ASCE Standards
  • ASCE7-16
  • Wind and Wind Loads
Keith Macbain's profile image
Keith Macbain posted 08-03-2022 09:19 AM
Hi all,

My question is about ASCE 7-16, Part 4 of Chapter 30 (C&C) and particularly how to apply Notes 3 and 4 of Table 30.6-2.  As an example, let’s consider a gable roof with the following details:

  • h = 70’
  • Θ = 10 deg
  • Exposure C
  • V = 100 mph
  • Zone 3e (edge corner of roof)
  • Effective Area = 10 ft2

Note 3 directs me to Figure 30.4-1, where I find a negative pressure of pnet30 = -48.5 psf (page 352, ignore positive pressure for now).  I then return to Table 30.6-2 and per Note 5 I find the adjustment factor for height and exposure to be λ = 1.676.  Assuming Kzt = 1.0, I then compute a total net pressure of (-48.5 psf)(1.676) = -81 psf (terms similar to Equation 30.4-1).

This seems to be complete to me, i.e., not for a total design of course but simply one C&C pressure for one selected zone.  However, I also see that Notes 3 and 4 of Table 30.6-2 both refer to Note 6 of Fig 30.5-1.  I have three questions, all centered around the latter part of Notes 3 and 4:

  1. Is the process and calculation above correct? (i.e., ignoring the reference to Note 6 in Table 30.5-1)
  2. How/when does Note 6 of Fig 30.5-1 relate to Part 4 or 6 of Chapter 30, particularly when referenced by Note 3 or 4 of Table 30.6-2?
  3. What is qh mentioned in Notes 3 and 4 of Table 30.6-2?  Is this a loose adaptation of Equation 30.4-1 as performed above?  I am aware of the usage of qh in other sections but strictly speaking I am unable to relate it to anything in Section 30.6 or the referenced potion of 30.4.  In other words, I try to follow the writing and notation very strictly/explicitly and am wondering if I’ve missed something or if this is just a typo or oversight of sorts (should it be maybe e.g., pnet similar to Equation 30.4-1?).  I searched the errata but did not find anything on this.  It seems to me that qh would/could relate to Figure 30.5-1 but there’s no opportunity to use either of these when a pressure is taken from a table (i.e., as in Figure 30.4-1). 

Thanks for your help. 

 Keith

Christian Parker's profile image
Christian Parker posted 08-09-2022 02:20 PM
Hello Keith,
I can't answer your question with any authority, but I'll share my thoughts and provide context.  You probably already know some of this, just trying to cover all bases.  Drawing heavily from two webinars presented by Emily Guglielmo: SEA-MW September 2021 Meeting, and Session 1 of AISC Night School 29, which was two weeks ago.

Note 6 is a mess.  Breaking it down:
Coefficients are for roofs with angle theta <= 7 degrees. <This tells us that we're here because our roof slope is less than 7 degrees, which is a funny thing to say to someone you just sent here for exactly the opposite reason (notes 3 and 4 on 30.6.1 are for roof slopes greater than 7 degrees).>  For other roof angles and geometry, use GCp values from Fig 30.3-2A-2I and Fig 30.3-5A,5B... <This tells us where to get GCp values, which Part 4 doesn't use.> ...and attendant q_h... <This is to make sure we're using q_h, which Part 4 also does not use.> ...based on exposure defined in Section 26.7. <Linking back to the definition of Exposures B, C, and D in Ch. 26.>
If we were using Part 3, we would have been referred here by section 30.5.2, which tells us where to get our GCp values for walls and flat roofs from T30.5-1, and refers us to Note 6 for any other geometries.  Part 4 is the simplified method based on Part 3 equations and coefficients.  Since we're using Part 4, none of this is directly applicable.

My take is that the Part 4 authors are referencing this note to document how they got the tabulated values.  Parts 1 and 3 have a paper trail of section references to textbook definitions of exposure categories and "h", but not Part 4.  I think this note is a reminder that you're supposed to know that exposure is per section 26.7, and height should be the Chapter 26 definition you would use for q_h: "h = mean roof height of a building or height of other structure, except that eave height shall be used for roof angle theta less than or equal to 10 degrees, in ft (m)."

That's the best apologetics I can come up with, but it's still a bad look for 7-16.  Even if I'm right, this should've been in the commentary not the code.  What do you think?
Here is the code logic I would follow for this problem in ASCE 7-16:
1. Equation 30.6-1 refers us to Table 30.6-2 for EAF, RF, and p_table.
2. Table 30.6-2 notes 3 and 4 refer us to Table 30.4-1 for pressures because our slope is low, but remind us to use the roof zones from T30.6-2, on the previous page.
3. Because of notes 3 and 4, Note 5 also applies.  This is a heads up that Table 30.4-1 will try to gaslight us into using its lambda values, but we shouldn't listen to it.  Lambda shall be from T30.6-2, ignore the siren song and stay the course.
4. We're going to need a roof height to get our pressure out of Table 30.4-1.  Notes 3 and 4 also directed us to Note 6 in T30.5-1, which wants us to close our eyes and pretend we're looking for q_h so that we realize that "h" is the eave height, not the average.
5. Check the exposure category definitions in chapter 26 and pick one.  Now we can flip to Table 30.4-1 to get that pressure, which T30.4-1 calls p_net30.
6. Sure enough, T30.4-1 note 1 refers to eq. 30.4-1, which in turn references back to T30.4-1 for lambda.  Note 5 in T30.6-2 warned us about this so we'll ignore it and get lambda from pg 367 instead.
7. We need p_table for eq. 30.6-1; Note 5 from T30.6-2 tells us this is actually lambda x p_net30 from 30.4-1.  In your case p_table = 1.676 x -48.5 psf = -81 psf.
8. Luckily we're Exposure C, because otherwise we would have to multiply in the Exposure Adjustment Factor from pg. 366.  Note 1 under the EAF table lets us off the hook.  Whew!
9. Luckily our effective wind area is 10', so RF = 1 for all Exposure Categories.
10. That does away with all our terms in eq. 30.6-1 except for K_zt.  If our topo is normal, our design pressure ends up being the same 81 psf as p_table.

In my opinion, there is nothing simple about this "simplified" method.  ASCE 7-22 authors seem to agree: they removed Parts 2 and 4 from Chapter 30, reducing the C&C page count by a third.  My advice is to use Part 3 instead: it's more accurate and harder to mess up.  I have never met an engineer who uses the simplified methods regularly, but I would be interested to hear from anyone who does.
Keith MacBain's profile image
Keith MacBain posted 08-12-2022 12:45 PM
Hi Christian,

Thanks for your excellent and thoughtful input.  I especially enjoyed your style which makes it entertaining to read so thanks for that also!  I hadn't considered items in your third paragraph, which offer a great perspective.  In fact that's currently the best explanation and understanding I have for this and yes, if correct I agree that it would be best placed in the commentary. 

I see you arrive at the same design pressure which I take as a 'yes' to question 1.  For the sake of completeness, the potential answers I expected are somewhat like:

  1. Yes it's computed correctly.  The reference in Notes 3 and 4 can be ignored because there’s nothing actionable there when you're following Part 4. 
  2. It doesn't.  Note 6 appears anecdotal at best - you may skip it entirely if you're looking to obtain a pressure.  The note directs you to other figures which contain GCp values appropriate if you weren't using Part 4, however despite the reference these are not to be used with Part 4 where the pressure is taken from a table and factored with λ, EAF, RF, and Kzt.  The term GCp is not used in this computation and the reference is misleading. 
  3. The term qh is not used in Part 4 and should be ignored.  In Part 4 pressures are taken from a table and factored with λ, EAF, RF, and Kzt only (i.e., without any use of qh).  

If correct, these potential answers would seem to indicate an error in Part 4.  However, a strong argument against it being an error is that after 12 years this is not found in the errata (this apparently first surfaced in 7-10).  Could this be an indication of how (in)frequently this section is used?  Or perhaps it's not an error at all but instead just my failure to understand it correctly?  I generally don't use Part 4 and happened upon it when following a design example.

Yes agreed, the term 'simplified method' is a bit of an oxymoron.  Unfortunately, the concept actually is pretty simple however the presentation of it seems to be where the difficulty is introduced. 

Still hoping an ASCE person can/will chime in on this and tell us if it truly is an error.   

Best regards,
Keith