IBC vs. ASCE (7-10 to 7-05 vs Ult to ASD)

[wb.c]

From IBC 2012:

From Guide to the wind Load Provisions of ASCE 7-10:

From ASCE 7-10 Commentary:

The difference between sqrt(1.6) and sqrt(0.6) is something that a lot of engineers seem to accept as an acceptable 2% difference, why make a big deal?. Fair enough, the calculated values are very close.
However, insignificant as this difference might seem, the reason behind it should not be a mystery.

On the one hand, IBC makes it clear that the conversion they specify is to be only used when using the provisions of specified standards. Standards that directly refer to ASCE 7-05 basic wind speed. For example, TIA-222 has an exact copy of the ASCE 7-05 basic wind speed map in their code, and they also use a 1.6 factor on the wind. So, based on this one example, I would conclude that IBC is attempting to convert the ultimate wind speeds to allowable wind speeds (ASCE 7-10 to ASCE 7-05). Seems clear enough. This conversion is done by multiplying Vult by sqrt(0.6), according to the IBC.

Now, on the other hand, ASCE 7 is using sqrt(1.6) and they fortunately provide some additional explanations for why sqrt(1.6) is used. The 1.6 comes from the load factor for strength design in ASCE 7-05 and is the wind load ratio between 7-10 and 7-05.
The ASCE 7-10 Wind Guide makes it clear that designs using 7-10 Fig. 26.5-1A wind and designs using 7-05 Fig 6.1 are the same, assuming we are talking about strength design and each uses the corresponding load factors.

By this logic, 1/sqrt(0.6) = 0.36+0.1ln(12T) gives a value of T = 920, a little bit more than the 709 we get with sqrt(1.6).

So, why does IBC 2012 use the sqrt(0.6) and ASCE 7 use sqrt(1.6)?

Per ASCE 7-10:
1.0W_(7-10)= 1.6W_(7-05)
Since W=V²
1.0V²_(7-10) = 1.6V²_(7-05)
√(1.0V²_(7-10)) = √(1.6V²_(7-05))
V_(7-10) = √(1.6)V_(7-05)

It looks like IBC is trying to do this:
0.6W_(7-10) = 1.0W_(7-05)
0.6V²_(7-10) = 1.0V²_(7-05)
√(0.6V²_(7-10)) = √(1.0V²_(7-05))
√(0.6)V_(7-10) = V_(7-05)

The logic being here that if you can compare 7-10_ult to 7-05_ult you should be able to compare 7-10_asd to 7-05_asd.
This assumption can be seen as a little erroneous based on the Peterka and Shahid equation shown above.

Did the IBC gang miss a memo from the ASCE 7 gang or what?

 

[271828]

That's a lot to dig through and you're looking at pretty old references.

What does ASCE 7-16, IBC 2021, and ASCE 7-22 say about this?

 

[wb.c]

Not exactly old if you consider what has been legally adopted. A lot of jurisdictions still use IBC 2008 and ASCE 7-05. In any case, IBC 2021 still has the same conversion. ASCE 7-16 has the same wind speeds as ASCE 7-10, this is noted in 7-16. 7-16 also has the same table with the same conversions. 7-16 also still directly says that you can get 7-05 wind speeds by dividing 7-16 ultimate by sqrt(1.6).

 

[Rabbit12]

A lot? I haven't seen a building department use anything that old in years and years. Where are you practicing?

 

[phamENG]

The use of 1.6 was an attempt by the ASCE 7 committee to maintain continuity in design reliability as they shifted from an ASD based wind speed. Since the importance factor in 7-05 for a Risk Category 2 building is 1.0, that's the most sensible place to make the conversion. Since the goal is to get to a factor of 1.0 in the load combination, using sqrt(1.6) to convert from MRI 50 makes sense. That gives us an equivalent MRI of 700 years. Great. But what about everything else? What about RC 1, 3, and 4 structures? That sqrt(1.6) is only a valid conversion between MRI 50 and MRI 700 wind speeds.

In 7-05, everything was based on MRI 50 with importance factors. In 7-10, each risk category (except III and IV) are based on their own MRI. This allows for a more consistent statistical reliability across designs. But it also means the sqrt(1.6) is worthless on its own. If you take the 7-10 wind speed and use your 1/sqrt(1.6) conversion, you'll find that you need to then divide those by the sqrt(I) to get back to the 50 year MRI.

But then, why would you do that? There's no need. The V_asd is essentially useless anyway. The only value I have ever found in it is, as your reference suggests, comparing old certification reports to new code requirements. You can use it to see what the old reporting method would say about current wind speeds. The references in the IBC that allow use of Vasd are very limited. Flagpoles designed to NAAMM FP 1001 and TIA-222 antenna supporting structures and antennas (sometimes).

The real issue, I think, is that they use V "asd". If anything, it should be V^50-mri or something. After all, a similar conversion existed between 7-98 and all following versions to go from the 3-s Gust back down to fastest mile for similar comparisons. And 7-16 and now 7-22 are changing a little more. Not so much in the math behind everything, but the size of the data set. With more data comes new statistical analysis comes new wind speeds for much of the country, as well as an additional map for RC 4 structures.

 

[dauwerda]

Without digging into the details, my first thought is that ASCE was providing the conversion between 7-10 and 7-05. IBC was/is not. They are providing a conversion from 7-10 Ultimate wind speeds to 7-10 ASD wind speeds - not the wind speeds that were used in previous versions of the IBC/ASCE, but "equivalent" nominal wind speeds all within the same revision.

So in short, they are doing this:

0.6W_{(7-10, ult)} = 1.0W_{(7-10, asd)}
0.6V²_{(7-10, ult)} = 1.0V²_{(7-10, asd)}
√(0.6V²_{(7-10, ult)}) = √(1.0V2_{(7-10, asd)})
√(0.6)V_{(7-10, ult)} = V_{(7-10, asd)}

not this:

It looks like IBC is trying to do this:
0.6W_(7-10) = 1.0W_(7-05)
0.6V²_(7-10) = 1.0V²_(7-05)
√(0.6V²_(7-10)) = √(1.0V²_(7-05))
√(0.6)V_(7-10) = V_(7-05)

 

[wb.c]

A lot? I haven't seen a building department use anything that old in years and years. Where are you practicing?

https://www.iccsafe.org/wp-content/uploads/Master-I-Code-Adoption-Chart-AUG-2021.pdf

Most jurisdictions lag behind on the adopted codes... but that's besides the point.

 

[wb.c]

But then, why would you do that? There's no need. The Vasd is essentially useless anyway. The only value I have ever found in it is, as your reference suggests, comparing old certification reports to new code requirements. You can use it to see what the old reporting method would say about current wind speeds. The references in the IBC that allow use of Vasd are very limited. Flagpoles designed to NAAMM FP 1001 and TIA-222 antenna supporting structures and antennas (sometimes).

Right, as ASCE 7 says, the conversion is needed when implementing a product that has been designed consistent with unfactored load effects (7-05 wind). This I assume would include the Importance factor, which is why ASCE 7 doesn't just convert V_ult to V_asd using 7-10 and beyond wind loads. With 7-10, the wind speed maps included the importance factor, which would basically mean that the speeds shown in 7-10 are not really unfactored. To me it is clear why this would be needed. I guess in theory you could convert from 7-10 Vult to 7-10 Vasd and also remove the importance factor and you might come to the same values as directly going from 7-10_ult to 7-05_asd. The reality is a lot of equipment is designed using completely unfactored wind loads. There has to be a way to connect the equipment/product design to structure design.

As for the references in IBC allowing for very limited use, I don't think IBC is limiting the requirement to use converted values only sometimes in TIA-222. Basically all antenna-supporting structures and antennas would meet this reequipment. IBC adds the requirement for the escarpment criteria, in addition to that of TIA-222.

From the IBC-2012 commentary:

So if you are doing a design with TIA-222, you SHALL convert Vult to Vasd because it is applicable to TIA-222? As already mentioned, TIA-222 has an exact copy of the 7-05 map, and uses the same factors of 1.6 and 1.0 as 7-05. So how is using what is basically 7-10_asd wind speeds applicable?

What is even more interesting is the commentary in IBC 2012 regarding the conversion from V_ult to V_asd.

As some have already noted, it seems evident that IBC was not trying to convert to 7-05 wind, although the SHALL convert seems a bit out of place, as seen in the example of TIA-222.
The IBC's V_asd will have an importance factor embedded in it, which makes no sense for TIA-222 that then uses a factor to factor the wind. You would basically end up applying I twice and maybe not even the right I that's embedded in IBC. And it's not like IBC is saying that the V_asd is only for comparison purposes. They straight up say you shall use converted values of V.

So, in the end, what is the point I want to make? I think IBC messed up. The sad part is that even the 2021 version still has the same conversion requirement.
As for me, whenever I work with unfactored design wind speeds, √(1.6) is the way to go, as long as you understand that you are going from 700yr MRI to 50yr MRI.
In the end, the 700yr MRI to 50yr MRI conversion gives you the same (single map) of ASCE 7-05, which in this case TIA-222 actually uses.

 

[phamENG]

Importance Factor and Load Combination Factored (what everyone means when they say 'factored') are two very different things. And the new maps don't really have an importance factor built in. The importance factor was a ham fisted method of recognizing the importance of those structures and the need for them to resist higher loads, but that is now handled directly through statistical analysis of the wind speed records.

If I need the 50 year MRI wind load for something, I just look it up using the ATC Hazard Tool. It's faster and cleaner than messing with approximate conversions.

Have you reached out to ICC to ask them about this? I've found that when I think a mistake has been made in the development of a procedure by a team of engineers with significantly more experience than me that has also been used by essentially the entire engineering community for over a decade without the issue coming up, I'm usually the one who's wrong. While it's always possible they did, in fact, make a mistake...I doubt it.

And, as you alluded to in your first post, the values are so close it really does not matter when you're looking at something so incredibly varied as wind load. For a lot of buildings, we could all be using the tabulated pressures from the building codes of the 1940s, save 90% of the time currently spent on wind analysis, and have buildings that are at least as strong as using current wind procedures. It's the old measure with a micrometer, mark with a crayon, cut with a nuclear bomb metaphor. Attempting to be an order of more precise than we are accurate is a bit silly, I think.

 

[wb.c]

Yeah... I guess some might be confused about the difference between load factors and wind speed factors.
After all, the load (pressure) is a function of speed.

And it is the speed that can be factored, well before we even get to a load factor, by quite a few factors (K_zt, K_d, K_e, and G) as well as a "factoring of sorts" with the various maps for different risk categories, the reasons for which are nicely documented in the commentary of ASCE 7.
Keep in mind that adjustments for risk categories are probably pointless when talking about specific product design since that is specifically related to fire and Life Safety protection.

There are two instances in my mind where converting from an ultimate 700yr MRI speed to an unfactored 50yr MRI allowable level wind are needed.
[ul]
[li]When you need to consider a product that was developed with unfactored "allowable" level wind speeds[/li]
[/ul]
[ul]
[li]When you would need to get ASCE 7-05 equivalent wind speeds[/li]
[/ul]

As for the latter, I would also use a much quicker method like the online ASCE 7 Hazard tool, which is even a step better than the maps as far as resolution.
If you are working with an ASD design, then you would just apply the load factors as specified in the combinations. Really no need to get to a V_asd.

That being said, IBC seems to mention in the commentary that it has maintained specified "basic wind" requirements in place within the code and simply noted them as V_asd.
In that case, the IBC conversion would probably be useful. That does make me wonder, if the "original" basic wind speeds in IBC were the same as ASCE 7-05, wouldn't the √(0.6) still be the wrong conversion?

This is not the first time a well implemented code has had mistakes in it, and it probably won't be the last. ASCE 7 still has mistakes in it. That is why it is important to apply engineering judgment in order to understand why something is, and not just blindly follow along. Whether there is a fundamental error or just a typo. ICC will not take on any liability, because standards and codes are just an attempt to compile the latest research, and research changes. Now this specific example with the conversion of V, is quite inconsequential, as was already mentioned by myself and others, but the fundamental point is not insignificant. Know what you know.