ASCE 7 Wind Load Terminology
ASCE 7 Wind Load Terminology
(OP)
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?
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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RE: ASCE 7 Wind Load Terminology
RE: ASCE 7 Wind Load Terminology
RE: ASCE 7 Wind Load Terminology
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.
RE: ASCE 7 Wind Load Terminology
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.
RE: ASCE 7 Wind Load Terminology
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.
RE: ASCE 7 Wind Load Terminology
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?"
RE: ASCE 7 Wind Load Terminology
The IBC requires construction documents to include both, the "Ultimate Design Wind Speed," Vult and the "Nominal Design Wind Speed," Vasd (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.
RE: ASCE 7 Wind Load Terminology
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.
RE: ASCE 7 Wind Load Terminology
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.
RE: ASCE 7 Wind Load Terminology
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."
RE: ASCE 7 Wind Load Terminology
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.
RE: ASCE 7 Wind Load Terminology