NBCC 2005 Wind Speed-Up over Hills & Escarpments

[AE2970]

Hi all,

With regard to the 2005 NBCC Commentary/Supplement, I am looking for a clarification on terms used in the exposure factor modification equation (Eq. 3) for buildings on a hill or escarpment. I would appreciate an opinion or directions to a worked example or clarification from an official source.

1. It does not appear that "L" is defined. Do they mean "Lh"?

2. The "x" term is not clearly defined. It looks like it represents the local horizontal distance of the structure from the crest.

3. The "z" term appears to be the local height of interest on the structure measured from the base of the structure at the top of the hill.

Thanks in advance.

 

[Teguci]

Since I don't have the commentary, this is what ASCE has:

Not sure about L, but if its used for calculating K1, 2 or 3 then it seems like you have a misprint.
x = distance from crest of terrain feature to building site (either direction)
z = Local height of building (this variable messes everything up)

 

[charliealphabravo]

Thanks Teguci,
The NBCC formula looks like a very close cousin to the ASCE7 formula but it wasn't close enough to be clear what the "L" term represents. I think "x" and "z" are the same.

 

[AE2970]

Ok, I think I own everything that NBCC has published on the matter and I can't seem to raise a human in Montreal.

Here are the relavent pages from the NBCC structural commentary/supplement/user guide. It looks to me like there is a mistake or omission with respect to the term "L" in equation 3 and paragraph 14...either that or i'm going blind from staring at it.

At this point I am reduce to calculating the Kzt term from ASCE7 and then reverse engineering the mystery "L" term from NBCC equation 3. The topographic multiplier seems to be used the same way in both codes and should be transposeable. Any more tasteful suggestions for a workaround would be appreciated.

Regards,

 

[Teguci]

Looks like there are a whole bunch of typos as follows (smile):

k = Mu (identical values)
Alpha = Gamma (identical values)
Ce star = Kzt (close approximation for expected values)
Delta Smax and Ce relates to K1

I mean really, why change a good thing? ASCE applies to you guys too right?
American, check
Society, check
Civil, for the most part, check
Engineers, check!

 

[charliealphabravo]

Your preachin to the choir brother. I was raised on ASCE-7. It looks like it was changed for national pride? or copywrite???

So no word on what L might mean? I don't see an equivalent expression in ASCE-7 which defines the limits of formula application from the crest of the escarpment.