IBC (Eq 16-33) Wind Speed Conversion is INCORRECT

[larcix]

I couldn't find any references to this online, and this is a huge oversight by the IBC committee. Any engineers working with wind, especially with older standards or software, or anything that references this very familiar equation, should be aware of the correct calculation, just incase it matters for your situation.

The incorrect equation:
V.asd = V.ult * sqrt(0.6), (IBC Section 1609.3.1 Wind peed conversion, (Equation 16-33))

The correct equation:
V.asd = V.ult * sqrt(1/1.6) = V.ult(2016) * sqrt(0.625)

This is trying to convert 1.0 Ultimate wind speeds to the 1.6 factored Allowable wind speed. This equation is easily derived from the calculation of wind pressure, which includes a V^2 factor. So, all other things being equal, converting 1.0 Ultimate wind speeds to Allowable wind speeds that requires a 1.6 factor (ASCE7-05, 2.3.2), requires dividing by a factor of 1.6^(1/2), which is equal to 0.625^(1/2), not 0.6^(1/2).

This may seem relatively minor, and it typically is, but many MANY pieces of software and other codes use this same wrong equation, creating UNCONSERVATIVE situations where the actual code-specified wind speed is greater than the equation provides.

 

[phamENG]

I have to disagree with you on this.

V in codes using ASCE 7-10 and later is the ultimate/basic wind speed based on the mean recurrence interval associated with the risk category of the building. The wind load calculated with it is used with ASD factor 0.6 and LRFD factor 1.0

V in codes using ASCE 7-05 and earlier is the basic wind speed based on a 50-year mean recurrence interval and is adjusted using an importance factor based on the risk category of the building. The wind load calculated with it is used with an ASD factor of 1.0 and an LRFD factor of 1.6.

V[sub]asd[/sub] is only used in conjunction with NAAMM FP 1001 and TIA-222. The IBC specifically says that you only use it when using Exceptions 4 or 5 of 106.9.1.1.

 

[EngineeringEric]

I think you may be off Larcix. the equation IBC has is correct.

ASCE 7-10, using V_ult has an "ASD factor of 0.6W" or "LRD of 1.0W". there is no more 1.6W to get to ultimate, ultimate is 1.0 now... when using the ultimate wind that is.
the conversion is based on 0.00256*K*V_baisic^2 = 0.6 0.00256*K*V_ult^2 => V_baisic^2 = 0.6*V_ult^2 => V_basic = sqrt(0.6)*V_ult.

And as usual, PhamEng made it sound better!

 

[Aesur]

I agree with phamENG - I had this come up with a PEMB manufacturer recently (our drawings used one method, theirs the other) and had to explain to them that basically there are two ways to calculate ASD equivalent depending on if you use ASCE or IBC and that since wind design is based on ultimate with the most recent codes, it shouldn't actually change their design. In the end, if you take 90% of the US, you only see about 2 mph difference in the ASD wind speeds which imo is negligible since ASD level is rarely used with newer load combinations.

 

[larcix]

I'm not entirely sure what you mean by you "disagree" as I am simply noting that 1/1.6 is not 0.6, but 0.625. This is a mathematical statement, and it will alter the results from that equation.
(NOTE: The 0.6 ASD LOAD factor you discussed is a factor applied to actual force values, like 1000lb*0.6 = 600lb; this is similar to the ASCE7-05 1.6 wind load factor to bring those allowable loads up to ULT. The wind SPEED factor of sqrt(1/1.6) is just taking out that baked-in factor from originally translating ASD to ULT)

Let me give you a specific example where this comes into play:
tnxTower uses modern codes to analyze telecommunication structures, but it allows for calculations to be completed in previous code standards (this is very useful, some jurisdictions are even still using old standards, other times you need to recreate results from previous reports, etc.). If you input allowable wind speeds directly, you will have accurate results, but, the software also allows you to input ultimate wind speeds and let the software factor it down per that IBC equation. This will give you the wrong wind speed, off by quite a bit.

This is not just my opinion, my office has had to deal with this discrepancy many times. We've had to explain to contractors and owners why this value is wrong or different from previous reports and calculations, as this equation is commonly used. Thus, this post.

 

[larcix]

@EngineeringEric
My same reply, the 0.6 LOAD factor from ULT wind loads to ASD wind loads is not the same as the 1/1.6 factor (don't forget the square root) that needs to be pulled out of the wind speeds to complete a back-conversion, for those speeds to be used in ASD calculations.

 

[phamENG]

I don't disagree that 1/1.6 does not equal 0.6, I disagree that it matters.

V[sub]asd[/sub] is not meant to convert 7-10 wind loads back to 7-05 wind loads. If you want to use wind speeds from a previous standard, you use those speeds. It's really easy to get them. The maps are written with different statistical and reliability goals AND with different data sets. There is no conversion from one to the other.

Unless I'm mistaken, V[sub]asd[/sub] is actually a fastest mile wind speed and not a 3s gust at all. If you look at the 2009 IBC (the last edition to reference 7-05), you'll see a very similar conversion chart from 50 year MRI basic wind speeds back to fastest mile. Looking at a city I do a lot of work in, we're in a 120mph wind zone for 7-10 (call it V[sub]7-10[/sub]. Using the the sqrt 0.6 equation, V[sub]asd[/sub]=93mph. Then if you go back to 7-05, V[sub]7-05[/sub]=110mph. V[sub]fm[/sub]=95mph from Table 1609.3.1 in IBC 2009.

So you can see that V[sub]asd[/sub] does not equal V[sub]7-05[/sub] even if we used .625 instead of .6 (and I'm pretty sure this is what you're trying to equate), V[sub]asd[/sub] is very close to V[sub]fm[/sub].

Or am I completely missing the mark on what you're trying to convey?

 

[JStephen]

ASCE 7-05 uses 3-second gust wind speeds, but yes, they have tweaked the basic wind maps themselves, in addition to the 1.6 change.

 

[dauwerda]

This is trying to convert 1.0 Ultimate wind speeds to the 1.6 factored Allowable wind speed. This equation is easily derived from the calculation of wind pressure, which includes a V^2 factor. So, all other things being equal, converting 1.0 Ultimate wind speeds to Allowable wind speeds that requires a 1.6 factor (ASCE7-05, 2.3.2), requires dividing by a factor of 1.6^(1/2), which is equal to 0.625^(1/2), not 0.6^(1/2).

This is not the intent of the IBC provision. The conversion is not attempting to convert it to a wind speed used in previous codes (where a 1.6 factor was used), it is converting it to an equivalent "asd" wind speed following the current code provisions. The commentary makes it pretty clear that this is what the intent is (I am referencing the 2012 IBC as that is the most recent version I have access to the commentary in):
"This conversion equation is the result of the wind load being proportional to the square of the velocity pressure and the ASD wind load being 0.6 times the strength level wind load."

The commentary goes on to say:
"It should be noted that the term "basic wind speed" remains in ASCE 7, but it corresponds to the "ultimate design wind speed" in the code. For a comparison of ASCE 7-93 fastest-mile wind speeds to ASCE 7-10 3-second gust wind speeds, refer to Table C26.5-6 in the ASCE 7-10"

Here is a snip of the referenced table, which does use the 1.6[sup]1/2[/sup] conversion as it is actually showing the conversion between code editions.

 

[Deker]

As others have said, I'm not sure this amounts to a being a real issue. The wind speeds from each edition of ASCE 7 are meant to be used with the respective load combinations from that edition. You should not be taking an ASCE 7-16 ultimate wind speed, converting it to ASD, and then factoring it back up using ASCE 7-05 LRFD load combinations. And even if you did, I think you're overstating any unconservatism that may result as the two methods produce values within 4% of each other.

Edit: Looks like a few others beat me to the punch .

 

[phamENG]

Hmmm...I see now that I was mistaken about the fastest mile comparison. Seems that's just an interesting quirk in my area, and possibly others along the coast due to the upgraded hurricane modeling used for 7-10 and 7-16.

 

[larcix]

Thank you all for the replies. I believe that dauwerda was the most helpful here. The funny part is, if that equation were to be used for what I've seen it used for, it would be wrong. But as you correctly point out, it's not SUPPOSED to be used for that, but it is.

This all comes back to what, exactly, are the nominal, ultimate, and basic wind speeds, the fastest mile, 3-s sgust, and all the other names for wind speeds that have been defined.

The equation can be used INSIDE ASCE7-10 or ASC7-16 for references to an ASD wind speed, but should never used to calculate wind speeds for use outside of that specific code. But people do this. I guess that is the crux of my post. I'll edit my OP.

 

[Settingsun]

Is 4% in highly approximate wind load a huge issue, but using thrice superseded codes not?

 

[phamENG]

But as you correctly point out, it's not SUPPOSED to be used for that, but it is...I guess that is the crux of my post.

I certainly agree with that. I think if anything is wrong, it's that the IBC uses "V[sub]asd[/sub]" as the symbol. I think it leads to a lot of confusion. V[sub]nom[/sub] might work better, as it wouldn't draw people to it quite the same way and they may look into what it's actually used for - which is an incredibly narrow scope (flag poles and antennas, and even those standards appear to have been updated to use wind design in line with ASCE 7-10 and -16).

 

[Aesur]

This all comes back to what, exactly, are the nominal, ultimate, and basic wind speeds, the fastest mile, 3-s sgust, and all the other names for wind speeds that have been defined.

Funny enough, Nominal = ultimate = basic = 3 second in newer codes and nominal was ASD in older codes as that is the level of wind that was used in the combinations/calculations. Very redundant and the reason I don't design anything using Vasd (unless still using older codes). My understanding of the word "nominal" is the force or load given by the code, so for 7-10 and newer for wind, this is Vult. Additionally, figure 26.5-1A of ASCE 7-16 (as an example) is the "basic wind speeds" for "nominal design 3-s gust speeds" as noted in note 1. I only give Vasd because IBC says I have too.. I'm not sure what if anything it is actually used for in buildings since everyone should be using ultimate.