ASCE 7-16 Commentary Wind Exposure Open Patches

[JSMHA]

I have a building that is between exposure B and C. I am trying to check the open patches section of the ASCE 7-16 commentary to make a rationale determination. Note 4 of the figure in the commentary is not reading clear to me. The note says if the open patches within any of the three radii is less than 25% of the sector area than the sector should be considered exposure B. But then says if any of the sectors have open areas within any of the radii exceeding 50% then it should be exposure C. It does not make sense that both of these say within ANY of the three radii as then you can have a sector from the 500 foot radii that is exposure B and a sector in the same direction from the 1500ft radii that is exposure C. Are you supposed to take the more stringent exposure? In that case why wouldn't the first sentence read if "ALL of the radii have less than 25%"? If anyone has another reference for this procedure that would be greatly appreciated.

 

[StrucPatholgst]

Be conservative. The last thing you want is a call in 3 years saying their building shakes in the wind.

 

[driftLimiter]

Yep, If I find myself struggling to get the wind loading down to the last 0.01psf I just be conservative. If it results in something crazy then sure dial it in more. Most of the time typical framing/connections works even when being conservative.

 

[RWW0002]

The difference in Expoxure B and C is far from 0.01 psf (Try ±30% on all wind pressures) and in some structures this may be the single decision that makes the biggest cost impact on the structural design. I admit that many of us have not given exposure its fair share of thought or engineering judgement in the past due to lack of clear guidance from the code. Some engineers, especially in competitive industries like PEMB and Wood Truss industry seem to always arrive at exposure B. I have ended up being conservative and using C in the past anytime a large open field is near the site, but not without some gnashing of teeth about lack of clear guidance.

I have not dug into the new commentary yet, but I am glad that some updated guidance exists. I for one am tired of throwing Exposure C at borderline sites to be conservative due to lack of backing.

Sorry, I am not much help on your specific question, but I will dig in to this the next time I have a project that is on the B/C threshold.

 

[driftLimiter]

Im not meaning to suggest the difference is 0.01 psf. I'm suggesting that it is an illusion to think just cause you have computed the wind pressure down to the gnat's ass that it is correct. I think its a fair point that the result of this decision could be a primary cost component of the structure but its not like you wont have shear walls under one assumption then all of a sudden you have shear walls. Its more a function of how many fasteners your going to have, what type they are, and where they are located.

 

[slickdeals]

My understanding is that you evaluate each "pie" separately including all the open patches within those pies. This is also graphically represented in the diagram on the top right corner of the figure.

 

[RWW0002]

@driftLimiter - Understood and I agree. Moment frame buildings are where this can really have cost implications. Superprecise wind load calcs (where wind loads pressures are shown to .01 or even .001) is a pet peeve of mine as well.

@JSMHA - I took a look at the commentary last night, and I agree with you that it makes logical sense for the first sentence to read "ALL of the radii have less than 25%" for an exposure B condition.

 

[DanKile]

I think you take the sum area of the open patches within a radii and divide by the radii wedge area. You'll need to check this for each radii. If any one of these radii changes the exposure, then the direction considered will be the higher exposure.

So for the 2600ft radii you would need:
2654645 / 2 = 1327322
about 1,330,000 ft^2 of open patches to be exposure C and
2654645 / 4 = 663661
more than about 664,000 ft^2 to take the average of B & C

 

[Aesur]

To me what it says is as follows: if open patches are less than 25% in any radii then you are B, if you are between 25% and 50% but less than 100' then you interpolate Kz and if in any radii you are more than 50% then you are C.

 

[RWW0002]

@ Aesur - But what if, in a given sector, you are <25% in one radii and >50% in another. What are exposure is this sector then? I assume it is supposed to be C, but the wording of the code could be argued wither way. (i.e. code says if <25% in any one radii is open then it is B - so exposure B it is. or code says if >50% in any one radii is open then it is C - so exposure C it is. )

If the intent of the code is that a sector can only be B if all of the radii are <25% then it should read if the open patches within all of the three radii is less than 25% of the sector area than the sector should be considered exposure B. Likewise, if it is permissible for the segment to be B so long as at least one radii is <25%, then the wording should be adjusted to clarify.

I agree with OP that the wording "within any of the three radii" for all three conditions (<25%, 25%-50%, and >50%) leads to ambiguity for a case where the radii differ.

 

[Aesur]

I believe the intent here is, if any sector is <25% (the image has 8 sectors in 3 radii), then that sector is B, if it's between 25% and 50% in any sector then that sector (not all sectors) is interpolated, however if any radii is more than 50% then it is all C. Basically you are allowed openings in any sector, even more than 50%, but that direction must be C, the last statement is if over 50% of the total radii area is open then the whole structure is C.

Example:
Sector 1, all three radii completely open and sectors 2-8 have no open patches, then you are C for wind from sector 1, (>50% open in that sector), but you are B from all other directions because the open area of each radii is 1/8 of that radii (12.5% of that radii area).

2nd Example: Sector 1, 3, 5 and 7 radii 1 open, radii 2 and 3 no open patches and the remaining sectors have no open patches, then you are C. Because >50% of radii 1 is open, even though the remaining rectors have no open patches.