ASCE 7-05 vs ASCE 7-16 Wind Loading w/ Canadian wind pressures

[StrucPEng]

Hi All,

This has probably been covered before but I have not been able to find a succinct answer. I am in a situation where I sometimes need to reference ASCE 7 wind loading equations to determine loading on elements in Canada. I convert a wind pressure from the Canadian National building Code into a 3s gust for use with ASCE 7 wind provisions per the appendix equations. My main question is since Canada does not use an ultimate level wind speed but the ASCE 7 does is there any additional conversion that needs to be done? More specifically, the equation for developing your wind pressure in ASCE-7 has not changed between 05 and 16 (except for the importance factor) but have the GCp values been adjusted for the ultimate wind speeds or have those only changed to reflect new research? My current understanding is its only the actual wind speed that was changed to eliminate the 1.6 load factor but nothing else was adjusted materially.

Ultimately, I would think there should be no problem converting to a 3s gust, calculating a wind pressure and then using that with the applicable NBCC load factors to determine an ultimate load given that the Canadian wind speed is a nominal wind pressure.

Let me know if I am miss understanding here.

 

[STpipe]

I find it strange that you need to reference ASCE 7 to calculate wind loads for elements in Canada, can you provide a explanation where you would need to do this?

Generally, I don't like to mix and match with different codes. They are generally tuned such that loads vs. resistance provide a desired level of reliability, which is where the different load factors and material resistance factors come into play and these are intended to work together to provide a specified statistical level of safety for that type of structure or element.

 

[StrucPEng]

One example would be wind loads on attached canopies. Just being introduced as part of 7-16 there is no specific direction in Canadian codes to calculate wind loads on attached canopies. That would be a simple example of where it would be useful. If for no other reason than to have a comparison to confirm an assumption.

I understand your point and to some extent agree, but I would say that really doesn't change the intent, you would still use the Canadian resistance factors for the materials and the Canadian load factors so the reliability is no different. That is kind of the point of the question to confirm that there is no built in adjustments, to the GCp values for example, that are exclusive to ultimate wind speeds that would complicate the transport of the wind load value.

 

[Enable]

...you would still use the Canadian resistance factors for the materials and the Canadian load factors so the reliability is no different.

I'm still pondering your original question, but I just want to point out that what I have quoted here is very much incorrect (CSA S408 Guidelines for the Development of Limit State Design Standards has guidance for things like this). The reliability index is derived, at least partially, from the mean and variance of an assumed distribution for resistance. In Canada, that distribution is taken to be normally or log-normally distributed with mean = output of code resistance equations, and the variance is entirely derived from the variance of the inputs (such as compressive strength of concrete, yield of steel, etc). If your mean resistance changes (which it will given you are calculating it utilizing a different code), and so do the inputs (potentially so but even if not the distribution is shifted by the difference in the means), so does the reliability index for a given load distribution.

Here is a paper by my former professor that goes through the process for welded wire fabric. Take a look at section 4.2 where it should make it a bit more clear how altering mean resistances factors into the reliability index.

 

[jayrod12]

For attached canopies, although there is no direct section on that, I typically use the roof components and cladding values for overhangs and the appropriate area. Figure I-11 maybe, maybe I-9 in the structural commentaries