ASCE 7 Wind Load Terminology

[JAE]

For some years the "standard" wind speed for most of the US has been 90 mph.

This wind speed has essentially become sort of a standard vernacular for building design.
Despite the wind speed going to 115 mph for the central non-coastal US, with the advent of the ASCE 7-10 and the revised wind speed maps, it seems that there is still a prevalent use of 90.

The 90 mph number shows up on various websites for municipalities, counties, etc. as a design requirement.
I've seen city websites where the current code is IBC 2012 (and ASCE 7-10) but the city "law" states that 90 mph is to be used...totally contradictory. Or is it?

I initially put this off as just some city employees not understanding the newer codes, but then got to wondering if the 115 mph wind "design speed" was really still just a 90 mph wind with some safety factor/importance factor added in.

So should we still be speaking in terms of 90 mph when discussing this with non-engineer types, media, etc. or is it now wrong to use 90 and 115 is the correct value?
I can see a lot of people getting confused over the "change" from 90 to 115. Is the 115 mph really based on a 90 mph 3 second gust wind and is it OK to refer to them interchangeably?



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[Shu Jiang PEng SE]

The wind design speed bumps to 115mph from 90mph in ASCE7-10. Also you can see that the wind load combination factor is modified in the ASCE7-10. I do not see much differences in wind load if both the changes of design speed and load combination factor are considered.

 

[structSU10]

As I understand it ASCE just built in the importance factors and safety factors into the wind maps to make wind get treated similar to seismic - at a 'strength' level. It does just add more confusion than anything. I guess they just needed to find something to revise for the next code cycle, as besides some added clarifications, the design loads are generally the same.

 

[MotorCity]

I am guessing that we will still need to reference the 90 mph wind speed when speaking to non-structural engineers since they probably are not aware of the change in ASCE 7. Heck, most lay people probably don't even know that ASCE 7 exists.
I can see a scenario where you might mention in a conversation that the design wind speed has increased to 115 mph to an owner and they immediately sound the alarm that their building is in danger.

 

[WARose]

Funny this comes up because I was just explaining it to a customer today. (Hoping I understood it myself.) He was curious why he was in a 90 mph wind zone all these years and now he's in 115.

I refer to the 115 mph (or whatever) wind speed with the qualifier "ultimate". That (sometimes) cuts down on the confusion. For people who stay confused I will figure it at ASD level and quote it that way.

 

[hawkaz]

It used to be 70 mph in the UBC days.
I agree- this can get very confusing to they layman. 115 mph (Ult) is roughly equivalent to 90 mph (ADS) 3-sec gust, which is roughly equivalent to 70 mph fastest-mile wind speed

And- don't forget that if you are in a different risk category, 115 mph could be 105 mph or 120 mph (Ult)

As WARose noted, defining ULT or ASD is critical now.

 

[JLNJ]

Add to it the FM Global Data Sheet calculations where they use service pressures to determine required roof rating and you have a bit of a mess.

The industry will have to move off of talking in terms of service pressures, but it will take a while. I still have vendors asking me, "What seismic zone is this in?"

 

[wannabeSE]

JAE,
The IBC requires construction documents to include both, the "Ultimate Design Wind Speed," V_ult and the "Nominal Design Wind Speed," V_asd (2015 §1603.1.4). Pick your poison when talking to layman. It might be easiest to call it a 90 mph nominal wind speed with the ASD values.

 

[dianium500]

I will never understand why ASCE 7-10 decided to go this route. I live in Florida and I have clients ask me if they can stay in their homes during a CAT 4/5 storm because their plans list the wind speed (in my area it's 150 mph ult or 116 mph ASD), at which point I have to explain LRFD vs ASD. They don't need to have access to the ASCE 7 to see the general design notes in their drawings.

ASCE should have left the wind speed alone and down grade it as needed. Let the engineers factor the loads when we are designing like w/ everything else.

 

[dozer]

ASCE's need to be more scientific or rational or whatever has just lead to confusion, especially among lay people but even among engineers. I work in a multi-discipline office so I often try to explain to mechanical or electrical or chemical engineers the difference between LRFD and ASD. After I watch their eyes roll back in their head I wonder why we had to make it so difficult. I know, I know, LRFD is the greatest thing since sliced bread. Trouble is, I don't think anybody ever thought about the pipe stress engineer who suddenly is told his pipe that used to have to withstand a 90 mph wind now has to handle a 115 mph. Panicked, he designs beefier pipes not realizing that what really changed is the mean recurrence interval, so you get a use a factor in the combinations to bring everything back to being more or less the same.

It's got to the point that when people ask me what the wind speed is for a certain locality I just shake my head and say, "I have no idea." But seriously, I say, "Using what code?" I have given up trying to explain code wind speeds to folks. But I do tell them to just be sure the wind speed they are using goes along with the code they are using. Better yet, just tell me where your structure or piece of equipment or whatever is and what code you want it to conform to and I'll do the rest.

 

[WinelandV]

Trouble is, I don't think anybody ever thought about the pipe stress engineer who suddenly is told his pipe that used to have to withstand a 90 mph wind now has to handle a 115 mph.

In the ASCE's defense, the ASCE 7 document is "Minimum Design Loads for Buildings and Other Structures", and it appears that the building portion is what they are most interested in.

To the main topic, I've had the most trouble with wind terminology when dealing with other engineers. For so long for wind, "un-factored" meant "service level" (like with dead, live and snow loads). So, I will see reactions for wind called out as "un-factored reactions", I always send off an email to determine if they mean that they used a LF = 1.0, or if they actually meant "service level". This is why I don't specify factored/un-factored anymore. In my calcs I specify "service" or "ultimate", and for any reaction reporting I just state what load factor I used (whether 0.6 or 1.0).

As far as the "what's the design wind speed" question, I always answer, "it depends."

 

[JedClampett]

It's just like when AISC went from all ASD to all LRFD to the current ASD/LRFD mix. The goal is trying to use the same load combinations for all materials. Until you get to the subgrade, then it's all ASD, all the time.
We've got codes in our office (1982 UBC) that show the design windspeed as 70 mph. Somehow engineers explained the change from 70 mph to 90 mph without tearing down buildings. We'll get by.

 

[UcfSE]

I always reference the new wind speed when talking with anyone. If they balk at it, I explain the results haven't changed much but the terminology did, and we can talk in more detail from there if the person is interested in more explanation. It's not like we're going to go back to the old speeds, so I don't see any point in prolonging the use of them.