Using ASCE Commentary to "override" the standard (CH 29 Case B wind on freestanding signs

[ReverenceEng]

General question followed by a specific example:


When and how can we use the ASCE 7 Commentary to our advantage when it might conflict with or provide "further clarification" as to the actual standard language?

For example, in Chapter 29, when we look at freestanding signs, we need to evaluate Case A and B, and then sometimes C. For Case B, the code requires a 20% offset of the resultant wind load from the centroid. The Commentary states that this value roughly aligns with some data from 1990 and 2001, but it is only for studies on single-faced signs. It states that more current studies that are more applicable show that this value is overly conservative.

COMMENTARY EXCERPT:
Mehta et al. (2012) tested a variety of aspect ratios (B/s) and clearance ratios (s/h) for double-faced signs with all sides enclosed to address current industry practice. The study included both wind tunnel testing and a full-scale field test to calibrate the wind tunnel models (Zuo et al. 2014; Smith et al. 2014). These sign configurations exhibited an average reduction of 16% in mean force coefficients with a range of 9% to 22% as compared to single-faced sign force coefficients given by the equation above. These tests also showed that the eccentricity of 0.2 times the width of the structure is overly conservative. Eccentricities reported in the study ranged from 0.039 to 0.105 times the width of the structure, with an average of 0.061. Testing by Giannoulis et al. (2012) supports the findings in Mehta et al.(2012).[/tt][tt][/tt][/tt][/pre]

So, my question:

When, if ever, can we use the commentary to our advantage? Here, it seems the data would allow us to reduce the 0.2 offset from Chapter 29, but the provisions have not been updated to reflect the data. Can we reduce the Case B offset ever using judgment?

What are our thoughts?

 

[phamENG]

The commentary is a formal discussion of the state of the art at the time of publication. It does not allow you to override any portion of the standard. If the code says to apply cases A, B, and C in a particular way, then you need to do it that way. The commentary just helps you understand where it's coming from.

If you're asked to do a forensic evaluation or evaluate an existing structure, being able to look at it from the view of the data rather than the code is a big plus but it doesn't change the fact that the code is the code and it must be followed.

One example of where the commentary in the wind chapters really helps in a substantive way is with regard to serviceability. The code itself is silent on serviceability and leaves to engineering judgment. The commentary supplies a lot of good information to help make those judgements.

 

[ReverenceEng]

I figured someone was going to tell me what I already felt I knew...but thought...mayyybeee there is some sort of "thing" out there somewhere...thanks!

 

[phamENG]

No problem. Sorry I couldn't give you the out you were looking for.

 

[TLHS]

I think there's an argument that if you have some sort of edge case that potentially doesn't match the intent of the standard writers then you should use rational engineering judgement and potentially could decide that portions of the standard don't apply based on what's in the commentary and other sources.

This, however, is potentially the opposite. If the commentary says that they know a provision is conservative, but they've left the provision in anyway, it implies that the standard writers' intent was for the provision to be conservative. I think the commentary's language probably makes it less justifiable if you were to take exception to it than if you took the same information from your own research. If you had found new studies on your own you could at least argue that the code wasn't current with the state of the profession. In this case, they're aware of the state of the profession, but have decided that this is not yet sufficient to change requirements.